Re: IIgs RGB to VGA continued

["TomCh" <tomcharlesworth@blueyonder.co.uk>]

Michael J. Mahon wrote:
> jsnospam@cimmeri.com wrote:
> > The basic setup is:
> > AL875 ADC
> > AL250 scan doubler
> > ICS1523 clock generator in a PLL
> >
> > We tried changing the dot clock sampling rate.   We noted many
> > interesting effects, but no overall image improvement.
> >
> > Also considered phasing: it's done by the ICS clock generator chip.
> > The phasing is adjustable and we tried changing it throughout its range
> > and it made no difference.   Not during dot clock adjustment.
> >
> > Now look at:
> > http://members.aol.com/mdnttrain/vga6.jpg
> >
> > In this latest photo, a first order low pass filter was added on each
> > of the 3 color inputs.  This reproduces the lower bandwidth of the
> > older monitors and makes the modulation bars disappear.   We are
> > beginning to believe that the modulation bars are really coming out of
> > the machine like that and not a defect of the converter itself.    A
> > supporting observation is that the beat bars are quite "solid" looking
> > (real beat bars we've seen are soft).
> >
> > Another is that they don't show up on the black and white portions of
> > the image as Vince noted.   That means that if the bars ARE present,
> > they are always white bars on white backround, and black bars on black
> > backround .. which seems inconsistent with the other bars we're seeing.
> >
> > Do you all agree with our theory, or think something is still wrong in
> > the converter's phasing circuit / dot clock rate?
>
> Your results prompt a little application of sampling theory.  ;-)
>
> If non-synchronous sampling is to work, the bandwidth of the incoming
> signal(s) must not exceed the Nyquist frequency, or half the sampling
> frequency.
>
> Since the RGB signals coming from the IIgs are not band-limited, I
> would expect some pretty sharp edges, and therefore frequencies well
> over 14MHz.  The patterns you see are the result of aliasing of those
> frequencies to lower frequencies (the difference between the dot clock
> frequency of the IIgs and the sampling frequency).
>
> If you're sampling around 15MHz, then the bandwidth of the RGB signals
> should not exceed 7.5MHz, or, for practical filters, 6.5MHz.
>
> The bad news is that using a filter also kills the bandwidth required
> for sharp 80-column text.  ;-(
>
> That's why I still recommend *synchronous* sampling.
>
What would you use as your sync?
You have to use the video CLK from the motherboard, right?

Tom