OT: Composite/Component [was Re: [mythtv-users] Re: Image quality, what effects it?]

Cory Papenfuss papenfuss at juneau.me.vt.edu
Wed Aug 11 08:20:19 EDT 2004

>> Composite.  The composite video signal is on the cable that plugs into
>> the yellow connector.  Your right and left audio go through the red and
>> white connectors.
> One step more complicated answer:
> Composite video is the black-and-white (or monochrome) signal, and the
> 3.58MHz color subcarrier signal, all going through one cable, usually
> with a yellow RCA (Phono) plug on consumer equipment.
> S-Video is kind of a low-rent version of component, which preceded it
> on consumer equipment: the color subcarrier signal is kept separate on
> all the wires, which permits the removal of the filter usually
> necessary to keep it out of the luminance (or monochrom) signal -- this
> gets you better sharpness, and less color crawl.  It commonly moves
> around on 4-pin mini-DIN connectors on consumer gear (and 7-pin locking
> ones on pro-gear).

 	I think I would call S-video a high-end composite signal rather than a 
low-rent component.  SVid still has to be NTSC (or PAL if you prefer?... or is 
it just SCART for PAL?).  The main difference is the separation of Y/C.  Having 
looked at the two on a scope, it's pretty nifty to see a nice, simple, clean 
B&W signal on the Y channel with a colorburst fuzz and phase-modulated sinusoid 
afterwards.  I am curious as to whether or not the Y signal is then allowed 
more bandwidth than the 6MHz of composite?  The lack of color-crawl is the 
biggest plus to s-vid.  The sharpness isn't necessarily a function of Svid 
(unless >6MHz is allowed), but rather tv's that cut off bandwidth where the 
Pb/Pr comb filter goes.

> Component usually refers, as David says, to a three-cable video system
> where the luminance and the I and Q components (as they're called in
> pro circles (ok, Pr and Pb aren't *exactly* I and Q, but they're close
> enough for NTSC ;-)) travel on three separate cables; about the only
> place you currently find that on consumer gear is on "progressive" DVD
> players and upscale monitors (and scan-doublers).

 	It's almost exactly the same as the old "sync-on-green" three-coax 
workstation monitors.  Oversimplified, Y=R+B+G, Pb=B-Y, Pr=R-Y and H/V sync is 
also added to Y.  Realistically, they're weighted by their perceptual 
luminance, but for illustration it's not important.  It's basically the same 
thing as RGsB with the colors mixed.
 	The big deal with it is that it's NOT limited to NTSC specs.  There's 
no color subcarrier, colorburst, frequency limiting, comb filtering, 
blahblahblah.  Once you loosen that restriction, you can just put three colors 
down the wire with H/V sync and draw a picture.  Just like VGA... not 
specified.  Now, to make life easier for everyone, a few standard modes were 
made up (480p, 1080i, 720p, etc).

> Component originated on Sony Betacam professional recorders, originally
> as a deck-to-deck dubbing format (as, incidentally, did S-video; the
> pro version derives from the old U-matic dub cable format, which was
> 4-wire split multiplexed chrominance, but at the 729kHz on-tape
> subcarrier frequency).

 	Interesting. What do you mean exactly?  Did the U-matic remix a 
"baseband" chrominance (demoded from the 3.58 subcarrier) onto 729kHz.  Then 
send a differential I and Q down 2 pairs?

> At the moment, so far as I'm aware, there are no consumer grade capture
> cards, and certainly nothing with a tuner on it, which do Y-Pr-Pb
> component input.  You have to get up to things like the Matrox
> DigiSuite (and possibly RtX) cards to get that.

 	I suspect that it's a bus limitation problem.  I've been looking at 
trying to abuse a BTTV card into being a generic ADC.  They generall have 2 40 
MHz, 8-bit ADCs in them to do SVID.  Because it's stuck at NTSC, however, the 
requirement is basically for 1 40MHz ADC downsampled to 20MHz.  Putting more 
than 1 card in a machine capturing raw pushes the PCI bus pretty hard.  Trying 
to nab 3 channels at something fast enough to adequately capture 720p would 
probably take a serious machine to do raw over PCI.  Might have to be fancy 
PCI.  Now, a realtime MPEG capture like ivtv would be quite interesting.

> Ok, I think it's time for someone to snipe at me now for trying to be
> informative.  ;-)

 	Me too... :)


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