[Openal-devel] AL_SOFT_direct_channels

[Eric Wing]

>From Section: 5.3.4. Specifying Buffer Content

Buffers containing audio data with more than one channel will be
played without 3D spatialization features – these formats are normally
used for background music.



On 3/2/12, Jason Daly <jdaly at ist.ucf.edu> wrote:
>
> Chris,
>
> I guess I thought that it had been specified (i.e.: in the spec) that
> multichannel sources wouldn't ever be spatialized, but I can't seem to
> find that now.  Traditionally, at least the Loki and Creative
> implementations followed this concept.  Only mono sources were
> spatialized and stereo/multichannel sources were passed through.
>
> Is this basically intended to formalize that idea?
>
> --"J"
>
>
> On 03/02/2012 10:12 AM, Chris Robinson wrote:
>> This and the loopback extension are what I'm hoping to have done before
>> releasing OpenAL Soft 1.14.
>>
>> OpenAL Soft somewhat recently got support for HRTF filters for when using
>> headphones, however some stereo audio tracks may already be designed to
>> play
>> with headphones which makes it not sound as good when fed through HRTF
>> filters. Something like this was requested as a way to let such audio pass
>> through to the output without going through spatialization.
>>
>>
>> Name
>>
>>      AL_SOFT_direct_channels
>>
>> Contributors
>>
>>      Chris Robinson
>>
>> Contact
>>
>>      Chris Robinson (chris.kcat 'at' gmail.com)
>>
>> Status
>>
>>      In-progress
>>
>> Dependancies
>>
>>      This extension is written against the OpenAL 1.1 specification.
>>
>> Overview
>>
>>      This extension allows a multi-channel source to play without
>> virtualized
>>      output speakers. By default, OpenAL requires buffer channels to be
>> down-
>>      mixed to the output channel configuration, possibly using HRTF
>> techniques
>>      to give a sense of speakers that may not be physically present.
>> Sometimes
>>      audio tracks are authored with their own spatialization effects,
>> where the
>>      AL's spatialization methods can cause a notable decrease in audio
>> quality.
>>
>>      This extension provides a mechanism for applications to specify
>> whether
>>      audio should be filtered according to the AL's channel spatialization
>>      methods for multi-channel buffers.
>>
>> Issues
>>
>>      Q: Should this be a buffer property or source property?
>>      A: Source property. This gives more flexibility to the app to decide
>>         whether some piece of audio should be filtered or not.
>>
>>      Q: Should this work on mono (3D) sources?
>>      A: No. Besides most people not having a mono speaker, or possibly
>> even a
>>         front-center speaker, its main intent is for stereo tracks that
>> have
>>         spatialization effects pre-applied.
>>
>>      Q: Are environmental effects, provided by EFX for example, still
>> applied?
>>      A: Yes. There's no compelling reason to disable them, particularly
>> when
>>         the application has to enable them on a per-source basis in the
>> first
>>         place.
>>
>> New Procedures and Functions
>>
>>      None.
>>
>> New Tokens
>>
>>      Accepted by the<paramName>  parameter of alSourcei, alSourceiv,
>>      alGetSourcei, and alGetSourceiv:
>>
>>          AL_DIRECT_CHANNELS_SOFT                  0x1033
>>
>> Additions to Specification
>>
>>      Append to Section 4.3.2, Source Attributes
>>
>>      Table 4.x Channel Virtualization
>>
>>      Name                       Signature  Values               Default
>>      -------------------------  ---------  -------------------  ----------
>>      AL_DIRECT_CHANNELS_SOFT    i, iv      AL_TRUE, AL_FALSE    AL_FALSE
>>
>>      Description:
>>      AL_DIRECT_CHANNELS_SOFT set to AL_TRUE indicates the audio channels
>> do not
>>      go through virtualization or spatialization and plays directly on the
>>      matching output channel if it exists, otherwise it is dropped.
>> Applies
>>      only when playing non-mono buffers.
>>
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>>
>
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