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AUDIO_SYSTEM(9) Kernel Developer's Manual AUDIO_SYSTEM(9)

audio_system — the NetBSD in-kernel audio mixer specification

This document aims to describe all aspects of the in-kernel audio mixer included with NetBSD 8 and onwards, describing its current behavior as of 2018.

This is the most fundamental element to the mixer. The vchan has all of the properties of the traditional single open NetBSD audio channel. It consists of playback and record rings along with audio_info structures.

Upon opening of /dev/audio or /dev/sound, a new vchan and mixerctl structure is created. In the case of /dev/sound, audio_info structures are inherited from the last open of /dev/audio or /dev/sound.

All vchans are up or down sampled into the mix ring (intermediate) format before being sent to hardware.

It is described in the following diagram:

	VCHAN1---------\
			\   VCHAN0
	VCHAN2-------------MIX RING ---- HARDWARE
		...     /
	VCHANn---------/

In the case of sysctl(8) usemixer=0 (see below), there is only one vchan whose play and record rings are the hardware play/record rings.

User accessible vchans are numbered starting at one (1). Vchan 0 is used internally by the mixer for the mix ring and its ring buffers are not user accessible.

The only limit to the number of open vchans is the speed of the computer and the number of free file descriptors.

A block of audio data is the basic unit for audio data. Audio applications will not commence playback until three (3) blocks have been written - this is the source of latency in the mixer along with the size of the audio data block.

For normal uses of audio read/write there will be three blocks of audio data before playback commences one in the vchan, one in the mix ring and one in the hardware ring.

The size of the audio data block is dependent on the audio format configured by the application the latency sysctl(8) and the underlying audio hardware.

Some audio hardware devices only support a static block size, as such the overall latency of the mixer for these devices cannot be changed. Other devices such as those supported by hdaudio(4) allow the hardware block size to be changed, allowing the latency of the mixer to change from 4 milliseconds (ms) to 128 ms with the mixer intermediate format being 16 bit, stereo, 48 kHz.

With regard to mmapped audio, blocks are played back immediately so the latency presented to applications is one third of the latency sysctl(8) value.

Latency can be calculated by the following formula:

	Latency (ms) =	 blocksize(bytes) * num blocks * 1000
			--------------------------------------
			freq(Hz) * bytes per sample * channels

Latency in the mixer and latency presented to audio applications is consistent, it will be the same regardless of the audio format requested by the audio application.

The default latency configured at boot time is 150ms and is subject to the above constraints.

Two new ioctls have been added to accommodate mixing of multiple vchans:

:
Allows setting the target vchan to operate on for subsequent ioctl(2) calls.
:
Returns the current vchan number.

These ioctls were necessary as some audio applications like to open an audio(4) device and an audioctl(4) device so to check on buffer usage and samples played etc.

As opening an audioctl(4) device would represent vchan 0 (the mix ring), these ioctls allow setting the target vchan and audio_info structure to that of an existing vchan.

Mixerctl structures are allocated when a new vchan is created. The mixer control structure allows for setting the software volume for playback - vchan.dacN or recording - vchan.adcN. These are 8 bit values and the this value is applied during mixing into the mix ring.

The software volume is applied to all channels (1, 2, 4 etc.) in the vchan and at present (2018-05-04) there are no balance controls for user accessible vchans.

The first vchan corresponds to the vchan.dac1/adc1 mixer controls.

All vchan mixer controls only have effect upon its own volume and writing to outputs.master (or equivalent) control is required to change the volume of the hardware.

Mixer controls are only present whilst the chan is in use and numbering starts at one (1). Mixer control numbers i.e. dac/adc1 correspond to their vchan number.

Audioctl allows access to the audio_info structure of a given device. Due to the audio mixer a -p flag was added to allow access to a given vchan's audio_info structure. The values for -p are numbered starting at zero (0).

Not specifying -p is the same as specifying -p 0 and will result in working with vchan 0 (the mix ring). This will display the audio parameters of the mix ring and allow setting the hardware gain and balance.

This is for compatibility with existing applications and shell scripts that are unaware of the -p switch.

The parameters for playback and recording only effect the particular vchan being operated on (gain, sample rate, channels, encoding etc), except -p 0 (the mix ring).

With the introduction of the audio mixer the following sysctl(7)s have been added:

:
 
:
 
:
Intermediate mixing format. (see below)
:
Expressed in milliseconds. (see above)
:
Off/On (0/1), defaults to off. This sysctl(7) determines if multiple users are allowed to access the sound hardware. The root user is always allowed access (i.e., for wsbell). The first user to open the audio device has full control of the audio device if this sysctl is set to off. There currently is an outstanding PR about affecting a privileged process - PR/52627.

Ideally if root intervenes with the audio device, it should do so unaffected.

If this control is set to on, then all users' audio data are mixed and all users have access to the audio hardware.

:
Off/On (0/1), defaults to on. This sysctl(7) enables or disables the audio mixer. When set to off, the audio device can support only one vchan. This vchan's play and record ring buffers are the hardware ring buffers.

This option was added to aid older/slower systems where the extra overhead of the audio mixer might pose a problem.

The initial concept was to handle incoming audio data similarly to that of a superheterodyne radio receiver:

RF -> IF -> AF

So the corresponding mixing concept is:

vchan -> mixing format -> hardware

The sysctl(7)s described above determine the format for mixing. All vchans are up or down sampled to this format before mixing takes place.

On most systems this defaults to 16 bit stereo 48kHz. The sysctl(7)s governing the mixing format may only be changed when there are no vchans in use.

On faster systems the precision (8, 16, 32 bits) may be changed along with the sample rate and number of channels (mono, stereo, 4 etc.).

On older/slower systems utilizing audio mixing, it may be required to lower the quality of this format to ease the amount of data processing whilst mixing.

All possible audio formats (mulaw, alaw, slinear, ulinear, 8, 16, and 32 bit precision) are converted for use by the audio mixer.

It is possible to use mmap for audio playback, achieving reduced latency. However the audio applications selected format must match the mixing/intermediate format (see above).

It is possible to obtain the audio_info for vchan0 which contains the intermediate/mixing format to ease applications configuring for mmapped audio.

At present most applications don't use the mix ring's audio_info structure to obtain the requiredplay back parameters and some user intervention is required to set the audio format for the application.

Audio mixing requires signed linear support in the host's endianness. Driver authors should support slinear_le and slinear_be formats.

If the audio hardware is intended to be used with the mixer disabled, mulaw 1ch 8000 hz needs to be supported also.

This is easily achievable with the auconv framework/filters. All new drivers should consider the use of auconv where possible.

audioctl(1), mixerctl(1), audio(4), audio(9)

Nathanial Sloss

Great appreciation goes to Onno van der Linden, isaki@, maya@, jmcneill@, pgoyette@, mrg@, riastradh@ and christos@ — without their input, this code would not be what it is currently.

May 28, 2018 NetBSD-8.1