[amsat-bb] Turn off AGC when receiving BPSK-1000

[Phil Karn]

I forgot to offer some advice when receiving the ARISSat-1 BPSK-1000
telemetry beacon: turn off your receiver AGC if at all possible. If you
can only choose between fast and slow, pick slow. If this causes a large
variation in audio level, reduce the gain to avoid clipping on the
peaks. A sound card A/D is 16 bits so you have plenty of dynamic range;
don't be afraid to use it.

Ideally the background noise level should be constant with the signal
going up and down.

This greatly helps the demodulator and decoder to distinguish signal
from noise. The error correction uses the Viterbi algorithm, and one of
its big features is the ability to distinguish between "strong" and
"weak" bits; a strong '1' or '0' is considered less likely to be in
error than a weak '1' or '0'. The decoder can even accept "I don't know"
for a limited number of bits.

The decoder can still fix errors in strong bits. But it can fix more of
them in the weak bits and still more in the "I don't knows" (known
technically as "erasures").

This is especially important when the signal fades deeply, as it often
does with ARISSat-1. With the AGC off, the audio signal level falls
during a fade and the decoder can recognize it as a burst of erasures or
near-erasures.

As with many questions in life, "I don't know" or "I think it's X but
I'm not sure" are better answers than being sure of the wrong answer.

73, Phil