[pulseaudio-discuss] Why should we set multichannel-input and multichannel-output to a fallback map

[Tanu Kaskinen]

On Fri, 2021-04-02 at 17:49 +0800, Chengyi Zhao wrote:
> Hi guys,
> 
> I don't know why we should set multichannel-input and 
> multichannel-output to fallback map
> in the configuration file default.conf.
> 
> I think these have no actual usage scenarios, we should delete these 
> configurations.
> 
> Do you agree with me？
> 
> ----------------
> ./src/modules/alsa/mixer/profile-sets/default.conf：
> 
> -[Mapping multichannel-output]
> -device-strings = hw:%f
> -channel-map = left,right,rear-left,rear-right
> -exact-channels = false
> -fallback = yes
> -priority = 1
> -direction = output
> -
> -[Mapping multichannel-input]
> -device-strings = hw:%f
> -channel-map = left,right,rear-left,rear-right
> -exact-channels = false
> -fallback = yes
> -priority = 1
> -direction = input
> -

These are used with sound cards that can't be opened with the regular
mappings. Some pro audio cards can have e.g. 12 channels and refuse to
be opened with any less channels. The "exact-channels" option is set to
false with the multichannel mappings, which means that no matter how
many channels the card requires, these mappings successfully open the
device.

-- Tanu