Re: Difference in luma-chroma delay of C64/C128 compared to standard S-video

On 01.09.2017 00:19, Mia Magnusson wrote:
>
> For some reason the decoding process in a TV anyway needs a delay. It's
> likely that the s-video standard were set to make a S-VHS player as
> simple as possible, i.e. bypassing any delay that's needed for a VHS to
> do composite -> separate chroma/luma -> FM modulate luma and frequency
> shift chroma -> record to tape, playback from tape -> FM demodulate
> luma and frequency shift chroma -> combine luma and chroma.
>
> So therefore it makes sense that the signals could have different
> timing specs for composite v.s. s-video in general.
Hi Mia,

The delay is only needed for PAL (Phase-Alternating Line), and some of 
the first PAL receivers didn't bother with having the delay. In PAL the 
phase of the color-difference signal V is inverted on every line. The 
delay is used to "perfectly separate" U and V by calculating 
(u+v)-(u'-v') = 2V and (u+v)+(u'-v') = 2U.

"perfectly" only if the two successive lines of a field contain the same 
color information. They are also separated by the line from the other 
interlaced field in the final frame, so it's seldom the case. However, 
the main feature of PAL is that it corrects the phase issues of color, 
so no tint control is needed.

S-VHS stores luminance and chrominance separately on the tape, so it is 
easy to provide them also separately in the S-VIDEO connector.

There can still be timing difference between luma and chroma, causing 
color information to shift in addition to having smaller bandwidth, and 
thus not naturally co-inciding with the edges of the luminance signal. 
Digital televisions have methods to look for the edge on the luminance 
and then correct the edge of the chrominance to match.

In theory composite is produced by summing the Y and C of S-VIDEO. In 
practice you may get time shift whenever they are combined or separated.

-Pasi


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