UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Converting component to S-Video

ppppenguin · 15th Sep 2006, 12:17 pm

(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.

15th Sep 2006, 12:17 pm	#3
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	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.