Can we discuss DC restoration circuits?

[old_tv_nut]

Originally Posted by ChrisW6ATV

Your description is correct for the problem.

No, it's stated backwards, as I explained.

Originally Posted by ChrisW6ATV

Years ago, I had a Sears portable color TV with lousy black-level retention, but I could make it pretty good if I cranked the contrast way down. (I had to have the room lights off and it was essentially unwatchable, it was so dim, but it really did keep its black level consistent within that low range of contrast.)

If you look at the CTC-7 contrast control circuit, you can see that it changes only the AC gain of the video ouput. AC gain is greatest when the contrast is set to max and a large capacitor shunts the video amplifier cathode resistor. Therefore, the DC coupling percentage is greatest at minimum contrast (where the AC gain is small and closer to the small DC gain). Probably something similar was involved in your Sears portable.

[miniman82]

Studied the -9 schematic some more over the past few days, and made some realizations.

1. the video output tube is DC coupled to the CRT cathodes
2. the video output tube is AC coupled to the video amp tube via .1uf capacitor, with DC bias being set by the brightness control
3. between the amp and output is where the DC resto of the signal needs to happen

I going to be looking into this more in depth in the coming weeks, so if anyone has circuit specific input I'd love to hear it.

From what I've read, the black level must be reset before the beginning of each horizontal scanning line, and it's usually done during horizontal retrace. This is what makes the diode a bad choice, because it clamps the entire signal (including any sync pulses) to ground. What this means is that with a diode, the black level is being inluenced by the amplitude of the sync pulses, so you are not seeing 'true black'.

[old_tv_nut]

Originally Posted by miniman82

Studied the -9 schematic some more over the past few days, and made some realizations.

1. the video output tube is DC coupled to the CRT cathodes
2. the video output tube is AC coupled to the video amp tube via .1uf capacitor, with DC bias being set by the brightness control
3. between the amp and output is where the DC resto of the signal needs to happen

I going to be looking into this more in depth in the coming weeks, so if anyone has circuit specific input I'd love to hear it.

From what I've read, the black level must be reset before the beginning of each horizontal scanning line, and it's usually done during horizontal retrace. This is what makes the diode a bad choice, because it clamps the entire signal (including any sync pulses) to ground. What this means is that with a diode, the black level is being inluenced by the amplitude of the sync pulses, so you are not seeing 'true black'.

1) There is some loss of DC at the coupling between the first and second video amps, but not completely.
2) There is also significant loss of DC coupling in the video output due to the capacitor-coupled contrast control in the cathode
3) DC needs to be restored at the output of the video output - but doing it there may negate the brightness control action, so something may need to be done to provide a brightness control - I can't remember how DC restoration was added in the Porta-color fix that was posted to Videokarma, but that may provide the solution - suggest looking it up.
4) you are right, theoretically, clamping to sync tips is not the best in case sync level varies, but it may be a practical solution if you don't mind readjusting the brightness control occasionally when you change channels. Back porch clamps are much more complex and require pulse drivers and very accurate timing to make sure they do not drift into clamping either partially on sync tips or partially on active video. The sync tip clamping is automatically self-gated on the most negative part of the signal.

[miniman82]

Originally Posted by old_tv_nut

1) There is some loss of DC at the coupling between the first and second video amps, but not completely.

You are correct. For proof, I just inserted a diode before the video output peaking coil and ground. It made no difference at all in the picture, so I can assume DC restoration will have to be done elsewhere. Drat, there goes my hope of a simple solution!

2) There is also significant loss of DC coupling in the video output due to the capacitor-coupled contrast control in the cathode

Looks like it's set up as a cathode bypass cap of sorts. Any way to improve this part of the circuit, or am I right in assuming that cap has to stay there? My guess is that any changes would require a complete redesign of the video output...

3) DC needs to be restored at the output of the video output - but doing it there may negate the brightness control action, so something may need to be done to provide a brightness control - I can't remember how DC restoration was added in the Porta-color fix that was posted to Videokarma, but that may provide the solution - suggest looking it up.

I am under the impression that can't happen, because the output and CRT are directly coupled? I can't find the porta color thread.

4) Back porch clamps are much more complex and require pulse drivers and very accurate timing to make sure they do not drift into clamping either partially on sync tips or partially on active video. The sync tip clamping is automatically self-gated on the most negative part of the signal.

That's why I got excited by that integrated circuit I first posted, but it does not come in a high enough voltage rating for use on something like my CTC-9. Highest rated version I think was 220v or something like that, and the cathodes of the -9 sit at somewhere around 350...

[ChrisW6ATV]

Originally Posted by old_tv_nut

No, it's stated backwards, as I explained.

Oops, you are right, good catch. Somehow I missed the description of black level going UP in higher-contrast scenes, which is something I have not ever seen.

Miniman82-

Yes, you would need to build a circuit that would be driven by the LM1881.

Thanks for the posts, Jerome.

[miniman82]

I figured.

That being the case, the problem now becomes where to get the correct voltage to clamp the signal to? I read somewhere that to do it accurately/reliably enough, you would need a DAC. Idea is that you use a microcontroller to set the DAC to an accurate black level, then use the LM1881 to clamp the signal to the DAC generated voltage. The McGyver in me says there's another way, though.

The article I referenced in my first post mentions a comparator in the feedback loop of the video amp: why not use the LM1881 to trigger the comparator, thereby stabilizing the black level before the beginning of each horizontally scanned line?

[cbenham]

Originally Posted by miniman82

why not use the LM1881 to trigger the comparator, thereby stabilizing the black level before the beginning of each horizontally scanned line?

This may work, however you might need to experiment with the value of R set on the LM 1881.

Its nominal value is 680K for NTSC signals, but I have had problems with the resulting pulse being slightly too wide to properly drive a clamp. I've reduced R set to 470K or 560K with better results.

The pulse must start and end during the back porch time and must not extend past the end of the H back porch.

If it does, the clamp will be clamping to a voltage other than ground, the usual voltage level of the back porch portion of the waveform.

Hope you are successful.

Cliff

[miniman82]

Originally Posted by cbenham

This may work, however you might need to experiment with the value of R set on the LM 1881. Its nominal value is 680K for NTSC signals, but I have had problems with the resulting pulse being slightly too wide to properly drive a clamp. I've reduced R set to 470K or 560K with better results.

Cool, thanks for the heads up! Can you tell me what set you are working with, and what circuit you drove with the LM1881?

The pulse must start and end during the back porch time and must not extend past the end of the H back porch.

The LM1881 datasheet lists a 4µs gate time, which is shorter than the 4.7µs alotted for the entire pulse. I assume this is so what you are describing does not happen, but naturally there will be some setup with an o-scope involved to get things perfect.

If it does, the clamp will be clamping to a voltage other than ground, the usual voltage level of the back porch portion of the waveform.

See, this is where I get confused. If clamping to ground were sufficient, everyone would be doing it. However, my understanding is that the true black level is not at ground potential, but rather slightly above it.

http://en.wikipedia.org/wiki/File:Composite_Video.svg

In the above illustration, you can see that there is a blank level at .285v (the so-called 'blacker than black' level), and then slightly above that you have the 'true' black level at .339v (7.5 IRE). This is where studio monitors are calibrated to:
http://www.outside-hollywood.com/200...tudio-monitor/
So is it also the voltage you would ideally clamp to, or am I wrong?

[old_tv_nut]

Originally Posted by miniman82

Cool,
See, this is where I get confused. If clamping to ground were sufficient, everyone would be doing it. However, my understanding is that the true black level is not at ground potential, but rather slightly above it.

http://en.wikipedia.org/wiki/File:Composite_Video.svg

In the above illustration, you can see that there is a blank level at .285v (the so-called 'blacker than black' level), and then slightly above that you have the 'true' black level at .339v (7.5 IRE). This is where studio monitors are calibrated to:
http://www.outside-hollywood.com/200...tudio-monitor/
So is it also the voltage you would ideally clamp to, or am I wrong?

I'm sure Cliff meant "blanking (back porch)" rather than ground. Fact is, there is no place in the signal where there is an NTSC black to clamp to. PAL solved this by making black equal to the back porch level. In an NTSC receiver, you must also clamp to back porch, and then adjust the black level down slightly to be correct. I repeat: it is not totally terrible to clamp to the sync tips, it may just require occasional adjustment when changing channels/sources. The back porch clamp is necessary in pro gear, since there will not be someone constantly adjusting for changes in incoming sync amplitude.

Regarding the clamping pulse width: the value in the IC is likely a compromise between burst gate and clamping pulse width. Ideally, the burst gate should be wide enough to include the whole burst, while the clamp pulse should be narrow enough to never include sync or active video. So, changing the value as Cliff suggests is a good idea.