On my NUC and perhaps on other Linux computers, configuring the system for efficient, dropout-free, low-latency real-time audio can be tricky. It may be desirable to disable PulseAudio, and then to make Csound and all other audio software on the computer use low-level ALSA alone.
Edit /etc/pulse/client.conf and reconfigure autospawn (turn it off):
; autospawn = yes
autospawn = no
Then kill the pulseaudio daemon: killall -9 pulseaudio.
Audio configuration for Ubuntu is not well documented, especially not for musicians or programmers. However, I have learned some things.
It is indeed possible to permanently turn off PulseAudio and use only plain ALSA for all audio interfacing. This provides audio with less overhead and fewer glitches, but only for one application at a time. This is the best choice for me and probably for most serious musicians.
The basic thing in ALSA is a "card," and their names should be determined thus:
mkg@xenakis:~/csound-extended/WebAssembly/examples$ cat /proc/asound/cards
0 [PCH ]: HDA-Intel - HDA Intel PCH
HDA Intel PCH at 0xdf240000 irq 138
1 [UA25EX ]: USB-Audio - UA-25EX
EDIROL UA-25EX at usb-0000:00:14.0-4, full speed
Cards in turn have devices, which can be discovered thus ([card number - device number]):
mkg@xenakis:~/csound-extended/WebAssembly/examples$ cat /proc/asound/devices
1: : sequencer
2: [ 1] : control
3: [ 1- 0]: digital audio playback
4: [ 1- 0]: digital audio capture
5: [ 1- 0]: raw midi
6: [ 0] : control
7: [ 0- 3]: digital audio playback
8: [ 0- 7]: digital audio playback
9: [ 0- 8]: digital audio playback
10: [ 0- 9]: digital audio playback
11: [ 0-10]: digital audio playback
12: [ 0- 2]: hardware dependent
33: : timer
More information can be obtained thus, which lists all available cards and their devices:
mkg@xenakis:~/csound-extended/WebAssembly/examples$ aplay -L
null
Discard all samples (playback) or generate zero samples (capture)
pulse
PulseAudio Sound Server
default
hdmi:CARD=PCH,DEV=0
HDA Intel PCH, HDMI 0
HDMI Audio Output
... und so weiter.
The difference between hw and plughw is important; hw identifies a device that can use only its native sample rate and format, plughw identifies the same device, but with the addition of a plugin that automatically handles sample rate and format conversions. For our purposes plughw should always be used. The audio format conversions in ALSA behave as though they are considerably more efficient than those in PulseAudio.
For most pieces, the Csound options should be:
-d -f -m195 -+rtaudio=alsa -odac:plughw:1,0 [ -iadc:plughw:1,0 ]
In the browser, there is only 1 input channel, outside the browser there are probably 2 input channels.
It is vital to understand that with plain ALSA, only one audio stream can be active at a time. Hence, in the browser, only one tab can use audio at a time. Other audio tabs should be closed before running a WebAssembly piece.
The Chromium browser is invoked by the shell script /usr/bin/chromium-browser. Normally, Chromium uses PulseAudio. The script has comments saying that the browser audio configuration can be overridden to use plain ALSA in an initialization file or in an environment variable. Indeed, I got ALSA to work in Chromium by putting the following into my ~/.chromium-browser.init file:
export CHROMIUM_FLAGS="${CHROMIUM_FLAGS} --alsa-input-device=plughw:1,0 --alsa-output-device=plughw:1,0"
Chromium is now a snap but it does not correctly use ALSA. Chromium does not even work with apulse (see next). May be related to (this bug)[https://bugs.launchpad.net/snapd/+bug/1835290].
It is possible to use Firefox with apulse as follows; apulse is a set of shared libraries that provides ALSA in the form of a simplified pulseaudio interface.
Install apulse:
sudo apt install apulse
Set environment variables to tell apulse what ALSA audio interfaces to use:
export APULSE_PLAYBACK_DEVICE=plughw:1,0
export APULSE_CAPTURE_DEVICE=plughw:1,0
Run firefox with apulse:
apulse firefox
You can and should use a high-precision format for rendering to soundfiles. All playback software of which I am aware can handle any format and play it back properly. I recommend using the following Csound options: --sinesize=65536 --0dbfs=2 --sample-rate=96000 --ksmps=100 --wave. These options are consistent with contemporary professional practice for digital audio.