The sampling clock is 14.318 MHz, 4x the color frequency. I use the positive edge of vertical sync from LM1881 to reset the memory's write address to zero and let the sampling does its work until the next positive edge of vertical sync. However, the amplitude of the colour burst after D/A is almost half of the original. I suspect that it's because the video signal before A/D has become out of phase with the sampling clock, but I don't know to test on that.
For YCrCb, as long as sampling rate is 4x colour frequency and the decoder outputs 8-bits eg 4:2:2, it should require the same amount of memory space as what I am doing now. And for RGB, the memory required should be tripled. Am I correct?
btw, what i trying to build is commonly known as 'video digitizer' or 'frame grabber', but I just found out yesterday. foolish me