On 29.05.2014 23:12, David Rowe wrote: > Very cool that demo can run on the command line! Such a great way of > prototyping. A lot of the work in FreeDV (e.g. sound cards to modems to > codecs, sample rate changes, fifo buffering) is simply to reproduce what > the Linux command line does for us. > > Good point from others re bit errors and encryption. However I imagine > there are cipher systems that can work over HF, after all encrypted > speech is a major application for HF DV. Once you get a few dB over the > min SNR the bit error rate drops to zero with FEC. > > There is another application for SmartMic - secure telephony over the > GSM voice channel. The idea is we develop a modem that can pass 1200 > bit/s or so through the GSM _voice_ codec. This channel is pervasive > around the world, reliable, and low latency (unlike mobile data). >
Hi David. Thanks for pointing out some issues. It's correct bit errors might propagate - worst case: indefinitely. But there are keystream modes like RC4 or CFB block cipher modes which allow flipping a bit in the ciphertext resulting in a flipped plaintext bit at the same location. This should allow error correction or the FDM modem to work normally without any further modification. I recently tried adding fec and the fdm modem to my encoding chain: Adding fec works as expected: ./c2enc 1200 ../../raw/hts1a.raw - |openssl enc -rc4 -k Test -nosalt |./fec_enc - -|./fec_dec - -|openssl enc -rc4 -d -k Test -nosalt| ./c2dec 1200 - - |play -t raw -r 8000 -e signed-integer -b 16 - Adding the modem to the pipe breaks the whole thing - so IMHO the modem is unfortunately not fully bit transparent: ./c2enc 1200 ../../raw/hts1a.raw - |openssl enc -rc4 -k Test -nosalt |./fec_enc - -|./fdmdv_mod - -|./fdmdv_demod - -|./fec_dec - -|openssl enc -rc4 -d -k Test -nosalt| ./c2dec 1200 - - |play -t raw -r 8000 -e signed-integer -b 16 - Regards, Andreas ------------------------------------------------------------------------------ Time is money. Stop wasting it! Get your web API in 5 minutes. www.restlet.com/download http://p.sf.net/sfu/restlet _______________________________________________ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
