RE: [AV Media Matters] headswitch

[Peter.Copeland@bl.uk]

Dear Heather,
    I shall start the ball rolling with my personal slice of ignorance.
    Video recordings are made by a head which rotates upon a head drum.
The
tape is usually made to go round approximately a semicircle when this
happens (there are some exceptions), and there are two heads fixed to
the
head drum, so that when one leaves the tape the other head takes over.
Meanwhile the tape falls by its width (slightly less than half an inch,
in
the case of half-inch tape), so the spiral which is traced by a head is
called "helical". The head rotates at the video frame rate, thirty times
per
second in America and twenty-five in Europe (with some minor
variations). 
    When everything is working well, the "horizontal synchronisation
pulses"
in the video waveform occur on each line, and there are 525 lines in
each
video frame (in America - 625 in Europe). Furthermore, the video frame
is
divided into two "fields", with the odd-numbered lines occupying one
field
and the even-numbered lines the other. Thus the entire screen is
refreshed
60 (or 50) times per second, reducing flicker (which is always more
noticeable with bright images).
    The fields are terminated by a large synchronisation pulse; I don't
know
its size in America, but in Europe it is about 25 lines per field. (This
is
where Ceefax lives in Britain, plus other information which does not
appear
on the screen unless something goes wrong). But both the "vertical
syncs"
and "horizontal syncs" are "blacker than black", and the television set
will
recognise them, and change its scanning action.
    I'm gradually coming to the point! When the helical-scan video
recorder
was first invented, the two heads scanning every frame had to be
switched
when one left the tape and the other made contact. On record, both heads
are
connected, so for nearly half the time they are magnetising the air! But
there was a problem upon playback. After much experiment to minimise the
television's picture jumping, it was decided to switch the heads a
couple of
lines before the bottom of the picture. This was where most viewers
wouldn't
notice it, but it didn't corrupt the vertical syncs. So television sets
wouldn't object (there are considerable energy-saving devices in the
scanning coils of a cathode ray tube, and real damage might be caused
otherwise).
    In the analogue domain, some helical scan video recorders had
mechanical
controls to reduce the disturbances before the days of timebase
correctors.
Nowadays, digital timebase correctors will correct the relationship
between
the picture information and the synchronisation pulses by re-clocking
the
latter; but the headswitching at the bottom of the frame is likely to
remain
visible, because it occured at essentially random inftervals between two
horizontal sync pulses.
    I plead for mercy when everyone else shoots me down!
Peter Copeland

-----Original Message-----
From: heather@bavc.org [mailto:heather@bavc.org]
Sent: 23 May 2002 23:25
To: AV Media Matters
Subject: [AV Media Matters] headswitch


Hi!
What causes visible headswitching and why is it more apparent on some
formats than on others?
just curious..
Heather Weaver, BAVC


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