🚀 go-pugleaf

"Lionel" <usenet@imagenoir.com> wrote in message 
news:feoou21mgqn0k5na7ifoi2mp1aqsmr6l3f@4ax.com...
> On Mon, 5 Mar 2007 09:13:34 -0500, "Tam/WB2TT" <t-tammaru@c0mca$t.net>
> wrote:
>
>>
>>"Lionel" <usenet@imagenoir.com> wrote in message
>>news:ksknu25mu0q4p10p0qhi0u56uihln1ndde@4ax.com...
>>.....................................that a good starting place might be 
>>to
>>use a CPU clock
>>> that's based on the PAL subcarrier frequency (4.43MHz) multiplied with
>>> a PLL+VCO+Divider to give me phase reference points, & directly
>>> generate the composite colour signal via an R2R ladder.
>>>
>>
>>You might want to consider the converese of that. 20+ years ago it was
>>common in cheap computers here to start with an oscillator at 14.318 MHz,
>>which was then counted down to the NTSC color subcarrier frequency of 
>>3.579
>>MHz. 14.318 is also an integral multiple of the horizontal and vertical 
>>sync
>>frequencies. I see that 17.734475 is a standard crystal frequency. Is that
>>4x the actual PAL frequency?
>
> Yes, it is. However, I don't know if it's high enough to give me
> enough samples to create a clean Chroma, etc. I'm pretty sure that
> I'll need to sample at a minimum of 8x (35.468950MHz) the base colour
> subcarrier frequency. I /hope/ I won't have to go as high as 16x
> (70.937900MHz). ;)
> That said, using 17.734475MHz means I'd only have to multipy it by 2
> or 4 to get my higher multiples, which may make life easier.
>

*****************
That would still simplify things because you could use a frequency doubler, 
or two in series, and not have to worry about phase noise and such of a PLL. 
A 2N2369A, or the like, biased class C will work nice as a doubler at these 
frequencies.  You would need a limiter on the output.

Tam
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