Re: Newbie Apple IIgs questions

[Eric Smith <eric@brouhaha.com>]

"Michael J. Mahon" <mjmahon@aol.com> writes:
> The deviations from NTSC are minor timing variances, particularly
> in the vertical sync pulse.  One effect of which is the collapsing
> of the the normal interlaced raster to a non-interlaced one.
> 
> In all other respects, the Apple composite video signal is
> "acceptable" as an NTSC signal.

Acceptable in that a TV or monitor will display it.  Certainly not
in the sense of being compliant with any standards.  There's a very
major deviation in that the standard requires there to be exactly
227.5 cycles of the color carrier per scan line, but the Apple II
produces 228.0 cycles.  Woz designed it that way deliberately so that
the color encoding of consecutive scan lines would be identical.  He
was concerned about use with televisions, not standards compliance.

Note that this deviation also interferes with the ability of newer
televisions and monitors with comb filters to do the best possible
chroma/luma separation.  When Woz designed it, comb filters were not
yet in common usage.

> The video overlay card does two things:  it creates standard sync and
> blanking signals (correcting the minor timing differences) and it does
> anti-aliasing of the chroma signal, so that there is less color artifact
> on, for example, dot pairs.

It does more than that.  It generates a correct, standard-compliant
signal such that there are 227.5 cycles of color carrier per scan
line, and the color phase of alternate lines is inverted to compensate.

Eric