El 28/05/17 a las 05:25, Tom Schuessler escribió:
OK I'm NOT a Linux guy. Let's see if we can do LilacSat-1 for the rest of us.
What I have been able to figure out is that Codec2 is a digital voice codec that creates audio speech out of some input data. If I read things right, LilacSat-1's concept here is to take analog FM (PL tone of 67HZ to open it's squelch), input and digitize the audio and then encode what it hears to digital data using the Codec2 process. That data is transmitted on a 9600 data stream BPSK decoded by an application. I think that is a good basic explanation of the system. A unique concept.
Hi Tom,
Your understanding is almost correct. To be precise, LilacSat-1 has an SDR, so RF is digitized and then FM demodulation is done digitally (on an ARM processor, I think).
Now back to not being a Linux person. I see the posts on the BB dealing with folks using GNU radio to decode the data. GNU Radio which is a Linux Digital voice program apparently can be ported to Windows, but the install process is very touchy feely. My looking at pages about Codec2 I get references to the "FreeDV" program. I found they do have a Windows installer for FreeDV. Is that the accepted program for this purpose using CODEC2? After installing the FreeDV program, I see that there are different flavors of the Codec it uses, but only some of them seem to match what is noted as flavors on the Codec2 development page. Which actual flavor is the LilacSat-1 system using?
Codec2 is an open-source audio codec which was developed because all the other low bitrate codecs that one can use for radio are covered by patents. It is used in FreeDV, which is a complete solution (digital voice mode) for HF (and a mode for VHF/UHF is now in development).
FreeDV and LilacSat-1 use the same codec, but other than that, the rest (the modem) is different.
The Codec2 variant used in LilacSat-1 is Codec2 1300bps, which is the same that is used in FreeDV 1600. LilacSat-1 uses 9600baud BPSK and FEC following the CCSDS standard. It uses custom framing to multiplex the digital voice with telemetry and the image downlink. FreeDV 1600 uses multicarrier BPSK/QPSK at a total of 1600bps.
So the FreeDV software can't be used for LilacSat-1.
Currently, the only solutions to receive LilacSat-1 involve GNU Radio. You have a decoder in gr-lilacsat
https://github.com/bg2bhc/gr-lilacsat
which was developed by Wei BG2BHC (he is in the satellite team) and another in gr-satellites which I developed
https://github.com/daniestevez/gr-satellites
Probably it's easier to get up and running gr-lilacsat, but the decoder in gr-satellites has some extra features.
I intend to give this a listen sometime this long weekend to see if I can decode anything. Should be interesting. Does anyone have raw downlink data samples that I could use to test what the FreeDV program can "hear"?
You have a sample here:
https://github.com/daniestevez/satellite-recordings
73,
Dani EA4GPZ