Brightness and Contrast

[TinCanAlley]

I don't know if it's just me, but I've used a calibration DVD via RF converter on my Zenith and did based on the instructions (pluge pattern). When I do this, the image is a bit too dark, so I adjust by eye from there. Only issue is when I do this, black areas are really black on well lit scenes, but washed out and on the gray side in dark scenes. During scene transitions, the screen is not black, but gray and what seems a bit too bright as it is distracting.

Is this a limitation of 1974 tech, am I adjusting it wrong or is there an issue in the brightness/contrast of my TV?

[old_tv_nut]

Your set is probably from the era before sets had 100% DC restoration, so floating of the black level opposite to the average scene brightness is normal for it.

[kf4rca]

More info here:
https://www.analog.com/en/resources/...s/an-1603.html

[TinCanAlley]

When reading up on DC restore, it makes me think it would be about unstable brightness without it. I could be understanding it incorrectly as the issue I'm looking into is about how in low light scenes it's almost impossible to get true blacks and instead all black levels are more or less levels of grey with some very close to black. Where as in bright scenes, anything supposed to be black, is black. I figured that in night scenes, it would be easier for the set to produce blacks as there is less brightness overall.

I have the schematics and was hoping someone could point out the section where DC restore would be. I know I'll be looking for a clamping diode and capacitors. The best I could find was it would be somewhere in the video output going to the grids.

Thanks for the help.

[old_tv_nut]

Color sets designed before the introduction of analog ICs usually did not have explicit DC restoration circuits, but rather would have a degree of DC coupling through the circuits from detector to video ouput. Typical for these color sets might be 85% DC coupling, meaning there is a 15% variation in the black level depending on the average scene brightness.

[old_tv_nut]

"...the issue I'm looking into is about how in low light scenes it's almost impossible to get true blacks and instead all black levels are more or less levels of grey with some very close to black. Where as in bright scenes, anything supposed to be black, is black."

Conversely, if you set the "brightness" control for correct blacks on a dark scene, shadows will be clipped on a bright scene.

Some later sets that had full DC restoration relabeled the controls:
The DC bias control that had always been labeled "Brightness" was relabeled "Black Level", and the gain control that used to be called "Contrast" was relabeled "Picture."

[old_tv_nut]

I think it comes down to a question of whether you want to modify the set to improve the DC coupling/restoration or keep its historical performance. It's up to you.

[TinCanAlley]

Originally Posted by old_tv_nut

I think it comes down to a question of whether you want to modify the set to improve the DC coupling/restoration or keep its historical performance. It's up to you.

I can live with it, I just thought the Chromacolor 2 on the 25EC58 chassis would have all the bells and whistles for the time.

[old_tv_nut]

Originally Posted by TinCanAlley

I can live with it, I just thought the Chromacolor 2 on the 25EC58 chassis would have all the bells and whistles for the time.

It probably does, but not all the bells and whistles of a later chassis.