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