improve audio support

[khusmann]

Current audio support is awkward because there's no good SBC encoder / decoder for Python. (So example uses pyav instead).

I'd like to get python bindings for google's libsbc -- this will need to be its own separate project...

[Ylianst]

Do you think it would be possible to run VARA FM or other digital modes over BT audio if software was written to do that? It looks like you have to mess with audio cables, add-on cards, virtual audio devices, etc to make some digital modes work. Seems with audio support in BT, this would be nice and direct without any user configs.

[khusmann]

Do you think it would be possible to run VARA FM or other digital modes over BT audio if software was written to do that? It looks like you have to mess with audio cables, add-on cards, virtual audio devices, etc to make some digital modes work. Seems with audio support in BT, this would be nice and direct without any user configs.

@Ylianst 100% -- this is in fact how the HT app does all its digital image modes, etc. It just generates the audio clip, converts to SBC and then transmits over RFCOMM

[Ylianst]

I imagine that for APRS send/receive, it uses it's internal TNC... but yes, for the rest, it's a great way to the app to be super flexible and you can probably get better speeds.

[khusmann]

@Ylianst yes, the app uses the internal TNC for APRS.

I imagine if you did your own APRS decoding via audio (e.g. via direwolf) you could get better decoding than whatever they're doing onboard the radio (at the cost of more power / processing on the device you're using)

[Ylianst]

You are right that full software decoding would likely be best, especially if you can add error correcting codes. Do you have an idea what is the sampling rate of the Bluetooth audio? also, is any compression applied to the BT audio link? Obviously, if the BT link is compressed, that would be a problem.

[khusmann]

@Ylianst audio is at 32khz compressed with the SBC codec. I have a working send and receive example in benlink here: https://kylehusmann.com/benlink/benlink/audio.html check out benlink.examples.audiotransmit and benlink.examples.audiomonitor

The python code will be much simplified with libsbc bindings for python as I mention above, for now I'm using pyav for decoding. But you can get the idea...

[Ylianst]

@khusmann Ha! Got it. Yes, the SBC codec will add some noise to the digital signal for sure. This is interesting stuff, I am going to be reading up on this.

[khusmann]

@Ylianst yes, not ideal but good enough for basic use of the image modes it seems.

I haven't experimented with it much but it did seem there was some flexibility for the radio to play different bitrates (via different bitpool settings). So you could potentially go higher than the default audio compression quality it uses.

So yeah, definitely worth experimenting with. I suppose a good first step would be to do a side-by-side comparison of direwolf decoding of signals at different SBC compression rates.

[gretel]

allow me to throw data modes of m17 into the mix, i.e. https://github.com/xssfox/freedvtnc2

[Ylianst]

@gretel - Oh! That is interesting! I got plenty on my hands just getting basic packet radio working in HT Commander, but this is very interesting work that will be useful when I start working on audio.

[Ylianst]

@khusmann - I am trying to get your audiomonitor.py to work, but I am not very familiar with python and when I run it, I get:

Can I get really dumb down instructions on running it? I want to try to get audio frames going in/out and needs to figure out the basics.

[khusmann]

Ah, you need to install the package -- from the root repo dir:

pip install -e .

Then run (from anywhere):

python -m benlink.examples.audiomonitor

[Ylianst]

Thanks you, now I get this:

Ok, I needed to also do:

pip install pyaudio
pip install av

Now, I get this:

If I use 1 as the channel, I get this:

Apologies for the really basic questions.

[khusmann]

Yup, I don't include those deps in the package level bc they're only needed to run the example. (And I'll remove pyav once I make my own python sbc bindings)