How can I record AMR audio format on the iPhone?

Question

A voice recorder doesn't need uncompressed Linear PCM audio. Compressed AMR would do fine. The iPhone framework built for recording audio is simple enough, but the only examples I've found for setting up the audio format (which come from Apple) use LinearPCM. I've tried various other combinations of values, but can't seem to get anything to work.

Does anybody have any code that actually records AMR?

Edit: The AMR format is one of the options for setting the data type, but the other options (packet size, frame size, etc.) don't seem to match up no matter what I set them to.

Edit: Here's what I have for the PCM version:

/*
 If we want to use AMR instead of PCM:
 AMR Format:
 Sampling Frequency: 8 kHz/13-bit (160 samples for 20 ms frames), filtered to 200-3400 Hz
 eight source codecs :  12.2, 1.2, 7.95, 7.40, 6.70, 5.90, 5.15, 4.75 kbit/s
 generated frame length: 244, 204,  159,  148,  134,  118,  103,   95 bits per frame
 */
format->mFormatID = kAudioFormatLinearPCM;
format->mSampleRate = 8000.0;    //8 kHz
format->mFramesPerPacket = 1;    //1 frame per packet
format->mChannelsPerFrame = 1;    //Mono
format->mBytesPerFrame = 2;        //8/bits per frame (round up)
format->mBytesPerPacket = 2;    //Same as bytes per frame
format->mBitsPerChannel = 16;    //16-bit audio
format->mReserved = 0;            //always 0
format->mFormatFlags = kLinearPCMFormatFlagIsBigEndian |
                       kLinearPCMFormatFlagIsSignedInteger |
                       kLinearPCMFormatFlagIsPacked;

Answer 1

AMR codec is NOT supported for encoding/recording on the iPhone, albeit it is supported for playback: this is the reason the kAudioFormatAMR constant exists.

Official api says that supported encoding formats are:

• ALAC (Apple Lossless) ~> kAudioFormatAppleLossless
• iLBC (internet Low Bitrate Codec, for speech) ~> kAudioFormatiLBC
• IMA/ADPCM (IMA4) ~> kAudioFormatAppleIMA4
• linear PCM ~> kAudioFormatLinearPCM
• µ-law ~> kAudioFormatULaw
• a-law ~> kAudioFormatALaw

You may try one of these formats or use an open source AMR encoder as goldenmean suggests.

edit: Updated Official api link