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

[Chengyi Zhao]

On 2021/4/3 上午3:48, Tanu Kaskinen wrote:
> 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.
> 
I think there are two issue about these configurations:

1. If the sound card has two channels, this multichannel mapping
may be mapped.
2. Because there is no paths-output and paths-input items,
multichannel ports will not be added.

Thanks,
Chengyi