dya@unccvax.UUCP (York David Anthony) (08/22/88)
In article <7652@cit-vax.Caltech.Edu>, stevel@tybalt.caltech.edu (Steve Ludtke) writes: .. > I was doing on the ouput from the PLL. The next thing I need to accomplish > is to strip the colorburst off of a video signal. I've thought of a couple > of really messy ways of accomplishing this, and I'm aware of a couple of > expensive chips that will do the job, but I'd rather not resort to that. Stripping the colorburst is easy. A 74C221 and an FET switch (use an Elantec EL2020 current-mode opamp as a buffer because it is both fast-n-easy) with the appropriate trigger will separate the burst. Generating a CW carrier from the burst (what I think you really want to do) is a little tougher. One easy (if you don't care about good phase linearity across the horizontal line) way to strip the colorburst is to 1) filter the composite video through a very high-Q tuned circuit at 3.58 mc and 2) amplify the bejeevers out of this and ring an ordinary color TV s/c generator crystal with the resulting signal. You should gate the burst part only to the crystal, and a 74C221 or such can handle the burst gate generation. The former approach (ring the crystal and apply the resulting waveform to a limiter, say, the pentode half of a 6GH8-yukk) is classic circuitry from General Electric "Portavision" and other cheap ilk. It works, but not really well; the phase adjustment range is poor and the phase linearity across the line of the recovered subcarrier leaves much to be desired. Some people take the crystal and incorporate it into an "almost an oscillator" circuit of the injection-locked ilk. Better still, why not just copy the AFPC circuit out of a '70s color TV set, using an entertainment receiver chip (or chipset). The CA 3170/E should show up in every entertainment replacement catalog with an ECG or SK replacement, and is very cheap. It also will work with TTL logic for the gate and a +12 or +24 supply, generally. Zenith, RCA, and virtually everyone else used this chip. The best part of this chip are DC phase controls (very useful) (i.e. the "hue" control on a receiver) and buffered outputs. The worst part is the 105 degree demodulation angle output, said to provide "better" color rendition. You should be able to complete this part of the project with an expenditure under $10, assuming your junk box is reasonably well stocked. The complete CA3170 circuit appears in RCA Databook SSD-240B. I posted this to the net as it might be of general interest to others doing frame buffers and their associated circuits. York David Anthony DataSpan, Inc