Hello,
I've been working steadily on FreeDV 700D, porting from Octave to C, and
working through some tricky sync problems. I can now simulate the
end-end system on the command line, and listen to the results. It's
outperforming FreeDV 1600 by about 10dB on fast fading channels, and is
error free at -2dB SNR on AWGN channels.
The main differences from FreeDV 700C are a new OFDM modem with rate 1/2
LDPC code and the option of a long interleaver (seconds of latency). It
uses the the 700 bit/s codec, although with a few tweaks we could run
other codecs and LDPC codes using a few dB more SNR. More inf on the
FreeDV 700D mode here:
https://www.rowetel.com/?p=5573
For anyone who would like to play along, here's README_ofdm.txt for the
mode:
https://svn.code.sf.net/p/freetel/code/codec2-dev/README_ofdm.txt
This includes references to blog posts, lots of command line examples,
and specification at the end. There's enough information for the super
keen to run it over the air using the cmd line tools, which I haven't
done yet.
Next steps are some over the air tests, write a freedv_api.c "mode" for
it, integration into the FreeDV GUI program, and lots of tweaking.
- David
On 16/04/18 08:52, Glen English wrote:
I will write more abt this when I return, however a) the quality of the
capacitor influences notch depth, b ) getting the area and proximity of
the loop to the line matters. for a 6" filter, anything over 33db for
600k bp-reject is very good.
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