How do color signals work? An interactive explanation

[miniman82]

Been wanting to get around to this for some time now, and I think I've finally found enough material to be able to do this in layman's terms- or at least as close to Barney style as I am likely to get!

So if you're an amateur TV repairman such as myself, you probably already grasp the concept that the RF signal carrying the actual TV information isn't any different than a regular radio transmission. The amplitude of the transmitted signal (once detected after the tuner and IF strip have done their part) corresponds to brightness information amplified by the receiver, which is sent to the CRT as video. This signal also contains various timing signals related to proper scanning of the CRT. But beyond that, how did they manage to squeeze coloring information into a signal that already had defined characteristics?

The answer is: they had the space to send the information, literally in between the lines!

In actuality, what the engineers did was to place coloring information in the form of quadrature modulated sine waves in between the spaces used by the brightness signals. By using otherwise 'wasted space' in the existing signal, they were able to include coloring information without upsetting the millions of black and white sets already in operation.

Whoopty doo Basil, what does it all mean you must be asking? Quadrature? Sine waves? What's all this madness? I will explain.

Because a black and white receiver lacks the circuitry to detect this coloring information, it will display a color program in black and white and you would never even know there was color information hidden in the signal. But in a color receiver, something almost magical happens:

The coloring (chroma) signal is separated out of the brightness information right before the video amplifiers, and sent to a chroma amplifier. Then it is split into 2 paths, to recover the coloring information. These 2 paths are called 'I' and 'Q', referring to the fact that one signal is 'In phase' with the transmitted signal and the other is in 'Quadrature' or offset 90 degrees with the transmitted signal. I found a neat video that does a great job of explaining quadrature signals below.

https://www.youtube.com/watch?v=h_7d-m1ehoY

What are these I and Q signals being compared to, you may ask? Well, it's rather sneaky! At the transmission site there was a local signal that they were referenced to, but this signal (called the subcarrier) was not included in the actual transmitted video information! Instead a small sample of it is included after the horizontal timing pulse in the normally transmitted signal, and this short sample is used to bring a local oscillator in the receiver into phase before the start of each horizontally scanned line. This local oscillator is then used as the reference signal to which the I and Q signals will be compared. In TV lingo, the subcarrier is 'suppressed' at the transmitter, but the small sample that is transmitted is used to 'regenerate' that same signal locally in the receiver- which saved a ton of signal space.

Those I and Q signals that are recovered by the color receiver are compared to the signal recovered from the local oscillator, which runs at 3.58Mhz. Differences in the phase of the I and Q signals with respect to the 3.58Mhz local oscillator represent hue information and amplitude represents intensity of the transmitted color, which are then passed to a matrix to drive the CRT. Thus by sensing the phase and strength of the recovered color information, the receiver is able to determine what color should be on the screen at any given moment and how strong that color should be. The last interesting step is that the coloring information is usually provided at the CRT grids, and the brightness information is normally provided at the CRT cathodes. Thus the CRT itself becomes a sort of mixer for the signals arriving at its respective elements, the final signal arriving on the screen in its correct proportion to provide a vivid color image.