Re: QA1562 offline audio rendering advice
Re: QA1562 offline audio rendering advice
- Subject: Re: QA1562 offline audio rendering advice
- From: William Stewart <email@hidden>
- Date: Thu, 5 Mar 2009 19:28:28 -0800
On Mar 5, 2009, at 5:13 PM, Bruce Meagher wrote:
Hi Doug,
Thanks for the tip on version 2.2. I was able to get aac -> lpcm
offline rendering working on the device as described in the tech
note against the iphone version 2.2 but not against 2.1. A couple
questions:
1) The aqrender.cpp file includes two header files "CAXException.h"
and "CAStreamBasicDescription.h" that I couldn't find in any of the
audio frameworks. I can't seem to find the missing pieces to make
the code run "as is" on the target, but I'm probably just missing
something simple. I ported the sample code to an Objective C class,
but as Bill mentions below it would be good to run the original
code. The sample code appears to be more of a command line tool
with the file names coming in from main's argv and agrc. Is there a
shell app I can run to download and test command line utilities to
the phone?
These files are from the CoreAudio Public Utility folder (it is
example code and not shipped as part of any framework) - you should
use the versions that come with the iPhone SDK (not the Leopard ones)
2) Are there any published performance specs w.r.t. offline
rendering? On the macbook pro running the iphone simulator my app
decompresses 128kbps 44.1k 2ch aac audio file to lpcm about 100x
faster than realtime (e.g. 120 second AAC audio file decompresses in
1.2 seconds to 2ch lpcm including writes to the disk). On same app
the iphone I see a little over 2.5x faster than realtime (e.g. same
120 second AAC audio file decompresses in ~48 seconds). Removing
the writes to disk on the phone only saves a few percent. If I drop
the sample rate of the file 32kHz it runs in 38 seconds (~3x faster
than realtime). I'm not comparing the iphone to a mac, but should I
expect a 2.5x aac ->lpcm rendering speed on the iphone h/w? I'm
sure there are many variables but any guidelines would be great to
know I'm on the right track.
you can't use the sim to gauge any notion of performance on the devices.
The h/w codecs are optimised around power savings, not "cpu time", so
the only real metric for real time is that it has to be faster. We
haven't characterised the times to the usage case you are interested in.
3) Any tips how I might try to get this to work with version 2.1?
Have any of you developers out there had success offline rendering
aac ->lpcm with iphone version 2.1 (I thought there were many)?
I don't think so - alot of people were having problems with this in
earlier iphone releases and we made it a priority for 2.2 to make sure
we could support this usage case.
Bill
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