PortAudio M1 Latency

October 4th, 2023
music, tech
I recently tried to get my whistle-controlled bass synthesizer working on my Mac, as a backup to my Raspberry Pi version. [1] It's written on top of PortAudio, a cross-platform audio library, so getting it to compile and run was mostly a matter of getting the library installed, which went quickly. Unfortunately, the latency was far too high to be useful as a real-time musical instrument.

Testing with PortAudio's demo paex_read_write_wire.c [2] with suggestedLatency changed from defaultHighOutputLatency to defaultLowInputLatency I see:

Input device # 2.
    Name: MacBook Pro Microphone
      LL: 0.0528542 s
      HL: 0.0621875 s
Output device # 3.
   Name: MacBook Pro Speakers
     LL: 0.0187083 s
     HL: 0.0280417 s
Num channels = 1.

This is 72ms of round-trip latency, or 3,156 samples. You can easily hear the latency, and it's pretty painful for live music. It's even higher than the 67ms I see in the web browser. [3] Changing FRAMES_PER_BUFFER from the default of 512 to 128 doesn't have any effect, so I think this is lower level buffering.

I also tried with a Scarlett 2i2 audio interface, and latency wasn't any lower. Setting PA_MIN_LATENCY_MSEC also doesn't do anything. I'd try turning off ambient noise reduction but this option doesn't exist for M1 Macs.

I don't have this problem when running the Reaper DAW so it's definitely possible to get low latency on this machine. I wonder if there's some kind of default processing running on the audio system, and Portaudio doesn't ask to turn it off?

(I could go back to running my code as a Reaper plug-in, but I'd much rather have something stand-alone.)

[1] In setting up at Adirondack Dance Weekend I couldn't find one of my two Pis. It turns out I'd missed that it was under my mixer and forgotten to pack it:

[2] Built with gcc -I/opt/homebrew/include/ -L/opt/homebrew/lib/ -lportaudio paex_read_write_wire.c -o paex_read_write_wire after brew install portaudio to get the library.

[3] Suspiciously this is quite a bit higher than I used to see on my Intel Mac. I used to get 19ms in Chrome and 14ms in Firefox, and I'm now getting 67ms in Chrome and 64ms in Firefox.

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