modulating carriers

rokicki

So I'm curious: how would we go about modulating a carrier (presumably generated by one of the magic counters) with the output of another COG?
I suppose FM modulation is just manipulating the phase register, right? But I'm not sure how to go about amplitude modulation . . .

cgracey

To realize FM, you must configure a counter to run in NCO or PLL mode, then output your samples to the FRQ register. These +/- samples must be scaled for bandwidth and biased to the center frequency before being written to FRQ. This will cause the continuous-wave signal to go up and down in frequency. It works great!

For AM, set either NCO or PLL mode and write the appropriate frequency value into FRQ. Then, perhaps using the other counter,·PWM the same pin at a much lower rate (or maybe toggle the DIR bit to modulate the drive), so that over brief periods you will get·averages of continuous-wave·vs DC output. This will demodulate to analog AM levels.



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Chip Gracey
Parallax, Inc.

rokicki

Oooh, that's evil! PWM a carrier! I never thought of that. Cool. Hehe. Putting square waves into the demodulation stage. A good test of the filtering circuitry in the demod.

How about the broadcast TV demo? For this, the output frequency is probably much too high for this to work; you must have come up with something else in order to drive the phase-shifted chroma modulation onto the broadcast carrier.

Man, this propeller plus some limited external circuitry may *possibly* even make a pretty cool software-controlled radio. Think superheterodyne with a completely software-driven IF. Of course you'd probably have to be very careful so the EMI from the processor didn't kill your signal, but what fun!

Paul Baker

A resistor ladder is used for the broadcast TV, theres few threads in this forum discussing it.

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1+1=10

cgracey

The video circuit in each cog has a 4-bit counter that is used for chroma. With each tick, the counter increments and it is added to the 4-bit chroma value. The top bit of this sum is the modulation that jumps the 3-bit DAC (3 I/O pins with three resistors summed to make a 1V 75-ohm signal) up and down by 1 to generate the chroma modulation. This baseband signal can be further AM'd at the channel frequency to produce an RF signal. A second COG can be used to generate a 4.5MHz +/-50KHz aural subcarrier that can get AM'd, too, by the first COG. This makes a complete audio/video channel.

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Chip Gracey
Parallax, Inc.