Can we discuss DC restoration circuits?
 

Now that I've gotten my CTC-9 up and running pretty well, my thoughts have already turned to improvements in the circuit. DC resto comes to mind first, so I did some searching. Turns out there's an IC (LM2480 triple bias clamp) which will serve this purpose, but I have no idea if it could be adapted to my set. Problem lies in the fact that the IC does DC resto at the CRT cathode, where my CTC-9 drives the grids. So the question then becomes: is it possible to use a device like this on my set, or would the conventional diode setup still be the best choice?

http://www.national.com/ds/LM/LM2480.pdf

I never got this in depth with TV's before, so I'm gonna need some guidance on this one. Replacing caps and general troubleshooting I'm fine with, but I have no idea how modding the chassis in this way would turn out.

You picked a good time to start this topic. I have been wondering how to improve the picture on the RCA color TV I recently finished restoring. It is a GG589M with a CTC-16X chassis. It has poor or nonexistent DC restoration. I also have an RCA combo radio/phono/CTC-16 set that I thought had pretty good DC restoration, so I briefly looked for differences between their circuits and have not found anything obvious yet.

Depending on what comments other VK members have, I would rather avoid using an IC in an old TV set, but if a silicon diode would be suitable, that would be OK since they are commonly used to replace selenium rectifiers already.

There have been several discussions of DC restoration in older color sets in the last few years, including one that really improved a GE Portacolor TV if I remember right. That circuit used a diode and a couple other components, I think. It seems to me that the discussions have all been in the rectangular-screen forum. I will add links to anything I can find.

I saw the GE Porta Color thread, and I'm thinking about doing it to my own '76 P-C set. Only reason I'm looking at an IC is pure simplicity; if people don't want to use it, they can always do the diode thing. I guess I just want an elegant (read:simple) solution.

Just remember I saw lots of guys try to make improvements in things and sometimes end up screwing them up. The engineers were pretty smart guys. You are restoring technology from the early times. And example would be trying to get a good picture out of any Motorola color television, impossible. Those sets were design the way they were and it would take an act of god to get them to produce a decent picture.

My suggestion would be to see if you can find any service bulletins addressing the issue. I fould service bulletins to be very helpful. And sometimes they kept techs from chasing around on very weird problems

Ok, good topic. As Chris had mentioned, I have seen good DC restoration on a CTC-16, modt likely the one Chris had mentioned in his thread. I have a CTC-17 (Zenith CRT)with decent DC restoration and my other CTC-17 (RCA CRT) with non existant DC restoration. The first chassis I believe is an XAB and the other chassis is an XAD IIRC. Im curious as to what causes this as well. Were there modifications done at the factory, or was this pissibly a field service call somewhere later. There has been to circuit modifications I can see to the XAD chassis at all. I have two 23" Zenith consoles from 1967. One has decent DC restoration, and one has none. I dont recall the chassis numbers off hand, but the one with decent DC is an SC600.

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Well, it's not a 'problem' per se. Like I said, I'm looking to improve the picture quality of this already well running set. If I have to experiment on the Porta Color before moving on to the CTC-9, then that's what I'll do- for the experience I guess.

I just looked at the schematic for the -9, and it looks like it has video directly coupled from the video output tube to the picture tube cathodes, with chroma capacitively coupled on the grids from the R/G/B-Y amps. So does that that mean I don't need DC resto, or can I still do something? My understanding is that the signal on the cap coupled grids causes DC 'sag', which messes with the black level. Perhaps I could just use voltage regulator IC's to 'prop up' and stabilize the DC on the grids or something. Just throwing out ideas at this point.

What exactly are you looking at in the picture to determine the quality of the dc restoration?

Maybe a weak crt is being interpreted as poor restoration?

My CRT tests new...

As I understand it, during bright scenes any black (or otherwise dark) objects will become lighter. It's really obvious if you're watching something in letterbox format, because the black boxes at the top and bottom of the screen will get 'lighter' as opposed to staying black like they should. Really ought to be called black level restoration instead of DC restoration, that would make more practical sense.

Weak crts do that too. I have had sets with semi weak crts and after replacement the blacks were black.

Good to know, but I'm trying to focus more on the nitty gritty electrical aspects of this. I want to engineer something for my set, test it, and possibly come up with some kind of circuit that others could use. If this goes well enough, I may be able to offer tube based PCB retrofitting kits that people can buy.


Here's some reading, to get the juices flowing: http://www.scribd.com/doc/7351543/Crt

A snippet reads:

"DC restoration of the video signal can be done in two ways. The first method is to use a simple diode to clamp the
signal at the reference black level. Diode clamping can be done either at the AC coupled input of the preamplifier or at
the AC coupled output of the CRT video amplifier. The disadvantage of diode clamping is that the black level is sensitive
to fluctuations in the power supply as well as noise coupling and temperature drift of the diode's forward drop.
A more effective approach for DC restoration is to do a dynamic black level clamping at the back porch of each
video signal. This requires the use of comparator within the feedback loop of the CRT video amplifier and the preamplifier.
During the horizontal retrace period, the comparator compares the DC feedback taken from the CRT video amplifier's
output with the voltage set by the brightness control potentiometer. Depending on the CRT video amplifier's output
voltage, a clamping capacitor at the output of the comparator is either charged or discharged so that the feedback
loop is stabilized and the video signal is restored to the black level."


This would seem to suggest that the restoration method employed in a set such as the CT-100 is inferior, since it uses a diode:

http://www.antiqueradio.org/art/RCAC...nSchematic.jpg


Instead of doing DC resto in the amplifier section, they chose to do it individually for each color at the outputs.

Reflecting on the above article, I'm gonna say that using the IC I first posted about is probably not the way to go about this. I think a better circuit can be designed which could be inserted into the video amp section of a set like my -9, and it could be done cheaply. Thoughts?

Ok. let's clear up some things;

1) lack of DC restoration means on a bright picture, the blacks get too black (your black bars in letterbox are black and dark objects go too black and lose detail) (also, colors get over-saturated); on a dark picture, the opposite happens - your black bars in letterbox turn gray (also, colors get washed out).
2) this problem is in the luminance circuit only - you do not have to worry about the color difference drive on the CRT grids. Chroma may appear to be AC coupled when you look at the circuit, but it is not. There is a DC restorer action in the outputs so that the color balance does not drift depending on scene content. (Imagine if having a large red object in the scene caused the color to shift over-all toward cyan - this would be the color equivalent of the lack of luminance DC coupling, and would not be acceptable.)
3) different sets have different amounts of DC coupling depending on designer's choice
4) the amount of DC coupling can vary in some designs depending on the settings of the contrast and brightness controls

5) Adding DC restoration is a worthwhile experiment to improve picture quality

Your description is correct for the problem. This video parameter has been called black-level retention as well as DC restoration.

Ctc17 may have a good point about CRTs, but maybe related to brightness/contrast or setup adjustments in general, and this could help explain some of our similar models with and without decent restoration. I am going to try my CTC-16X chassis on my test jig and see if I can affect it by trying different settings of the CRT bias switch among others. 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.)

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

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.

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.

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! :sigh:


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...


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.


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...

Quote:
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!

Sorry about that!

Quote:
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...
Probably. In the CTC-5 , it's a DC control, which unfortunately runs the brightness up and down along with the contrast because the cathode (and plate) current is not zero at black. Several people with CTC-5's have modified their sets to be like your CTC-7 to fix that problem, but it reduces the DC coupling

Quote:
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.Keep looking and I will too -it's around here (or maybe audiokarma?) somewhere. The brightness could still work if it changes the height of the blanking interval and you end up clamping to the bottom of that - but not sure if that is the case.



Quote:
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...

Interesting - maybe doing something to clamp at the grid of the video output and changing the cathode circuit back to DC coupled like the CTC-5 could work. But I would find the Portacolor thing first, as I recall it being very simple as well as effective.

I jus tlooked at the LM2480. It does not have any pulse circuits to establish back porch clamping - its main claim is to provide a low impedance (in combination with C4) for the diode clamp D3. So, it is a sync tip (or blanking pulse tip) blanker anyway.

Um, I have a CTC-9.... :scratch2:

Will do! :thmbsp:

Sorry CTC-9. I better put on my glasses!

Portacolor fix was here, but the illustration has disappeared. Can someone help?
http://www.videokarma.org/showthread...ght=portacolor

Also, I've been meaning to ask: what's the deal with AK asking for a password to view certain threads? There's an AK thread linked in the first response in the above URL, but I cannot access it...

Good Day gentlemen,

here is the GE DC restoration improvement info - see included document.

Personally, i use the NS (National Semiconductor) LM-1881 sync separator, Back Porch sampler IC for DC restoration - see included pdf.

The chip has a Burst/Back porch output, perfect for DC restoration processing.

I use this IC as French TV AM carrier modulated video uses positive modulation and many TVs sold here are lousily-adapted Euro B/G (Negative Video mod.) to French standards. Background brightness and video content contrast vary with program content: unacceptable!

Having a "BackPorch clamped" solid video reference is the only way to go for correct DC restoration, be it in B&W and more essentially for color.

If you don't want a scene where the headlights of a car at night turn "black/negative" instead of brilliantly illuminating the stage, something must be done. IC LM-1181 + some added circuitry, depending on the TV does the job.

The LM-1881 gives a stable true back porch sampled output and takes into its stride varying RF input signal intensity coming from the antenna/tuner/IF strip assembly.

Just to inform you of how complex the problem is, i learned the hard way that having a DC restored full Y/RGB chain wasn't enough: you also need to control the RF & IF AGC gain controls to keep video demodulation within the accepted signal level limits, but that is another story...

I hope this will help you,

Best Regards

jhalphen
Paris/France

Interesting...

So if I'm reading the directions of the LM1881 correctly, it will provide only a timing pulse for clamping. Does that mean I still need to provide the correct clamping voltage through something like a transistor, which would be triggered by this IC?

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.

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?

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

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 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.

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.

The phrasing we (I) have been using is a bit misleading. You do not clamp TO back porch. You clamp the back porch to some level. In a PAL receiver, you clamp the back porch to the DC level of black in the video amp chain. In an NTSC reveiver, you clamp the back porch to a level below black (7.5 IRE units below) so that black is at the right level in the video amp chain.