GB2160059A - Digital video tape recorders - Google Patents

Abstract

A digital video tape recorder comprises a head drum 1, four transducer heads A to D mounted in pairs on the head drum 1 for recording oblique tracks on a magnetic tape 2, a demultiplexer 5 for distributing digital data to be recorded and corresponding to a video signal such that each field of the video signal is divided into 50-line segments, the digital data corresponding to each field are evenly distributed between all four of the heads A to D for recording, and each track comprises a block of digital data relating to one 50-line segment and a block of digital data relating to another 50-line segment, the two blocks of digital data being separated by an edit gap. <IMAGE>

Description

SPECIFICATION Digital video tape recorders This invention relates to digital video tape recorders (DVTRs).

One previously proposed form of DVTR has, as indicated in Figure 1 of the accompanying drawings, four rotary recording and reproducing heads A, B, C and D mounted on a rotary head drum 1, the heads A to D being arranged in two pairs A and B, and C and D. An input analog video signal is sampled and the resulting samples are pulse code modulation coded to form digital data for recording by the DVTR in oblique tracks on a magnetic tape 2. For recording, the data are divided into 50-line segments and the samples within each of these segments are equally distributed between the two heads of a pair.For a 625-line 50Hz (50 fields per second) television system there are assumed to be three- hundred active lines per field, and for a 525-line 60Hz (60 fields per second) television system there are assumed to be twohundred-and-fifty active lines per field, so in 625line operation a field occupies twelve tracks, while in 525-line operation a field occupies ten tracks.

This is indicated diagrammatically in Figure 2 of the accompanying drawings, which shows spatially the oblique tracks designated A, B, C and D to correspond to the heads A, B, C and D that recorded them.

It will be seen from Figure 2, and possibly seen more clearly from Figures 3A and 3B of the accompanying drawings, which show on a time scale which of the heads A to D are recording a 525- line and 625-line operation, respectively, that in 625line operation, because each field occupies twelve tracks, the first fifty lines of each field are always allocated to the same pair of heads A and B or C and D. In 525-line operation, however, because each field occupies only ten tracks, the first fifty lines of each field are alternately allocated to the pairs of heads A and B, and C and D.

In both 625-line and 525-line operation some problems arise in severe error conditions. Thus if, for example, the head A suffered a head clog causing total drop-out, then in 625-line operation alternate 50-line segments are affected, and this results in a banding effect with three bands per picture.

On the other hand, in the event of a similar head clog in 525-line operation, the banding effect changes position from field to field and this results in flicker.

According to the present invention there is provided a digital video tape recorder comprising: a head drum; four transducer heads mounted in pairs on said head drum for recording oblique tracks on a magnetic tape, said pairs of heads being at an angular spacing of 180"; and a demultiplexer for distributing digital data to be recorded and corresponding to a video signal such that each field of the video signal is divided into segments, each said segment comprising the digital data corresponding to a group of successive lines of the field; wherein the digital data corresponding to each field are evenly distributed between all four said heads for recording, and each said track comprises a block of digital data relating to one said segment and a block of digital data relating to another said segment, said two blocks of digital data being separated by a gap.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: Figure 1 shows diagrammatically a rotary head drum of a DVTR; Figure 2 shows diagrammatically tracks recorded on a magnetic tape by the DVTR of Figure 1; Figures 3A and 3B show diagrammatically 525line and 625-line operation respectively of four recording heads of the DVTR of Figure 1; Figure 4 shows diagrammatically an embodiment of DVTR according to the present invention; Figures 5A and 5B show diagrammatically the division of a 525-line field and a 625-line field into five and six segments, respectively; Figure 6 shows diagrammatically the various parts of a single track recorded on a magnetic tape by the DVTR of Figure 4;; Figures 7A and 7C show diagrammatically which heads are recording in respective different modes of 625-line operation; Figures 7B and 7D show diagrammatically which heads are recording in respective different modes of 525-line operation; and Figures 8A and 8D show diagrammatically tracks on a magnetic tape and correspond respectively to Figures 7A to 7D.

Referring to Figure 4, the DVTR to be described comprises a head drum 1 which in operation is rotated one-hundred-and-fifty times per second, and on which are mounted four recording and reproducing heads A, B, C and D arranged in two pairs A and B, and C and D, the two pairs being disposed 180 apart. A magnetic tape 2 is wrapped around the drum 1 over an angular range of some 240 to 330".

An input analog video signal is supplied by way of an input 3 to a coder 4 where the input signal is sampled and the resulting samples are pulse code modulation coded to form digital data for recording. These digital data are supplied from the coder 4 to a demultiplexer 5 which distributes the data to four outputs A, B, C and D corresponding respectively to the heads A,B,C and D.

As shown in Figures 5A and 5B, in 525-line 50Hz operation each field is assumed to have twohundred-and-fifty active lines and these are divided into five segments each of fifty lines, while in 625line 50Hz operation it is assumed that each field has three-hundred active lines and these are divided into six 50- line segments. The number of lines within each segment can of course be varied without changing the number of segments per field, if a slightly different number of active lines per field are required. The digital data within each segment may be shuffled.

In recording the oblique tracks on the tape 2, each track recorded by each of the heads A to D is recorded as two half- tracks with a gap therebetween comprising a mid-track edit point, and each track contains recorded data relating to two different segments in the two half-tracks respectively.

Such a track is shown diagrammatically in Figure 6 and comprises two video data blocks VI and V2, the video data blocks V1 and V2 being preceded and followed at the beginning and end of the track by audio data blocks Al and A2. Each data block is preceded by a run-in period R1, R2, R3 or R4 respectively to provide the necessary indication on decoding that a data block follows. Additionally, at the three points between data blocks in the track, that is between the data blocks Al and V1, V1 and V2, and V2 and A2, there is an edit gap El, E2 or E3, respectively. Each of the edit gaps El to E3 can be used after the data have been reproduced where a gap is required for editing purposes, and in particular where a gap is required to enable signals being edited to be synchronised.If required, more than two audio data blocks may be provided in each track, and our one or more audio blocks may be provided between the video data blocks.

Referring again to Figure 4, the operation of the demultiplexer 5 will now be described for 525-line and 625-line operation. Basically the operation of the multiplexer 5 is such that the data relating to each 50-line segment are distributed to all four of the heads A to D. Thus, rather than the data of each 50- line segment occupying two oblique tracks as in the previously- proposed DVTR, the data of each 50-line segment occupy four halftracks in this embodiment, so a 525-line field occupy twenty half-tracks and a 625-line field occupies twenty-four half- tracks.

The operation of the demultiplexer 5 will now be considered in more detail with reference to Figures 7A to 7D and 8A to ED. One mode of 625-line operation will first be considered with reference to Figure 7A. In this figure the top line indicates the recording operation of the heads A and B, while the lower line indicates the recording operation of the heads C and D. Associated with each of these lines the numbers relate to the 50- line segments 1 to 6 of a 625-line field as indicated in Figure 5B.

Thus, for example, the data relating to segment 1 are evenly distributed between all four of the heads A to D, so that in each field they occupy a first half-track scan by the heads A and B and a first half-track scan by the heads C and D. As indicated, the data of one field are evenly distributed over twenty-four half-track scans. The same mode of operation is indicated in Figure 8A which shows diagrammatically a series of pairs of oblique tracks with the numbers thereon corresponding to the segments 1 to 6 and the letters corresponding to the heads A to D.

Figures 7B and 8B show a similar mode of 525line operation. In this case the data of each 50-line segment are again evenly distributed to all four of the heads A to D and again the data for each 50line segment occupy four half-tracks. Thus the data of segment 1, for example, in a first field occupy a first half- track scan by the heads A and B and a first half-track scan by the heads C and D, and in a second field occupy a second half- track scan by the heads A and B and a second half-track scan by the heads C and D.

An alternative mode of 625-line operation is similarly illustrated in Figures 7C and 8C. The difference relative to the mode of Figures 7A and 8A is that the data relating to segment 1, for example, occupy a first half-track scan by the heads A and B and a second half-track scan by the heads C and D.

In practice, the main difference between this mode and the mode of Figures 7A and 8A is that the total time occupied in recording or reproducing a field is slightly less in the mode of Figures 7C and 8C, as compared with the mode of Figures 7A and 8A.

This has the advantage that preceding and subsequent processors which are to handle the data need less storage capacity.

Likewise, an alternative mode of 525-line operation is illustrated in Figures 7D and 8D. The difference relative to the mode of Figures 7B and 8B is that the data relating to segment 1, for example, in a first field occupy a first half-scan track by the heads A and B and a second half-scan track by the heads C and D, and in a second field occupy a second half-scan track by the heads A and B and a first half-scan track by the heads C and D.

In all the cases described, for both 625-line and 525-line operation, the even distribution of the data between all four of the heads A to D means that in error conditions an improved reproduced picture can be obtained as compared with the previouslyproposed DVTR. Thus if, for example, the head A suffered a head clog, then in all cases there would be a twenty- five percent loss of reproduced data, but the lost data would be evenly distributed over the whole of each field. This would enable optimum operation of any correction and concealment processes, and in particular there would be no banding or flickering, but merely a possible overall degradation of the picture, which is much less noticeable or objectionable to a viewer. Moreover, the provision of the half-track scans results in convenient field edit points in both 525-line and 625line operation.

Claims (9)

1. A digital video tape recorder comprising: a head drum; four transducer heads mounted in pairs on said head drum for recording oblique tracks on a magnetic tape, said pairs of heads being at an angular spacing of 180"; and a demultiplexer for distributing digital data to be recorded and corresponding to a video signal such that each field of the video signal is divided into segments, each said segment comprising the digital data corresponding to a group of successive lines of the field; wherein the digital data corresponding to each field are evenly distributed between all four said heads for recording, and each said track comprises a block of digital data relating to one said segment and a block of digital data relating to another said segment, said two blocks of digital data being separated by a gap.

2. A digital video tape recorder according to claim 1 wherein said head drum is rotated onehundred-and-fifty times per second, and in 525-line 60Hz operation said demultiplexer is operative to divide each field into five segments and to divide the digital data corresponding to each said segment into four said blocks of digital data so that said digital data relating to a field are recorded in twenty half-tracks.

3. A digital video tape recorder according to claim 2 wherein in 625-line 50Hz operation said demultiplexer is operative to divide each field into six segments and to divide the digital data corresponding to each said segment into four said blocks of digital data so that said digital data relating to a field are recorded in twenty-four halftracks,

4. A digital video tape recorder according to claim 3 wherein each said segment comprises fifty active lines of a field.

5. A digital video tape recorder according to any one of the preceding claims wherein said gap comprises an edit gap followed by a run-in period followed by one of said blocks of digital data.

6. A digital video tape recorder according to claim 1 wherein each said track comprises in sequence a block of digital audio data, a gap, a said block of digital video data, a gap, a said block of digital video data, a gap and a block of digital audio data.

7. A digital video tape recorder according to claim 6 wherein each said gap comprises an edit gap followed by a run-in period.

8. A digital video tape recorder substantially as hereinbefore described with reference to Figure 4 of the accompanying drawings.

9. A digital video tape recorder substantially as hereinbefore described with reference to Figures 4 to 8 of the accompanying drawings.