[alsa-devel] How to understand capabilities of a mixer control output by "amixer"?

[Lin, Mengdong]

I'm new to ALSA. Could anybody help explain the capabilities values of an ALSA mixer control?

-          What's the difference between pvolume and pvolume-joined? Only 'Master' control has pvolume-joined, but I cannot see any GUI difference between 'Master' and 'Headerphone' by alsamixer.


-          What does the penum and cenum mean? What are they designed for? I think "p" means playback and "c" means capture. But it's confusing to see control 'Capture' also has a capability "penum"?


Here is output of amixer on my destop:

Simple mixer control 'Master',0
  Capabilities: pvolume pvolume-joined pswitch pswitch-joined penum
  Playback channels: Mono
  Limits: Playback 0 - 64
  Mono: Playback 51 [80%] [-13.00dB] [on]

Simple mixer control 'Headphone',0
  Capabilities: pvolume pswitch penum
  Playback channels: Front Left - Front Right
  Limits: Playback 0 - 64
  Mono:
  Front Left: Playback 60 [94%] [-4.00dB] [on]
  Front Right: Playback 60 [94%] [-4.00dB] [on]
....

Simple mixer control 'Capture',0
  Capabilities: cvolume cswitch penum
  Capture channels: Front Left - Front Right
  Limits: Capture 0 - 31
  Front Left: Capture 0 [0%] [-13.50dB] [on]
  Front Right: Capture 0 [0%] [-13.50dB] [on]

Simple mixer control 'Capture',1
  Capabilities: cvolume cswitch penum
  Capture channels: Front Left - Front Right
  Limits: Capture 0 - 31
  Front Left: Capture 9 [29%] [0.00dB] [off]
  Front Right: Capture 9 [29%] [0.00dB] [off]

Simple mixer control 'Input Source',0
  Capabilities: cenum
  Items: 'Mic' 'Line'
  Item0: 'Line'

Simple mixer control 'Input Source',1
  Capabilities: cenum
  Items: 'Mic' 'Line'
  Item0: 'Line'

Thanks & Best Regards
Mengdong