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Speaker setup + possible typo in CoreAudioTypes.h label value?
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Speaker setup + possible typo in CoreAudioTypes.h label value?


  • Subject: Speaker setup + possible typo in CoreAudioTypes.h label value?
  • From: Derk-Jan Hartman <email@hidden>
  • Date: Wed, 23 Mar 2005 17:51:14 +0100

I have been looking at the issue of channels, layout, speakers and mixing.

ACL = defines channel layout trough either ACLT, ACBm or ACB

ACLT = defines a standard speaker setup or says a bitmap or description is used
ACBm = defines which channels are available trough the use of ACB
ACB = bitlabels for channels defines which speakers are present.
ACD = defines for each channel what the speaker setup is with ACLabels.
AClabel = predefined speaker labels can also set that the channel is defined in coordinates.


Now as far as I can tell none of this is obligatory to ALWAYS be present. you can choose what you want to use.
This poses the following problem for VLC. AUHAL does not support downmixing. Therefore VLC has to do it. But to me it's sheer impossible to find out what channels should be downmixed to. I mean how am i supposed to know what channel is LFE, what channel is the Center speaker. If I'm VERY out of luck the things can even be specified by coordinates.
It seems this system is more designed to account for channel layout and positioning and not so much for keeping score of speaker presence and speaker nature.


My sonica theater seems to use ACD with AClabels, but other devices can simply have the ACLT set and nothing else?
Am I missing something here? Is there an easier way to figure out which speakers are available for output?
Is it REQUIRED for instance that drivers set a proper ACBmap? If so than this is not clear from the headers (let alone the documentation :D ) and should be clarified, if it is not then maybe it would be a good idea to have that as a requirement?


Also, i spotted the following in my list of ACLabels in the headers:
kAudioChannelLabel_RightCenter                          = 8,
kAudioChannelLabel_CenterSurround                       = 10,
kAudioChannelLabel_LeftSurroundDirect           = 10,

That centersurround value seems a bit strange to me. a value of 9 would seem more logical?

DJ
---
Universiteit Twente
Derk-Jan Hartman (d.hartman at student.utwente dot nl)
http://home.student.utwente.nl/d.hartman

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  • Follow-Ups:
    • Re: Speaker setup + possible typo in CoreAudioTypes.h label value?
      • From: William Stewart <email@hidden>
References: 
 >AUHAL output questions (From: Derk-Jan Hartman <email@hidden>)
 >Re: AUHAL output questions (From: William Stewart <email@hidden>)
 >Re: AUHAL output questions (From: Derk-Jan Hartman <email@hidden>)
 >Re: AUHAL output questions (From: Derk-Jan Hartman <email@hidden>)

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