[amsat-bb] Re: Anyone using an FT-847 and SignaLink USB to copy ARISSat-1 BPSK telemetry?
karn at philkarn.net
Sat Aug 6 12:39:25 PDT 2011
On 8/6/11 6:09 AM, Alan P. Biddle wrote:
> made/purchased, due to the numerous running production mods. There have been
> a few reports of people being unable to "print" the BPSK telemetry, even
> though the signal is good, and the CW decodes well.
I just thought of another possibility. I wrote my demodulator to work
with a sample rate of exactly 48 kHz. Check to make sure your A/D
converter supports that rate and it's actually set to it. Most computer
A/Ds do, but because 44.1 kHz is so common as the sample rate of the CD
while 48 kHz is the professional audio standard, it's possible that some
don't support 48 kHz.
In theory I could modify my code to work with any sample rate that is an
exact integer multiple of the symbol rate (1 kHz) but it's not trivial.
But there are programs that will perform arbitrary sample rate
conversion for you (e.g., the open source program 'sox').
My demodulator is rather sensitive to errors in that A/D sample rate. It
processes audio in 'chunks' of about 1/2 second, and it can easily
handle a slip of up to 1 sample per 'chunk'. That corresponds to a
sampling frequency error of 2 Hz. More than that gets dicey, and it's
possible that some sound cards have bigger errors than this.
Do you have any way to check the accuracy of your A/D clock? One way
would be to digitize WWV, run a FFT on the recording, and look at the
audio tones that should be exactly 440, 500 and 600 Hz.
73 Phil, KA9Q
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