DESIGN VIEW is the summary of the complete DESIGN SOLUTION contributed article, which begins on Page 2.
Why should you ac-couple your video signals? In many cases, it's because the application has a single supply. Or you may have dual supplies, because going to a single supply will involve ac-coupling the video, and that will compromise the video quality.
Whatever the case, ac-coupling a video signal creates a problem. The signal's dc voltage level must be re-established afterward to set image brightness and ensure the signal falls within the linear region of the next stage. That operation, called "biasing," employs different circuits, depending on the video signal's waveform and the accuracy and stability required of the bias point. Sine-wave signals, like audio, use resistor-capacitor (RC) coupling to establish a stable bias voltage.
Unfortunately, only the chroma signal (C) of S-Video even approximates a sine wave. Luma (Y), composite (Cvbs), and RGB are complex signals that vary in one direction from a reference level, below which a sync waveform may be attached. Such signals require a form of biasing unique to video that's called "clamping," because it clamps one extreme of the signal to a reference voltage, leaving the other extreme to vary. The classic form of this is a diode clamp, where the diode is activated by the sync on the video.
But there are others, such as "keyed clamp" for color difference signals (Pb and Pr) and graphics RGB. Another biasing method is called a "dc restore," which adds feedback to a keyed clamp to improve the bias point's accuracy, prior to an analog-to-digital converter.
This article explains how each method works, the appropriate signals for each, the merits and drawbacks of dual and single power supplies, and why some circuits are inherently better for some applications.
Full article begins on Page 2