[openal] How to use AL_EFFECT_COMPRESSOR

Mariusz 'MX' Szaflik mxadd at o2.pl
Thu Apr 3 13:38:33 EDT 2014

W dniu 2014-04-03 18:01, Chris Robinson pisze:
> On 04/03/2014 07:43 AM, Mariusz 'MX' Szaflik wrote:
>> Hi
>> stupid (?) question - how to use AL_EFFECT_COMPRESSOR ? (openALSoft,
>> latest version)
>>         | |->[COMPRESSOR]->OUT
>>         +->DF2------------------------------->/
>> how the COMPRESSOR fits into this ?
>> it should be where I wrote it above, but there is no chaining of global
>> effects - I'm right ?
> Unfortunately the way EFX effects work doesn't allow for using a 
> compressor on the final output mix. It will only act on a given effect 
> send, and further, OpenAL Soft will mix those effect sends to a mono 
> stream (all sends mix to a mono buffer in the effect slot, without 
> panning, then the effect itself will spread it to all output channels 
> as appropriate).

ooohhh --- that was the 'wrong' thing testers were hearing on linux
on XAUDIO the sounds that go to the 'master' effect are 3d positioned 
(if my system is 7.1 then 8 channels go to the master effect and 8 
channels goes out to the final submix)
but on OpenAL  this will be downmixed to mono - right ?
(it was not that doticeable because only about a half of the gain goes 
thru the wet part, the dry path is still positioned i hear)

is there any chance in the future to fix this ? :>

so for now there is no way to use compressor ;(

seem that I need to go with hard route - fetch OALSoft and add some 
'hacks' to the mixer to be able to query average volume before clipping 
for each mixed packet
(with that feedback I will be able to lower gain of individual sources 
to not produce clipping in next step)
then ofcouse use only my hacked version of openAL with the project

>> How OpenALSoft is dealing with clipping ?
>> I ask because there is much less clipping heard compared to XAudio2
>> version of sound engine.
>> (let's say i have 10 cannons on the ship firing at once - OpenAl version
>> sounds much better without compressor than the XAudio one - is there
>> some 'anti clipping' code in mixer ?)
> I don't know how XAudio works internally, but OpenAL Soft adds a bit 
> of attenuation to mono sounds that get played centered, according to 
> how many output channels there are. This means that centered mono 
> sounds are slightly quieter, so they won't overflow as quickly/badly 
> and the subsequent clamping won't be as noticeable.
> This attenuation is added so mono sounds will (roughly) keep the same 
> perceived volume as when it's panned to one speaker. Otherwise, if a 
> centered mono sound played 100% on all speakers, there would be up to 
> 700% more power (for 7.1 output) compared to when it's panned to play 
> on one speaker only. And IMO that's not very desirable for sounds that 
> freely move around in 3D space.
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