Re: Turning on 8.24 audio for iPhone
Re: Turning on 8.24 audio for iPhone
- Subject: Re: Turning on 8.24 audio for iPhone
- From: William Stewart <email@hidden>
- Date: Sat, 17 Jan 2009 01:17:26 +1100
To answer your other question.
On iPhone, the 8.24 fixed format is the canonical format we use for
audio unit processing. We chose 8.24 because the 24 bits gives us
better resolution than 16, and the 8 bits should give enough
headroom for common uses. With 16.16 you don't really have enough
bits on the right side.
On the desktop, the canonical format is float32 for the same use
cases (and others actually). We don't currently support 8.24 as a
desktop format.
We provide definitions for sample types in CoreAudioTypes.h for the
2 different platforms so you can at least have generic code using
these types - which we do in AUBase* classes (as an example)
Bill
On Jan 16, 2009, at 9:24 PM, omenie.com wrote:
OK, got it - anyone else having problems with this, there's a 'how
many fraction bits' field - I somehow missed it, the document 'Core
Audio Data Types Reference' was refreshed in November -
#define AUDIOFORMATFLAGS kLinearPCMFormatFlagIsSignedInteger |
kLinearPCMFormatFlagIsPacked | (24 <<
kLinearPCMFormatFlagsSampleFractionShift)
And all is now well down on the farm.
On 15 Jan 2009, at 18:01, omenie.com wrote:
Hi all.
I'm mixing multiple audio streams in an iPhone / iTouch app and
clamping the result to 16 bits. Everything works fine right now,
but since I'm generating intermediate results in 16.16 I could
trivially convert to 8.24 and use iPhone's native resolution and
save some CPU on the clamping code's over/underflow testing. What
changes do I need to make to the AudioStreamBasicDescription and
AUDIOFORMATFLAGS to enable this? Also, once 8.24 is enabled, will
the same code run on Mac OS X, or is the 8.24 audio format not
supported on the desktop?
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