mrapple@quack.sac.ca.us (Nick Sayer) (06/29/91)
In article <9106281438.AA14158@ws-30.cae.wisc.edu> kolstad@CAE.WISC.EDU writes: > > I'm a "EE in training" (Junior year...) and would like to find some >information on what composite TV siganals are made of. At the risk of exposing my true ignorance, let me start the ball rolling. 1v - - |xxxxxxxxxxxxxxxxxxxxxxxxxxxxx| | | 0v ------- ---/\/\/\/\/----- |---- | | -.4v ----- That little waveform is one line of NTSC video. The excursion to -.4v is the horizontal sync pulse. It is followed by a 3.579545 MHz sine wave .5 volts peak-to-peak amplitude. Shortly after that the visual information is sent. The portion of the picture not normally visible is the horizontal blanking interval. The timing is related to the 3.579545 MHz burst, called the "color burst" (more later). The horizontal scan rate is exactly 1/227.5 of that frequency, or about 15734.26 kHz. The blanking interval is about 11 usec, and the visible part about 52 usec. I don't know off hand what constitutes the vertical sync signal, though I suspect it is a very long -.4v pulse. The first few lines after the vertical sync are non-visible lines, as are the last few lines on the screen. This constitutes the vertical blanking interval. Vertical sync occurs at 1/525 of the horizontal sync frequency, or a tad less than 30 Hz. I may have this one wrong. Vertical sync might happen twice as often and the scanning interlaces. I don't know. During the picture portion, the lower the voltage, the darker the spot will be. That's the luminance of the spot. If the spot has any color, then a 3.579545 MHz sine wave will be amplitude-centered at the appropriate luminance. The amplitude of that sine wave constitutes the color saturation, and the phase difference between the wave and the original color burst determines the resultant hue. I also don't know the relationship table between phase and hue, though the primary colors (Red, Green and Blue) will be 120 degrees apart. I suspect that green is 0 degrees and red is 120 degrees ahead of the burst, but that's just a guess. I suspect PAL (Phase Alternating Lines) is the same way, except that it's based on 25 Hz instead of 30, and instead of a color burst, it shifts the color-to-phase relationship 180 degrees each line. I don't have the first clue about SECAM. -- Nick Sayer | Official Scapegoat for the | "And now my mrapple@quack.sac.ca.us | MC68HC11 Mailing List. | post is done!" N6QQQ | To subscribe, send mail to | --buster@acme.edu 209-952-5347 (Telebit) | mc68hc11-request@quack.sac.ca.us |