Re: vDSP and iOS render callback
Re: vDSP and iOS render callback
- Subject: Re: vDSP and iOS render callback
- From: Kevin Dixon <email@hidden>
- Date: Wed, 25 Apr 2012 13:42:16 -0700
Ok great.
So let's say I want to perform an FFT in my Render callback.
Which option would be more performant?
1. Set my client type to Float32, and use vDSP
2. use the canonical type 8.24, and use a NEON-optimized FFT
implementation (which I already have on hand)
Thanks!
-Kevin
On Tue, Apr 24, 2012 at 8:23 PM, Brian Willoughby <email@hidden> wrote:
>
> On Apr 24, 2012, at 18:25, Paul Davis wrote:
>>
>> On Mon, Apr 23, 2012 at 4:16 PM, Kevin Dixon <email@hidden>
>> wrote:
>>>
>>> Maybe I'm missing something, I was looking at this question stack
>>> overflow
>>> (http://stackoverflow.com/questions/3398753/using-the-apple-fft-and-accelerate-framework/3534926#3534926).
>>> I was under the impression that the render callback supplied sample
>>> data in 8.24 fixed format, yet I don't see any sort of type conversion
>>> going on in the example autocorrelation implementation when they call
>>> the FFT.
>>> Can anyone shed some light on this?
>>
>>
>> this is wrong. CoreAudio (like most audio SDKs) uses 32 bit floating
>> point as its only "native" data type. this absolutely includes the
>> render callback for an audio unit. unless iOS is totally different, in
>> which case forgive my emphasis.
>
>
>
> Hey, Paul, the second thing you said is true: iOS is totally different.
>
> CoreAudio for OSX does default to Float32 unless the code specifically
> requests otherwise.
>
> CoreAudio for iOS defaults to Q8.24, a fixed-point format that is more
> efficient on the older ARM processors without fast float support. In that
> world, I/O devices are 16-bit integer, and DSP is run at 8.24 for speed.
>
> The confusing thing is that the ARM chips are now getting to where they
> support float as well as int, and so CoreAudio/iOS is moving towards
> Float32.
>
> Brian Willoughby
> Sound Consulting
>
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