Application of diode clampers? Why exactly we need to add DC to signal?

Hello everyone,
I am studying diode clampers which are used to add DC value to the signal. I really cant figure out why exactly we need to add DC to the signal. I searched quite a bit on google but couldnt find the answer. In some sites, it is given that we use clampers in circuits used in TV's, but not mentioned in detail. Can anyone please help me in this regard.
Best regards
LK

Suppose you have an AC signal that goes above and below 0V but you want the signal to never go negative. A diode clamp of that signal will do that.

In old NTSC analog system the signal is AC as it passes through the various processing circuits. Thus the most negative point of that signal (the black level ) will vary in its relative value to ground, depending upon the picture signal level (bright areas versus dark areas). If uncorrected, the black level displayed on the display will vary with signal level. Thus, for example, a scene with a lot of white area or black area will tend to appear gray and a bright spot will have a black horizontal tail. A lot of old B&W TVs displayed these characteristics.

This proved intolerable for color sets so they used a technique called "DC Restoration". This used a clamp circuit to clamp the negative sync pulse voltage (defined as the full black level) to ground. This eliminated the DC shifting black level with the scene characteristics. Of course all the circuitry after this clamp circuit to the picture tube had to be DC coupled to maintain this proper DC level.

crutschow said:

This proved intolerable for color sets so they used a technique called "DC Restoration". This used a clamp circuit to clamp the negative sync pulse voltage (defined as the full black level) to ground. This eliminated the DC shifting black level with the scene characteristics. Of course all the circuitry after this clamp circuit to the picture tube had to be DC coupled to maintain this proper DC level.

Click to expand...

It was used in B&W sets as well

Nigel Goodwin said:

It was used in B&W sets as well

Click to expand...

True, but I think it was only the high end sets. I remember my first seeing of the term "DC restoration" was in one of the old Heathkit B&W sets where it was stated to give a significantly improved picture over sets without that feature.

Analog video:
The really low cost sets don't have DC restoration or black lever clamping.
The next level up does clamp the sync level but that was never the intended way. There are some errors in that method.
After horizontal sync there is a time called back porch or black lever. That is what should be clamped to "black" and the sync should be blacker than black.

The original question is why. Video is shipped around the world through 1000s of amplifiers and repeaters. Noise sneaks in. Most of the amplifiers are AC coupled. By the time you get the video, what level is black is totally lost. If "white level" is not certain then the picture is a little too light or dark. Not a big problem. If "black level" is lost, your eye is very sensitive to low light errors. So video is always sent where right after sync a "reference black lever" is sent to tell your TV, this is what black looks like.