Re: HE-AAC playback on iPhoneOS, does it use the HW decoder
Re: HE-AAC playback on iPhoneOS, does it use the HW decoder
- Subject: Re: HE-AAC playback on iPhoneOS, does it use the HW decoder
- From: William Stewart <email@hidden>
- Date: Tue, 19 May 2009 11:01:46 -0700
On May 19, 2009, at 9:15 AM, Nathan Peel wrote:
Perhaps the decoder doesn't like the encoder I've chosen? afconvert
doesn't appear to support HE-AAC yet.
We don't have HE-AAC encoders/decoders shipping on the desktop yet -
these will be available with Snow Leopard.
On May 19, 2009, at 9:27 AM, Nathan Peel wrote:
Thanks Bill. Our app is taxing the device, I don't want spare
cycles spent on decoding audio.
Can I use AudioQueue to decode HE-AAC? Tried playing a file back
with the code in the SpeakHere demo and it played at low fidelity.
Yes. There's an "extra hoop" that you have to go through
HE is a layered format, with AAC-LC at its core. If you just ask a
file for the format of its data, then it will give you back the "base
layer". If you use that to then construct an audio queue, it will just
decode the "base layer" and ignore any of the enhancement layers.
There is an AudioFile (and AudioFormat) FormatList property to resolve
this - this gives you back the full list of the different formats that
are contained in a file (or described by a magic cookie in the case of
AudioFormat). You then go through this list, pick out the format that
you want, and use that format (ASBD) to construct an audio queue object.
There's a tech note available on how to do this, with code, and I
think the latest version of aqplay shows you how to do this as well.
Have a search around the developer site (if you can't find it ping me
and I'll track it down)
Bill
On May 18, 2009, at 5:47 PM, William Stewart wrote:
On May 18, 2009, at 3:19 PM, Nathan Peel wrote:
I just learned that as of 2.2 iPhone OS supports HE-ACC via this
tech note.
http://developer.apple.com/iphone/prerelease/library/technotes/tn2008/tn2236.html
When the device is decoding an HE-ACC file is it using the
hardware decoder?
Yes - why?
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