Re: Ampex Quad scanning
By 'comet tail', do you mean white dots in a regular pattern which is what a quad can produce, or on video highlights?
The term usually applies to highlight smearing on tubed cameras and arises from a bright light/reflection not being fully discharged.
In a quad, there's a potential big problem with head switching because it's in the visible frame time (unlike, say, a VHS two head machine where the switching can only be seen at the bottom of the picture). If the head switch timing is wrong, there can be no video during the head changeover and the FM limiter produces noise, seen as white. There's twenty segments per field on 625/50 quad, so each field has 312.5/20 lines = 15.6 lines (double that per frame). The visible artefact is usually a zig-zag pattern of dots, usually 'trembling' a little as the servos can never be 100% stable.
Best regards,
Paul M
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