Re: IIgs video modes...

["Bryan Parkoff" <nospam@nospam.com>]

"Michael J. Mahon" <mjmahon@aol.com> wrote in message 
YvCdnQdTCLY8RHjYnZ2dnUVZ_oupnZ2d@comcast.com">news:YvCdnQdTCLY8RHjYnZ2dnUVZ_oupnZ2d@comcast.com...
> Michael J. Mahon wrote:
>> David Empson wrote:
>>
>>> Linards Ticmanis <ticmanis@gmx.de> wrote:
>>>
>>>
>>>> Michael J. Mahon wrote:
>>>>
>>>>
>>>>> The answer is in the schematic--the NTSC color signal is created
>>>>> not by the multiple-of-3.58MHz pulse scheme that is the Apple II
>>>>> standard, but by a chrominance modulator driven by the RGB signals.
>>>>> Therefore, its color rendition is not dependent on its pixel rate.
>>>>
>>>>
>>>> Does this apply only to SHR modes, or also to IIe compatible modes?
>>>
>>>
>>>
>>> Probably all modes.
>>
>>
>> Agreed.
>>
>>> The IIgs RGB output emulates the colour behaviour of 8-bit Apple II
>>> video modes, e.g. alternating green/violet pixels in HGR mode, with an
>>> odd coloured pixel every 14th (yellow or brown).
>>>
>>> The NTSC composite output can generate a proper colour burst derived
>>> from the emulated colour, producing the right effect on NTSC, but with
>>> proper colour encoding rather than a simulated colour burst and having
>>> to rely on timing tricks.
>>
>>
>> Yes, that's my take on it, too.
>>
>> Just for fun, I think I'll take a look at the IIgs composite
>> video with a scope...
>
> Well, *that* was interesting!
>
> I should have *started* by looking at the video, not the schematic!
>
> First, the composite signal:
>
> In SHR mode (the GSOS desktop), the chrominance signal is a phase-
> modulated 3.58MHz sine wave--produced by the chroma modulator.  For
> the light blue background, the luminance signal is dominated by an
> 8MHz signal, the result of dither at a 16MHz pixel rate.  White and
> black are constant levels.  The dark blue (the "standard" HGR color
> 6) shows no dither, and is a constant luminance level plus the 3.5MHz
> chrominance subcarrier for "blue".
>
> In HGR mode, the composite signal is *not* the standard digital
> Apple II video, but is also luminance plus a 3.5MHz chrominance
> subcarrier--so all composite video is generated in this way.
>
> I also took a look at the analog RGB signals for the desktop, and
> they are clearly dithered at a 16MHz pixel rate.  (For colors that
> have a constant component, like "traditional" HGR colors, there is no 8MHz 
> dither signal.)
>
> So for any conversion of either digital or analog RGB signals to
> another scan standard, it will be necessary to sense the presence
> of the 8MHz dither signal and switch the pixel clock to 16MHz to
> avoid a nasty 8/7 beat in the sampling that will produce 40 vertical
> bars across the screen in regions of constant dithered color.
>
> Apparently the VGC *does* contain a PLL to synthesize 16MHz from the
> 28MHz clock.
>
> What an amazing and, yes, *alien* video generation the IIgs has!

Michael,

    I quoted earlier.  Would you prove that Apple II modes like LGR, HGR, & 
DHGR are using 1.143 pixel out of 560 pixels to fit into 640 pixels at 8MHz 
/ 16MHz instead of 7MHz / 14MHz?

Bryan Parkoff