GB2115595A - Magnetic tape recording - Google Patents

Abstract

A transverse-scan or helical-scan magnetic tape recorder may have its playback head adjustable axially of the drum, so as to ensure that it can be fully aligned with the track it is supposed to be reading, and also for other reasons. In this invention the playback head 22 can be moved to a position 22c ahead of both the record head 20 and the erase head 18 such that while the record head is recording, the playback head plays back signals recorded on the tape prior to their being erased by the erase head. The played back signals can be subjected to a processing operation, e.g. mixing or special effects, and the processed signals then applied to the record head for re-recording on the same tape. In this way mixing and special effects can be achieved with simple operating procedures and with one less recording than hitherto required. <IMAGE>

Description

SPECIFICATION Magnetic tape recording This invention relates to magnetic tape recording, and is concerned with a magnetic tape recorder and with a method of operating a magnetic tape recorder. Magnetic tape recording is used in a variety of fields, particularly for recording video signals, for audio recordings, or for storing data generally in data processing and computing.

In all these applications there is a frequent need to edit the signals recorded on the tape. That is to say, the signals are played back from the tape, subjected to some processing operation, and rerecorded on tape, in the same relative position as that which was read. The type of processing operation is not particularly material but will involve modification of the signals in some way.

For example in video signal processing it may be desired to add captions to the picture. In data processing it may be desired to amend or up-date some of the data.

Hitherto this editing has required either two tape recorders (e.g. for video or audio recordings) or intermediate storage of the data while the tape is reversed to permit rewriting (e.g. in computing).

Thus considerable cost or complexity is involved.

In video recording it is known to use a recorder of the type having separate record and playback heads with a tape path such that the heads make repeated passes across the tape in an obtique or transverse direction to the direction of tape movement. Such recorders are known as transverse-scan or helical-scan recorders, and in principle could be used for other types of signal storage than just video recording. In video recording, however, it is known to provide a degree of relative movement to the playback head in a direction transverse to the length of the tracks. This is required for the following reasons.

The first reason is that it is necessary to move the playback head slightly to ensure that it is fully aligned with the track it is supposed to be reading.

The second reason arises because the playback head is used differently when recording from when playing back. When playing back from the tape the playback head is synchronised correctly with the tape. However, during recording it is the record head which is thus synchronised, but the playback head is still used to play back the just-recorded signal to provide confirmation that it has recorded correctly. There is a finite spacing between the record and playback heads, and during the time interval that the playback head takes to reach a position vacated by the record head, the tape will have moved slightly; typically by one-third of the distance between adjacent tracks. In the record operation therefore the playback head has to move by about one-third of the track spacing relative to its position during normal playback.

If the recorder is designed to allow the tape to move backwards, the playback head then has to be moved by one-third of the track spacing in the other direction.

The recorder may be designed to read from a single television field with the tape stationary. As the tape is not moving the apparent slant of the track changes by one track spacing at one end of each pass. Furthermore the exact track position may be anywhere along the tape so the head must be movable by up to one track spacing to be sure of being capable of lying exactly over one track. A total movement of two track spacings may therefore be provided, with suitable control systems for the playback head to track a stationary tape so as to be exactly on one of the recorded tracks.

An example of such a recorder is the Ampex VPR2. In this recorder the playback head is mounted for movement by means of a crystal controlled automatically to perform the various requirements described above.

This invention is based on the appreciation that such a playback head mounting system can be used to provide editing and like facilities at reduced cost and complexity relative to the existing equipment.

In a first aspect the invention provides a method of operating a magnetic tape recorder, the recorder being of the type comprising a record head, an erase head preceding the record head, and an adjustable playback head separate from the record head, and the heads making repeated passes across the tape in an oblique or transverse direction to the direction of tape movement, in which the playback head plays back signals prior to their being erased by the erase head, these signals are subject to a processing operation, and the processed signals are applied to the record head for re-recording on the tape in the same position as the played back signals without interruption of the tape movement.

Normally the processing operation will incorporate a delay which compensates for the spacing between the playback and record heads.

In a second aspect the invention provides a magnetic tape recorder comprising a record head, an erase head preceding the record head, an adjustable playback head separate from the record head, and means defining a tape path such that the heads make repeated passes across the tape in an oblique or transverse direction to the direction of tape movement, and further including control means operable selectively to move the playback head transversely of the track direction so as either to play back signals just recorded by the record head or to play back signals prior to their being erased by the erase head.

The invention will now be described in more detail by way of example with reference to the drawings,in which: Figure 1 is a schematic view of the drum of a video tape recorder of the C-format type; Figure 2 is a view illustrating the tracking path over the tape in the recorder of Figure 1; Figure 3 illustrates a section of tape carrying a plurality of tracks; Figure 4 illustrates in more detail the operation of the recorder in accordance with this invention; Figure 5 is a schematic block diagram of a known video editing method; Figure 6 is a schematic block diagram of a video editing method embodying the invention; and Figure 7 is a schematic block diagram showing how a single recorder can be used for editing in accordance with the invention.

Figure 1 illustrates a video tape recorder of the C-format type. The recorder has a cylindrical drum 10 around which passes a magnetic tape 12 which passes round almost the entire periphery of the drum. The tape 12 is guided onto the drum by an entrance guide 14 and off the drum by an exit guide 16. The drum 10 contains three video heads, namely an erase head 18, a record head 20, and a playback head 22. The drum rotates in the direction of the arrow A which is against the direction of tape movement indicated by the arrows B.

As seen in Figure 1, the tape is fed onto the drum at a different height from that at which it is taken off the drum. This is more clearly seen in Figure 2 which shows the tape 12 between the entrance guide 14 and exit guide 16, or in other words is a view taken round the circumference of the drum, through to a different scale from Figure 1. As the drum rotates the video heads will form oblique tracks 24 on the tape 12. The recorder is thus known as a helical-scan recorder in view of the somewhat helical path which the tape follows around the drum. The recorder lays down a plurality of parallel tracks on the tape as indicated schematically in Figure 3.

The recorder is so constructed that each pass or track can accommodate one television field. Thus the field blanking interval falls in the period where the relevant head passes between the entrance guide and the exit guide. It is therefore vital that the rotation of the drum is synchronised to the television signal which is being recorded or played back.

As shown in Figure 1 , the three video heads are equally spaced around the drum periphery at 120 degree spacings. In principle such even spacing is not essential, though practical considerations dictate that the heads cannot be superimposed on each other and so there is bound to be some spacing between the heads. The order of the three heads is (1) erase (2) record (3) playback.

During playback of a signal recorded on the tape, the playback head 22 is synchronised to the television field scan and is aligned with the recorded tracks.

However, during the record operation it is the record head 20 which is synchronised with the television field scan and is aligned with the recorded tracks. The erase head 18 precedes the record head 20 by one-third of a rotation. Between the instant when the erase head erases a given point on a track and the instant when the record head passes over that same point there is a duration of one-third of a drum rotation. In this interval the tape will have moved longitudinally by one-third of the inter-track spacing. Thus, referring to Figure 4, if the record head 20 passes over a track 30 on the tape, the erase head will have to transverse a path 32 which is one-third of the inter-track spacing x ahead of the track 30. Thus the erase head 18 must be axially displaced on the drum 10 relative to the record head 20 by a fixed amount equal to one-third of the track spacing.

During the record operation, the playback head 22 may be used to confirm that the recording operation has worked properly. The playback head 22 follows the record head by one-third of a drum rotation, so in this instance the playback head 22 adopts a position 22a on a path 34 which is onethird of the track spacing x after the path 30 followed by the record head. Thus the playback head is axially displaced on the drum relative to the record head by one-third of the track spacing but in the opposite direction from the erase head.

During normal playback when the playback head 22 is itself synchronised to the television signal, the playback head 22 follows the track 30 and thus will be at the position shown in 22b in Figure 4. In known recorders such as the Ampex VPR2 the playback head is adjustably mounted on a crystal to accommodate this movement.

We have appreciated, however, that the playback head 22 could alternatively be moved by two-thirds of the intertrack spacing (2xl3) in the opposite direction, to the position 22c shown in Figure 4. Then the playback head would read the immediately preceding track 36, it being seen that it is exactly one track spacing (x) ahead of the position 22a. Thus instead of monitoring the signal which has just been recorded, the playback head will now monitor the signal which is about to be erased by the erase head 18.

It is assumed here that the normal full-width erase head which wipes the whole tape clean prior to reaching the drum 10 is switched off.

Figure 5 illustrates a conventional video editing system. Three video tape recorders (VTR's) are used, namely VTR 40 which carries the main signal, VTR 42 which carries the material to be added or edited in, and VTR 44 which carries the edited or combined signal. A mixer 46 is used to mix the outputs of VTR's 40 and 42. The cost of three VTRsis very substantial. If an external source is being used, this takes the place of one of the VTRs 40 and 42.

Figure 6 shows an editing system embodying the invention. The same facilities are provided as in the system of Figure 5 but only two VTRs are required. A first VTR 50 is arranged to play back the signal on the tape immediately prior to its being erased. That is, the playback head is moved forward of the erase and record heads to the position 22c shown in Figure 4. The output of VTR 50 is passed through a synchroniser 52 to one input of a mixer 54, which also receives the output of the second VTR 56. The mixer output is then applied back to the VTR 50 and is recorded on the tape. In this way the signal is read from the tape, modified, e.g. to include captions, and put back onto the same tape again, in the same track position, with interruption to the continuous running of the tape.

It will be appreciated that the signal read by the playback head is two-thirds of a drum rotation, i.e.in this instance two-thirds of a field, ahead of the corresponding position of the record track.

Thus the playback head output must be delayed by two-thirds of a field before being applied to the record head of recorder 50. The purpose of the synchroniser 52 is to provide this delay. The synchroniser can in principle be located anywhere in the loop between playback and record heads.

Preferably the recorder timebase corrector circuits would be expanded to provide a sufficient delay period, but it could for example be positioned after the mixer 54 if the VTR 56 is appropriately resynchronised.

Another advantage of this system over that of Figure 5 arises when several different subsidiary tapes or external sources are to be edited in turn into a main tape. With the system of Figure 5, after each effect has been added, the tape must be removed from recorder 44 and replaced on recorder 40 before the next source can be edited in. Alternatively all the switching and synchronising circuits must be changed over so that recorders 40 and 44 affectively interchange their functions. With the Figure 6 arrangement none of this is necessary as the principal tape always remains on recorder 50.

Figure 7 illustrates the use of a single VTR 60 to produce special effects. A synchroniser 62 is connected to the output of the playback head of the VTR 60 and provides a two-thirds field delay.

A special effects source 64, such as a Quantel 3001, is also connected to the VTR output. This also provides the necessary delay function. A mixer 66 then combines or selects from the outputs of the synchroniser 62 and source 64 to provide a signal for re-recording on the tape. As indicated, the output 68 of another external source can also be applied to the mixer.

The audio and timing tracks on the tape have not been discussed above. An advantage of the system is that such tracks as run longitudinally at the tape edges can remain undisturbed by rhe playbackieditirecord sequence.

It will be seen therefore that by providing for adjustment of the position of the playback head when the record head is in operation the single VTR can be made to provide functions which have previously required two different VTRs, at the additional cost only of a synchroniser circuit.

The system can be adapted for use with other recording formats which involve a plurality of parallel tracks, including the transverse scan format, and also the B-format in which each field is made up of 16 separate tracks. The signals need not be video signals but can be audio signals, or can be any data stream such as may be encountered in computer or data processing applications.

Claims (6)

1. A method of operating a magnetic tape recorder, the recorder being of the type comprising a record head, an erase head preceding the record head, and an adjustable playback head separate from the record head, and the heads making repeated passes across the tape in an oblique or transverse direction to the direction of tape movement, in which the playback head plays back signals prior to their being erased by the erase head, these signals are subject to a processing operation, and the processed signals are applied to the record head for re-recording on the tape in the same position as the played back signals without interruption of the tape movement.

2. A method of operating a magnetic tape recorder substantially as herein described with reference to Figure 6 or Figure 7 of the drawings.

3. A magnetic tape recorder comprising a record head, an erase head preceding the record head, an adjustable playback head separate from the record head, and means defining a tape path such that the heads make repeated passes across the tape in an oblique or transverse direction to the direction of tape movement, and further including control means operable selectively to move the playback head transversely of the track direction so as either to play back signals just recorded by the record head or to play back signals prior to their being erased by the erase head.

4. A recorder according to claim 3, in which the record, erase and playback heads are accommodated in a rotating drum and the tape path comprises a helical path around the drum periphery, the playback head being movable axially of the drum.

5. A recorder according to claim 4, in which the record, erase and playback heads are substantially equi-spaced around the drum circumference and the playback head is movable between a position one-third of a track spacing behind its normal playback position to a position two-thirds of a track spacing ahead of its normal playback position.

6. A magnetic tape recorder constructed and arranged to operate substantially as herein described with reference to Figures 1 to 4 of the drawings.