View Full Version : Color from a CBS field sequential camera

Steve McVoy
09-14-2015, 07:15 AM
We have produced the first (crude) pictures from a CBS Vericolor camera since 1951. Work will continue to improve the picture:

09-14-2015, 06:24 PM
Very interesting.

Thank you for the post.

09-15-2015, 02:48 AM
I have been following this on EarlyTelevision site.... fascinating... great to see the progress

09-15-2015, 02:35 PM
Keep up the great work!

Steve McVoy
10-10-2015, 08:03 AM
See our latest progress: better resolution and color bars:

10-10-2015, 09:04 AM
Looking better all the time - thanks for posting. I was surprised to hear that the IO response rolls off so strongly. I had always assumed that the output of the IO electron multiplier had only a small roll off due to stray C, like a photomultiplier tube circuit.

Is R20 (24k ohms) the IO output load? That combined with the stray capacitance must be the cause of the roll off. Of course, you need a certain size resistor to get the low frequency gain, but the capacitance rolls off the highs. In that case, the overall design is a trade off between needing more overall gain in later stages vs. more peaking (high frequency gain) in the later stages. The peaker circuit (R193 and C16) looks like it has a break point of 200 kHz. This would correspond to about 5 pF across 24k ohms, a reasonable sounding number. It also implies about a 20x boost at 4 MHz, not 80x.

The .05 coupling cap effect on low frequency response is not a problem if it is followed by a black level clamp somewhere further along. Is there a DC restorer (with a horizontal pulse) somewhere later in the circuit? There should be, otherwise the R, G, B black levels will vary with respect to each other.

Steve McVoy
10-10-2015, 04:07 PM
The 80x number comes from the Vericolor manual. I measured the actual difference between 10 kHz and 3 mHz through the video amp (not the preamp) and found about a 80x difference in gain with C16 at about midpoint. Yes, 24 k is the load resistance.

So far no DC restorer, but you are right, and I'll add one later.

10-10-2015, 04:42 PM
Duhh -- screwed up the math somewhere, plus, I calculated for 4 MHz, not 12 MHz.

With the peaker cap set to 12 pf, the 12 MHz gain would be 56 times the low frequency gain. Stated strays of 45 pf across the 24k load implies a nominal peaker setting of 17 pf across the 62k, and indeed the gain ratio is 80x.