Composite video decoding: Theory and practice

After a few busy weeks, I’ve finally arranged enough time to cover the details behind my color composite video decoding project I featured recently. Before you proceed, I suggest you read the previous post, as well as the one before that covers B/W decoding (this post builds on that one), and watch the video, if you haven’t already:

So, feel ready to learn about composite color coding, quadrature amplitude modulation (QAM)? Let’s get started! Before you get going, you can grab the example excel file so you can view the diagrams below in full size, and study the data and formulas yourself.

Combining luminance and color information

If we look at one line of video information (“scanline”), it’s basically a function of two things: luminance (brightness) and chrominance (color) information, combined to a single waveform/signal like this one I showed in my first article.

If we’d like to just have a black-and-white image, encoding it would be easy: Maximum voltage for white, minimum voltage for black, and the values between would simply be shades of grey. However, if we’d like to add color information to this signal, we need to get clever. What the engineers in the 1960’s did to get two things stuffed into one signal was to add color in sine wave modulated form on top of the luminance signal. With proper analog electronics, these two signals could then be separated from the receiving end.


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