Re: Color Reference of NTSC Formula?

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

Mark McDougall wrote:
> Bryan Parkoff wrote:
>
> > It is only a question why Deep Red appears in the left of this pixel. The
> > answer might be two zero bits between one bit because of no luminance.
>
>
> Here's what *I* understand about colour artifacting, although I've never 
> studied the specifics on the Apple 2 in any detail. So take this with a 
> grain of salt..
>
> As you know, colour (chrominance) is encoded as a phase shift from the 
> colour burst reference modulated on top of the monochrome signal 
> (luminance).
>
> In an ideal world, this colour component might consist of a sine wave 
> whose phase can change instantaneously for each 'pixel', and by any 
> multiple of an infinitesimal amount. Naturally, circuits have bandwidth 
> limits as does the transmission spectrum so the phase changes have a 
> finite 'resolution' and the phase can't actually change instantaneously. 
> Obviously, these limitations still allow a reasonable quality picture to 
> be displayed.

Actual chroma subcarrier bandwidths are on the order of 1MHz, so the
band limiting is pretty severe.

> The Apple II (as did other computers of that era) generate what should 
> be analogue video signals using digital approximations. Indeed, the very 
> reason computers have discrete 'pixels' is a by-product of this fact, 
> whereas a TV picture raster line has no such horizontal delineation.
>
> You can, for example, crudely approximate a sine wave using a simple 
> square wave of the same frequency. If that square wave is passed through 
> a low-pass filter, the higher frequency components are filtered out and 
> the resulting output more closely resembles a sine wave.
>
> Now, you can't change the phase of a sine wave using a square wave of 
> the same frequency. But if you chose, for example, a frequency 4 times 
> higher, and approximated the sine wave using 4 consecutive 1's followed 
> by 4 0's, then the resulting square would be exactly the same, but you 
> can now vary the phase by +/- 45 degrees by inserting or removing an 
> extra 1 or 0 into the stream.

I think you mean 90 degrees...

And note that 1110 has the same phase as 0100, but three times the
luminance.

> It gets more complicated when you start moving away from 4 consecutive 
> 1's and 0's. For example, if you toggled 1's and 0's every two clocks 
> (rather than 4), then you'd think that you've simply doubled the 
> frequency of the colour signal. However, colour is encoded as a phase 
> shift - it's not frequency modulated - so that, and the fact that the 
> decoder is band limited - means the decoder 'sees' the 'double 
> frequency' as a constantly changing phase. You'd no doubt end up with 
> groups of repeating pixel colours.

No, as soon at there is no 3.58MHz component in the signal, there is
no color.  Alternating bit values of a 4 x 3.58MHz clock would produce
a 7MHz signal, and display as 50% gray.

> Also, the resolution of your 'clock' also limits how *quickly* you can 
> encode phase changes. If your stream changes from 0 to 1, the value is 
> held at 1 for the entire pixel, and the decoder can't 'see' how the 
> waveform is going to vary in future, so your next pixel is limited in 
> some way by the colour of the previous pixel. Depending on your clock 
> resolution, it may take 2 or more pixels to get from 1 colour to the next.
>
> I don't know the specifics of the frequencies involved on the Apple 2, 
> but I *suspect* the artifacting is a result of being able to change the 
> phase of the signal by +/- 90 degrees only? Can anyone confirm?

Yes--but the artifacting results from the video stream containing
3.58MHz components.

> Hopefully I haven't sold you a crock of sh*t here... I'm pretty sure 
> it's the gist of the mechanism if not 100% accurate.
>
> So if you want to understand how to 'emulate' artifacting I think you 
> need to understand both (1) how colour is encoded on NTSC/PAL and (2) 
> how the apple generates the video signal.

The actual mechanism of color encoding is rather complex, and requires
an understanding of vector algebra and the response of analog filters.

For a complete understanding, I'd recommend reading some books on
analog TV and electronics.

Many years ago, D. G. Fink's "TV System Engineering" (IIRC) was the
definitive text.

-michael

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