Loopback on Linux

[PrimmR]

Would it be possible to support audio loopback on Linux devices, the same way as with Windows (or macOS #876)?

It seems that it should be possible on at least ALSA with a little work, and maybe even easier with Pipewire (#887), but beyond that I'm not sure how it could be implemented.

[wgxh-cli]

Hi! I'm doing things similar to yours. I use alsa host for both input and output devices. But suddenly I found that cpal's data_callback calling frequency is a bit lower than regular one, causing audio input to have a little latency. This really worried me.

BTW I'm sure this is a cpal problem. Recently I use pure C and libalsa to write another loopback program, it works well, with almost no latency.

The biggest difference between a loopback program using alsa and one using cpal is that the buffer_size property in alsa's stream setup is explicit, that is, you have to set a proper buffer_size for your stream. But in cpal, most of its users would use default buffer size, that is BufferSize::Default, which, explained in hosts/alsa/mod.rs, will cause a latency.