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Re: IGS Monitor from Composite Source?
Alex Freed wrote:
"Michael J. Mahon" <mjmahon@aol.com> wrote in message
RoydnZnvkKYZHKDbnZ2dnUVZ_r6vnZ2d@comcast.com">news:RoydnZnvkKYZHKDbnZ2dnUVZ_r6vnZ2d@comcast.com...
mdj wrote:
There are plenty of chips out there that'll decode NTSC into component
forms. I doubt you'd need a lot in the way of support circuitry to
drive it.
No, it's a mess. There are filters and delay lines involved, so any
chip you find will have lots of pins and lots of external components.
Chroma separation and demodulation is either an analog process or a
pretty complex DSP process, so there's no free lunch.
Not quite true. PAL decoding requires a delay line, NTCS does not.
Good chroma/luma separation is done by "comb filters" - get one from an old
but good TV set. Or just use simple filters with band limiting.
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.
BTW, a comb filter *is* a delay line. ;-)
Decoding NTSC is not rocket science. There are chips with not too many pins
to do it. I can make a circuit out of about 10 discrete transistors and 4
diodes
to do the job - like in a TV from the 1960s. Less transistors if I can steal
the
color carrier reference (3.58 MHz) from the computer.
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.
That's not a trivial design, and it involves several filters.
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.
-michael
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Home page: http://members.aol.com/MJMahon/
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