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Could anybody help explain the capabilities values of an ALSA mixer control?

What's the difference between pvolume and pvolume-joined? I cannot find the difference from alsamixer GUI output.

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

Here is some output of amixer:

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

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