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

Alan Cresswell alancresswell at xtra.co.nz
Wed Aug 17 16:24:14 PDT 2011

Hi Mark, Phil,

That's interesting.  I have collected all my passes on the TS2000 with the
AGC on and set to the longest setting.  This is mainly because I often
record the signal level every 0.5 seconds during a pass which requires the
AGC to be on and the longest setting irons out any short fades.  I will turn
the AGC off and look at the statistics over a few passes but given that with
the AGC on I get almost all the available frames I don't expect to see much
difference.  It will be interesting to see.


-----Original Message-----
From: amsat-bb-bounces at AMSAT.Org [mailto:amsat-bb-bounces at AMSAT.Org] On
Behalf Of Mark L. Hammond
Sent: Wednesday, 17 August 2011 20:36
To: Phil Karn; Amsat - BBs
Subject: [amsat-bb] Re: Turn off AGC when receiving BPSK-1000

Hi Phil,

This is a great reminder.  Thus far, my data has been collected with the
TS-2000x using a "mid" setting for AGC; and with the HDSDR software set to
"AGC Med" when using/playing back Funcube Dongle data.   I've set both to
OFF now, since it's possible 

I wonder if anybody has experimented with AGC settings and the HDSDR
decoding?  Might be worth running a recording through a few times at various

In any event, Phil...THANK YOU for making this code real.  I have seen it
print data when the signal was visibly "in the dirt" which is impressive and
fun to see.


Mark N8MH 

At 02:31 PM 8/16/2011 -0700, Phil Karn wrote:
>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
>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
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