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

[Jay R. Ashworth]

On Wed, Aug 11, 2004 at 08:20:19AM -0400, Cory Papenfuss wrote:
[ quoting me: ]
> > 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 knee for luminance bandwidth in a traditional NTSC signal is
actually 4.2MHz, and, if your filtering wasn't good enough, you got
less.  In *theory*, it's possible to move more through an S-video link,
but you have enough bandwidth on the tape to store it.  S-VHS and Hi-8
can make use of more of what's already there, but I don't know that you
can usefully extend it any.

In any event, Yves Faroudja has proved in his Super-NTSC labs that you
can fit into an NTSC channel a picture that would knock most
network-affil chief engineers *flat* on their ass; most NTSC gear is
truly garbage.  But then they wouldn't be able to *sell* HDTV.

Cf. Pogue Carburetor.  :-)

>                        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.

Functionally the same result, though you're right, not exactly the same
course of action.

> > 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.

Well... yeah.  And you're right; I and Q are *modulated* chroma, where
Pr and Pb are luminance signals, so there's more difference than I let
on.

> 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).

Yeah; this is an issue I've always wondered about; those component
cables can carry signals that are *not* RS-170A timed, then, right?

> > 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?

No, it was like S-video: the chrominance was quadrature modulated, but
at 729kHz, instead of 3.58MHz; they shipped it out as Y/C; you can get
frequency heterodyne boxes to convert U-Matic Dub Out to "proper"
S-video.

Remember, even on VHS, the NTSC color subcarrier is *not* recorded at
3.58MHz.

> > 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.

I understand that the problem is specifically the Bt878 chip, which
wants to bus-master its PCI transfers; the video4linux at redhat list
archives for the last month might be informative on this.

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

Cheers,
-- jra
-- 
Jay R. Ashworth                                                jra at baylink.com
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	"You know: I'm a fan of photosynthesis as much as the next guy,
	but if God merely wanted us to smell the flowers, he wouldn't 
	have invented a 3GHz microprocessor and a 3D graphics board."
					-- Luke Girardi