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
http://www.oz9aec.net/index.php/funcube-dongle/423-funcube-dongle-on-the-air... 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 http://sox.sourceforge.net 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?
73
Patrick