Mike Jordan
When I was watching an old video obviously transferred from VHS on YouTube about Wood Norton in the 1980s (ah memories!!), it suddenly occurred to me that the line tearing always seems to be at the bottom of frame.
Why is this? Surely, if the tearing is caused by start of picture relative to sync pulses, it should be at the top of frame.
Chris Woolf
It depends on what effect you are describing as "tearing".
The picture and the line sync pulses are never grossly mistimed relative to each other on helical scan tape, since the frame recording is continuous. Most of the pulling of lines off to one side – is that what you are seeing? – was due to the monitor having a flywheel sync, and the tape producing syncs that creep out of time with that.
Flywheel syncs enabled monitors/TVs to keep acceptable pictures even if the sync pulse was a little noisy and tricky to decipher. But helical scan VTs (beforeTBCs) have clean syncs but inherently poor frame timing. Besides the drum needing to be running at a correct frame sync speed the length of tape stretched round the drum needed to be identical in replay to what it was in record. That required a tension control. In VHS it was fixed; in posher machines it was a manual control, and in fancy ones a servo compared the line sync timing at the start and end of picture and wibbled a spring arm to get everything to match.
Other effects on basic machines were the problem of switching from one field/frame to the next. The video head would run off the end of the helix at the "bottom" and have to start again at the "top". On half-wrap machines with two heads the switch would cause a ragged line which would have a different timing. On full-wrap machines the single head would leave a hole at the bottom of frame. Both would upset sync circuits and cause pulling or tearing.
And lastly, the velocity errors – intra-line timing – were pretty horrendous and some fussy monitors (or the sync circuits of downstream recorders) would struggle to follow. This would produce the small-scale sideways jagging of lines.
Of course as soon as cheap digital TBCs (really cheap memory) became available all such analogue complexities were consigned to oblivion…. thank goodness.
