[Openal] ADPCM

[William Stewart]

Garin, (and others interested)

AAC - the main audio codec in MPEG-4, makes a very attractive format  
for games I think. There were two explicit design goals with AAC - it  
was developed by the same people who developed MP3.
(1) Decode CPU requirements should be minimal
(2) Quality of encoding should be better - thus smaller files for the  
same quality.

So, when we compare AAC with MP3, the two areas we compare are:

(1) Quality:
     AAC at 128kbps is about the same as MP3 at 160kbps
         - from public listening tests, Ogg is about the same as AAC

(2) Decode CPU Usage:
     AAC's decode is extremely efficient - On a G4 1GHz iMac decode  
is less than 1% CPU (about 0.8%). On a G5 it is about 0.2 - 0.3 %
     MP3 decode - On a G4 1GHz iMac decode is less than 2.4% CPU  
(about 0.8%). On a G5 it is about 1 %

     I've no idea what Ogg's performance characteristics are


There are some open source AAC efforts (faac) as well (though I don't  
know what their performance/quality characteristics are), as well as  
various options for hardware based decoders for embedded devices.

It is an open standard.

Bill

On 24/06/2005, at 2:46 PM, Garin Hiebert wrote:

>> Speaking of audio compression, I was wondering about ADPCM.  It is  
>> my understanding that the decoding is done by the main processor  
>> for all (??) OpenAL backends. This is the main reason I'm shying  
>> away from it.  Even the old Playstation would do this on the SPU/ 
>> DSP... so I was wondering if Creative has any plans to support  
>> this in their native drivers?
>>
>
> It's been considered, but ADPCM decompression is extremely  
> efficient, so it hasn't been a major issue.  ADPCM as well as more  
> CPU-intensive formats such as Ogg Vorbis or MP3 are certainly still  
> being considered...
>
>
>> I suppose in lieu of this we need the equivalent of the OpenGL  
>> shading language for sound DSPs; but I won't hold my breath.
>>
>
> Even if we had such plans, it still wouldn't be a good idea to hold  
> your breath until it's done.
>
> Garin
>
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