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> On the oscilloscope, I assume the front and back porch is clearly visible on the left and right hand side. Not quite. The front porch is actually the black level on the right hand side (I know, this confused me at first, too), at the end of the line, when the signal drops back to the black level. The sync pulse is the lower-than-black bit in the middle. The back porch is the portion thereafter, containing the fuzzy bit. Collectively these are called the horizontal blanking interval. The fuzzy bit is the color burst. Color TV was a hack on the original B&W design. To put color information into the signal without making a whole new signal that existing sets couldn't decode, the color information is ̶p̶h̶a̶s̶e̶-̶m̶o̶d̶u̶l̶a̶t̶e̶d̶ quadrature-modulated upon the luma signal. You need a phase reference to decode that information, and that's what the color burst is there for. If this were a color demo, rather than the nice, neat square waves, we'd see the rest of the signal line looking similarly fuzzy. Wikipedia has a nice diagram[1]. Why is the sync pulse below the black level? Well, consider the original B&W signal. The brightness of any portion of the picture was controlled by how strong the cathode was emitting when it swept across that portion of the screen. The intensity of the picture tube's cathode is controlled directly by voltage level of the video signal[2] during the active display period (the portion of the line not part of the horizontal blanking interval). So great, we've come to the end of a line, shut off the cathode after it has swept one line across the display tube, and need to reposition it to sweep another line. Why can't we just keep the signal at the black level instead of dipping below black? Well, analog signals and circuits can be kinda fiddly. Components drift or go bad. People think they know better than the warning of no user-serviceable parts. So it's entirely possible that due to a bad or misconfigured circuit, the cathode could actually not be quite off when fed a signal at the black level. In one of those cases, if the cathode were still on during retrace, the user would see slanting lines drawn across the picture, and might rightly get annoyed at this. So, to ensure that, no really, the cathode is actually off for reals during retrace, the signal is pulled below the black level so there will be no retrace lines drawn on the picture on any sets where the brightness levels are iffy for whatever reason.[3] 1. https://en.wikipedia.org/wiki/Analog_television#Structure_of.... 2. You could demonstrate this by raising/lowering the level of the square wave from/to the black/white level on the signal in this demo, and get a neat gradient effect. 3. On some sets, you actually can still force the cathode on during retrace and see the retrace lines by cranking the brightness all the way up. Probably not the best thing for the life of the tube, though. Edit: update with corrections in anyfoo's reply, below. Also added the link to the Wikipedia diagram I had originally intended to include. More Edit: Added an explanation of why the sync pulse is below the black level (blacker-than-black), because even though parent didn't ask, I'm pretty sure someone else is wondering (I know I used to). |
The two signals in the quadrature amplitude modulation are the two color difference signals. While that is the intent and also usually how encoders and decoders are actually implemented, most elegantly the math works out to a different interpretation as well: You can see the phase as the hue (i.e. what color), and the amplitude as the saturation (i.e. how much color).
That alternative interpretation works extremely well to explain the whole scheme without having to explain how and why quadrature amplitude modulation works (which needs a lot of math).