[aprssig] Clock/timing accuracy when generating an APRS signal

[Richard Corfield]

Thanks for the info. The receiving PC is a Netbook and was using internal
sound card. As I type this some process is hogging the system or it's
running out of RAM so I watch the text follow a few characters behind me
typing it. (Windows Update is a bad one, as is the virus scanner. I guess
the real issue here is Google Mail doing something big in javascript).

I am now using a crystal in my transmitter (a PIC microcontroller). I had
been trying to use its internal RC clock which is more like 2000ppm, though
I wonder if that is stable enough over short time periods that I could train
it using the accurate 1Hz signal from my GPS module.

The waveform generation is based on interrupts but they are high priority
and very predictable. The bigger problem is that I have to round the amount
of clock ticks per sample to a whole number so lose just under 1% at 2200Hz,
and measure the length of a bit in terms of samples which is not a whole
number. If needed I could use a second timer to improve that. If I can't
assume that the receiver can adjust to small drift in timing by looking at
when the tone switches then I'll need to keep the length of my bits quite
accurate. Because the error will be different for each tone it will look
like jitter rather than a constant error factor.

Thanks

 - Richard
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