Dropout
In a video tape, the magnetic coating is not completely homogenous and with video track widths smaller than the diameter of a human hair; even a microscopic blemish in the magnetic coating will delete some picture information. As the tape ages, slight contamination by dust and metallic particles and oxide-shedding effects will aggravate the situation. The effect of these tiny blemishes is a momentary loss of replay signal known as a dropout. Unless dropouts can be ‘masked’ in some way, a disturbing effect will take place in the form of little ragged black or noisy ‘holes’ in the reproduced picture.
In practice, the video information on one TV line is usually very much like that on the preceding line; so that if we can arrange to fill in any dropout ‘holes’ with the video signal from the corresponding section of the previous line, the patching job will pass unnoticed. What’s required, then, is a delay line capable of storing just one TV line of 64 fls duration, so that whenever a dropout occurs we can switch to the video signal from the previous line until it has passed. This is known as DOC. For delay-line bandwidth reasons, this is difficult to achieve at video baseband frequen- cies, so it is carried out on the FM signal before demodulation. To avoid disturbance on the picture, the switching has to be very fast, virtually at picture-element rate. One form of the DOC technique outlined above breaks down when any dropout exceeds one TV line-duration, and this happens often – the tape area occupied by one line is microscopic. If the dropout period exceeds one line, the disturbance will be visible on the TV screen. To prevent this, further steps must be taken. Modern designs recy- cle the last ‘good’ TV line around a delay line and read it out continuously for the duration of the dropout.
Crispening
Following demodulation and de-emphasis, a technique known as picture crispening or picture sharpening is carried out. The 3 MHz capability of a standard VCR means that replayed pictures will lack the sharpness and definition of an off-air picture; and fine detail, for example the frequency gratings of a test pattern, will not be reproduced. Very little of the content of an average picture consists of fine repetitive detail and the subjective effect of limited HF response is a blurring of sharp vertical edges. The crispening circuit goes some way to compensate for HF roll-off by artificially ‘sharpening up’ vertical edges in the picture. The technique has been used in broadcast and CCTV for many years to compensate for the finite scanning spot size in cameras and fixed satellite services systems. In such applications, it is known as aperture correction.