Understanding audio type volume controls

Each audio concurrency management context defined in your system has an associated mixer device created under an audio management card. The mixer device for your audio concurrency management context contains mixer groups for each audio type defined in the context’s audio policy configuration file. You can use the standard snd_mixer_group_*() APIs to read and write volume and mute settings of these mixer groups, which allows for synchronous adjustments of the volume and mute levels of all audio streams of the specified audio type.

If you have two multimedia subchannels running, adjusting the volume level of the multimedia mixer group synchronously affects the volume levels of both multimedia subchannels.

Additionally these audio management mixer groups have the ability to have their settings saved and restored across power cycles or restart of the io-audio service. The volume and mute settings the user had specified can be saved and restored provided you have the following configured on your system:

config_write_delay is set to a non-negative number
the path /etc/system/config/audio is writable on your system
deva-util-restore.so is in the library path on your system

Audio type volume controls are in series with their respective subchannels volume controls, so the levels of both volume controls are factored together to determine the overall user volume level applied to a given audio stream. For example if the volume level for an audio type is set to 50% and the individual subchannel’s volume level is set to 25%, the overall volume level of the audio stream would be 12%.

It's important to note that the ducking level applied to a subchannel is scaled by the overall volume level of the ducking subchannel. For example, if the user used the audio type volume controls to reduce the overall volume level of the ducking subchannel by 50% and the configured ducking level was 50%, the final ducking level applied to the ducked subchannels would be 25%.

To illustrate how the various volume levels (subchannel, audio type, and audio ducking) work together, you can use the mix_ctl utility as shown below and cat the state of the card. In this example, there are two audio streams of audio type default and multimedia running with the user volume controls (Subchn and AudioType) configured to full volume (100%). The multimedia audio type is ducking the default audio type to 50%, so if you use the cat command to see the output, you see that the subchannel of default audio type has its Ducking and Current volume level at 50%, while the multimedia subchannel is at 100%.

Note: When you use the cat command, the channel's information is shown for a card. Here's what some of the parameters mean:

Subchn — The user volume control setting for the specific subchannel.
AudioType — The volume control setting for the specific audio type
Control — The amount of volume control calculated by multiplying the value from the Subchn and the AudioType.
Ducking — The ducking volume applied to the channel
Current — The calculated resulting volume level based on multiplying the Control by Ducking

# cat /dev/snd/stateC1
McASP @ 0x48468000, McASP
pcmC1D0p
        ocb = 0x10141c38 - subchn = 0x1014ed30
        pid = 1773597  wave @ localhost
                RUNNING - not block - not block - not block
                scount=7483392 used=258048 free=0 frag_size=64512 frags_total=4
                Rate=48000 Format=s16l Voices=2
                NOT MMAPED   CACHED   INTERLEAVED
                pcm_link mode: ASYNC
                Audio Type=default
                Ducking State=ACTIVE
                Volume Ramp Parameters:
                                Subchn  AudioType  Control  Ducking  Current
                   Channel 0     100%       100%     100%     50%      50%
                   Channel 0     100%       100%     100%     50%      50%
        ocb = 0x10150528 - subchn = 0x1014eb78
        pid = 380962    wave @ localhost
                RUNNING - not block - not block - not block
                scount=7483392 used=258048 free=0 frag_size=64512 frags_total=4
                Rate=48000 Format=s16l Voices=2
                NOT MMAPED   CACHED   INTERLEAVED
                pcm_link mode: ASYNC
                Audio Type=multimedia
                Ducking State=ACTIVE
                Volume Ramp Parameters:
                                Subchn  AudioType  Control  Ducking  Current
                   Channel 0     100%       100%     100%     100%     100%
                   Channel 0     100%       100%     100%     100%     100%

Now, if you configure the multimedia volume level to 50%, you'll see the AudioType, Control and Current values of the multimedia subchannel go to 50% and the Ducking and Current values of the default subchannel scaled down to 25%.

# mix_ctl -a0:0 group "multimedia" volume=50%
#  cat /dev/snd/stateC1
McASP @ 0x48468000, McASP
pcmC1D0p
        ocb = 0x10141c38 - subchn = 0x1014ed30
        pid = 1773597  wave @ localhost
                RUNNING - not block - not block - not block
                scount=7483392 used=258048 free=0 frag_size=64512 frags_total=4
                Rate=48000 Format=s16l Voices=2
                NOT MMAPED   CACHED   INTERLEAVED
                pcm_link mode: ASYNC
                Audio Type=default
                Ducking State=ACTIVE
                Volume Ramp Parameters:
                                Subchn  AudioType  Control  Ducking  Current
                   Channel 0     100%       100%     100%     25%      25%
                   Channel 0     100%       100%     100%     25%      25%
        ocb = 0x10150528 - subchn = 0x1014eb78
        pid = 380962    wave @ localhost
                RUNNING - not block - not block - not block
                scount=7483392 used=258048 free=0 frag_size=64512 frags_total=4
                Rate=48000 Format=s16l Voices=2
                NOT MMAPED   CACHED   INTERLEAVED
                pcm_link mode: ASYNC
                Audio Type=multimedia
                Ducking State=ACTIVE
                Volume Ramp Parameters:
                                Subchn  AudioType  Control  Ducking  Current
                   Channel 0     100%       50%      50%     100%     50%
                   Channel 0     100%       50%      50%     100%     50%

Lastly, if you configure the subchannel volume level of the multimedia subchn to 50%, you'll see its Subchn value go to 50%, and the Control and Current values of the multimedia subchannel go to 25%. The Ducking and Current values of the default subchannel are further scaled down to 12%, maintaining the overall system audio policy of default audio types not exceeding 50% of the multimedia volume level.

# mix_ctl -a1:0 group "Wave Playback Channel,2" volume=50%
# cat /dev/snd/stateC1
McASP @ 0x48468000, McASP
pcmC1D0p
        ocb = 0x10141c38 - subchn = 0x1014ed30
        pid = 1773597  wave @ localhost
                RUNNING - not block - not block - not block
                scount=7483392 used=258048 free=0 frag_size=64512 frags_total=4
                Rate=48000 Format=s16l Voices=2
                NOT MMAPED   CACHED   INTERLEAVED
                pcm_link mode: ASYNC
                Audio Type=default
                Ducking State=ACTIVE
                Volume Ramp Parameters:
                                Subchn  AudioType  Control  Ducking  Current
                   Channel 0     100%       100%     100%     12%      12%
                   Channel 0     100%       100%     100%     12%      12%
        ocb = 0x10150528 - subchn = 0x1014eb78
        pid = 380962    wave @ localhost
                RUNNING - not block - not block - not block
                scount=7483392 used=258048 free=0 frag_size=64512 frags_total=4
                Rate=48000 Format=s16l Voices=2
                NOT MMAPED   CACHED   INTERLEAVED
                pcm_link mode: ASYNC
                Audio Type=multimedia
                Ducking State=ACTIVE
                Volume Ramp Parameters:
                                Subchn  AudioType  Control  Ducking  Current
                   Channel 0     50%       50%       25%      100%     25%
                   Channel 0     50%       50%       25%      100%     25%