CA1209693A - Helical scan recording and playback systems - Google Patents

Abstract

Abstract of the Disclosure Helical scan magnetic recording system of the type where time compressed audio signals are recorded in extensions of the video tracks. In order to avoid a time delay of the reproduced audio relative to its original relationship to the video when played back, additional audio recording heads are located on the helical scanner so as to record the audio information on tracks that when played back are reproduced sufficiently in advance of the video to allow restoration of the original audio/video synchronism. The arrangement can thus can be employed to produce pre-recorded tapes having time advanced audio such that synchronous audio/video is reproduced on conventional playback reproducing systems.

Description

~ 27332-26 Background of the Invention Field of the Invention The present invention is concerned in general with helical scan magnet~c tape recording and reproducing systems, particularly with those in which signals are recorded on a magnetic medium by a plurality of rotating magnetic heads along a series of sequential discontinuous tracks. Information of one type (such as the video portion of a television signal) is recorded and later reproduced continuously, but information of another type (such as the correspond;ng audio portion of the television signal) is compressed along the time axis and record-ed non-simultaneously. Before the two types of information are recorded and repEoduced, they have a ,, ~,.' [19693 certain time relationship between them. After being recorded and reproduced, one type of information (the audio portion, for example) is time delayed with respect to such time relationship. Although the invention is directed principally to overcoming certain problems in the recording of time compressed audio signals in the extension of video tracks in a helical scan television recording and playback system, the principles of the invention are applicable to the recording of other dissimilar signal types in which the synchronization of the signal types is of importance.

~escription of the Prior Art So called "helical scan" magnetic recording and playback systems for use with television and other high bandwidth signals are well known in the art. HeliCal scan systems for television are widely used both pro~essionally and by consumqrs. Magnetic tape is transported around a rotating drum or scanner which carries record/playback heads such that a series of parallel obliquely angled tracks are recorded in succession along the length of the tape. These helical scan tracks carry the television picture information.-`~The accompanying sound or audio information in most systems is recorded by a stationary head and carried in a longitudinal track typically outside the helical scantracks. Because of the drawbacks resulting from narrow audio track width and the slow longitudinal tape speed, ~`particularly in consumer systems, other techniques for recording the sound information have been developed. One alternative approach is to modulate a subcarrier with the sound information and to record the resulting spectrum along with the modulated television information spectrum in the helical scan tracks. A disadvantage of this approach is that the audio information is not separable _ J

~20g~S~3 from the video information prior to demodulation and thus the audio cannot be easily handled by itself as is desirable in editing or re-recording with added audio information.
~nother approach to the recording of sound signals in a helical scan television recording and playback system is set forth in US-PS ~,303,950; US-PS 4~353J098 and the article "A New 8-Bit PCM Audio Recording Technique Using an Extension of the Video Track" by N.~lakano et al, IEEE Transactions on Consumer Electronics, Vol. CE-28, No. 3, August 1982 (manuscript received June 6, 1982), pp.
241-248. Each of these references proposes compressing the audio along the time axis and recording it in an extension of the helical scan video track. The video lS information fills 180 degrees of each track such that with two heads the video recording and playback is continuous, whereas the tims compressed audio is recorded and reproduced in discontinuous segments that are non-simultaneous and time delayed with respect to the corresponding video information. According to the ~ Nakano et al proposal the time-compressed audio is in ; digital form (pulse code modulation) and the two audio channels are interleaved so as to reduce susceptibility ;~ to burst errors resulting from tape drop outs, for example.
In helical scan magnetic tape recording systems of the type disclosed in the above cited references, in which the video information is recorded and reproduced continuously but the audio information is compressed along the time axis and recorded in segments non-simultaneously with the video information with which it was originally associated, there are some specific problems limiting the utility and ease of application of such a system.
The time compression of the audio signals and their ~3 non-simultaneous recording (with respect to the video signals) causes a distinct time delay on playback between the video portion of the signal and its corresponding audio portion. This time delay may be tolerated, i~ it does not exceed approximately 50 milliseconds. The system of US-PS 4,303,950, for example, is described as having the audio signal lag the video signal by approximately 16 milliseconds. In other systems of this type, the delay may be even greater. With respect to the original recording (e.g., the first generation) in such systems where both audio and video information arrived at the input to the recorder simultaneously (such as with a television signal input in which the audio is in synchronism with the video~, the delay may be tolerable or perhaps only marginally acceptable.
I~, however, an aktempt is made to copy this recording on another recorde~ of the same type, then the time delay of the two tape generations will add and become unacceptable.
One solution to the time delay or lag problem is set forth in US-PS 4,353,098 which suggests storing the video information and reading it out in time synchronism with the delayed audio. This approach has the drawback that means for holding and playing back video signals, such as frame stores, are expensive and would be commercially unacceptable in a con-sumer product.
It is also desirable to provide recording equipment for the production of magnetic tapes in which the time compressed audio is advanced in time with respect to the corresponding video information such that magnetic tapes produced on such : equipment can be played on prior art ~2~ 33 systems with synchronous audio/video. The present invention also contemplates magnetic tapes recorded with such a characteristic.
It is thus an object of this invention to provide a helical scan recording and playback system, in which video and time compressed audio information are treated in such a way as to provide playback without time delay.

Summary of the Invention In accordance with the teachings of the present invention/ the above-mentioned problem is soived and the general utility and applicability of such helical scan recording s~vstems are ~reatly increased.
~ ccording to the present invention, means are provided for recording those portions of the recorded signal containing the time compressed information in track positions such that the normal playback heads scan the time compressed information in advance of the time that the other information is scanned in order that the relative timing of the information can be restored. More particularly, additional record heads are provided that are carried by the rotating head assembly at positions offset from the normal record heads. The offset is at leas-t as great as the offset required to restore the original timing of the signals prior to their application to the recording system when played on a prior art playback system that does not incorporate the ~eachings of said Todd application. Thus, for example, the invention restores the synchronism of the audio and video portions of a television signal.

~096~3 Thus, in accordance with a broad aspect of the invention, there is provided a helical scan recording system for recording a plurality of signals on a magnetic medium in a series of sequential discontinuous tracks, comprising signal processing means receiving said signals, said means including means for compressing at least one of said signals along the time axis, and recording means receiving the signals from said signal processing means including said at least one time compressed signal, said recording means including a rotating assembly carrying a plurality of recording heads scanning said tracks for recording signals along said tracks, said means recording said at least one time comp.ressed signal on tracks spaced apart from tracks on which said other signal or signals is or are simultaneously recorded such that when the magnetic medium is played back said at least one time compressed signal is advanced in time with respect to said other signal or signals received by the recording means by virtue of being recorded in a track that is played back earlier than the track in which said other signal or signals were simultaneously recorded.
In accordance with another broad aspect of the invention, there is provided a method of recording signals in a series of sequential discontinuous tracks on a magnetic medium using a plurality of rotating magnetic heads, the signals including at least one signal compressed along the time axis, comprising recording said at least one time compressed signal on tracks spaced apart from tracks from which said other signal or signals is or are recorded such that said at least one time compressed signal is advanced in time with respect to said other signal or siynals by virtue of being reproduced earlier than the track in which said other signal or signals were simultan-eousl~ recorded, and proces~iny said siynals, including -5a-expanding said at least one time compressed signal.

srief Description of the Drawings Figure l is a block diagram of a prior art helical scan television recording system employ:ing two heads in which audio information is recorded along with video -5b-information ~n separate portions of each helical sean traek.
Figure 2 is a bloek diagram of a helieal sean television playback system according to the prior art.
Figure 3 is a block diagram of a helical scan television playback system.
Figures 4A and 4B show a record and playbaek timing diagram and the assoeiated time delays for a helieal sean recording system of the type shown in Figure 1 using two record/
playbaek heads.
Figure 5 shows a playback and xerecording timing dia-gram of a helieal sean television playback system.
Figure 6~ shows a top view of the rotating head drum with the respeeti~e loeations of pr~mary and seeondary heads, and Figure 6B of the traek, eonfiguratlon on the magnetie tape for the system diseussed in eonneetion with Figures 3 and 5.
Figure 7 is a bloek diagram of a helieal sean television recording system aeeording to the present invention.
Figure 8 is a representat~on of the track eonfigu~ation on the magnetie tape reeorded us~ng the reeording system of the present invention.

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Detailed Description of the Invention ~ eferring now to the drawings, Figure 1 shows a simplified block diagram of a prior art helical scan television recording system of the type described in which the audio information is compressed along the time axis and recorded in non-continuous segments in extended portions of the helical scan tracks. Systems of this type are described in said VS-PS 4,303,950; 4,353,098 and article by K. Nakano et al. Analog audio channel 1 and channel 2 are applied to respective analog-to-digital converters 2 and 4 which in turn apply the digitized audio signals to time compression means 6 and 8 that each include buffer memories and means for reading the data in and out at the appropriate times. The time compressed lS signals are applied to error encoding means 10 that includes means for interleaving the two digitized chan-nels. The processed digital audio information from block 10 is a~plied along with the video information to be recorded to switching means 12 that connects the audio and video information to the two helical scanner recording heads Pl and P2 in the desired sequence. The switch can be actuated in the conventional way by a signal derived - from the scanner drum rotational position. The time compression means 6 and 8 can also be controlled by a signal derived from the drum position. Details of the prior art system of Figure 1 are well known in the art.
In the prior art system, playback is simply the reverse of recording as is shown in the block diagram of Figure 2. The output from heads Pl and P2 is switched by a signal derived from the drum position. The off-tape digital audio information is applied to error detection and de-interleaving means 14, the two-channel output of which is applied to the time expanding means 16 and 18 which also can be controlled by a signal derived from the 35 drum position. The expanded signals are applied to , -respective digital-to-analog converters 20 and 22 that provide the two analog audio channels of information, which are delayed with respect to the reproduced video.
Figure 3 shows a playback system. The heads Pl and P2 are applied to a switching means 24 that operates under the control of a signal derived from the drum position ~o provide the video signal information as in the prior art. However, the audio information is reproduced by a further set of heads Sl and S2 that are offset from the heads Pl and P2 such that heads Sl and S2 read the audio information off tape sufficient-ly in advance of the time when the audio information would be read by the Pl and P2 heads that the oxiginal relative timing of the audio and video signals (e.g., the relative timing prior to the;r application to the recording system) can be restored.
In other words, the synchronism of the audio and video portions of the telev~sion s~gnal can be restored. The signals from heads Sl and S2 are applied to a switching means 26 of the same type as used to switch heads Pl and P2 and the switching is also controlled by a signal derived from the drum posit~on, but such that the heads Sl and S2 read the respective audio portions of the helical tracks. The sw~tching means output is applied to processing c~rcuitry that ~ncludes error detection and de-interleaving means 28 and time expansion means 30 and 32.
5torage means such as a buffer memories 34 and 36 to provide precise adjustment of audio/video synchronism can be located either after blocks 30 and 32 as shown, or alternatively, a single buffer can be located before block 28. Buffering of digital audio information can be accomplished vexy inexpensively, whereas buffering of video information is very costly. The processed and stored audio information is applied to respective digital-to-analog con~erters 38 and 40 which provide the J

~Læo~.~3 two channels of analog audio information.
In Figures l through 3, additional processing may be provided in practical embodiments, for example, the audio signals will usually be modulated onto a carrier ~rioc to recording and demodulated after playback. Such details are well known in the art and are osnitted here for clarity.
A four head helical scan video tape recording and playback system is described in US-PS 4,358,799.
However, the second pair of heads is for the purpose of reducing the head drum diameter and also for use in providing playback monitoring during recording.
The operation of the Todd invention will be better appreciated by reference to the remaining Figures.
Figure 4A shows an example of a timing diagram of a helical scan audio/video recording system of the t~pe shown in Figure l, where time compressed audio signals are recorded, for example/ on a first portion oE each track (e.y. the first 30 degrees of rotation oE the head assembly), and video information is recorded on a secon~
~ortion of the track (in this example, 180 degrees of rotation). The rotating head system in this embodiment contains two heads (referred to herein as primary heads Pl ~i and P2), one recording all even fields and the other recording all odd fields, the two together making up one picture frame. If information begins entering the recoeder at time T=0, then, in the example shown, primary head Pl records video scan n, an odd field onto the first track. One head rotation later, it records odd field scan n+l onto track 3, and another head rotation later, odd field scan n+2 onto track 5, etc. Primary head P2 records the even fields of scan n, n+l, n+2, etc. onto tracks 2, 4 and 6 etc. On each oE the scans following the first one, each of the two heads records time compressed 35 audio at the beginning of each track. In order to see the ,: - ,.. .. ..

exact time relationships it is best to work backward in time from an assumed audio recording.
If it is assumed that audio is recorded in time compressed form at the beginning of scan n+l (even) by primary head P2, then this recording is 2.8 milliseconds long in a 60 ~z system with a total scan of 210 degrees.
This amount of recorded audio information requires some prior processing time, for example preconditioning for later error correction, which in this example is assumed to take 2 milliseconds. Prior to this time, audio information was accumulated and stored in a buffer for a time period of 1~.7 milliseconds (1/60th of a second).
These 16.7 milliseconds are highlighted in Figure 4A as the example auaio block. If this sample audio block is 15 now followed through to the playback diagra~ of Figure 4B, it can be seen that playback of the even field scan n+l,~of which the example audio block is a part, cesults in a time dela~ with respect to the corresponding video information, of 31.5 milliseconds. This total time delay is composed of the basic inherent time delay and the addition o output processing time (for example, error correction and de-interleaving) which is assumed in this example to be 10 milliseconds.
`' It can easily be seen that this time delay is an 25 inherent ~art of the system configuration and can be greater or smaller in magnitude than in the example given, depending on the processing chosen for time compression.
The time compression can comprise, for example, analog timè compression, digital signal processing such as pulse 30 code modulation, or delta modulation, or any other suitable method. It is also clear that the sequence o~
audio and video information on the track can be reversed without significant changes in these basic relationships.
The example time delay of 31.5 milliseconds can probably 35 be tolerated on a single playback pass, but will result in :

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excessive time delay if the information is to be copied onto another recorder of the same type which results in addition of time delays.
Figure 5 shows the play~ack and re-record timing rela-tionships with respect to the same example audio block if the playback system described in connection with Figure 3 is used.
The recording timing diagram would be identical to Figure 4A
and the prior art recording arrangement such as shown in Figure 1 is employed for recording and re-recording in connection with the present invention. A plan view of an embodiment of the head configuration of the present invention is shown in Figure 6A, which will be described below.
In Figure 5 thesc~n n~l (containing the example audio block) is played first by secondary head S2 which retrieves only the time compressed audio information and ignores the ~ideo information~ Due to the physical location of secondary head S2 on the head drum, this audio in~ormation is retrieved at a time preceding the retrieval of the video information on the same track by primary head P2. There is thus sufficient time avail-able to compensate the delay in the relative timing of the audio information.
The secondary heads Sl and S2 are offset fro~ the primary heads Pl and P2 angularly, as indicated in Figure 6A, and also in veritical height on the scannex drum as is apparent from in~
spect;on of Figures 5 and 6B. In practice the offset is not critical: the offset need only be great enough to pro~ide at least the required time advance relatiYe to heads Pl and P2.
The buffer storage then can be adjusted to provide the desired synchronism.

, -lla-The location of the secondary heads at a different vertical height than the primary heads (e.g., the secondary heads lying in a plane offset with respect to the plane of in which the primary heads lie, the planes i . ,~, - ~2 ~ fi93 being perpendicular to the axis of the helical scanner) may result in the secondary heads scanning less than the entire full length of each track. However, if the secondary heads are offset in the direction such that any loss of track scan is at the end distal from the time compressed audio segment this causes no problem.
Suitable design of the scanning geometry would allow the heads in both planes to scan the entire track length.
Figure 5B shows an example of the pattern of tracks 10 as they are recorded onto the magnetic tape. In practice, the track angles are small with respect to the t~pe edges. The example head positions of Figure 6B
shown on the track diagram correspond, in time, to the arrow marked "examyle head position" in Figure 4A and 15 Figure 5t i.e. 14 milliseconds after t=0.
The exack head position for the secondary heads ha~s, in the preerred embodiment, been chosen such that secondary head S2 will read ("preview") signals which have been recorded by primary head P2 and secondary head 20 Sl will preview signals recorded by primary head Pl. The secondary heads are switched so that during playback they read only one type of the dissimilar signal (audio in the example) and the other type of dissimilar signal (video in ~A~I the example) is read~by the primary heads.
As stated above,~the offset of the secondary heads with respect to the primary heads is not critical.
~nother possible arrangement is to provide dual gap heads at the primary head angular positions, such that the secondary heads are vertically offset by one or, 30 preferably two, tracks. A two track ofEset is preferred because the the fabrication of a head with only a one track spacing would be more costly and because a one track advance would not provide the sufficient tlme advance likely to be required. Thus in the case of dual gap heads 35 offset by two tracks, a two field advance could be ` ::
. -obtained instead of the one and a half field advance asshown in Figures ~A and 6B.
In accordance with the teachings of tAe present invention magnetic tapes can be recorded with time advanced audio segments such that such tapes can oe played with synchronous audio and video on prior art recorder/reproducers of the type described in connection with Figures 1 and 2. Thus pre-recorded videotapes could be produced which would be playable without delayed audio on the prior art two-head playback machines. In order to accomplish this a similar electro-mechanical arrangement is employed as has been described in connection with the playback aspects of the Todd application. However, the functions of the primary and secondary heads are reversed, such that the audio portion of a television signal to be recorded is applied to the primary heads and the corresponding video portion is applied to the secondary heads. If t~e record and playback delay timings are the same as described, then the secondary to primary head offset is the same as for the playback embodiments of the Todd application. While pre-recorded tapes could be produced with time advanced audio by other means, such as the use of a video store to delay the video, ~`~ the use of the secondary heads to advance the audio timing with respect to the video on the tape is substantially less expensive.
Figure 7 shows a simplified block diagram of the recording system according to the present invention.
Blocks functioning the sal~e as those in the prior art system of Figure 1 use the same reference numerals with a prime mark ('). As in Figures l through 3, additional processing may be provided in practical embodiments, for example, the audio signals will usually be modulated onto a carrier prior to recording and demodulated after playback. Such details are well known in the art and are ., .

lZ~3 omitted here for clarity.
Unlike the prior art system of Figure 1, the recording system of the embodiment of Figure 7 includes a scanner having four heads. Although the scanner could have an angular head position configuration such as shown in Figure 6A, the conEiguration is shown as discussed above with dual gap heads located 180 degrees apart, the pairs of heads Sl/S2 and Pl/P2 being in vertically separated planes with respect to the scanner axis of rotation. The video information is applied to a switching means 42 that applies that information to the Sl and S2 heads. The time compressed audio information from block 10' is applied to a switching means 44 that applies that information to the Pl and P2 heads~ The vertical ofeset between the S and P head pairs is two tracks such that time compressed audio is recorded two tracks spaced apart ~rom the video information that is si~ultaneously recorded. The head offset is such that when the magnetic tape is played back the time compressed audio is advanced in time with respect to the video information by virtue o~
being recorded in a track that is reproduced earlier than the track in which the video was simultaneously recorded.
Figure 8 depicts the track pattern of a magnetic tape recorded with an arrangement such as the embodiment of Figure 7. The time compressed audio information is recorded in a first segment along each consecutive track.
Video information is recorded in the following segment.
The time compressed audio in each track is recorded two tracks or scans earlier than the video in the same track.
For example, the left hand track carries audio from scan n+l, odd and the video from scan n, odd. Thus when any track is played back the audio information is advanced in time with respect to the relative timing of the audio and video information that was simul-taneously applied to the 3S recording heads. The switching means 42 and 44 under ~2~

control of a signal derived from the scanner rotational position switch the audio and video signals as necessary to achieve the desired pattern and as explained above, a signal derived from the scanner position also causes the time compressed audio to be read out at the appropriate times.
It is anticipated that tapes recorded with time advanc-ed audio will contain some information (such as an identifica-tion bit) to signify the fact so that playback mac~l~nes capable of correcting the normal time delay can change modes and still reproduce the time advance recording properly. O~herwise, the playback machines which can correct ~or the normal time delay would reproduce the advanced audio recorded tapes with the audio advanced approximately 30 MS with respect to the video.
It will be appreciated that the invention is not limited to the specific track layout as shown in Figure 8 in which there is an audio segment followed by a video segment. For example, the audio information can be recorded in two seg~ents at the beginning and end of each track.
Optionally, buffer memories can be included in the embodiment of Figure 7, either by locating one such memory after block 10' or one in each line from blocks 6' and 8'.
By providing more than the required time advance with the pairs of heads P and S, exact synchronism in playback on machines having known delay characterisiics can be provided by the delay in the buffer memories.

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Claims (10)

Claims

1. A helical scan recording system for recording a plurality of signals on a magnetic medium in a series of sequential discontinuous tracks, comprising signal processing means receiving said signals, said means including means for com-pressing at least one of said signals along the time axis, and recording means receiving the signals from said signal processing means including said at least one time compressed signal, said recording means including a rotating assembly carrying a plurality of recording heads scan-ning said tracks for recording signals along said tracks, said means recording said at least one time compressed signal on tracks spaced apart from tracks on which said other signal or signals is or are simultaneously recorded such that when the magnetic medium is played back said at least one time compressed signal is advanced in time with respect to said other signal or signals received by the recording means by virtue of being recorded in a track that is played back earlier than the track in which said other signal or signals were simul-taneously recorded.

2. A helical scan recording system according to claim 1 wherein said plurality of recording heads includes first and second pairs of recording heads, the pairs of recording heads carried by said rotating assembly offset with respect to each other such that one pair of heads records at least portions of tracks earlier in time than when the other pair of heads records at least portions of the same tracks.

3. A helical scan playback system according to claim 2 wherein the offset of said pairs of recording heads is at least substantially equal to the offset required to restore the relative timing of the signals when played back.

4. A helical scan playback system according to claim 2 wherein said signals are the audio and video portions of a television signal and the audio portion is the signal compressed along the time axis.

5. A helical scan playback system according to claim 4 wherein the offset of said pairs of recording heads is at least substantially equal to the offset required to restore the synchronism of the audio and video portions of the television signal.

6. A method of recording signals in a series of sequential discontinuous tracks on a magnetic medium using a plurality of rotating magnetic heads, the signals including at least one signal compressed along the time axis, comprising recording said at least one time com-pressed signal on tracks spaced apart from tracks from which said other signal or signals is or are recorded such that said at least one time compressed signal is advanced in time with respect to said other signal or signals by virtue of being reproduced earlier than the track in which said other signal or signals were simultaneously recorded, and processing said signals, including ex-panding said at least one time compressed signal.

7. A recording method according to claim 6 wherein said time advance is at least substantially equal to the time advance required to restore the relative timing of the signals prior to their application to the recording system.

8. A recording method according to claim 7 wherein said signals are the audio and video portions of a tele-vision signal and the audio portion is the signal compressed along the time axis.

9. A recording method according to claim 8 wherein said time advance is at least substantially equal to the time advance required to restore the synchronism of the audio and video portions of the television signal.

10. A recorded video tape having a plurality of adjacent tracks which bear video information and time compressed audio information, the time compressed audio information in each track being advanced in time with respect to the video information.