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Re: [AV Media Matters] headswitch

All modern videotape formats use helical scanning, i.e., the tape is
wrapped around a spinning head-drum in a helix.  As the drum spins, the
heads in the drum trace diagonal lines across the tape.  The wrap of the
tape around the drum can be very close to 360 degrees (as in the
one-inch Type C format), but it cannot be a full 360 degrees, or there
would be no place for new tape to get written on by the heads.  The head
switch occurs when the head leaves the tape and a new head (or a new
pass of the same head) takes over.

In most professional formats, the head switch occurs within the vertical
blanking interval, a part of the signal that does not appear on the
screen; therefore, the head switches for those formats are not visible.

In such formats as VHS (which has a 180-degree wrap and two heads per
frame), the head switch takes place in the active video, although in a
portion of it that might not be visible on some TVs.  As there is a fair
amount of leeway as to where the head switch may occur, two machines of
the same standard can provide head switches in different scanning
lines.  Depending on the overscan of the display, one set of switches
might be visble while the other is not.


heather@bavc.org wrote:
> Hi!
> What causes visible headswitching and why is it more apparant on some
> formats than on others?
> just curious..
> Heather Weaver, BAVC

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