How to save just raw PCM to file with iOS SDK (Core Audio)?

1
votes

I'm converting an MP3 file into raw PCM, and I need to save it as just raw PCM. (Note, am using Java/RoboVM to port to iOS.)

I'm using the coreaudio package, and the relevant part of my code looks like this:

// Define the output PCM format.
AudioStreamBasicDescription outputFormat = new AudioStreamBasicDescription();
outputFormat.setFormat(AudioFormat.LinearPCM);
outputFormat.setFormatFlags(AudioFormatFlags.Canonical);
outputFormat.setBitsPerChannel(16);
outputFormat.setChannelsPerFrame(1);
outputFormat.setFramesPerPacket(1);
outputFormat.setBytesPerFrame(2);
outputFormat.setBytesPerPacket(2);
outputFormat.setSampleRate(22050);

// ...

outputFile = ExtAudioFile.create(outputFileURL, AudioFileType.CAF, outputFormat, null, AudioFileFlags.EraseFile);

I then run through a loop, reading from the MP3 file and writing to the output file.

Upon importing this raw file into Audacity, I notice it always has a spike at the start, indicating that it's not actually a raw PCM file but instead is inside of a wrapper with a header (whether it be WAV or CAF headers, etc).

I understand I can just take the file and strip the header off and get the raw PCM data, but in terms of space/performance of this part of my app, I'd love if I can just keep it simple and save the raw PCM data as-is without a wrapper, but I don't know how to go about doing that.

The issue arises here:

outputFile = ExtAudioFile.create(outputFileURL, AudioFileType.CAF, outputFormat, null, AudioFileFlags.EraseFile);

There aren't many choices for AudioFileType, I've tried WAVE and CAF. Ideally there would be a PCM or RAW option but there's not. Is there a specific AudioFileType I should choose, or do I need to go about this another way?

1
iosaudiocore-audiorobovm

1 Answers

0
votes

The extended audio file services framework doesn't support a "raw" PCM format.

For an application to understand a PCM format it needs to know data stuff like:

  • How many channels are there
  • Are they interleaved or not
  • What is the sample rate
  • Is the data floating point or not
  • What is the bit depth
  • etc...

In fact, on iOS and OS X the AudioStreamBasicDescription is a struct which tells you what is required to interpret a PCM stream. For this reason, a "raw PCM" format doesn't really work, it needs at least some metadata. The closest formats to raw PCM are WAV, AIFF and CAF. If these don't serve your purposes you'll have to create a custom file format. But this doesn't need to be difficult.

The extended audio file services APIs are quite configurable. After opening an audio file to read (ExtAudioFileOpenURL) you can set various properties on the ExtAudioFileRef handle.

In your case consider setting kExtAudioFileProperty_ClientDataFormat. This property controls the format of the PCM data read from the file. As ExtAudioFileRead decodes the input file, it will convert the data it sends back to the format you specify. There are some limitations to this method. IIRC, it does not support doing sample rate conversion and things like that.

As you read the properly decoded data, you can then use something like NSOutputStream to write the "raw PCM" format of your choice directly to a file with no metadata at all.