Sine wave demodulation with PIC

[Kyle Miller]

hi,
I am trying to demodulate a video color chroma signal (3.58 mhz) that
has been stored in digital memory at a sufficient high sampling rate
and have done the demodulation with a desktop computer and the color
pictures are excellent using 32-bit sine calculations, but now I
really want to do it using an 8-bit microcontroller, preferrably a PIC
as speed is not a concern. Right now I am averaging the voltages of
several sequential samples to approximate the "zero point crossing" of
the chroma waveform and waiting until a data sample is captured within
about 15 degrees of being "in-phase" or "out-of-phase" with the chroma
reference oscillator (instead of working hard to calculate phase
differences) and then taking the voltage difference from the zero
point to approximate the red and blue modulation. From the picture (I
may be able to post a sample) the color gets very noisy where the
luminance is fluctuating, but large solid objects have fairly good
color. Could I be measuring the "zero-point" differently or do you
guys know of any good resources on the 'net for measuring waves with
microcontrollers?

Thanks,
Kyle

 

[Tom]

hi,
I am trying to demodulate a video color chroma signal (3.58 mhz) that
has been stored in digital memory at a sufficient high sampling rate
and have done the demodulation with a desktop computer and the color
pictures are excellent using 32-bit sine calculations, but now I
really want to do it using an 8-bit microcontroller, preferrably a PIC
as speed is not a concern. Right now I am averaging the voltages of
several sequential samples to approximate the "zero point crossing" of
the chroma waveform and waiting until a data sample is captured within
about 15 degrees of being "in-phase" or "out-of-phase" with the chroma
reference oscillator (instead of working hard to calculate phase
differences) and then taking the voltage difference from the zero
point to approximate the red and blue modulation. From the picture (I
may be able to post a sample) the color gets very noisy where the
luminance is fluctuating, but large solid objects have fairly good
color. Could I be measuring the "zero-point" differently or do you
guys know of any good resources on the 'net for measuring waves with
microcontrollers?

Thanks,
Kyle

Sounds like you need a PLL maybe.

Tom