[amsat-bb] Re: Decoding wideband recordings

Patrick Strasser OE6PSE oe6pse at wirklich.priv.at
Thu Aug 18 08:15:55 PDT 2011

Am 2011-08-10 00:54, schrieb Phil Karn:
> It's been suggested that I modify my ARISSat-1 BPSK-1000 telemetry
> demodulator/decoder to accept wideband quadrature (I & Q) recordings
> like those produced by most of the software defined radios out there.
> This is fundamentally not that hard, but first I need some information.
> How many people could actually use this?

I guess all people that use SDRs in the style of FCD, like SDR Widget,
or generally all DDC receivers that work with soundcard interfaces.

> What is the format of the
> recorded files? Is there a standard, or does each make of SDR produce
> its own?

Often the I and Q channel are recorded via Left and Right of a sound
interface, either built-in in the computer or via USB. FCD for example
is not more than a very powerful DDC chain, an USB audio device with
96kHz bandwidth and a USB HID interface to control all the settings,
like filters and gains. So for a recording you'd probaly end up in
whatever your recoding program produces, likely WAV, but any format with
16bit and 2 channels would do.

For decoding please be aware that I/Q via sound interfaces has a weak
spot at the centre frequency. The interfaces all have a high pass
characteristic below like 30Hz, which means the resulting spectrum has a
notch in the middle. You usually would not like to exactly tune the
center frequency to the frequency of your interest, but a little below
or above. You can see that notch for example at

With all oscillators being not perfect this can probably be ignored, as
you wouldn't end up at the center frequency anyway.

For converting audio files I can recommend sox
which is quite common in Linux distributions, but as cross platform tool
also available for Windows and Mac OS X. It's also available as library.

What I wonder is how good BPSK1000 survives speech encoders, like MP3.
Is there any experience already?



Engineers motto:   cheap, good, fast: choose any two
QTH: JN77rb                       http://sat.mur.at/
Patrick Strasser OE6PSE <oe6pse at wirklich priv at>

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