Track configuration
A typical track layout for a two-head helical VCR appears in Figure 21.16. Here we can see the video tracks slanting across the tape, shaded for head A, white for head B. At the edges of the tape, further tracks are present: the lower carrying a control track (serving a similar purpose to the sprockets in a cine film, and described later) and the upper carrying the sound track. Sound is recorded longitudinally in the same way as in an audio recorder, but with limited frequency response due to the low linear tape speed in the sorts of VCR we are dealing with. Tape is 12.65 mm (VHS) or 8 mm (Video-8) wide. It progresses at speeds between 22.4 and 10.06 mm/s, depending on format and mode.
Scanning systems
In the original VCR plan, a guard band was left between video tracks on the tape. This was true of the first machine to appear on the domestic scene, the Philips N1500. Linear tape speed here was over 14 cm/s, and the track configuration is shown in Figure 21.17. It can be seen that each video track is spaced from its neighbours by an empty guard band, so that if slight mist racking should occur, cross-talk between tracks could not take place. Each video track was 130 flm wide and the intervening guard bands 57 flm wide. This represents relatively low packing density of infor- mation on the tape, and it was soon realised that provided the tape itself was up to it, a thinner head could be used to write narrow tracks; if the lin- ear tape speed was also slowed down the tracks could be packed closer together, eliminating the guard band. Using both ideas, tape playing time for a given spool size could be doubled or trebled. First, the problem of cross-talk had to be solved. Even if the mechanical problems in the way of perfect tracking could be overcome so that each head always scanned down the middle of its intended track, cross-talk would occur due to the influence of adjacent tracks, and the effect on the reproduced picture would be intolerable. A solution to this problem was found in the form of azimuth recording.
The azimuth technique
For good reproduction from a tape system, it is essential that the angle of the head gap on replay is exactly the same as was present on record with respect to the plane in which the tape is moving. In an audio system, the head gap is exactly vertical and at 90° to the direction of tape travel. If either the record or replay head gap is tilted away from the vertical, even by a very small amount, tremendous signal losses occur at high and medium frequencies, the cut-off point travelling further down the fre- quency spectrum as the head tilt or azimuth error is increased. If the same head is used for record and replay (as is usually the case in audio tape
recorders), the azimuth error will not be noticed, because there is no azimuth difference between record and replay systems.
This azimuth loss effect is the key to successful recording and replay of video signals without a guard band. Let’s designate our video heads A and B, and skew A’s head gap 15° clockwise and B’s head gap 15° anti- clockwise as in Figure 21.18a. This imparts a total 30° difference in azimuth angle between the two heads, and the result is video tracks on the tape like those in Figure 21.18b.
With the built-in error of 30°, head A will read virtually nothing from head B’s tracks, and therefore the guard band can be eliminated. This was the modus operandi of the Philips VCR-LP format, using the same cassette and virtually the same deck layout as the original VCR format, but with linear tape speed reduced by 50% , and video track width down to 85 flm. It worked, and the 2-h machine was a reality. Subsequent formats use a smaller azimuth tilt: 6° for VHS and 10° for Video-8. The offset between heads is double this figure in each case.