ALIGNMENT TAPE
For each format a reference cassette, called an alignment or interchange tape, is available from videorecorder manufacturers. It is recorded on a specially produced and aligned ‘model’ machine at the factory, and all its characteristics are centre-tolerance. It contains colour-bars, monochrome step-wedge, and test-card images for checks on all aspects of the replay machine. An important feature is an r.f. sweep signal in which the entire tape-frequency spectrum is covered in one 20 ms field period. An oscilloscope connected to the f.m. replay signal and triggered from the head flip-flop waveform will thus display a graph of overall replay frequency response; suitably- positioned markers facilitate the check and evaluation of the response of the video heads and preamplifers. An oscilloscope-type display of the r.f. sweep pattern is reproduced in Fig. 14.12.
Audio aspects of the alignment tape include an l.f. tone for audio level checking and adjustment, at an accurately specified frequency for replay capstan speed checks. An h.f. tone checks A/C head azimuth setting. Test tapes are also available with Hi-Fi sound tracks; and recorded to LP specifications for VHS and V8 formats.
Alignment tapes are expensive, and the test sections on them very short. To avoid wear, and particularly to eliminate the risk of dam- age, they should never be used where their particular qualities are not essential, and should never be placed in a machine which has not been proved mechanically safe. Much work which requires the use of an alignment tape can be carried out with a locally recorded substitute: take a high-grade tape and record colour-bars, grey-scale step-wedge and test card, with tones from an audio generator or the broadcast signal. Use a new, well set-up machine on which the real alignment tape plays perfectly at tracking-control centre. For all applications (except sweep-checking of video f.m. preamps) use this tape, followed if necessary by one pass of the alignment tape as a final check. In the text that follows the term ‘test tape’ refers to the alignment tape or the above-described substitute.