General_Failure wrote:
Michael J. Mahon said:Since the video pulses for lores graphics and DHR are sent at 14MHz, each cycle of 3.58MHz covers 4 pulses. Any "asymetrical" pattern will contain a 3.58MHz component (in a Fourier sense).So the patterns 1100, 0110, 0011, or 1001 are maximum amplitude (saturation) 3.58MHz signals, while the patterns 1000, 0100, 0010, 0001, 1110, 0111, 1011, and 1101 are half-amplitude chroma signals. The patterns 1010, 0101, 1111, and 0000 do not contain any 3.58MHz component, and so are colorless.You're right. It is an elegant exploit of a technology. And as realised by many, this method does not work in PAL because the standard is designed to neutralise a lot of abhorrent signals. The standards of NTSC and PAL aren't really too different. However it is the little differences that make all the difference. A good explanation of how the video signal works exists on Rickard's PIC site on this page I think <a href="http://www.rickard.gunee.com/projects/video/pic/howto.php";>http://www.rickard.gunee.com/projects/video/pic/howto.php</a> Velleman sells a kit that seems to be a direct rip or Rickard's PIC Pong too. I bought one cheaply recently. Anyway, back to topic. The translation would either have to be done by a nice whack of logic, or a massive lookup table, as Bryan seems to have implemented. That must have taken a very long time.
Bryan is still "in process". The "sliding window" approach I described can use as few as 4 bits for its lookup, or can use a wider window, to accomodate the typically low bandwidth of an analog chroma channel. The resultant "smearing" of the hue and saturation information could be adjusted to produce a more accurate rendition of what an analog NTSC monitor would display. In today's hardware, a 4Kx16-bit ROM addressed by a shift register would not be a big problem, and could output 5x5x5 RGB data for the DACs. Of course, the 14MHz operating rate might be harder. ;-) -michael Parallel computing for 8-bit Apple II's! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it is seriously underused."