Re: where to start with converting an mp3 file to pcm
Re: where to start with converting an mp3 file to pcm
- Subject: Re: where to start with converting an mp3 file to pcm
- From: Brian Willoughby <email@hidden>
- Date: Mon, 19 Sep 2011 19:57:57 -0700
Agreed that expanding MP3 audio to an uncompressed files gains
nothing. Agreed that ExtAudioFile is the best API for the original
task. Also agreed that CAF is the best format choice unless you need
Windows compatibility.
However, CAF is more than just a container file format. IMA4 is 4-
to-1 compressed and surely has to be lossy, so you're better off
using the 'native' sample format of CAF (I don't know whether there
is a unique name for the native sample format of CAF, but it
certainly isn't IMA4). CAF is also a container format, just like IFF/
FORM is the chunk container format for AIFF and RIFF is the chunk
container format for WAV. CAF happens to use 64-bit chunks instead
of 32-bit chunks.
IMA4 is just the typical example for Apple sample source, but even
they do not recommend using it for production code.
Brian Willoughby
Sound Consulting
On Sep 19, 2011, at 15:30, Steven Winston wrote:
ExtAudioFile is definitely a good candidate to do this too. I pointed
at Audio Queue due to the existing online doc.
I'd also point out that .caf is a container format; I think what
you'd want is IMA4.
On Mon, Sep 19, 2011 at 3:21 PM, Chris Adamson
<email@hidden> wrote:
Agreed with Steven that the premise seems dubious in the first place.
That said, directly using the Audio Conversion functions would be
more direct than an Audio Queue, although it's not as easy to find
good docs or sample code. We do have an example in the book.
You'd then have to write the PCM sample buffers to a file with
Audio File Services.
The cleanest way to do it would be with an ExtAudioFile, which
would give you MP3-to-PCM conversion and file I/O to the container
of your choice(*) in one fell swoop.
(*) - if your app is the only thing that's ever going to read the
converted files, I'd argue that .caf would be superior to .aiff
or .wav.
--Chris
On Sep 19, 2011, at 5:06 PM, Steven Winston wrote:
Erm... why do you want to expand the file to PCM on the user's hard
drive? Why not keep it as mp3? You get the same quality as the
fact
that it was mp3 at some point means there's already a degradation of
signal all you could possibly get is less time decoding it in
software; but nearly every device I can think of has hardware mp3
decoders; so that doesn't make sense either.
In any case; it's actually quite trivial to do. You can use offline
render from audio queue services. Documentation for that is online.
On Mon, Sep 19, 2011 at 1:59 PM, Morgan Packard
<email@hidden> wrote:
Hello list,
I'd like to ship my app with mp3s and expand them to pcm files
(aif or wav)
on the user's hard drive. I'd be grateful if anyone could point
me toward
the most appropriate technologies for the job. Even better would
be to point
me toward some sample code.
thanks!
-Morgan
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