[Design View / Design Solution]
Get A Grip On Clamps, Bias, And AC-Coupled Video Signals
Achieving a proper dc restore is the name of the game when dealing with RGB, component, S-Video, and composite video signals.
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.
HIGHLIGHTS:
AC-Coupling Of Video Signals
When any signal is ac-coupled, the coupling capacitor stores the sum of the (signal's) average value, and the difference in dc potential between the source and load. Figure 1 (shown below) illustrates how this affects the stability of a bias point for different signals.
Single-Supply Bias Circuits For Video
RC-coupling works on any video signal, as long as the RC product is large enough. Also, the op-amp supplies must have sufficient range to handle the negative and positive excursions around the signal's average value. Though most ac-coupled applications are built this way, the advent of digital video and battery-operated devices has made the negative supply a cost and power burden. Less expensive alternatives are discussed.
Video Clamps
If luma, composite, and RGB signals are ac-coupled, the bias voltage will vary with video content and the brightness information will be lost. A diode clamp helps hold the black level constant. If the sync level varies, the diode can be replaced by a switch (a keyed clamp). By adding a dc voltage source, bias can be set for signals like chroma, Pb, and Pr, as well as composite and luma.
I'm an R&D engineer in Portugal and I'm working on this subject. It's hard to find reliable and detailed technical information on this topic. This is a very good article. I'm really impressed! Thank you.
Gonçalo Costa -July 19, 2004
Congratulation! This article is very interesting and helped me very much. Thank you! Regards
Boleslaw Polus -March 12, 2004
it is good article. I would have loved it in PDF format for reference. It was referred by Mike from Maxim Video discussion forum.
Shyam -March 10, 2004
No junction dots on schematics with the use of 4-way ties is not good for clarity, especially if not in pdf format where you can take a good look...
dugan oahu -March 01, 2004
Found a bug: the Diode in Figure 4a is connected with the wrong polarity. It would not allow positive voltages the way it is connected now.
But maybe I am seeing this wrong since there is no PDF to print out? :)
Greg Jackson -February 12, 2004
Two comments:
Some Video encoders need to be DC coupled, viz. the current-DAC driven parts from Philips such as SAA7120 and the like. They will not work if there is not a DC 75Ohm resistor to GND at the receiving side. AC coupling can be done after this termination resistor.
Also, not having PDF is ridiculous, take a look at EDN's articles in PDF for example.
Greg Jackson -February 12, 2004
I agree entirely with Timothy Ryan, and I have posted similar comments myself in the past. This website has superb content, but it should be available in PDF; viewing and saving articles as HTML is a poor subtitute. C'mon Electgronic Design -- give us PDF!!
Anthony Smith -February 06, 2004
I would avoid reading articles that are not PDF. I like to minimize mouse usage and not required to click on a link in order to view FIG-1 etc... All of the content should be inside one PDF file, and I can read it or print it easily.
I know you are trying to steer people toward E-book, but this "Drill Deeper" teaser and the number of clicks I need to do just to view an article is just too much.
jerry chow -February 05, 2004
Article was very good. I echo reader Timothy Ryan's comments: please give option for PDF version of articles.
al florida -February 04, 2004
Please print the full article in the hardcopy of the magazine.
Todd J Cottle -February 02, 2004
Incorporate the figures into the text, ala desktop publishing.
Horace Smith -January 29, 2004
I agree with Tim Ryan. A PDF option would be great. This also keeps the illustrations near the text & allows the reader to more easily cross-reference. Since virtually everyone has Adobe Acrobat on their systems, the PDF becomes a desirable option.
Now, about the article! I did enjoy it, particularly when it shows a number of techniques for dealing with the level shifts that occur with R/C coupling.
Dave Telling -January 29, 2004
This isn't about the article, but the whole series of articles. A number of publishers have a web version of their magazine. The ones that I enjoy the most are those who give you the option of a PDF version of the article. They are the easiest to read and have the best illustrations. Whenever I print an article to keep, I really appreciate the PDF prints over the text prints. Thanks.
Timothy Ryan -January 28, 2004
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