Re: How does the apple2 RGB card work?

[pausch@saaf.se (Paul Schlyter)]

In article <1148744609.916946.15710@38g2000cwa.googlegroups.com>,
General_Failure <tristan.mumford@gmail.com> wrote:

> Michael J. Mahon said:
>> Since the video pulses for lores graphics and DHR are sent at 14MHz,
>> each cycle of 3.58MHz covers 4 pulses.  Any "asymetrical" pattern
>> will contain a 3.58MHz component (in a Fourier sense).
> 
> >So the patterns 1100, 0110, 0011, or 1001 are maximum amplitude
> >(saturation) 3.58MHz signals, while the patterns 1000, 0100, 0010,
> >0001, 1110, 0111, 1011, and 1101 are half-amplitude chroma signals.
> >The patterns 1010, 0101, 1111, and 0000 do not contain any 3.58MHz
> >component, and so are colorless.
> 
> You're right. It is an elegant exploit of a technology. And as realised
> by many, this method does not work in PAL because the standard is
> designed to neutralise a lot of abhorrent signals.
> The standards of NTSC and PAL aren't really too different. However it
> is the little differences that make all the difference.

True.  PAL requires a delay line, to be able to sum up the color
signal from the current line and the previous line - that's the way
the phase shift cancellation is performed.  This delay line was quite
expensive in early color TV sets.  For awhile, the japanese TV
manufacturer Sony used something called "Poor man's PAL" -- basically
it worked by converting the PAL signal to NTSC so the delay line
wasn't needed.  Otoh then you also lost the phase shift cancellation
property of PAL.  Therefore, "Poor man's PAL" TV sets reintroduced
that hue knob, to adjust the color hue of the image - that knop was
present on most NTSC TV sets but was absent on all "full PAL"
receivers since it wasn't needed there.  Check this link for more info:

http://stjarnhimlen.se/tv/tv.html#PAL-SIMPLE


> A good
> explanation of how the video signal works exists on Rickard's PIC site
> on this page I think <a
> href="http://www.rickard.gunee.com/projects/video/pic/howto.php";>http://www.rickard.gunee.com/projects/video/pic/howto.php</a>
> Velleman sells a kit that seems to be a direct rip or Rickard's PIC
> Pong too. I bought one cheaply recently.
> Anyway, back to topic.
> The translation would either have to be done by a nice whack of logic,
> or a massive lookup table, as Bryan seems to have implemented. That
> must have taken a very long time.
> 
> At any rate, from all this information gained, many different methods
> of implementation seem to be possible.

-- 
----------------------------------------------------------------
Paul Schlyter,  Grev Turegatan 40,  SE-114 38 Stockholm,  SWEDEN
e-mail:  pausch at stockholm dot bostream dot se
WWW:     http://stjarnhimlen.se/