[amsat-bb] LilacSat-1 and Codec2

[Dani EA4GPZ]

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