Analog RGB Video black level and constrast

[Fusion916]

From what I understand "black level" is the "0V" level sent to the monitor, and contrast is the peak to peak voltage swing on the RGB line.

Two questions here (based on my understanding, correct me if I am wrong).

1. If the RGB lines ac coupled into the monitor, isn't it impossible to externally control black level?

2. If I'm ok with the black level, should adjusting contrast (RGB voltage swing) make the picture appear brighter?

The reason why I am asking is because I'm trying to compensate for a dark picture on a monitor (the tube isn't strong anymore) and want to build an external box to adjust black level, contrast and rgb levels.

 

[Enzo]

HAve you tried rejuvenating the tube? I used to service arcade game video monitors, and many of them ran 16 hours a day. I often found weak pictures, and a simple session with a CRT rejuvenator brightens them right up almost every time. And even tubes that were not dim, many times benefited from a gun cleaning.

And you don't mention it, but did you adjust the screen drive to the CRT? Screen and black level tend to interact too.

I guess my main thought here is make the monitor as close to 100% before looking to modify it or make some external gadget. Kinda like putting a new air filter and fuel filter into your car before looking to start using premium gas.

 

[Pano]

The overall levels are called "Drive" and the black levels "Cutoffs", but they go by various names. Typically in a North American video signal, the black levels are at 7.5% of white. That's called "Setup"

The way I adjust Drive (contrast) and cutoffs (black) is to maximize the drive without clipping. I have made test patterns for that. Then set the black level so as to not crush the blacks too much. Also have test signals for that. The black level is very dependent on the ambient light. As Enzo says, they interact. Gamma mostly adjusts the mid-tones.

So far, that isn't much help for a dim tube. The rejuvenator may be the ticket. You can push the signal, but at some point the circuits will clip before the signal ever gets to the tube. That may be a halfway fix, but probably won't look great.

 

[abraxalito]

RGB lines aren't normally AC coupled. If there's a series capacitor then its charge is usually clamped at each sync pulse to effectively DC couple.

 

[DF96]

Older RGB can use 0.3V for black, 1V for white, and 0V for sync pulses (usually on the green channel). You need either DC coupling, or AC coupling with clamping during the 'flyback' time. The clamp can be active (driven from the sync circuits) or 'passive' (using just a diode).

If the monitor is correctly set up then increasing signal amplitude (i.e. constrast) should make the white/grey parts lighter and the black parts stay the same.

 

[Pano]

IME, that's rare. Black levels will usually change when you change the drives, unfortunately.

Is this a computer monitor? You are sending it an RGB signal, or some flavor of VGA?

 

[Enzo]

The drive and cutoff controls are usually (or at least often) on the neck board. They are controls for the guns. The screen control I refered to is typically on the flyback, along with the focus control.