Re: This can't be true...

["Michael J. Mahon" <mjmahon@aol.com>]

Don Bruder wrote:
> In article <1hedhs2.u6m8msbrjz5oN%bieling@terra.es>,
>  bieling@terra.es (Jorge Chamorro Bieling) wrote:
>
>
> > Don Bruder <dakidd@sonic.net> wrote:
> >
> >
> > > In article <1hebjf9.10ohgzb1kxg1ymN%bieling@terra.es>,
> > > bieling@terra.es (Jorge Chamorro) wrote:
> > >
> > > <snip>
> > >
> > > > Article ID: 5508
> > >
> > > Why can't it? It's absolute fact. And based on the tag at the end, it
> > > looks like it might even be "directly from the mouth of Apple Inc."
> >
> > Sure, it comes from :
> > <http://docs.info.apple.com/article.html?artnum=5508>
> >
> > What I think is that in the article, the two phrases that explain why
> > some recorders may not work are *not* true at all :
> >
> > 1.- "The Apple IIe does not contain the color burst element of the NTSC
> > video signal"
> >
> > and
> >
> > 2.- "By modulating other portions of the video signal, the Apple IIe
> > simulates this missing information and allows an image to be produced on
> > a television"
>
>
> Why would that be untrue? All it takes to make color from a B&W signal 
> is the proper "jiggling" of the signal timing. Nudge it a little in 
> either direction, and you've got color dots rather than B&W dots. No 
> need to "build" the NTSC colorburst part of the signal, and it ends up 
> working with any hardware that isn't super-picky. Granted, it might come 
> out looking like crap, like when you try to put 80-column text on a TV, 
> but it at least worked to some extent in most cases.

No, you have to supply the color reference burst, or nothing in the
video signal can be interpreted as color.  (In fact, the presence of
the burst is the "flag" that color is being sent.)

You are confusing the absence of a distinct chroma subcarrier ("artifact 
color") with the absence of the color burst, an 8-10 cycle burst of
3.58MHz on the back porch of horizontal blanking that supplies the
reference frequency and phase for interpreting the chroma component
of the composite video signal.

It is apparent that the author of the original Apple article had the
same confusion.

In Apple video, the burst is supplied when color graphics are displayed,
but the chroma subcarrier is "simulated" by switching the video between
0 and 1 at rates that produce 3.58MHz components in the video signal
at amplitudes and phases (relative to the burst) that correspond to the
"Apple colors".

> Nowdays, rather than being surprised that a piece of gear chokes on the 
> Apple II's "bastard NTSC" signal, I'd be more shocked to find gear that 
> *DOESN'T* barf - The display gear these days is built to a lot tighter 
> tolerances than "back in the day" to deal with all the new 
> digital/hi-definition/S-video/etc. signals that all our digital gear is 
> shipping to them for display. Back in the day, there was enough "slop" 
> in the display hardware that tricks like the Apple II used were barely 
> considered "tricks" at all unless you were the guy designing it.

Agreed.  Sweep circuits could sync (and were adjustable) over fairly
wide ranges, and, as long as the burst frequency was *very* close to
3.579545MHz (so that the crystal-stabilized PLL could lock to it),
you were golden.

As video equipment goes digital, this tolerance is not part of the spec
and is being lost.

-michael

Music synthesis for 8-bit Apple II's!
Home page:  http://members.aol.com/MJMahon/

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