Re: IGS Monitor from Composite Source?

["Alex Freed" <alexf@mirrow.com>]

"Michael J. Mahon" <mjmahon@aol.com> wrote in message 
mdqdnccRupRZuNzbnZ2dnUVZ_jadnZ2d@comcast.com">news:mdqdnccRupRZuNzbnZ2dnUVZ_jadnZ2d@comcast.com...
>
> No, NTSC requires a *shorter* delay line to delay the luminance signal
> to match the group delay of the chroma channel.  The combination of
> subcarrier bandpass filtering and chroma I and Q channel lowpass
> filtering introduces a few microseconds of delay relative to the
> luminance channel.

This is true: a short DL is used. One could argue that strictly speaking it 
is used
not for *decoding* NTSC but let's not split hairs.

The PAL Apples use the composite video delayed by one 14 MHz clock to get
the color difference signals.

>
> BTW, a comb filter *is* a delay line.  ;-)

It *contains* a delay line in most common implementations.

>
> So, separate horizontal sync to get the burst gate, build a crystal-
> controlled PLL to reconstruct the color reference, bandpass filter the
> chroma subcarrier, quadrature demodulate the chroma subcarrier, lowpass
> filter the I and Q demodulated signals, delay the luminance signal, and
> buffer and matrix the signals to RGB.

You make it sound difficult :)  Analog chips with 18 pins with a few 
external resistors and caps
do it quite well.

>
> That's not a trivial design, and it involves several filters.

"Trivial design" is a bit subjective I think. I did actually design and 
build multi-standard color
decoders decades ago.

>
>> A much better RGB signal can be obtained in Apple 2 by digital means :)
>
> True, but what it gains in bandwidth it loses in fidelity to NTSC
> color rendition.

If you mean that is doesn't look exactly as the Apple composite video on a 
TV you are right.
But then your TV may be different from mine. Don't forget Never Twice Same 
Color!

>
> -michael
>
> NadaNet file server for Apple II computers!
> Home page:  http://members.aol.com/MJMahon/
>
> "The wastebasket is our most important design
> tool--and it's seriously underused."