I need to somehow multiply a 15734.26Hz signal by 227.5 times. Those with a keen eye (or a calculator) will notice the result is very close to NTSC subcarrier frequencies. 3.579545MHz or close as damn it.
I realise I could just use a crystal that's that exact specification, and I have been, but the problem is, this crystal is not on the same domain as the rest of the video signal as I'm encoding RGB into composite video.
This is where the dreaded dot crawl comes in. If the subcarrier frequency is derived from the same domain, the dot crawl is gone completely.
I've no way to get on this clock domain BUT I know that the HSYNC x the line clock cycles = the subcarrier frequency. I just don't know how to get there.
Most ICs or circuits don't seem to be able to multiply that much and it's likely for a good reason looking at how they do it.
I assume I will need something that can sample the frequency and generate a clock, based on my criteria, from it.
Assumedly, I'm going to be looking at a microcontroller or FPGA which I'd really rather avoid.
Any ideas?
Best Answer
There (was) an IC for that: https://www.renesas.com/us/en/general-parts/1523-video-clock-synthesizer-i2c-programmable-delay. Obsolete now... no thanks to Renesas.
Basically, multiply up by 455 to get 2fsc or 910 to get 4fsc (that is 14.31818MHz.) The latter is available as a crystal.
Another option is to 'crash lock' a crystal oscillator to regenerate the subcarrier. That's how old TVs did it. This IC does that: http://synkie.net/wordpress/wp-content/uploads/2013/11/MC44144P.pdf
Another option is forget about making Fsc, and go straight to CCIR656 sampling (13.5MHz) using a chip called a Digital Multistandard Decoder (DMSD). Here's one from TI that I've used in the past: https://www.ti.com/product/TVP5150AM1. You'll need to add a microcontroller to initalize it, but that's not so difficult nowadays.