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 Upgrading the S-Video in the AV Famicom

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Drakon
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PostSubject: Upgrading the S-Video in the AV Famicom   Thu Mar 08, 2012 12:25 am

This is another interesting way to improve video quality in any system. Before when I was tinkering with using the av famicom video circuit kit in the sega genesis I discovered that I could either supply the colour sub carrier to the video encoder from the onboard crystal oscillator or from the subcarrier pin on the console. When I used the onboard oscillator the image had minor distortion and the picture wasn't as crisp / nice as when I fed it the colour subcarrier from the system. Looking at the sega genesis pcb I could see that the board only had one crystal on it meaning that the timing of everything on the pcb was being driven by the same clock signal. Pretty much all retro consoles use only one crystal to supply a clock signal to all the hardware on the system, using just one clock source makes everything run in sync leading to better image quality.

The way the av famicom kit works is it has an onboard 3.579 mhz crystal oscillator that feeds the colour subcarrier signal to the sony video encoder. I decided to try finding a way to get the video encoder to use the system clock circuit to get better image quality. Unfortunately the nes was never designed to use a video encoder since all the encoding was internalized into the ppu chip. The master clock signal on the nes is 21.477272 MHz which is nowhere near the 3.579 mhz signal that I needed to drive the video encoder. From reading documents on the nesdev wiki (crucially helpful information) the cpu and ppu both accept 21.477272 MHz and then internally divide this signal down to the speed needed to drive them. The cpu of the nes divides the signal by 12 I believe running it at 1.789 mhz, I can't remember what the ppu divides it down into. But since all the clock speed dividing is done internally inside of the chips I needed to build my own clock frequency dividing circuit to get the 3.579 mhz that I need. Luckily 21.477272 MHz divided by 3.579 mhz is 6.

Reading online various circuits of how to build a clock frequency division circuit confused me, so I went to my trusty friend tiido (the maker of the amazing sega genesis crystal clear audio mod). He pointed me to the right schematics which I'd also found and thought were right, he also told me what ic's I'd need and explained to me the very basics. I bought the chips today (two ics), wired them up from what he taught me...and it actually worked first try. The circuit is complicated so I'll have to draw the schematic later. Here's comparison screenshots of the s-video image quality when it's driven by a crystal oscillator vs when it's driving by the nes master clock signal that's been divided down:





The image quality is noticably cleaner so I'm really happy with how this turned out. So the moral of the story is driving the video encoder from the system master clock always produces the best image quality.
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PostSubject: Re: Upgrading the S-Video in the AV Famicom   Thu Mar 08, 2012 11:03 pm

Here's the schematic. I'm pretty sure this's correct

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PostSubject: Re: Upgrading the S-Video in the AV Famicom   Wed Mar 28, 2012 1:28 am

I've since upgraded the video circuit and fixed a major flaw but I won't post the schematic.
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