Anyone who has used VHS video tapes for time-shifting TV programs will have noticed a curious phenomenon.

The first 5-or-so frames of each and every thing you've recorded at the start of the tape will be visible. Watching a tape from the very begining will show a disjointed montage of adverts and continuity announcements in quarter-second bursts.

What can possibly cause this? It is as if, for every recording made, the machine starts recording fractionaly further into the usable area of the tape. However, if you rewind to the start of the tape for playback, you can see the snatch of non-overwritten material from each recording session.

To clarify, the start of the tape must look like this, if the tape has been used to record 4 programs in a row, rewinding to the start of the tape in before each recording:

     /    |    |    |                        /
     | p  | p  | p  |    p                  /
     | r  | r  | r  |    r                  \  
     | o  | o  | o  |    o                   \  
     | g  | g  | g  |    g                   /
     | 1  | 2  | 3  |    4                  /

       approx 0.2 seconds

What's up with that?

Do check this out for yourself. I've seen it happen on several UK PAL, VHS systems.

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