[sdiy] Video Synth...

[Ingo Debus]

Hi Nils,

Am 21.12.2007 um 22:21 schrieb np:

> First problem: Signal generation for a PAL BAS signal.
>

Since this is a German abbreviation, most here probably won't  
understand. BAS (Bild-Austast-Synchron) means composite video signal.

> For my video signal generator I need two sync-signals at relative  
> stable frequencies. VSync is at 50Hz (well - that's easy). HSync at  
> 15625 Hz.
>
> I did some math and found out that I can get both frequencies by  
> dividing down a 27Mhz crystal oscillator. That would be a cheap way  
> to do this, but I haven't ever done anything that runs at these  
> high frequencies. Will prototype board (the one with the dots, not  
> the strips) work at that high frequencies or am I asking for  
> trouble? For the frequency division job I plan to use the 74HCT4059.
>

I once used a 30 MHz crystal oscillator on prototype board to clock a  
microcontroller. Worked fine. Of course I kept the connection  
carrying that frequency as short as possible.

> Is there a simpler way to get my frequencies? Any idea that gets me  
> out of the need to do HF stuff is welcome. Maybe I've overlooked  
> another crystal that runs at a lower frequency, or I could do  
> things with PLLs, or there is a even simpler solution. I don't want  
> to use a microcontroller btw.

I think you can even use two free-running oscillators. It probably  
depends on the TV set you're using.
But if you use a master clock, can't it be much lower? 31250 / 2 =  
15625; 31250 / 625 = 50.

31250 is a familiar number if you're dealing with MIDI. I think back  
then they chose that rate because it could easily be derived from  
commonly used crystal frequencies.

In the old days, Elektor used to publish a lot of circuits that  
generated video signals. I can dig in my collection if you want.

Ingo