airwire

backstory

i have some speakers and a projector and it's too much effort to have a long cable to connect them. I also got a dac hat for one of my pis so I decided to roll my own audio transmitter/receiver.

features

• goes over udp
• simple command line interface
• maybe supports multiple channels with --channels (have not tested not stereo stuff yet)
• configurable buffer size
• cross platform and light (tens of mb of memory usage, currently a little over 1mb release compile size)
• stereo swap in case your channel order differs for stereo.
• written in rust
• optional opus for up to 2 channels

someday in the future

• configurable transports
• better handling of network conditions?
• stop stream to save power when no udp activity? not sure how to impl for now

usage

list devices for input and output

airwire enumerate

on linux this uses alsa, so it can't use pulseaudio/pipewire specific things.

my typical dev setup

For Windows users: I highly recommend this virtual loopback audio cable, it makes an output also shows up as an audio input (they also added a sep 16 channel loopback device which I might get around to testing),

cargo run --release -- transmit --target-device-name "CABLE Output (VB-Audio Virtual Cable)" --addr "192.168.68.96:6969" --stereo-swap --packet-pacing

cargo run --release -- can be replaced with just the command airwire if you want to use this outside of development

todo: on linux there's a bit of a hack cause cpal doesn't touch pipewire directly 1. cover your ears, 2. transmit the pipewire or pulse device depending on your audio server, 3. open pavucontrol and go to the recording tab and switch airwire to capture a monitor device 4. it seems like it'll choose the correct device next time?

then on a linux machine (in this case a pi, can leave buffer blank to automatically configure it):

RUST_BACKTRACE=full ./airwire recieve --addr "0.0.0.0:6969" --target-device-name pulse --buffer 480 --packet-pacing

if your network sucks I recommend removing the --buffer option because typically the default is greater.

side notes

windows

• may need to set process priority to be higher than normal if your scheduler is not working in favor, not sure how helpful this is.

out of scope

• encryption
• automatically matching codecs, frame sizes, and other protocol things with server (maybe in the future?)

compilation

linux

I compiled another copy of this project on a pi 5, and all I needed to do for dependencies at build time was sudo apt install libasound2-dev. By default opus support is enabled (btw I haven't tested without it), which requires cmake. So in addition you may need to run the following:

sudo apt install cmake

windows

it worked out of the box on my machine, might have some stuff already installed.

other platforms

it just needs the rust stdlib + cpal support, haven't tried

codec stuff

-b <bitrate> let's you set bitrate in kbps, defaults to 128kbps which is good for opus. see help for automatic selection which according to docs isn't very good --profile <profile> lets you set the opus profile, defaults to audio but can be set to lowdelay if you want. --fec enables forward error correction for opus codec --vbr enables variable bitrate for codecs that supported it --packet-loss-percent <percent> sets the packet loss percentage for some encoders, defaults to unset --gain <gain> sets the gain modifier in dB, may not be applicable on both sides, defaults to unset. does not work if opus is not used for now.

cookbook

Here are some of my personal scripts I use.

tranmission on linux desktop

airwire transmit --addr <target addr> --target-device-name "pipewire" --packet-pacing --repeat-packets 2

You may find it useful to create a null sink in Pipewire/Pulseaudio and then use pavucontrol to force capture from a monitor source.

recieve on pi

This uses chrt to set the process to a high priority, which is nice for getting a consistent latency.

ssh <pi creds> sudo chrt --rr 99 ./airwire recieve --addr "0.0.0.0:6969" --packet-pacing --buffer 480

gaming mode

Remove --buffer 480 and replace with --buffer 240 --frame-size 120. Makes most rhythm games playable.

special thanks

• claude for showing me a basic impl of a minimum viable product that this actually wasn't too hard if I used cpal.
• cpal for being awesome for crossplatform low latency audio
• @cutename from ssi for suggesting the extremely keep it simple solution of chucking pcm over the network.