donn@neurad.UUCP (Donn S. Fishbein) (03/29/85)
Could someone describe how to display a composite video signal (such as from a VCR) on a RGB monitor. A discussion of theory is ok, but I'd also like to see a circuit description or pointer to a commercial product. Please mail responses; I'll post a summary. -- Donn S. Fishbein, MD (N3DNT) ..!harpo!seismo!nbs-amrf!neurad!donn (301)496-6801
dsi@unccvax.UUCP (Dataspan Inc) (04/01/85)
This is not terribly easy, particularly if you want to get the benefits
of I/Q axis wide bandwidth demodulation....
What you need is:
1) A chrominance/luminance separator, which could be either
a comb filter (preferred) or simple bandpass filter from 3.08
to 4.08 mc;
2) A subcarrier regenerator to provide a 3.579545 mc sine wave
locked in phase to the colour burst signal in the composite
waveform;
3) Quadrature synchronous detection of the chromanance, to recover
the original I and Q sidebands;
4) Output filtering (DC-1.6 mc I, DC - 600 kc Q or to taste) as
well as a fixed delay to match I to Q channel because of the
lower bandpass of the Q channel;
5) Basis transformation matrix to convert I and Q into R, G, B
Note that you could perform demodulation among any arbitrary axes,
provided that the basis transformation matrix was correct. However, if one
selects R-Y, B-Y; that narrow bandwidth is only possible (both channels
limited to 500 kc/s. I think this is due to quadrature distortion caused
by VSB transmission of the I-channel and subsequent truncation of the
nonsymmetrical component contaminating the Q-channel. Another way to say
this is that high frequency crosstalk is minimized with I-Q axis demodulation.
Also, you must add back in the Y (monochrome) component unless your
monitor has a separate Y-input (which is linear, etc.)
There are devices which exist to do this (commercially black boxes)
but the ones which recover the full I-channel bandwidth are VERY VERY
expensive. The typical mid-line video store that caters to the industrial
and basement-Eyewitness News market probably has one.
If you had to build one, get the data sheet for the RCA CA 3217E. This
is narrow band but has potential; i.e. a quoted bandwidth of 900 kc for
the chroma demodulator and the basis transformation matrix built right in.
It also doesn't use flaky, hard to wind coils; just the Dale IR-2 types.
Be sure to ground pin 16 through a 470 ohm resistor if you like accurate
colour reproduction (the production receivers leave it open, for orangy
fleshtones!)
One question: why go through the effort for your colour under VCR?
Unless you have a type "C" or quadruplex in your living room, I think you
will be ** VERY ** disappointed in the spurious junk brought to light
from the video output jack.
David Anthony
DataSpan, Inc.