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Old 15th Sep 2006, 11:52 am   #1
arjoll
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Default Converting component to S-Video

I'm not sure of the right forum to post this, but figure I should be pretty close here (mods, please move if required).

Does anyone have any circuit ideas/designs for converting component video (YCbCr/YPbPr or whatever) to S-Video? I remember reading somewhere that the C component of S-Video (Y/C) is two C signals mixed together, and that composite is the whole lot mixed together.

Because of this I was just thinking of mixing the two chrominance signals together with a TL071 and seeing what happens, but I'm not sure about the bandwidth of the signals or if anything needs to be filtered etc.

The reason I ask is that I have just bit the bullet and built a media centre PC*. Most video cards have S-Video, but my Sony SW292 has component in, so I tracked down a card with component to run the TV. This is fine, but I have wired the house with composite and S-Video feeds between the lounge and living area to run the projector from work. The wiring was DVD player component -> TV; DVD player S-Video -> building cabling -> work's projector; VCR composite -> building cabling -> work's projector.

In theory I could split the component and use the three coax lines between rooms to run component to the projector, but that means I have to replace the S-Video leads etc already in place (and the S-Video connectors look so good beside the other connectors on the wall plates!).

Any ideas would be appreciated!

(media centre PC = Gigabyte motherboard w/Intel 915 chipset, Pentium-D 805, 1024 MB, 320 GB, ATI X1600 256MB video w/DVI and component out, Haupauge MCE500 dual analogue capture card, DVD+/-RW DL, Windows xp MCE)
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Old 15th Sep 2006, 12:15 pm   #2
Kat Manton
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Default Re: Converting component to S-Video

Hi,

Component contains luminance and colour difference signals; S-Video is PAL/NTSC encoded chrominance plus luminance.

So... you'd need a PAL encoder to turn the colour difference signals into chrominance. It's likely that there's an off-the-shelf IC that'll do it; I'm more familiar with encoders which have RGB input as that's more widely used here than 'component'.

Regards, Kat
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Old 15th Sep 2006, 12:17 pm   #3
ppppenguin
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Default Re: Converting component to S-Video

(All this is for PAL - the numbers for NTSC differ but the fundamentals are the same)

In the classic world of TV all signal are considered to originate as RGB. Typically each has an amplitude of 700mV and sync can be separate composite (mixed) sync or sync on green.

Component video is simply a colour space transformation of RGB to YCbCr (often incorrectly called YUV) where Y = 0.299R + 0.587G + 0.113B, Cb is B-Y weighted to 0.7Vp-p and Cr is R-Y weighted to 0.7Vp-p. The Y (luminance) signal has 0.3V of sync added and is equivalent to a monochrome signal.

In S video the Y signal is untouched. The Cb and Cr signals are quadrature modulated on to a 4.43MHz subcarrier to make the C signal. A colour burst is added at the start of each active line to synchronise a 4.43MHz crystal at the decoder. The subcarrier itself is suppressed so that for a monochrome picture the C signal is zero apart from the burst. I won't quote the full PAL equations here nor discuss the phase alternation in PAL.

In composite the Y and C signals are simply added.

That's all the theory. YCbCr gives highest quality. S is not quite as good becuase the chroma bandwidth is restricted. Composite is worse because it's difficult to fully separate the Y and C. Typical problems include reduced Y resolution and nasty lurid colour patterns on fine Y detail. Modern comb filter decoders do a better job but there are usually some artefacts.

You can buy PAL/NTSC decoders (often incorrectly called splitters) quite cheaply which will take composite or YC and produce RGB and/or YCbCr. Rather less availlable are encoders which do the opposite conversion. I'm talking about complete products here, not chips for rolling your own designs.

I have designed various coders and decoders over the years (and all sorts of other video stuff) but these have been aimed at broadcast grade applications. Nowadays equipment with video inputs will typically have a decoder chip that can take composite or S (sometimes also YCbCr) and give 8 bit digital component video.

I can recommend the book "Video Demystified" by Keith Jack.
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Old 15th Sep 2006, 12:46 pm   #4
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Default Re: Converting component to S-Video

Quote:
Originally Posted by Kat Manton View Post
Component contains luminance and colour difference signals; S-Video is PAL/NTSC encoded chrominance plus luminance.
Quote:
Originally Posted by ppppenguin View Post
In S video the Y signal is untouched. The Cb and Cr signals are quadrature modulated on to a 4.43MHz subcarrier to make the C signal. A colour burst is added at the start of each active line to synchronise a 4.43MHz crystal at the decoder. The subcarrier itself is suppressed so that for a monochrome picture the C signal is zero apart from the burst. I won't quote the full PAL equations here nor discuss the phase alternation in PAL.
Thanks, this is all sounding like a bit more than I intended - I didn't realise the PAL encoding was between component and S-Video. It is looking like splitting the component video is going to be a lot more straightforward.

My other option is tracing the pin connections are on the video card - it uses a 7 pin mini DIN connector and comes with a breakout cable for component and an S-Video to composite adapter, which would indicate all signals are present. If so (they could be on the three 'middle' pins) I'll look at just building a breakout cable if I can find a suitable 7 pin mini DIN plug.

Thanks for your help.

One other thought - in standard VHS VCRs are the Y and C signals ever separate at any stage? Just had a thought about capturing old VHS tapes with better quality...(sorry, drifting off topic)

Cya
Andrew
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Old 15th Sep 2006, 12:53 pm   #5
ppppenguin
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Default Re: Converting component to S-Video

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Originally Posted by arjoll View Post
One other thought - in standard VHS VCRs are the Y and C signals ever separate at any stage? Just had a thought about capturing old VHS tapes with better quality...(sorry, drifting off topic)
The Y and C are always separate on the tape. Hence you can get better quality by using a machine which has an S output.

The thing to remember is that the greatest damage is done by separating Y from C. Repeated combining and separating does even more damage. Modern combing decoders do it a lot better than older designs but it can never be perfect. I'm amazed at the performance of the low cost deoder chip in the newer Aurora standards converter. I know I've only seen the Y output but it's astonishingly good.
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