iOS：从音频文件获取原始PCM数据

Question

I'm investigating a straightforward task:

• open an audio file from the iPhone's 'iPod audio library'
• allows the user to select a chunk by setting two markers: start and end time
• time-reverse this chunk
• save it as a new file

What are my options?

I will list the results of a couple of hours of research: ( forgive the mess, I will as always tidy pu once I have figured it out )

• http://lists.apple.com/archives/coreaudio-api/2005/May/msg00096.html <-- 'I'm currently trying to create a program that plays back audio using an AUAudioFilePlayer AudioUnit plugin that streams the audio to an output AudioUnit'

• AUFilePlayer

• http://lists.apple.com/archives/coreaudio-api/2008/Dec/msg00156.html

• http://zerokidz.com/audiograph/docs/audiograph.pdf <-- this possibly links to code that does it, but it says it is in beta

• When reading audio file with ExtAudioFile read, is it possible to read audio floats not consecutively? <-- this leads to an OS X project that reads an audio file from disk into memory; looking through the code leads us to:

• https://developer.apple.com/library/mac/#documentation/MusicAudio/Reference/ExtendedAudioFileServicesReference/Reference/reference.html

  as far as I can see the audio Graph project attempts to stream the audio from file in real-time, whereas Stephan's Project just exposes the audio; however it looks like he is using obsolete API calls.

  this looks like the right code ( apart from the fact that there seems to be a bug in it ): https://stackoverflow.com/questions/8533143/decoding-mp3-files-by-extaudiofileopenurl

http://cocoadev.com/forums/discussion/499/core-audio/p1

https://developer.apple.com/library/ios/#samplecode/iPhoneExtAudioFileConvertTest/Introduction/Intro.html <-- here is an official Apple sample project that could probably be modified to get what I'm after

Answer 1

I believe you can use IphoneExtAudioFileConvertTest like you said to get what you need, after the user marks what times he wants, first thing you want to do is convert to PCM, next you need to find out which packets are the ones you need, write that to an audio file and then recompress... I wrote an answer here on how to get x amount of seconds from an audio file, ive only tested it with mp3 and m4a, but it can be adapted to PCM (pcm should be easier to do since its linear).

Daniel