Another RCA CT-100 discovery B8001599

[old_tv_nut]

Originally Posted by dtvmcdonald

Look carefully at both the CT-100 DC restorer circuit and the results on screen.
It does not do full DC restoration .. by intent! Its, if I remember rightly,
somewhere between 75 and 85%. This produces a bit extra brightness on
dark scenes, which the customers actually liked better. In fact, **I** still like it better, even in my dark room.

The CT100 DC restorer discharge path is 1M ohm, and the charge path looks like about 10K ohm, which appears to be 99% DC restored to me.
I stand ready to be corrected if someone can detail how this circuit could produce only 85% restoration.

Later sets with luma drive to the cathodes typically had deliberate ~85% DC coupling, which Consumer Reports complained about for years. Later solid state sets, especially those with video analog ICs and auto tracking, had 100% DC restoration, which stopped the complaints.

The CTC-5 super chassis (the cost-cutting disaster) had less than 100% DC coupling in the color difference outputs also, resulting in reduced saturation in an all-red image, for example. This was probably done to reduce tracking shift when replacing demodulator tubes.

[dtvmcdonald]

Originally Posted by old_tv_nut

The CT100 DC restorer discharge path is 1M ohm, and the charge path looks like about 10K ohm, which appears to be 99% DC restored to me.
I stand ready to be corrected if someone can detail how this circuit could produce only 85% restoration.

I'm actually not quite sure. I thought about it a lot and never decided.
What I report is what I measured with a 32 megohm resistive (and proper compensation capacitor) scope probe.

I think is some sort of combination of the presence of sync pulses and setup
in the signal and the presence of added blanking pulses.

But what I AM sure about, Consumer Reports notwithstanding, is that
with the brightness level of early CRTs, consumers did and should have complained about not seeing dark pictures well with 100% restoration.

Same thing applies to photos: I print some of my photos in a way designed
to be viewed in direct full sunlight. People ooooh and aaaaah about these when displayed properly ... but complain if viewed at "official museum light levels". That needs different (dodge and burn) processing. Such processing is clearly visible in sports programming in modern TV. Look at white parts of uniforms in direct sunlight.

[old_tv_nut]

Originally Posted by dtvmcdonald

I'm actually not quite sure. I thought about it a lot and never decided.
What I report is what I measured with a 32 megohm resistive (and proper compensation capacitor) scope probe.

...

Same thing applies to photos: I print some of my photos in a way designed
to be viewed in direct full sunlight. People ooooh and aaaaah about these when displayed properly ... but compain if viewed at "official museum light levels". That needs different (dodge and burn) processing. Such processing is clearly visible in sports programming in modern TV. Look at white parts of uniforms in direct sunlight.

Thanks - measurement trumps theoretical guessing, but my visual impression of CT-100s I have seen is that the restoration is much better than the 85% of later sets. On the other hand, I may not have been looking at a good selection of source material for this.
...
Comparison of monitor images and prints is a constant problem. With the fixed monitor calibration I use, prints can look too dark in dim room light and too bright in the competition viewing booth, so I do have to process differently. Monitors have been introduced that compensate for room brightness according to some psychophysical model - we'll have to see how well that works as time goes by.