[julie_m]

VHS can record 405-line video signals, because it is basically treating a field as the smallest unit -- it doesn't care what happens within each field, it just records the lines and sync pulses and reproduces them faithfully. And both 405 and 625 line systems used 50 fields per second. All VHS cares about is for the field to begin at the moment when the recording head has just begun its traverse across the width of the tape, so one field is recorded across the tape in exactly one half-turn of the drum.

DVD does care about the line standard, because it records each line separately and generates the sync pulses between one line and the next, and between one field and the next, at the time of playback. The field is compressed as a whole, taking into account the differences between each field and the previous one if it is not the first, and then recorded to disc. It probably won't know about 405-line pictures, and will make a mess of trying to record them as the timing of the sync pulses will be wrong.