SVHS vs S-Video


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NOTE: I found this on usenet in a Feb 11, 1998 post to rec.video.production by a fellow with the username Charlie Hand. It explains how svhs is basically about signal RECORDING while s-video involves signal TRANSMISSION.

Just want to add two things to the excellent post below. One is that I think degradation also happens when chroma and luma are combined and not only when they are separated (separation may cause most of it though). The second is I have seen mini-din4 s-video cables in an Italian department store with packaging that said "SVHS Cable", so don't argue with people too much about this issue. --John

Let me clarify the benefits of SVHS versus VHS, and S-video versus
composite.  The two are really quite unrelated, contrary to Scott's
understanding.

SVHS gives you more luminance bandwidth (= more luminance H
resolution) and, if I'm not mistaken, a little more luminance
signal-to-noise ratio than VHS. That's it. That's the only difference
between SVHS and VHS.  Both are color-under formats, meaning the
chroma is recorded separate from the luminance on the tape (separated
by spectrum - the luminance being frequency-modulated on a relatively
high-frequency carrier, and the chroma subcarrier being hetrodyned
down to a piece of spectrum below the luminance FM signal). In fact,
the chroma part of the format is identical for SVHS and VHS.

S-video versus composite is a completely different thing. S-video
differs in one way from composite, and one way only. S-video
carries the chroma separate from the luminance, while composite
mixes them together.

This is a key concept: if you connect the two signals in S-video
together, you get composite. Literally. Short the Y and C signals
from the S-video together and you have composite.

So what? Why bother? Well, by carrying the luminance and chroma on
separate wires in the S-video cable, you eliminate the need for a
chroma separator at the destination. Chroma separators are not
perfect, they leave their mark both on the luminance and the chroma.
How much of a mark? Not too much, but enough that you can reliably
see the difference between S-video and composite in many images,
assuming good quality video to begin with.

S-video has nothing to do with SVHS! S-video connections appeared on
SVHS VCR's because SVHS quality is sufficient to warrant it. S-video
connections are used to preserve video quality (as compared to
composite) everywhere, in all kinds of video equipment. The video
quality of VHS was already so poor that the benefits of S-video
interconnects would have been lost. SVHS is better, and the benefits
of S-video can be seen.

OK, so you go to SVHS in order to get improved luminance resolution
and SNR due to the higher carrier frequency and deviation of the SVHS
format. And you use S-video connections so your signal doesn't have to
go through a chroma separator, which might mess up your nice, improved
luminance and, even worse, your chroma which for SVHS or VHS (they are
identical) need all the help it can get.

Does a chroma separator (introduced by the use of composite cabling)
cancel out the benefits of SVHS over VHS? Not by a long shot!

Notice also that length is no more or less a factor for S-video than
for composite, except it might be harder to find high-quality S-video
cable than composite, but you could always run S-video on a pair of
RG59 cables.

So the ultimate situation is SVHS with S-video, but composite doesn't
cancel out the benefits of SVHS.

As a matter of fact, in many cases composite turns out to give
more pleasing results. Sometimes SVHS is used to record a signal with
significantly less luminance resolution than the SVHS format can
record (such as TV off the air, or a Beta SP signal), in which case
the extra bandwidth afforded by SVHS records nothing but extra noise.
In such a case, it can actually be better to have a chroma separator
mucking things up a bit. 

As a rule of thumb, with pro-sumer stuff, I always try both S-video
and composite, rather than assume S-video will always be better.

-Charlie
dictionary

RATTY STILL CAN'T FIGURE IT OUT

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