FreeDV 700E and Compression

FreeDV 700D is built around an OFDM modem [6] and powerful LDPC codes, and was released in mid 2018. Since then our real world experience has shown that it struggles with fast fading channels. Much to my surprise, the earlier FreeDV 700C mode actually works better on fast fading channels. This is surprising as 700C doesn’t have any FEC, but instead uses a simple transmit diversity scheme – the signal is sent twice at two different frequencies.

So I decided to develop a new FreeDV 700E waveform [8] with the following features:

1. The ability to handle fast fading through an increased pilot symbol rate, but also with FEC which is useful for static crashes, interference, and mopping up random bit errors.
2. Uses a shorter frame (80ms), rather than the 160ms frame of 700D. This will reduce latency and makes sync faster.
3. The faster pilot symbol rate will mean 700E can handle frequency offsets better, as well as fast fading.
4. Increasing the cyclic prefix from 2 to 6ms, allowing the modem to handle up to 6ms of multipath delay spread.
5. A wider RF bandwidth than 700D, which can help mitigate frequency selective fading. If one part of the spectrum is notched out, we can use FEC to recover data from other parts of the spectrum. On the flip side, narrower signals are more robust to some interference, and use less spectrum.
6. Compression of the OFDM waveform, to increase the average power (and hence received SNR) for a given peak power.
7. Trade off low SNR performance for fast fading channel performance. A higher pilot symbol rate and longer cyclic prefix mean less energy is available for data symbols, so low SNR performance won’t be as good as 700D.
8. It uses the same Codec 2 700C voice codec, so speech quality will be the same as 700C and D when SNR is high.

Over the course of 2020, we’ve refactored the OFDM modem and FreeDV API to make implementing new modem waveforms much easier. This really helped – I designed, simulated, and released the FreeDV 700E mode in just one week of part time work. It’s already being used all over the world in the development version of FreeDV 1.5.0.

My bench tests indicate 700C/D/E are equivalent on moderate fading channels (1Hz Doppler/2ms spread). As the fading speeds up to 2Hz 700D falls over, but 700C/E perform well. On very fast fading (4Hz/4ms) 700E does better as 700C stops working. 700D works better at lower SNRs on slow fading channels (1Hz Doppler/2ms and slower).

The second innovation is compression of the 700C/D/E waveforms, to increase average power significantly (around 6dB from FreeDV 1.4.3). Please be careful adjusting the Tx drive and especially enabling the Tools – Options – Clipping. It can drive your PA quite hard. I have managed 40W RMS out of my 75W PEP transmitter. Make sure your transmitter can handle long periods of high average power.

I’ve also been testing against compressed SSB, which is pretty hard to beat as it’s so robust to fading. However 700E is hanging on quite well with fast fading, and unlike SSB becomes noise free as the SNR increases. At the same equivalent peak power – 700D is doing well when compressed SSB is -5dB SNR and rather hard on the ears.

SSB Compression

To make an “apples to apples” comparison between FreeDV to SSB at low SNRs I need SSB compressor software than I can run on the (virtual) bench. So I’ve developed a speech compressor using some online wisdom [1][2]. Turns out the “Hilbert Clipper” in [2] is very similar to how I am compressing my OFDM signals to improve their PAPR. This appeals to me – using the same compression algorithm on SSB and FreeDV.

The Hilbert transformer takes the “real” speech signal and converts it to a “complex” signal. It’s the same signal, but now it’s represented by in phase and quadrature signals, or alternatively a vector spinning around the origin. Turns out you can do a much better job at compression by limiting the magnitude of that vector, than by clipping the input speech signal. Any clipping tends to spread the signal in frequency, so we have a SSB filter at the output to limit the bandwidth. Good compressors can get down to about 6dB PAPR for SSB, mine is not too shabby at 7-8dB.

It certainly makes a difference to noisy speech, as you can see in this plot (in low SNR channel), and the samples below:

FreeDV Performance

Here are some simulated samples. They are all normalised to the same peak power, and all waveforms (SSB and FreeDV) are compressed. The noise power N is in dB but there is some arbitrary scaling (for historical reasons). A more negative N means less noise. For a given noise power N, the SNRs vary as different waveforms have different peak to average power ratio. I’m adopting the convention of comparing signals at the same (Peak Power)/(Noise Power) ratio. This matches real world transmitters – we need to do the best we can with a given PEP (peak power). So the idea below is to compare samples at the same noise power N, and channel type, as peak power is known to be constant. An AWGN channel is just plain noise, MPP is 1Hz Doppler, 2ms delay spread; and MPD is 2Hz Doppler, 4ms delay spread.

Test	Mode	Channel	N	SNR	Sample
1	SSB	AWGN	-12.5	-5	Listen
	700D	AWGN	-12.5	-1.8	Listen
2	SSB	MPP	-17.0	0	Listen
	700E	MPP	-17.0	3	Listen
3	SSB	MPD	-23.0	8	Listen
	700E	MPD	-23.0	9	Listen

Here’s a sample of the simulated off air 700E modem signal 700E at 8dB SNR in a MPP channel. It actually works up to 4 Hz Doppler and 6ms delay spread. Which sounds likes a UFO landing.

Comments:

1. With digital when your in a fade, you’re a fade! You lose that chunk of speech. A short FEC code (less than twice fade duration) isn’t going to help you much. We can’t extend the code length because latency (this is PTT speech). Sigh.
2. This explain why 700C (with no FEC) does so well – we lose speech during deep fades (where FEC breaks anyway) but it “hangs on” as the Doppler whirls around and sounds fine in the “anti-fades”. The voice codec is robust to a few % BER all by itself which helps.
3. Analog SSB is nearly impervious to fading, no matter how fast. It’s taken a lot of work to develop modems that “hang on” in fast fading channels.
4. Analog SSB degrades slowly with decreasing SNR, but also improves slowly with increasing SNR. It’s still noisy at high SNRs. DSP noise reduction can help.

Lets take a look at the effect of compression. Here is a screen shot from my spectrum analyser in zero-span mode. It’s displaying power against time from my FT-817 being driven by two waveforms. Yellow is the previous, uncompressed 700D waveform, purple is the latest 700D with compression. You can really get a feel for how much higher the average power is. On my radio I jumped from 5-10W RMS to 40WRMS.

Jose’s demo

Jose, LU5DKI sent me a wave file sample of him “walking through the modes” over a 12,500km path between Argentina and New Zealand. The SSB is at the 2:30 mark:

This example shows how well 700E can handle fast fading over a path that includes Antartica:

A few caveats:

• Jose’s TK-80 radio is 40 years old and doesn’t have any compression available for SSB.
• FreeDV attenuates the “pass through” off air radio noise by about 6dB, so the level of the SSB will be lower than the FreeDV audio. However that might be a good idea with all that noise.
• Some noise reduction DSP might help, although it tends to fall over at low SNRs. I don’t have a convenient command line tool for that. If you do – here is Jose’s sample. Please share the output with us.

I’m interested in objective comparisons of FreeDV and SSB using off air samples. I’m rather less interested in subjective opinions. Show me the samples …….

Conclusions and Further Work

I’m pleased with our recent modem waveform development and especially the compression. It’s also good fun to develop new waveforms and getting easier as the FreeDV API software matures. We’re getting pretty good performance over a range of channels now. Learning how to make modems for digital voice play nicely over HF channels. I feel our SSB versus FreeDV comparisons are maturing too.

The main limitation is the Codec 2 700C vocoder – while usable in practice it’s not exactly HiFi. Unfortunately speech coding is hard – much harder than modems. More R&D than engineering, which means a lot more effort – with no guarantee of a useful result. Anyhoo, lets see if I can make some progress on speech quality at low SNRs in 2021!

Links

[1] Compression – good introduction from AB4OJ.
[2] DSP Speech Processor Experimentation 2012-2020 – Sophisticated speech processor.
[3] Playing with PAPR – my initial simulations from earlier in 2020.
[4] Jim Ahlstrom N2ADR has done some fine work on FreeDV filter C code – very useful once again for this project. Thanks Jim!
[5] Modems for HF Digital Voice Part 1 and Part 2 – gentle introduction in to modems for HF.
[6] Steve Ports an OFDM modem from Octave to C – the OFDM modem Steve and I built – it keeps getting better!
[7] FreeDV 700E uses one of Bill’s fine LDPC codes.
[8] Modem waveform design spreadsheet.

Command Lines

Writing these down so I can cut and paste them to repeat these tests in the future….

Typical FreeDV 700E simulation, wave file output:

./src/freedv_tx 700E ../raw/ve9qrp_10s.raw - --clip 1 | ./src/cohpsk_ch - - -23 --mpp --raw_dir ../raw --Fs 8000 | sox -t .s16 -r 8000 -c 1 - ~/drowe/blog/ve9qrp_10s_700e_23_mpd_rx.wav

Looking at the PDF (histogram) of signal magnitude is interesting. Lets generate some compressed FreeDV 700E:

./src/freedv_tx 700D ../raw/ve9qrp.raw - --clip 1 | ./src/cohpsk_ch - - -100  --Fs 8000 --complexout > ve9qrp_700d_clip1.iq16

Now take the complex valued output signal and plot the PDF and CDF the magnitude (and time domain and spectrum):

octave:1> s=load_raw("../build_linux/ve9qrp_700d_clip1.iq16"); s=s(1:2:end)+j*s(2:2:end); figure(1); plot(abs(s)); S=abs(fft(s)); figure(2): clf; plot(20*log10(S)); figure(3); clf; [hh nn] = hist(abs(s),25,1); cdf = empirical_cdf(1:max(abs(s)),abs(s)); plotyy(nn,hh,1:max(abs(s)),cdf);

This is after clipping, so 100% of the samples have a magnitude less than 16384. Also see [3].

When testing with real radios it’s useful to play a sine wave at the same PEP level as the modem signals under test. I could get 75WRMS (and PEP) out of my IC-7200 using this test (13.8VDC power supply):

./misc/mksine - 1000 160 16384 | aplay -f S16_LE

We can measure the PAPR of the sine wave with the cohpsk_ch tool:

./misc/mksine - 1000 10 | ./src/cohpsk_ch - /dev/null -100 --Fs 8000
ohpsk_ch: Fs: 8000 NodB: -100.00 foff: 0.00 Hz fading: 0 nhfdelay: 0 clip: 32767.00 ssbfilt: 1 complexout: 0
cohpsk_ch: SNR3k(dB):    85.23  C/No....:   120.00
cohpsk_ch: peak.....: 10597.72  RMS.....:  9993.49   CPAPR.....:  0.51 
cohpsk_ch: Nsamples.:    80000  clipped.:     0.00%  OutClipped:  0.00%

CPAPR = 0.5dB, should be 0dB, but I think there’s a transient as the Hilbert Transformer FIR filter memory fills up. Close enough.

By chaining cohpsk_ch together is various ways we can build a SSB compressor, and simulate the channel by injecting noise and fading:

./src/cohpsk_ch ../raw/ve9qrp_10s.raw - -100 --Fs 8000 | ./src/cohpsk_ch - - -100 --Fs 8000 --clip 16384 --gain 10 | ./src/cohpsk_ch - - -100 --Fs 8000 --clip 16384 | ./src/cohpsk_ch - - -17 --raw_dir ../raw --mpd --Fs 8000 --gain 0.8 | aplay -f S16_LE

cohpsk_ch: peak.....: 16371.51  RMS.....:  7128.40   CPAPR.....:  7.22

A PAPR of 7.2 dB is pretty good for a few hours work – the cools kids get around 6dB [1][2].

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