CA1137626A - Device for recording and playing back information on an endless magnetic tape - Google Patents
Device for recording and playing back information on an endless magnetic tapeInfo
- Publication number
- CA1137626A CA1137626A CA000312870A CA312870A CA1137626A CA 1137626 A CA1137626 A CA 1137626A CA 000312870 A CA000312870 A CA 000312870A CA 312870 A CA312870 A CA 312870A CA 1137626 A CA1137626 A CA 1137626A
- Authority
- CA
- Canada
- Prior art keywords
- tape
- heads
- gaps
- recording
- tracks
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/008—Recording on, or reproducing or erasing from, magnetic tapes, sheets, e.g. cards, or wires
- G11B5/00813—Recording on, or reproducing or erasing from, magnetic tapes, sheets, e.g. cards, or wires magnetic tapes
- G11B5/00817—Recording on, or reproducing or erasing from, magnetic tapes, sheets, e.g. cards, or wires magnetic tapes on longitudinal tracks only, e.g. for serpentine format recording
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B15/00—Driving, starting or stopping record carriers of filamentary or web form; Driving both such record carriers and heads; Guiding such record carriers or containers therefor; Control thereof; Control of operating function
- G11B15/18—Driving; Starting; Stopping; Arrangements for control or regulation thereof
- G11B15/1883—Driving; Starting; Stopping; Arrangements for control or regulation thereof for record carriers inside containers
- G11B15/1891—Driving; Starting; Stopping; Arrangements for control or regulation thereof for record carriers inside containers the record carrier being endless
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/008—Recording on, or reproducing or erasing from, magnetic tapes, sheets, e.g. cards, or wires
- G11B5/00813—Recording on, or reproducing or erasing from, magnetic tapes, sheets, e.g. cards, or wires magnetic tapes
- G11B5/00817—Recording on, or reproducing or erasing from, magnetic tapes, sheets, e.g. cards, or wires magnetic tapes on longitudinal tracks only, e.g. for serpentine format recording
- G11B5/00821—Recording on, or reproducing or erasing from, magnetic tapes, sheets, e.g. cards, or wires magnetic tapes on longitudinal tracks only, e.g. for serpentine format recording using stationary heads
- G11B5/00826—Recording on, or reproducing or erasing from, magnetic tapes, sheets, e.g. cards, or wires magnetic tapes on longitudinal tracks only, e.g. for serpentine format recording using stationary heads comprising a plurality of single poles or gaps or groups thereof operative at the same time
- G11B5/0083—Recording on, or reproducing or erasing from, magnetic tapes, sheets, e.g. cards, or wires magnetic tapes on longitudinal tracks only, e.g. for serpentine format recording using stationary heads comprising a plurality of single poles or gaps or groups thereof operative at the same time for parallel information processing, e.g. PCM recording
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/48—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
- G11B5/488—Disposition of heads
- G11B5/4893—Disposition of heads relative to moving tape
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/48—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
- G11B5/52—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with simultaneous movement of head and record carrier, e.g. rotation of head
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B5/00—Recording by magnetisation or demagnetisation of a record carrier; Reproducing by magnetic means; Record carriers therefor
- G11B5/48—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed
- G11B5/58—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following
- G11B5/584—Disposition or mounting of heads or head supports relative to record carriers ; arrangements of heads, e.g. for scanning the record carrier to increase the relative speed with provision for moving the head for the purpose of maintaining alignment of the head relative to the record carrier during transducing operation, e.g. to compensate for surface irregularities of the latter or for track following for track following on tapes
Abstract
A DEVICE FOR RECORDING AND PLAYING BACK INFORMATION ON AN
ENDLESS MAGNETIC TAPE.
ABSTRACT OF THE DISCLOSURE:
A device for recording and playing back information comprising an endless tape whereof one part has a transla-tional movement in a direction x and n magnetic heads integral with one another whereof the gaps face that part and are aligned in a transverse direction y. The heads have a slow movement in the direction y and form n continuous tracks forming parallel and equidistant portions in the direction y.
ENDLESS MAGNETIC TAPE.
ABSTRACT OF THE DISCLOSURE:
A device for recording and playing back information comprising an endless tape whereof one part has a transla-tional movement in a direction x and n magnetic heads integral with one another whereof the gaps face that part and are aligned in a transverse direction y. The heads have a slow movement in the direction y and form n continuous tracks forming parallel and equidistant portions in the direction y.
Description
~3~6~6 1 The present invention relates to devices for recording and playing back information on a magnetic tape by virtue of at least one magnetic head whereof the gap faces the tape, this head having a continuou~ movement relative to the tape.
Various types of video tape recorders~ generally comprising a plurality of magnetic heads, are used in order to record signals in the video-~requency range In order to obtain the desired pass-band without too many heads, the relative spee~ between the tape and the heads m~lst be very high. The solution generall~ used resides in imparting a rapid rotary movement to the heads combined with a translation of the tape. In video tape recorders of the "quadruplex" type, l~ magnetic heads are arranged on a drum rotating at high speed with a rotational axis parallel to the direction of translation of the tape, Their arrangement on the drum gives rise to portions of track paralle] to the transverse direction of the tape.
In video tape recorders of the "helical-scan" type, the tape i5 arranged so as to form a loop round the record-pl~yback member. A magnetic head rotates with respect tothis loop with an axis of rotation sllghtl~ off-centre with respect to the centre of the loop, thus formlng on the tape as it is unwound a helical track having parallel portions slanting with respect to the transverse directinn.
~'s~
~L~3~
1 In both cases the track obtained is discontinuous, and forms parallel portions. These two types of video-tape recorders require complex, heavy and expensive mechanical elements. The price of the "quadruplex" video tape recorder limits it to professional applications. Video tape recorders of the "helical-scan" type are less expensive~ but require complicated servo-controls since centring is critical~
An object of the present invention is to provide a simpler, less fragile and less expensive record-playbacX
device capable of forming part of a reporting unit which is easily transportable by virtue of lts low wei~ht. For this purpose, the device according to the invention enables rotary movement of the head to be avoided by using, instead of a magnetic carrier in the form of a ribbon transferred from a feed spool to a take-up spool~ a loop known as an "endless tape" such as that usually used for sound recor-ding. In order to use all the available width of the tape, a slow movement is imparted ~o the magnetic heads in a direction transverse to the tape, while the tape moves past indefinitely in the longitudinal direction. The slow movement is easily obtained by simple mechanical means~ and may be regulated above all during playback by a likewise simple servo-control loop in order to ensure the tracking.
The tracking of the tape by the magnetic heads is continuous, which eliminates the problems o~ synchronisat~on. The device is thus appreciably less sensitive to faults in the tape due to stretching ~or example, and to any possible vibration Finally, a variant of embodiment enables crosstalk between tracks to be considerably reduced, and the tracking servo-~3~3~6~6 control to be simplified by virtue of angular offset of the record~-playback gaps.
According to the present invention, there is provided a video tape recording and playback device comprising an end-less magnetic tape, magnetic head means having at least two gaps stacked across the width of said tape, tape driving means for continuously moving said tape past said magnetic head means, and head moving means for laterally moving said magnetic head means in a direction perpendicular to the direction of trans-port of said tape; said gaps having axes angularly offset by equal positive and negative values in relation to said direction;
said gaps recording along said tape a timing signal on two equally spaced tracks; said magnetic head means supplying two playback signals upon reading out said tracks with two gaps in azimutal alignment with said gaps; phase comparator means being provided for supplying in response to said playback signals a control voltage corresponding to the phase shift between said playback signals; said control voltage being fed back to a control input of said head moving means for cancel-ling out at playback any lateral offset of said gaps in rela-tion with said tracks.
Other features and advantages of the invention will appear by means of the following description and the appended Figures, wherein : I
- Fig. 1 shows a case containing an "endless tape";
- Fig. 2 and 3 show two embodiments of a s-tack of 4 magnetic heads;
- Fig~ 4 and 5 diagrammatlcally illustrate 2 ways of using the stack of heads accordiny to the invention;
- Fig. 6 shows a recording device according to the invention;
- Fig. 8 shows a variant in the arrangement of the 7~
record-playback gaps;
- Fiy. 9 is an explanatory diagram;
- Fig. 10 is a diagram of servo-control loop used :x with the variant of Fig. 8;
- Fig. 11 shows an embodiment of an information coder used for the invention;
- Fig. 12 is a set of chronograms illustrating the operating of the coder. r Figures 1 and 4 are disposed on the first sheet oE
the drawlngs, figures 2, 3, 5 and 3 on the second sheet oE the drawings, figures ~ and 7 on the third sheet o~ the drawings and figures 9 to 12 on the last sheet of the drawings.
Figure 1 illustrates the interior of a case contain-ing an endless tape. The magnetic tape 6 is woundaround a spool 13 having a rotational axis z. Its two ends have been joined in order to form an endless loop: the part 61 emerging from the interior of the winding is guideci by a series of rollers 9, 11, 12, and rejoins the part 62 situated outside the wind-ing. A roller ~ connected to a driving motor causes the tape to move past, and the tape winds on outside at the same time as it unwinds from the inside, which is made possible by the fact that the wound-on turns slide with respect to one another and that the feed zone is situated inside the winding. Record-ing-playback means 5 are provided in front of that part 63 of the fraction of tape 6 which is moving past between the rollers 11 and 12. Such a case is usually intended for recording sound.
It is then used at a speed of 10 cm/s. The length L of the tape is equal to 150 m, its wîdth 1 is equal to 6.3 mm for a "quarter-inch" tape, 12.6 mm for a "half-inch" tape, etc In altogether unexpected fashion, experience shows that, in spite of friction, a speed much higher than 10 cm/s may be reached without either breaking the tape or spoiling the "
~37~ 6 recorded signal. For example, the tape retains good quality of reproduction at the end of about a hundred hours of operation at a speed of 2.5 m/s. The invention provides for using this tape at the speed of 2.5 m/s for the purpose of recording signals having a wide frequency-band, such as video signals. At such a speed, a 9~376~6 1 signal having a pass-band of 2 Ml~z may be recorded with a signal-to-noise ratio of about 35 dB, using a magnetic head T~hereo~ the gap-height is about 50/um. The use of two heads enables a video signal o~ moderate quality to be recorded on two contiguous tracks, one of the tracks being reserved for luminance, and the other for chrominance and sound. The use of a stack of eight heads makes it possible to record on eight separate tracks a television signal having a pass-band of 16 MHz split up into eight components by a coding device. These two examples do not limit the invention. In the absence of any indication to the contra-ry, in the remainder of ~he description the recording-playb~ck means 5 will consist of a s~ack of 4 magnetic heads describing 4 separate tracks.
Referring to Fig. 2~ a group of four magnetic heads 100, 200, 300, 400 are stacked in the direction of the height of their respective gaps 1, 2, ~, 4. Each head comprises a winding, one only, 101, being visible in the Figure. For recording, a signal emanating, as will be seen later~ from a coding device having 4 ou~put channels is suppIied between the terminals of each winding. ~or playback, signals S1, S2, S3~ S4 respectively characterl-sing the items of information written in onto the 4 tracks and read out by the 4 heads 100, 200, 300, 400 are picked up at the terminals of their winding. The distance between the tracks recorded or read out depends on the height h of the heads9 which may be slightly greater than the height ~ o~ the gaps, which itself det,ermines the width of the tracks. The height ~ of the gaps may be around 1 ~0 to 50 /um and their a width ~ about 0O5 /um, It is nevertheless critical to obtain total heights h small enough, If it is desired that the tracks described by the 4 heads be very close together (about 50 /um apart), it win there~ore be preferable to use integrated magnetic heads such as that illustrated in Fig. 3. Conventional masking techniques are used in order to deposit on a substrate ~02 made of a non-conductive and non-magnetic material a layer of magnetic material 105 in the form of a ring intersected by a gap 1. The winding 101 of the head is obtained by de-positing conductive turns~ for example o~ copper, before and after depositing the layer 105~ The two ends of the winding are joined to two output terminals 103 and 104 at which the signal s1 is supplied or picked up. The desired gap-heights are thus very easily obtained. Similar heads having the desired total height h may be stacked.
Fig, 4 diagrammatically shows one way of using the stack o~ heads 5 according to the invention. For better understanding, the casing containing the stack of heads 5 has illustrated on it the re~pective gaps 1~ 2, 3~ 4 of the heads 100, 200, ~00, 400, these latter being embo-died in accordance with one of the two forms of Figs. 2 and 3. As the tape 6 is unwound in the direction x, each head tracks a track havlng a width A which will be consi-dered to be slightly equal to h. The portions 1OJ 20, 30, 40 respectively tracked by the gaps 1, 2, 3, 4 are there-~ore contiguous, and occupy a wldth H = ~xh of the tape 6.
The value of H ls much less than the total width l o~ the track. This is why the invention provides for lmparting to ~37~
1 the stack of heads 5 a transverse translational movement, in a direction y perpendicular to x in the plane of the tape 6, so that, if the total length of the tape were unwound and the tape were placed round a cylinder, four overlapping helical tracks would be obtained. For this purpose, the stack of heads 5 is mechanically connected to a motor 7J and its translational speed is such that, at the end of the unwinding of a complete length o~ tape, the stack 5 is in a position 5a at a distance from the initial position which ls greater than or equal to H. The tracked portions lOa, 20a, 30a, 40a follow in the direction y the portions 10, 20, 30, 40 tracked on the preceding revolu-tion. It is desirable to make maximum use of the width of the tape, and it is therefore preferable to choose a heli-cal pitch exactly equal to H = 4x h Under these conditions~the number of parallel portions of tracks following one another with a pitch h in the width 1 o~ the tape is , which is obtained at the end of ~h times the complete unwinding of the tape. V being the speed at which the tape moves in the direction x~ and v the transverse speed of the stack 5 ir the direction y, with~ by way of example h = 50 /um, L = 150 m, V = 2.5 m/s, each complete unwinding of the tape lasts 1 mm~ and v = 0.2 mm/mn. Signals having a pass~band of 2 MHz may be recorded on a tape having a width l of 12.6 mm along 240 parallel portions of track by making it run over its whole length 60 times if there are 4 heads~ and 30 times if there are 8 heads~ the recording then lasting for hal~ an hour. The product :
"duration x pass-band" of the device according to the in-~376~i;
1 invention is 8 hours x MHz for the chosen values of ~, l, V,This results in a device of high performance with respect to existing magnetic v-ldeo tape recorders with simple, cheap and strong mechanical means : cases containing a loop of magnetic tape which winds in endless fashion, a stack of7 heads 5, a motor 8 equipped with a roller associated with the cartridge and a motor 7 providing very slow feed and associated with the stack 5.
Fig. 5 illustrates a variant of embodiment in which the heads are not juxtaposed with one another as in Fig. 2, but are separated by a distance h, while remaining integral with one another. This arrangement enables non-integrated heads to be used while preserving the same track-pitch as ~70re, the movement of the stack of heads being the same.
; 15 Only the arrangement of the tracks on the tape is modified7 and i~ may be seen in the Fig. that the distance h separa-ting the gaps 1, 2, ~J ~ iS about equal to a quarter of the width l of7 the tape ~. The translational speed v o~ the stack 5 is such that, at the end of a revolution, each portion of track 107 20, ~0, 40 is shifted with a value at least equal to the head height h. The speed v is th~7r~-fore 4 times less great than before if7 the same track-pitch h is maintained. 4 helices of7 a pitch h are thus obtained, being independent instead of overlapping one another. Recording stops when the last portion of track followed by a gap (1 for example) is at a distance h from ~he first portion of track followed by the followlng gap (that is to say the gap 2). The stack of heads 5 will then have moved forwards by a distance hl, while in the 7~26 foregoing case it had moved forwards 4 times more quickly by a distance 1 = 4h1. Playback and recording must naturally be carried out with the same stack of heads in order to respect the arrangement of the tracks. ~he total number of portions of track is identical with the foregoing case, as is the duration of the recording.
A device such as that shown diagrammatically in Fig.
1 and one of Figs. 4 and 5 could be used without any other element for recording and then playing back information via a magnetic tape. However, because of instabilities of various types due either to the mechanical means or to the tape oscillating, it proves in practice to be necessary to supple-ment it with servo~control loops, enabling the conditions set out above relating to the speeds v and V to be respected on recording, and the tracks wri-tten in to be satisfactorily followed at the desired speed on playback in order to recon-stitute the information with as few errors as possible.
Referring to Fig. 6, an electrical signal S charac~
terising the information to be recorded is split up into n components ~n = 4 in the Fig.) by means of a coder 15 which will be detailed hereinafter. This coder makes available 4 signals Sl, S2, S3, S4 whereof the pass-band is compatible with the possibilities of the device (dimensions of recording heads 5 and speed of the tape 6). Each of these signals may according to circumstances consist of a carrier frequencyt modulated by the information or of a numerical signal. The 4 heads which make up the stack of heads 5 are respectively supplied by -the 4 signals for the ---~
.
....
~,~,' 10 ~7~
1 recording on 1~ tracks on the tape 6 in the form of modifi-ca~ions ~o the magnetic ch~racteristics of the tape The motor 8 drivin~ the tape 6 at the speed V and the motor 7 translating the stack of heads 5 at the speed v have respec-tively regulators ~8 and 37 controlled by a synchronisingsignals R8 and R7. The signal R8 emanates from an oscilla-tor 34 whereof the frequency f is so chosen as to obtain the desired speed V The signal R7 emanates from a frequency-divider 35 receiving the signal R8 and having a division ratio so chosen as to obtain the desired speed v The speed v must be very accurate in order to avoid any track over-laps.
Fig. ~f illustrates a device for playing back a magnetic tape 6, recorded by the dev:ice of Fig. 6. Playback requires much greater accuracy than recording. The playback device is provided with a servo-control of the radial position of the stac~ of heads by means of a signal characterising the radial trackino error, and a "longitudi-nal" servo-control by monitoring the tape speed with synchronising pulses recorded onto the tape itself upon recording. The operation of the device of Fig. 7 implies that the information signal S, a television signal for examp-e, comprises high-frequency synchronising pulses.
The line pulses, which have a frequency of 15625 Hz in the European televlsion standard and a ~requency o~ 15750 Hz in the U.S. standard, may be used for this purpose, the information being coded in the coder 15 in such a manner that these pulses may subsist in at least one Or the signals S1 to S4, S1 for example. Each head of the sta~k ~3762~
1 Of heads 5 delivers a signal (~1 to Sl~) which is a function of the magnetic characteristics of the portions of tape tracked by the heads. The l~ signals are processed by a decoder 150, which decodes in a manner corresponding to the coding carried out by the coder 15g for the purpose of delivering a signal S. A decoder 22 extracts the line pulses from the signal S1 amplified by an amplifier 1~ and a phase-comparator ~4 delivers an error signal ~T propor-tional to the phase-difference found between ~hese line pulses and reference pulses delivered by an oscilla~or 23 at the line-frequency. The error signal ~V enables the oscillator 34 described in reference with Fig. 6 to be controlled. This makes the translational speed V of the tape on playback equal to the speed on recording. The 5 radial servo-control of the stack of heads 5 is based on a wobble process : a low-amplitude transverse oscillation is imparted to ~he stack 5 at a frequency f above the frequency-band of the trackin~ errors whic~ i~ is desired to correct and below the frequency-band of the signals S1 to S4. For this purpose, the stack of heads 5 is made integral with the moving coil 171 of an electrodynamic motor 17 of the loudspeaker drive type whereof the base 172 is mechanicallY connected to the radial feed motor 7.
The moving coil 171 is moved by a signal supplying the terminals 17~ and 17~ of the moving coil and resulting from the sum, provided by a summation amplifier 25, of a signal ~ having a frequency f delivered b~ an oscillator 27 and an error signal av characterising the tracklng error. The periodic excurslon and radial correction are ~37~26 1 thus carried out by the same element 17. A group 151 of high-pass filters enables the component at the frequency f to be eliminated from the signals S1 to S~ before they are processed by the decoder 150. The servo-control loop com-prises : an envelope-detector 19 receiving the signal S1 after it has been amplified by the amplifier 1~, the oscil-lator 27 providing the signal ~ and the signal emanating from the detector 19, a low-pass filter 28 eliminating from the product signal the components at frequencies greater than or equal to f and supplying the error signal v, and the summator 25 referred to above.
~ hen the tracking made by the magnetic head 100 which provides the signal S1 is satisfactory, the oscillation at the frequency f set up in a direction radial to the track produces amplitude modulation of the signal S1 at the frequency 2f. IndeedJ the read out signal is maximal at the centre of the portion of track, and decreases when thls ~;~
centre is departed from. When the mean position of the head 100 departs from this centre by a value of a h, modulation of the signal S~ at the frequency f is added to the modu-lation at a frequency of 2f. The modulation level at the frequency 2f increases, while the modulation level at the frequency 2f decreases, when ~ h in~reases. Moreover, the phase of the modulation (positive or negative) depends on the polarlty of the error ~ h. After multiplication by the reference signal and filtering of the components at frequen-cies o~ f, 2f, ~f, the signal p v is obtained, of suitable amplitude and polarity to control the motor 17 and preserve tracking. A low-pass filter 26 extra~ts the do c ~ component ~ ' .
~37626 1 from the error signal ~ or from the signal ~ and controls the motor 7 in order to impart regular forward feed to the unit consisting of the motor 17 and the stack of heads 5.
It is clearly understood that the recording device illus-trated in Fig. 6 and the playback device illustrated in Fig. 7 are only examples of embodiment which do not limit the invention. The latter allows of numerous variants, more particularly of the servo-control loops of the play-back device. For the recording device, a variant of Fig. 6 ~0 resides in introducing radial servo-control of the position of the stack of heads 5 with respect to a portion o~ track already recorded. A magnetlc playback head is provided for this purpose o if n heads are required for recording, the stack of heads 5 comprises n ~ 1 heads, one of them being a playbac~ head (for example, referring to Fig. 4J the head having the gap 1). The speed v of radial feed is provided so that the playback head will follow previously recorded a track so that the portion of track being followed at each instant by the playback head w-lll coinci-de with the portion of track followed on the previousrevolution of the winding of tape by ~he recording head situated a'c the other end of the stack 5, that is to say, referring to Fig. 4, the head having the gap 4. This result is obtained when the displacement of the stack Or heads 5 during a complete unwind of the tape is equal to n times the track pitch h. The playback head delivers a signal corresponding to a previousl~ recorded information component Thls signal is used in ~he same manner as the signal S1 in the playback device of Fig. 7 for the purpose of servo-controlling the position of the stack of heads 5 with respect to the last recorded track. The servo-control loop may be the same as in Fig. 7, but not necessa-rilly so. This variant imparts greater regularity to the tracks. Playback is facilitated, and the servo-control loops of the playback device may be simplified. In particular, the risk of track overlap upsetting playback is avoided.
Without departing from the scope of the invention it is also possible to use a moving-coil electrodynamic motor capable of displacing the heads over the whole width of the tape. In this case, it is this motor which imparts continuous forward movement to the heads, and provides wobbulation and correction of departure in tracking errors.
A variant in the arrangèment of the gaps 1 to 4 of the stack of heads 5 is illustrated in Fig. 8. According to this variant, the gaps of the playback or recording heads, while remaining aligned on the same axis y, have their own axes angularly offset with respect to y at an alternately positive and negative angle having a value of ~. Thus two adjacent gaps, 1 and 2 for example, do not form an extension of one another. This arrangement makes it possible to reduce crosstalk between the tracks 10, 20, 30, 40 respectively described by the gaps 1, 2, 3, 4. Should there be any overlap between two adjacent tracks~ 10 and 20 for example, the gap 1 which is reading out an information element written in at a given instant onto the track 10, instead of reading out a part of an information element written in at the same instant onto the track 20, which ~-~
=
_ _ ~ ._ _ . _ .. _ .. _ ~L37~
1 would be the case ~,~rith the arrangement of Fig. 4, reads out some o~ the in~ormation ~Iritten in onto a certain len~th o~
the track 20, forming several information elements written in at di~erent instants. If this length, which depends on the angle ~ ) is suffi-icnt, the obtained mean error tends to cancel out. In practice, the problem o~ crosstalk is solved for small v~lues of ~ : around 15. It is there~ore thoroughly ad~antageous to adopt di~erentiated azimuth settings ~or the di~ferent heads of the stack. The arran-gement of gaps illustrated in Fig. 8 makes it possibleto embody a tracking servo-control loop on playback which is different from and more sensitive than that of Fig. 7.
An o~set a h of each head with respect to the track which must be followed produces a phase~shift in read out the signal with respect to the information as it was recorded.
In order to evaluate this phase-shift, it is necessary to have availab]e synchronising pulses recorded at the same time as the information on at least two o~ the tracks. In the case of recording a television signal, the pulses in question are ~or example the line synchronislng pulses re~erred to above. The speed o~ the tape 6 having been regulated on recording~ the pulses are simultaneously written in onto each track at regular intervals. Fig. 9 illustrat~ the location of two of these pulses on the tracks 10 and 20 : 110 and 120 on the track 10, 210 and 220 on the track 20. It has been assumed -ln this Fig. that the radial ~ollowing error is zero when these pulses 110 and 210 are read out. The gaps 1 and 2 then read out the pulses 110 and 210 at the same time. It will be seen ln the :~3~621~
1 Fig. th~t when the pulses 120 and 220 are being read out the the heads are offset with respect to the tracks by a value of d h. It will then be found that the pulse 120 is read out when the gaps are in position la and 2a, and the pulse 220 is read out, with a temporal phase-shift (after in the case of the Fig.), when the gaps are in position lb and 2b offset with respect to the positions la and 2a by a length of track a x. The pulses present in the signals Sl and S2 will therefore be phase-shifted~ ~ne value of the phase-shift increases with a h, and its polarity depends on thetracking error ~ h. The loop illustrated in Fig. 10 uses the results shown hereinbefore. It comprises two decoders 31 and 32 making it possible to extract the line synchro-nising pulses I1 and I2 respectively present in the signals S1 and S2 The pulses I1 and I2 are phase-compared by a comparator 33, which delivers a signal proportional to the phase-shif~ between I1 and I2 which, after ampllfication by an amplifier ~4, constitutes the error signal ~ v applied to the same electrodynamic motors 17 as that of Fig. 7.
The mean value of the signal ~ v obtained by means of the low-pass filter 26, as in Fig. 7, contro]s the motor 7, The diagram of Fig. 9 enables it to be established that~ V being the speed of the tape 6~ the time-interval separating the pulses Il and I2 has a value of ~ t = ~VX = 2 ~h sino~ .
For x = 15 and V = 2~5 m/s, the result is ~ t = 0.2 ~h.
For example, for h - 1/um, ~t = 0.2 /u~, the interval between two successive llne synchronising pulses being 64/us.
Such an interval can easily be detected, which gives good sensitivity of ~elo~ ~th an a~le~which is small enough to 3~3~6~6 1 have a negligible ef~ect on the recording pass-band and the track width, and therefore on the length of recordin~ and the radial speed of displacement of the stack of heads 5.
It is necessary to use a coder 15 (Fig. 6) and a deco-der 150 (Fig. 7) in order to split up the information signal S to be recorded into n components havin~ a pass-band adap-ted to the possibilities of the magnetic heads and to reconstitute this signal from the n signals delivered on playback. There are various coding processes, of the analo-10 gue or numerical type. Fig. 11 illustrates as an example acoder which carries out analogue coding for n = 4. It comprises 4 field-effect transistors 41, 42~ 43, 44 whereof the sources are connected to ground by way of capacitors C1, C2, C3, Cl~. Control pulses ~1' H2. H~ Hl~ o~ very short l5 duration, at the same frequency F and regularly offset from ~ one another in time are applied to the grids of the transis-tors 41 to 44 respectiv~y, causing them to pass successively from the cut-off state to the conductive state. The infor-mation signal S is applied to the drains of the 4 transis-20 tors. The charge volt.ages V1, V2~ V3, V4 appearlng across the terminals of the cap~citors C1, C2, C3, C4 respectively represent samples of the signal S during the respective durations of the control pulses. These pulses remain constant between two successive memorising pulses. The 25 control pulses H1 to H4 emanate ~rom a shi~t-register 46 controlled by a clock signal H at a frequency of 4F (more generally nF) emanating from a generator 45. The chrono-grams given in Fig. 12 show on an example the mechanism o~
splitting up a signal S into 4 components V1 to V4 The ~l~37~
1 pulses H1 to Hl~ occur respectively at the instants ~1 to t4 which are such that t2 ~ tl = t~ - t2 - 4 3 -~signal S having a pass-band of B, F is chosen to be of the same order of magnitude as ~. The signals V1 to V4 have a ass-band apprDximately equal to ~ (more generally B). They could be supplie~ directly to the 4 magnetic heads of the recording device. In order to improve the signal-to-noise ratio, it is preferred to use them in order to frequency- -modulate a carrier P emanating from an oscillator 55 at a 10 ~requency Fo higher than F but lower than or equal to the pass-band which can be attained with the heads used. This frequency modulation is carried out by 4 similar modulators 51, 52, 5~, 54, which deliver the signals S1, S2, S~, S4 respectively intended to be supp~el to the magnetic heads.
15 ~y way o~ example, the recording head according to the invention is intended to record video information with a pass-band of 10 MHz. For this purpose, 8 magnetic heads enabling tracks with a pass-band of 2 MHz to be traced are used. Fo = 2 MHz and F = 1.25 MHz are chosen, so that 8F =
20 10 MHz.
In order to decode the signals S1, S2J S~, S4 picked up across the magnetic heads after demodulation at the instant of playback, use may be made o~ the gates 41, 42, 43, 44 controlled by the same clock signals H1, H2, H~, H4.
25 Each demodulated signal is supplied to the input of one o~
the gates. Since the latter operate one a~ter the other,the signal S is obtained by combining the signals picked up at their outputs.
Various types of video tape recorders~ generally comprising a plurality of magnetic heads, are used in order to record signals in the video-~requency range In order to obtain the desired pass-band without too many heads, the relative spee~ between the tape and the heads m~lst be very high. The solution generall~ used resides in imparting a rapid rotary movement to the heads combined with a translation of the tape. In video tape recorders of the "quadruplex" type, l~ magnetic heads are arranged on a drum rotating at high speed with a rotational axis parallel to the direction of translation of the tape, Their arrangement on the drum gives rise to portions of track paralle] to the transverse direction of the tape.
In video tape recorders of the "helical-scan" type, the tape i5 arranged so as to form a loop round the record-pl~yback member. A magnetic head rotates with respect tothis loop with an axis of rotation sllghtl~ off-centre with respect to the centre of the loop, thus formlng on the tape as it is unwound a helical track having parallel portions slanting with respect to the transverse directinn.
~'s~
~L~3~
1 In both cases the track obtained is discontinuous, and forms parallel portions. These two types of video-tape recorders require complex, heavy and expensive mechanical elements. The price of the "quadruplex" video tape recorder limits it to professional applications. Video tape recorders of the "helical-scan" type are less expensive~ but require complicated servo-controls since centring is critical~
An object of the present invention is to provide a simpler, less fragile and less expensive record-playbacX
device capable of forming part of a reporting unit which is easily transportable by virtue of lts low wei~ht. For this purpose, the device according to the invention enables rotary movement of the head to be avoided by using, instead of a magnetic carrier in the form of a ribbon transferred from a feed spool to a take-up spool~ a loop known as an "endless tape" such as that usually used for sound recor-ding. In order to use all the available width of the tape, a slow movement is imparted ~o the magnetic heads in a direction transverse to the tape, while the tape moves past indefinitely in the longitudinal direction. The slow movement is easily obtained by simple mechanical means~ and may be regulated above all during playback by a likewise simple servo-control loop in order to ensure the tracking.
The tracking of the tape by the magnetic heads is continuous, which eliminates the problems o~ synchronisat~on. The device is thus appreciably less sensitive to faults in the tape due to stretching ~or example, and to any possible vibration Finally, a variant of embodiment enables crosstalk between tracks to be considerably reduced, and the tracking servo-~3~3~6~6 control to be simplified by virtue of angular offset of the record~-playback gaps.
According to the present invention, there is provided a video tape recording and playback device comprising an end-less magnetic tape, magnetic head means having at least two gaps stacked across the width of said tape, tape driving means for continuously moving said tape past said magnetic head means, and head moving means for laterally moving said magnetic head means in a direction perpendicular to the direction of trans-port of said tape; said gaps having axes angularly offset by equal positive and negative values in relation to said direction;
said gaps recording along said tape a timing signal on two equally spaced tracks; said magnetic head means supplying two playback signals upon reading out said tracks with two gaps in azimutal alignment with said gaps; phase comparator means being provided for supplying in response to said playback signals a control voltage corresponding to the phase shift between said playback signals; said control voltage being fed back to a control input of said head moving means for cancel-ling out at playback any lateral offset of said gaps in rela-tion with said tracks.
Other features and advantages of the invention will appear by means of the following description and the appended Figures, wherein : I
- Fig. 1 shows a case containing an "endless tape";
- Fig. 2 and 3 show two embodiments of a s-tack of 4 magnetic heads;
- Fig~ 4 and 5 diagrammatlcally illustrate 2 ways of using the stack of heads accordiny to the invention;
- Fig. 6 shows a recording device according to the invention;
- Fig. 8 shows a variant in the arrangement of the 7~
record-playback gaps;
- Fiy. 9 is an explanatory diagram;
- Fig. 10 is a diagram of servo-control loop used :x with the variant of Fig. 8;
- Fig. 11 shows an embodiment of an information coder used for the invention;
- Fig. 12 is a set of chronograms illustrating the operating of the coder. r Figures 1 and 4 are disposed on the first sheet oE
the drawlngs, figures 2, 3, 5 and 3 on the second sheet oE the drawings, figures ~ and 7 on the third sheet o~ the drawings and figures 9 to 12 on the last sheet of the drawings.
Figure 1 illustrates the interior of a case contain-ing an endless tape. The magnetic tape 6 is woundaround a spool 13 having a rotational axis z. Its two ends have been joined in order to form an endless loop: the part 61 emerging from the interior of the winding is guideci by a series of rollers 9, 11, 12, and rejoins the part 62 situated outside the wind-ing. A roller ~ connected to a driving motor causes the tape to move past, and the tape winds on outside at the same time as it unwinds from the inside, which is made possible by the fact that the wound-on turns slide with respect to one another and that the feed zone is situated inside the winding. Record-ing-playback means 5 are provided in front of that part 63 of the fraction of tape 6 which is moving past between the rollers 11 and 12. Such a case is usually intended for recording sound.
It is then used at a speed of 10 cm/s. The length L of the tape is equal to 150 m, its wîdth 1 is equal to 6.3 mm for a "quarter-inch" tape, 12.6 mm for a "half-inch" tape, etc In altogether unexpected fashion, experience shows that, in spite of friction, a speed much higher than 10 cm/s may be reached without either breaking the tape or spoiling the "
~37~ 6 recorded signal. For example, the tape retains good quality of reproduction at the end of about a hundred hours of operation at a speed of 2.5 m/s. The invention provides for using this tape at the speed of 2.5 m/s for the purpose of recording signals having a wide frequency-band, such as video signals. At such a speed, a 9~376~6 1 signal having a pass-band of 2 Ml~z may be recorded with a signal-to-noise ratio of about 35 dB, using a magnetic head T~hereo~ the gap-height is about 50/um. The use of two heads enables a video signal o~ moderate quality to be recorded on two contiguous tracks, one of the tracks being reserved for luminance, and the other for chrominance and sound. The use of a stack of eight heads makes it possible to record on eight separate tracks a television signal having a pass-band of 16 MHz split up into eight components by a coding device. These two examples do not limit the invention. In the absence of any indication to the contra-ry, in the remainder of ~he description the recording-playb~ck means 5 will consist of a s~ack of 4 magnetic heads describing 4 separate tracks.
Referring to Fig. 2~ a group of four magnetic heads 100, 200, 300, 400 are stacked in the direction of the height of their respective gaps 1, 2, ~, 4. Each head comprises a winding, one only, 101, being visible in the Figure. For recording, a signal emanating, as will be seen later~ from a coding device having 4 ou~put channels is suppIied between the terminals of each winding. ~or playback, signals S1, S2, S3~ S4 respectively characterl-sing the items of information written in onto the 4 tracks and read out by the 4 heads 100, 200, 300, 400 are picked up at the terminals of their winding. The distance between the tracks recorded or read out depends on the height h of the heads9 which may be slightly greater than the height ~ o~ the gaps, which itself det,ermines the width of the tracks. The height ~ of the gaps may be around 1 ~0 to 50 /um and their a width ~ about 0O5 /um, It is nevertheless critical to obtain total heights h small enough, If it is desired that the tracks described by the 4 heads be very close together (about 50 /um apart), it win there~ore be preferable to use integrated magnetic heads such as that illustrated in Fig. 3. Conventional masking techniques are used in order to deposit on a substrate ~02 made of a non-conductive and non-magnetic material a layer of magnetic material 105 in the form of a ring intersected by a gap 1. The winding 101 of the head is obtained by de-positing conductive turns~ for example o~ copper, before and after depositing the layer 105~ The two ends of the winding are joined to two output terminals 103 and 104 at which the signal s1 is supplied or picked up. The desired gap-heights are thus very easily obtained. Similar heads having the desired total height h may be stacked.
Fig, 4 diagrammatically shows one way of using the stack o~ heads 5 according to the invention. For better understanding, the casing containing the stack of heads 5 has illustrated on it the re~pective gaps 1~ 2, 3~ 4 of the heads 100, 200, ~00, 400, these latter being embo-died in accordance with one of the two forms of Figs. 2 and 3. As the tape 6 is unwound in the direction x, each head tracks a track havlng a width A which will be consi-dered to be slightly equal to h. The portions 1OJ 20, 30, 40 respectively tracked by the gaps 1, 2, 3, 4 are there-~ore contiguous, and occupy a wldth H = ~xh of the tape 6.
The value of H ls much less than the total width l o~ the track. This is why the invention provides for lmparting to ~37~
1 the stack of heads 5 a transverse translational movement, in a direction y perpendicular to x in the plane of the tape 6, so that, if the total length of the tape were unwound and the tape were placed round a cylinder, four overlapping helical tracks would be obtained. For this purpose, the stack of heads 5 is mechanically connected to a motor 7J and its translational speed is such that, at the end of the unwinding of a complete length o~ tape, the stack 5 is in a position 5a at a distance from the initial position which ls greater than or equal to H. The tracked portions lOa, 20a, 30a, 40a follow in the direction y the portions 10, 20, 30, 40 tracked on the preceding revolu-tion. It is desirable to make maximum use of the width of the tape, and it is therefore preferable to choose a heli-cal pitch exactly equal to H = 4x h Under these conditions~the number of parallel portions of tracks following one another with a pitch h in the width 1 o~ the tape is , which is obtained at the end of ~h times the complete unwinding of the tape. V being the speed at which the tape moves in the direction x~ and v the transverse speed of the stack 5 ir the direction y, with~ by way of example h = 50 /um, L = 150 m, V = 2.5 m/s, each complete unwinding of the tape lasts 1 mm~ and v = 0.2 mm/mn. Signals having a pass~band of 2 MHz may be recorded on a tape having a width l of 12.6 mm along 240 parallel portions of track by making it run over its whole length 60 times if there are 4 heads~ and 30 times if there are 8 heads~ the recording then lasting for hal~ an hour. The product :
"duration x pass-band" of the device according to the in-~376~i;
1 invention is 8 hours x MHz for the chosen values of ~, l, V,This results in a device of high performance with respect to existing magnetic v-ldeo tape recorders with simple, cheap and strong mechanical means : cases containing a loop of magnetic tape which winds in endless fashion, a stack of7 heads 5, a motor 8 equipped with a roller associated with the cartridge and a motor 7 providing very slow feed and associated with the stack 5.
Fig. 5 illustrates a variant of embodiment in which the heads are not juxtaposed with one another as in Fig. 2, but are separated by a distance h, while remaining integral with one another. This arrangement enables non-integrated heads to be used while preserving the same track-pitch as ~70re, the movement of the stack of heads being the same.
; 15 Only the arrangement of the tracks on the tape is modified7 and i~ may be seen in the Fig. that the distance h separa-ting the gaps 1, 2, ~J ~ iS about equal to a quarter of the width l of7 the tape ~. The translational speed v o~ the stack 5 is such that, at the end of a revolution, each portion of track 107 20, ~0, 40 is shifted with a value at least equal to the head height h. The speed v is th~7r~-fore 4 times less great than before if7 the same track-pitch h is maintained. 4 helices of7 a pitch h are thus obtained, being independent instead of overlapping one another. Recording stops when the last portion of track followed by a gap (1 for example) is at a distance h from ~he first portion of track followed by the followlng gap (that is to say the gap 2). The stack of heads 5 will then have moved forwards by a distance hl, while in the 7~26 foregoing case it had moved forwards 4 times more quickly by a distance 1 = 4h1. Playback and recording must naturally be carried out with the same stack of heads in order to respect the arrangement of the tracks. ~he total number of portions of track is identical with the foregoing case, as is the duration of the recording.
A device such as that shown diagrammatically in Fig.
1 and one of Figs. 4 and 5 could be used without any other element for recording and then playing back information via a magnetic tape. However, because of instabilities of various types due either to the mechanical means or to the tape oscillating, it proves in practice to be necessary to supple-ment it with servo~control loops, enabling the conditions set out above relating to the speeds v and V to be respected on recording, and the tracks wri-tten in to be satisfactorily followed at the desired speed on playback in order to recon-stitute the information with as few errors as possible.
Referring to Fig. 6, an electrical signal S charac~
terising the information to be recorded is split up into n components ~n = 4 in the Fig.) by means of a coder 15 which will be detailed hereinafter. This coder makes available 4 signals Sl, S2, S3, S4 whereof the pass-band is compatible with the possibilities of the device (dimensions of recording heads 5 and speed of the tape 6). Each of these signals may according to circumstances consist of a carrier frequencyt modulated by the information or of a numerical signal. The 4 heads which make up the stack of heads 5 are respectively supplied by -the 4 signals for the ---~
.
....
~,~,' 10 ~7~
1 recording on 1~ tracks on the tape 6 in the form of modifi-ca~ions ~o the magnetic ch~racteristics of the tape The motor 8 drivin~ the tape 6 at the speed V and the motor 7 translating the stack of heads 5 at the speed v have respec-tively regulators ~8 and 37 controlled by a synchronisingsignals R8 and R7. The signal R8 emanates from an oscilla-tor 34 whereof the frequency f is so chosen as to obtain the desired speed V The signal R7 emanates from a frequency-divider 35 receiving the signal R8 and having a division ratio so chosen as to obtain the desired speed v The speed v must be very accurate in order to avoid any track over-laps.
Fig. ~f illustrates a device for playing back a magnetic tape 6, recorded by the dev:ice of Fig. 6. Playback requires much greater accuracy than recording. The playback device is provided with a servo-control of the radial position of the stac~ of heads by means of a signal characterising the radial trackino error, and a "longitudi-nal" servo-control by monitoring the tape speed with synchronising pulses recorded onto the tape itself upon recording. The operation of the device of Fig. 7 implies that the information signal S, a television signal for examp-e, comprises high-frequency synchronising pulses.
The line pulses, which have a frequency of 15625 Hz in the European televlsion standard and a ~requency o~ 15750 Hz in the U.S. standard, may be used for this purpose, the information being coded in the coder 15 in such a manner that these pulses may subsist in at least one Or the signals S1 to S4, S1 for example. Each head of the sta~k ~3762~
1 Of heads 5 delivers a signal (~1 to Sl~) which is a function of the magnetic characteristics of the portions of tape tracked by the heads. The l~ signals are processed by a decoder 150, which decodes in a manner corresponding to the coding carried out by the coder 15g for the purpose of delivering a signal S. A decoder 22 extracts the line pulses from the signal S1 amplified by an amplifier 1~ and a phase-comparator ~4 delivers an error signal ~T propor-tional to the phase-difference found between ~hese line pulses and reference pulses delivered by an oscilla~or 23 at the line-frequency. The error signal ~V enables the oscillator 34 described in reference with Fig. 6 to be controlled. This makes the translational speed V of the tape on playback equal to the speed on recording. The 5 radial servo-control of the stack of heads 5 is based on a wobble process : a low-amplitude transverse oscillation is imparted to ~he stack 5 at a frequency f above the frequency-band of the trackin~ errors whic~ i~ is desired to correct and below the frequency-band of the signals S1 to S4. For this purpose, the stack of heads 5 is made integral with the moving coil 171 of an electrodynamic motor 17 of the loudspeaker drive type whereof the base 172 is mechanicallY connected to the radial feed motor 7.
The moving coil 171 is moved by a signal supplying the terminals 17~ and 17~ of the moving coil and resulting from the sum, provided by a summation amplifier 25, of a signal ~ having a frequency f delivered b~ an oscillator 27 and an error signal av characterising the tracklng error. The periodic excurslon and radial correction are ~37~26 1 thus carried out by the same element 17. A group 151 of high-pass filters enables the component at the frequency f to be eliminated from the signals S1 to S~ before they are processed by the decoder 150. The servo-control loop com-prises : an envelope-detector 19 receiving the signal S1 after it has been amplified by the amplifier 1~, the oscil-lator 27 providing the signal ~ and the signal emanating from the detector 19, a low-pass filter 28 eliminating from the product signal the components at frequencies greater than or equal to f and supplying the error signal v, and the summator 25 referred to above.
~ hen the tracking made by the magnetic head 100 which provides the signal S1 is satisfactory, the oscillation at the frequency f set up in a direction radial to the track produces amplitude modulation of the signal S1 at the frequency 2f. IndeedJ the read out signal is maximal at the centre of the portion of track, and decreases when thls ~;~
centre is departed from. When the mean position of the head 100 departs from this centre by a value of a h, modulation of the signal S~ at the frequency f is added to the modu-lation at a frequency of 2f. The modulation level at the frequency 2f increases, while the modulation level at the frequency 2f decreases, when ~ h in~reases. Moreover, the phase of the modulation (positive or negative) depends on the polarlty of the error ~ h. After multiplication by the reference signal and filtering of the components at frequen-cies o~ f, 2f, ~f, the signal p v is obtained, of suitable amplitude and polarity to control the motor 17 and preserve tracking. A low-pass filter 26 extra~ts the do c ~ component ~ ' .
~37626 1 from the error signal ~ or from the signal ~ and controls the motor 7 in order to impart regular forward feed to the unit consisting of the motor 17 and the stack of heads 5.
It is clearly understood that the recording device illus-trated in Fig. 6 and the playback device illustrated in Fig. 7 are only examples of embodiment which do not limit the invention. The latter allows of numerous variants, more particularly of the servo-control loops of the play-back device. For the recording device, a variant of Fig. 6 ~0 resides in introducing radial servo-control of the position of the stack of heads 5 with respect to a portion o~ track already recorded. A magnetlc playback head is provided for this purpose o if n heads are required for recording, the stack of heads 5 comprises n ~ 1 heads, one of them being a playbac~ head (for example, referring to Fig. 4J the head having the gap 1). The speed v of radial feed is provided so that the playback head will follow previously recorded a track so that the portion of track being followed at each instant by the playback head w-lll coinci-de with the portion of track followed on the previousrevolution of the winding of tape by ~he recording head situated a'c the other end of the stack 5, that is to say, referring to Fig. 4, the head having the gap 4. This result is obtained when the displacement of the stack Or heads 5 during a complete unwind of the tape is equal to n times the track pitch h. The playback head delivers a signal corresponding to a previousl~ recorded information component Thls signal is used in ~he same manner as the signal S1 in the playback device of Fig. 7 for the purpose of servo-controlling the position of the stack of heads 5 with respect to the last recorded track. The servo-control loop may be the same as in Fig. 7, but not necessa-rilly so. This variant imparts greater regularity to the tracks. Playback is facilitated, and the servo-control loops of the playback device may be simplified. In particular, the risk of track overlap upsetting playback is avoided.
Without departing from the scope of the invention it is also possible to use a moving-coil electrodynamic motor capable of displacing the heads over the whole width of the tape. In this case, it is this motor which imparts continuous forward movement to the heads, and provides wobbulation and correction of departure in tracking errors.
A variant in the arrangèment of the gaps 1 to 4 of the stack of heads 5 is illustrated in Fig. 8. According to this variant, the gaps of the playback or recording heads, while remaining aligned on the same axis y, have their own axes angularly offset with respect to y at an alternately positive and negative angle having a value of ~. Thus two adjacent gaps, 1 and 2 for example, do not form an extension of one another. This arrangement makes it possible to reduce crosstalk between the tracks 10, 20, 30, 40 respectively described by the gaps 1, 2, 3, 4. Should there be any overlap between two adjacent tracks~ 10 and 20 for example, the gap 1 which is reading out an information element written in at a given instant onto the track 10, instead of reading out a part of an information element written in at the same instant onto the track 20, which ~-~
=
_ _ ~ ._ _ . _ .. _ .. _ ~L37~
1 would be the case ~,~rith the arrangement of Fig. 4, reads out some o~ the in~ormation ~Iritten in onto a certain len~th o~
the track 20, forming several information elements written in at di~erent instants. If this length, which depends on the angle ~ ) is suffi-icnt, the obtained mean error tends to cancel out. In practice, the problem o~ crosstalk is solved for small v~lues of ~ : around 15. It is there~ore thoroughly ad~antageous to adopt di~erentiated azimuth settings ~or the di~ferent heads of the stack. The arran-gement of gaps illustrated in Fig. 8 makes it possibleto embody a tracking servo-control loop on playback which is different from and more sensitive than that of Fig. 7.
An o~set a h of each head with respect to the track which must be followed produces a phase~shift in read out the signal with respect to the information as it was recorded.
In order to evaluate this phase-shift, it is necessary to have availab]e synchronising pulses recorded at the same time as the information on at least two o~ the tracks. In the case of recording a television signal, the pulses in question are ~or example the line synchronislng pulses re~erred to above. The speed o~ the tape 6 having been regulated on recording~ the pulses are simultaneously written in onto each track at regular intervals. Fig. 9 illustrat~ the location of two of these pulses on the tracks 10 and 20 : 110 and 120 on the track 10, 210 and 220 on the track 20. It has been assumed -ln this Fig. that the radial ~ollowing error is zero when these pulses 110 and 210 are read out. The gaps 1 and 2 then read out the pulses 110 and 210 at the same time. It will be seen ln the :~3~621~
1 Fig. th~t when the pulses 120 and 220 are being read out the the heads are offset with respect to the tracks by a value of d h. It will then be found that the pulse 120 is read out when the gaps are in position la and 2a, and the pulse 220 is read out, with a temporal phase-shift (after in the case of the Fig.), when the gaps are in position lb and 2b offset with respect to the positions la and 2a by a length of track a x. The pulses present in the signals Sl and S2 will therefore be phase-shifted~ ~ne value of the phase-shift increases with a h, and its polarity depends on thetracking error ~ h. The loop illustrated in Fig. 10 uses the results shown hereinbefore. It comprises two decoders 31 and 32 making it possible to extract the line synchro-nising pulses I1 and I2 respectively present in the signals S1 and S2 The pulses I1 and I2 are phase-compared by a comparator 33, which delivers a signal proportional to the phase-shif~ between I1 and I2 which, after ampllfication by an amplifier ~4, constitutes the error signal ~ v applied to the same electrodynamic motors 17 as that of Fig. 7.
The mean value of the signal ~ v obtained by means of the low-pass filter 26, as in Fig. 7, contro]s the motor 7, The diagram of Fig. 9 enables it to be established that~ V being the speed of the tape 6~ the time-interval separating the pulses Il and I2 has a value of ~ t = ~VX = 2 ~h sino~ .
For x = 15 and V = 2~5 m/s, the result is ~ t = 0.2 ~h.
For example, for h - 1/um, ~t = 0.2 /u~, the interval between two successive llne synchronising pulses being 64/us.
Such an interval can easily be detected, which gives good sensitivity of ~elo~ ~th an a~le~which is small enough to 3~3~6~6 1 have a negligible ef~ect on the recording pass-band and the track width, and therefore on the length of recordin~ and the radial speed of displacement of the stack of heads 5.
It is necessary to use a coder 15 (Fig. 6) and a deco-der 150 (Fig. 7) in order to split up the information signal S to be recorded into n components havin~ a pass-band adap-ted to the possibilities of the magnetic heads and to reconstitute this signal from the n signals delivered on playback. There are various coding processes, of the analo-10 gue or numerical type. Fig. 11 illustrates as an example acoder which carries out analogue coding for n = 4. It comprises 4 field-effect transistors 41, 42~ 43, 44 whereof the sources are connected to ground by way of capacitors C1, C2, C3, Cl~. Control pulses ~1' H2. H~ Hl~ o~ very short l5 duration, at the same frequency F and regularly offset from ~ one another in time are applied to the grids of the transis-tors 41 to 44 respectiv~y, causing them to pass successively from the cut-off state to the conductive state. The infor-mation signal S is applied to the drains of the 4 transis-20 tors. The charge volt.ages V1, V2~ V3, V4 appearlng across the terminals of the cap~citors C1, C2, C3, C4 respectively represent samples of the signal S during the respective durations of the control pulses. These pulses remain constant between two successive memorising pulses. The 25 control pulses H1 to H4 emanate ~rom a shi~t-register 46 controlled by a clock signal H at a frequency of 4F (more generally nF) emanating from a generator 45. The chrono-grams given in Fig. 12 show on an example the mechanism o~
splitting up a signal S into 4 components V1 to V4 The ~l~37~
1 pulses H1 to Hl~ occur respectively at the instants ~1 to t4 which are such that t2 ~ tl = t~ - t2 - 4 3 -~signal S having a pass-band of B, F is chosen to be of the same order of magnitude as ~. The signals V1 to V4 have a ass-band apprDximately equal to ~ (more generally B). They could be supplie~ directly to the 4 magnetic heads of the recording device. In order to improve the signal-to-noise ratio, it is preferred to use them in order to frequency- -modulate a carrier P emanating from an oscillator 55 at a 10 ~requency Fo higher than F but lower than or equal to the pass-band which can be attained with the heads used. This frequency modulation is carried out by 4 similar modulators 51, 52, 5~, 54, which deliver the signals S1, S2, S~, S4 respectively intended to be supp~el to the magnetic heads.
15 ~y way o~ example, the recording head according to the invention is intended to record video information with a pass-band of 10 MHz. For this purpose, 8 magnetic heads enabling tracks with a pass-band of 2 MHz to be traced are used. Fo = 2 MHz and F = 1.25 MHz are chosen, so that 8F =
20 10 MHz.
In order to decode the signals S1, S2J S~, S4 picked up across the magnetic heads after demodulation at the instant of playback, use may be made o~ the gates 41, 42, 43, 44 controlled by the same clock signals H1, H2, H~, H4.
25 Each demodulated signal is supplied to the input of one o~
the gates. Since the latter operate one a~ter the other,the signal S is obtained by combining the signals picked up at their outputs.
Claims (6)
1. A video tape recording and playback device comprising an endless magnetic tape, magnetic head means having at least two gaps stacked across the width of said tape, tape driving means for continuously moving said tape past said magnetic head means, and head moving means for laterally moving said magnetic head means in a direction perpendicular to the direc-tion of transport of said tape, said gaps having axes angularly offset by equal positive and negative values in relation to said direction; said gaps recording along said tape a timing signal on two equally spaced tracks; said magnetic head means supplying two playback signals upon reading out said tracks with two gaps in azimutal alignment with said gaps; phase comparator means being provided for supplying in response to said playback signals a control voltage corresponding to the phase shift between said playback signals, said control voltage being fed back to a control input of said head moving means for cancelling out at playback any lateral offset of said gaps in relation with said tracks.
2. A device as claimed in claim 1, wherein said magnetic head means includes at least n magnetic heads, n being an integer at least equal to 2; said n heads being integral with one another, the centres of the gaps of said heads being aligned in said direction; said gaps following n continuous tracks forming, in said direction, parallel portions of tracks having a pitch h.
3. A device as claimed in claim 2, wherein the distance between the centres of two adjacent gaps is equal to h; the distance traversed by each of said heads in said direction during the unwinding of the total length of said tape being equal to n x h.
4. A device as claimed in claim 2, wherein the distance between the centres of two adjacent gaps is equal to p x h, p being an integer; the distance traversed by each of said heads in said direction during the unwinding of the total length of said tape being equal to h; the width of said tape in said direction being greater than or equal to n x p x h.
5. A device as claimed in claim 1, enabling an information signal having a pass-band B to be recorded by n magnetic recording heads in the form of n tracks on a magnetic tape so as to be able to be read out by scanning of said n tracks by n magnetic playback heads, n being an integer at least equal to 2, said recording device further comprising coding means for splitting up said information signal into n electrical recording signals having a pass-band less than B;
the two terminals of said n recording heads being simultaneously and respectively supplied by said n recording signals.
the two terminals of said n recording heads being simultaneously and respectively supplied by said n recording signals.
6. A playback device as claimed in claim 4, wherein said timing signal comprises two series of synchro-nizing pulses synchronously recorded at the same time as an information signal with a fixed frequency fo.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
FR7730244 | 1977-10-07 | ||
FR7730244A FR2405536A1 (en) | 1977-10-07 | 1977-10-07 | DEVICE FOR RECORDING-READING INFORMATION ON A LOOP OF MAGNETIC TAPE WRAPPED IN AN ENDLESS CASSETTE |
Publications (1)
Publication Number | Publication Date |
---|---|
CA1137626A true CA1137626A (en) | 1982-12-14 |
Family
ID=9196241
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
CA000312870A Expired CA1137626A (en) | 1977-10-07 | 1978-10-06 | Device for recording and playing back information on an endless magnetic tape |
Country Status (6)
Country | Link |
---|---|
US (1) | US4321634A (en) |
JP (1) | JPS5470816A (en) |
CA (1) | CA1137626A (en) |
DE (1) | DE2843734A1 (en) |
FR (1) | FR2405536A1 (en) |
GB (1) | GB2005458B (en) |
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1977
- 1977-10-07 FR FR7730244A patent/FR2405536A1/en active Granted
-
1978
- 1978-10-04 GB GB7839338A patent/GB2005458B/en not_active Expired
- 1978-10-06 JP JP12350078A patent/JPS5470816A/en active Pending
- 1978-10-06 DE DE19782843734 patent/DE2843734A1/en not_active Ceased
- 1978-10-06 CA CA000312870A patent/CA1137626A/en not_active Expired
-
1980
- 1980-05-05 US US06/146,412 patent/US4321634A/en not_active Expired - Lifetime
Also Published As
Publication number | Publication date |
---|---|
FR2405536B1 (en) | 1982-10-22 |
GB2005458B (en) | 1982-12-08 |
US4321634A (en) | 1982-03-23 |
GB2005458A (en) | 1979-04-19 |
JPS5470816A (en) | 1979-06-07 |
FR2405536A1 (en) | 1979-05-04 |
DE2843734A1 (en) | 1979-04-12 |
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