Hi all. This may not strictly be JUCE-related, but I’d be interested to get the views of any Linux audio people on why I’m witnessing the following phenomenon.
I’m writing a networked audio server. Taking the naive approach of sending packets directly from
AudioAppComponent::getNextAudioBlock I observed a lot of jitter at the client side. I moved my calls to
DatagramSocket::write to a dedicated thread, using
notify and observed the same thing.
Time::getMillisecondCounterHiRes to report the buffer period in ms:
... 0.026 0.027 0.027 0.028 10.413 0.088 0.061 0.042 0.044 0.043 ...
Wireshark reported comparable figures in the UDP timestamp deltas.
I’m using a buffer size of 32 frames, so I’d expect to see figures around 0.726 @ 44.1 kHz.
So this is with ALSA as the audio host, and the default device, which is “Playback/recording through the PulseAudio sound server.” I have JACK installed, and switching to “JACK Audio Connection Kit” I see intervals of:
... 0.041 1.402 0.035 1.463 0.065 1.339 0.034 1.417 ...
Finally, selecting JACK as the host, and “system” as the output device, the expected ~726 µs:
... 0.709 0.747 0.725 0.703 0.732 0.716 ...
I imagined that using ALSA as the host, calls to
getNextAudioBlock would be driven by audio hardware, and thus very evenly distributed in time. I can glimpse that pulseaudio might not be the most reliable output device for timing-critical purposes. How JACK can be used as an ALSA device, when it is itself a host backed up by ALSA, is a mystery to me. So is why I can’t see any device that both looks like a system soundcard and provides output channels in
AudioIODeviceType::getDeviceNames (e.g. “HD-Audio Generic, ALC294 Analog; Front output / input”, looks promising but has no channels). Given JACK is a host with ALSA as its driver (that’s how I have it configured anyway) it’s surprising to me that it’s the most temporally reliable here (particularly if, by default, ALSA is actually using pulseaudio at the backend). It’s not clear why JUCE finds the particular list of ALSA output devices, which differs from the lists I see in Audacity or Reaper, for example.
There are a lot of factors at play here – hardware, drivers, kernel perhaps, how pulseaudio/JACK/ALSA work, etc. – and a lot that I don’t know; if anyone can shed some light on the reasons for what I’m seeing I’d really appreciate it.