Re: Why no "comb fringes" on VCR still frame?

On Fri, 18 May 2007 16:15:29 +0100, "Martin Underwood" <a@b> wrote:

Forgive me if this is a really stupid and naive question...

When you grab a still frame using a PC from a video card, you get "comb
fringes" where there has been any movement between the two fields of a scene
that has been shot on video camera rather than telecined from film.

Why is it that you don't get that effect if you freeze a VHS tape or even a
professional video tape?


VHS machines have two video heads 180deg apart, and the tape is
wrapped slightly more than 180deg around the drum (just enough to
allow a bit of overlap for clean switching). One field is recorded by
each head, so a complete frame is recorded in two diagonal tracks in
one rotation of the drum. On playback, the tape has to be moving as it
was during recording, in order to ensure that as one head leaves the
end of one video track at the edge of the tape the other head entering
the opposite edge of the tape finds the beginning of the next video
track.

If the tape is stationary, the slope of the diagonal line swept by the
video heads on the tape is different from the slope of the recorded
tracks in such a way that whatever position the head occupies relative
to the track at the start of its sweep where it enters the tape, it
gradually crosses over throughout the sweep so when it leaves the tape
at the opposite edge, it occupies the same position relative to the
adjacent track. If it starts between tracks, it ends between tracks,
and if it starts on the centre of one track, it ends on the centre of
the adjacent track, crossing over halfway between. Because the signal
is FM, there is no amplitude variation as the signal rises and falls
as the heads cross the tracks, except near the crossover where it is
so low that it is only resolved as noise, and you get a horizontal
band of noise on the screen. By positioning the longitudinal position
of the tape, and hence the diagonal tracks on it, relative to the head
drum, the crossover noise band can be positioned to appear at any
vertical position on the frame. If the crossover is halfway down the
frame, the image displayed will be the top half of one field and the
bottom half of the adjacent one, and if the crossover can be
positioned off the screen, the image displayed will be one complete
field. Wherever the crossover is positioned, only one field's worth of
video can be displayed because the tape is only wrapped sufficiently
far round the drum for that much video to be accessible to the heads.
Both heads play the same material so in two 1/50 second intervals it
gets played twice, once by each head.

There are different sorts of professional recorder. The old 1"
C-format analogue ones recorded each field in a single sweep of 360deg
with the drum rotating twice as fast, but a similar argument applies.
Modern digital ones display still images by means of digital storage,
so can be set to display fields or frames, but are usually set for
fields because the lack of vertical resolution is less annoying than
the inter-field jitter that would otherwise result.

You may be able to tell your video card to do the same, i.e. grab a
single field instead of a frame.

Do VCRs just display one field and duplicate it into the other field when
switched to freeze frame? I'm not aware of a halving of vertical resolution
with VHS when you switch from play to pause.

Essentially yes. You may not be greatly troubled of the loss in
vertical resolution, but if the material is unadulterated interlaced
video, it really happens. On the other hand, quite a lot of material
is processed by the programme makers to derive both transmitted fields
from only one of the photographed fields, so it has only half the
possible vertical resolution anyway, even when the tape is moving.

Rod.
.

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