I've now tested the interleaver with FreeDV GUI on 700D (SVN rev 3579).
The tx and rx must have the same interleaver setting, which is set on
Tools-Options. The number (1,2,4,8,16) is the interleaver delay in
160ms modem frames, so 16(0.16) is 2.56 seconds (5.12 sec total from tx
to rx).
Especially at low SNRs, there can be a big delay (10 seconds or more)
for the Interleaver to sync. The modem can also lock up on the wrong
frequency offset, leading to false syncs, hence the ReSync button. I
need to improve acquisition at some stage. This low SNR world is
challenging - the raw bit error rate is 10% at -2dB SNR.
In terms of bit error rate, a 16 frame interleaver is worth 4dB in
simulated HF fast fading channels. Not sure how it will play out on
real channels for PTT radio. This is experimental radio, so lets try
and find out :-)
Here are some test files to play into FreeDV:
http://rowetel.com/downloads/codec2/700d_part4/
"fast" refers to a fading channel with 1Hz fade bandwidth, "intX" is the
interleaver depth.
Notes:
i) 700C doesn't maintain sync at these SNRs ....
ii) I can't actually hear the modem signal at -2dB
ii) You can make your own simulated files with:
./freedv_tx 700D ../../raw/ve9qrp.raw - --interleave 16 | ./cohpsk_ch -
- -24 --Fs 8000 --fast -f -20 | sox -t raw -r 8000 -s -2 -
~/Desktop/700D/ve9qrp_1600_int16_fast_snr2dB.wa
Tweak the cohpsk_ch third argument "-24" (Noise spectral density value)
to get the reported SNR you want.
-/-
I'm not sure if my model for SSB is correct. I'm just calculating the
total signal power and adding calibrated noise to get the desired SNR.
I suspect the peaks are higher than they should be. In a real SSB radio
ALC will raise the mic gain of low level segments of speech. The net
effect will be reducing the peak/average power ratio, taking the peaks
down closer to the noise. Need to measure a real SSB tx, and perhaps
model ALC.
- David
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