David’s Oct 2024 Update – Real Time RADE & BBFM

The working acronym for the Radio Autoencoder has been changed from RADAE to the more use-friendly RADE (pronounced”raid”).

This month I continued working on getting RADE V1 into a form where it can be used in real time. The hybrid Python/C model seems to be working out quite well, meeting our goal of enabling hams to use the waveform early, and allowing us to spot any significant bugs that we may have missed with the stored file test campaign. It also makes development really fast and efficient compared to coding in C.

To support the RADE release I wrote a RADE Introduction document, that covers the basics of the mode and a description of the waveform. Thank you to the test team for providing feedback on this document, and Yuichi, JH0VEQ, for a Japanese translation.

Initial reports of on air performance are encouraging, and match our experience from the stored file test campaign. This represents a significant step towards our goals for our ADRC funded project:

  • Improve speech quality to a level comparable to commercial codecs.
  • Develop a “rag chew” FreeDV mode with subjective speech quality comparable to SSB at high SNRs.
  • Improve low SNR operation such that FreeDV is superior to SSB over poor HF channels.

We are making good progress on all three goals, although it would be useful to perform some formal subjective tests to support the anecdotal reports. There is some work we could do to improve the usability of real world RADE, e.g. reduce PTT turn around delays, improved acquisition, and integration into SDRs.

RADE V1 is an interim step, and we need to keep moving forward. While a neat way to get the mode “on air” quickly – the hybrid Python model is not the end goal, nor is it intended for widespread packaging and distribution. Next step will be a C port of the core RADE encoder/decoder, which will significantly lower the CPU load and bring us one step closer to a more general purpose C library version of RADE, suitable for distribution and integration into SDRs.

The Baseband FM (BBFM) work (see demos from last month) is also moving along nicely. This project is aimed at high quality speech over VHF/UHF radio. This month I have been developing a single carrier PSK modem that can be used over DC coupled or bandpass filtered FM radio channels. This will support an on air experiments of high quality speech using off the shelf FM radios and handsets.

RADE Preview (FreeDV 2.0.0-20241018) Released

This is the first preview release of FreeDV containing the new RADE mode. For more information about RADE’s development, check out the blog posts on the FreeDV website:

As this is the first preview release, there are some limitations:

  • As RADE currently doesn’t return the signal’s signal to noise ratio (SNR), it’s not currently possible to receive it and the other FreeDV modes at the same time (as in, if you choose RADE and push Start, that’s the only mode you can work; you’ll need to stop, choose another mode and start again to work FreeDV with the existing modes).
  • Squelch cannot currently be disabled with RADE. It’s unknown at this time whether disabling squelch is possible.
  • Due to compilation problems, 2020/2020B modes are disabled.
  • There is currently no Windows ARM build; this will hopefully be included in a future preview build. You may be able to use the 64-bit Intel/AMD Windows build in the meantime.
  • Minimum hardware requirements haven’t been fully outlined, so your system currently may not be able to use RADE. Future planned optimizations may improve this.
  • FreeDV Reporter does not currently report receiving RADE signals, but will report that you are using it and when you’re transmitting.

Other notes:

  • These preview builds are significantly bigger than previous releases. This is due to needing to include Python and the modules that RADE requires. Planned porting to C/C++ will eventually negate the need for Python.
  • The Windows build includes Python but not the modules that RADE requires. As part of the install process, the version of Python built into FreeDV will go out to the internet to download the needed modules.
  • As development is expected to happen quickly, these preview builds have a six month expiry date (currently April 18, 2025).
  • 32-bit Windows is no longer supported due to its likely inability to work with RADE.

More information and download links can be found at https://github.com/drowe67/freedv-gui/releases/tag/v2.0.0-20241018.

David’s FreeDV update – September 2024

From mid-August to mid-September, we conducted a Radio Autoencoder (RADAE) test campaign in two phases (a) stored files and (b) a prototype real time system. Ten people joined our test group, with many submitting stored file and real time test results. In particular I would like to thank Mooneer K6AQ, Walter K5WH, Rick W7YC, Yuichi JH0VEQ, Lee BX4ACP, and Simon DJ2LS for posting many useful samples, and for collecting samples of voices other than their own to test.

We are quite pleased with the results, here is a summary:

  1. It works well with most speakers, with the exception of one voice tested.  We will look into that issue over the next few months.
  2. Some of the samples suggest acquisition issues on certain very long distance channels, but this issue seems to be an outlier, perhaps an area for further work.
  3. RADAE works well on high and low SNR channels.  In both cases the speech quality is competitive with SSB.
  4. It works on local (groundwave), NVIS, and International DX channels. It works well for (most) males and females, across several languages.
  5. Prototype real time/PTT tests suggest it also works well for real time QSOs, no additional problems were encountered compared to the stored files tests. Mooneer will tell you more about that in his September report!

Selected Samples

I estimate we collected around 50 samples, here are just a few that I have selected as good examples. I apologise that I don’t have room to present samples from all our testers, however your work is much appreciated and has contributed greatly to moving this project forward.

Our stored file test system sent SSB and RADAE versions immediately after each other, so the channel is approximately the same. Both SSB and RADAE have the same peak power, and the SSB is compressed to around 6dB Peak to Average Power Ratio (PAPR). In each audio sample below, SSB is presented first.

Here is a sample of Joey K5CJ, provided by Rick W7YC. The path is over 13,680km, from Texas, USA to New South Wales, Australia (VK2), on just 25W. Measured SNR was 4dB. Note the fading in the spectrogram, you can hear RADAE lose sync then recover through the fade.

Using another sample of Joey, K5CJ (also at 25W), Rick has provided a novel way to compare the samples:

He writes:

RADAE is in the (R) channel & analog SSB is in the (L) left channel. Listen using stereo speakers, and slide the balance control L-R to hear the impact. Or, listen to it on your smart phone & alternately remove the L & R earbuds – wow. It demonstrates how very well RADAE does over a 13,680 km path!

Here is Lee, BX4ACP, sending signals from Taiwan to Thailand in a mixture of English and Chinese using 100W. The measured SNR was 5dB, and frequency selective “barber pole” fading can be seen on the spectrogram.

Here is Yuriko (XYL of Yuichi JH0VEQ) using 100W over a 846 km path from Niigata Prefecture to Oita Prefecture in Japan. The reported SNR was just 2dB. From the spectrogram of the RADAE signal, the channel looks quite benign with no obvious fading. However I note the chirp at the start has a few “pieces missing”, which suggests the reported SNR was lower than the SNR experienced by the RADAE signal a few seconds later.

Next Steps for HF RADAE

Encouraged by these results, the FreeDV Project Leadership Team (PLT) has decided to press on with the real time implementation of RADAE, and integration into freedv-gui, so that any ham with a laptop and rig interface can enjoy the mode. This work will take a little time, and involves porting (or linking) some of the Python code to C. Once again, we’ll start with a small test team to get the teething problems worked out before making a general release.

ML Applied to Baseband FM

To date the Radio Autoencoder has been applied to the HF radio channel and OFDM radio architectures. We have obtained impressive results when compared to classical DSP (vocoders + FEC + OFDM modems) and analog (SSB).

A common radio architecture for Land Mobile Radio (LMR) at VHF and UHF is the baseband FM (BBFM) radio, which is used for analog FM, M17, DMR, P25, DStar, C4FM etc. For the digital modes, the bits are converted to baseband pulses (often multi-level) that are fed into an analog FM modulator, passed through the radio channel, and converted back into a sequence of pulses by an analog FM demodulator. Channel impairments include AWGN noise and Rayleigh fading due to vehicle movement. Unlike, HF, low SNR operation is not a major requirement, instead voice quality, spectral occupancy (channel spacing), flat fading, and the use of a patent free vocoder are key concerns.

We have been designing a hybrid machine learning (ML) and DSP system to send high quality voice over the BBFM channel. This is not intended to be a new protocol like those listed above, rather a set of open source building blocks (equivalent to vocoder, modulation and channel coding) that could be employed in a next generation LMR protocol.

It’s early days, but here are some samples from our simulated BBFM system, with an analog FM simulation for comparison.

Original

BBFM, CNR=20dB

BBFM, CNR=20dB, Rayleigh Fading at 60 km/hr

Analog FM, CNR=20dB

CNR=20dB is equivalent to a Rx level of -107dBm (many LMR contacts operate somewhat above that). The analog FM sample has a 300-3100Hz audio bandwidth, 5kHz deviation, and some Hilbert compression. For the BBFM system we use a pulse train at 2000 symbols/s, that has been trained using a simulation of the BBFM channel. As with HF RADAE, the symbols tend to cluster at +/-1, but are continuously valued. Compared to the HF work, we have ample link margin, which can be traded off for spectral occupancy (channel spacing and adjacent channel interference).

This work is moving quite quickly, so more next month!

Mooneer’s FreeDV Update – September 2024

This month was spent continuing the RADAE prototyping and testing efforts started last month. I focused primarily on creating a prototype of the FreeDV application that is able to route audio to/from separate processes that can actually handle the RADAE modulation and demodulation. By doing so, technically-inclined users can get an idea as to how RADAE would work with actual two way QSOs on the air.

One major challenge was getting transmit working reliably. With the prototype scripts, there was a few seconds delay on startup before a modulated signal could actually go out over the air. This was fixed by simply never stopping the TX or RX scripts. There’s still a delay at the beginning but for the current test effort, it’s tolerable.

Another challenge is that forking processes on Windows works significantly differently than on Unix/Linux (especially if you want to route stdin/stdout through your application). RADAE has a feature where on the end of transmission, a special signal is sent out that immediately causes squelch to close. This prevents the R2D2 sounding audio at the end of transmissions that’s common with the existing FreeDV software. Unfortunately, I wasn’t able to get this working on Linux during my testing as something was still keeping file handles open and not in an EOF state (despite my efforts to force the latter).

Regardless, I was able to get enough of a prototype working (along with instructions) to have a two way communication with Walter K5WH, audio of which is below:

Several other users in the test campaign were also able to successfully set up this prototype and have two way contacts as well, helping to prove out RADAE in real-world conditions. We did discover that some of us were accidentally using voice keyer files encoded at 8 kHz sample rate, which impacted audio quality on RADAE (since it was trained on 16 kHz samples).

Additionally, for the above contact (and other testing done during the campaign), Speex noise suppression was disabled. This caused some background noise to enter my side of the contact. One possible future avenue of investigation is the use of rrnoise for noise suppression instead of Speex, as it promises better performance than the latter.

Next up on the list is to actually integrate RADAE in a manner that doesn’t require significant setup (i.e. by just installing FreeDV as is the case today). This will require at least an API wrapper written in C/C++ to accomplish, with possibly some additional C code as required to maintain reasonable performance.

More information can be found in the commit history below:

(Note that all commit logs above were generated with the following command line:)

git log --author="member@email" --after "Month 1, 2024" --before "Month 31, 2024" --all > commit.log