Error setting ClientDataFormat

Question

I’m having some trouble encoding certain configurations of multi-channel Apple Lossless files. I have no problems with 2, 4 and 6 channel configurations, however I’m running into trouble generating a 12-channel file.

Using the ExtAudioFile APIs, I’m able to create the audio file with a 12-channel AudioStreamBasicDescription and a 12-channel FileChannelLayout. When it comes to setting the 12-channel ClientDataFormat (done before setting the client layout, which, obviously also fails), I get the “fmt?" error code.

My AudioStreamBasicDescription for the ClientDataFormat is generated as follows:

AudioStreamBasicDescription asbd;

asbd.mSampleRate = m_sampleRate;
asbd.mFormatID = kAudioFormatLinearPCM;
asbd.mFormatFlags = kAudioFormatFlagIsFloat;
asbd.mBytesPerPacket = m_channels * 64 / 8;
asbd.mBytesPerFrame = asbd.mBytesPerPacket;
asbd.mFramesPerPacket = 1;
asbd.mBitsPerChannel = 64;
asbd.mChannelsPerFrame = m_channels;
asbd.mReserved = 0;

The sample rate is 176.4 kHz and m_channels is correctly set. This has worked properly for 2, 4 and 6 channel setups, so I don’t think it is at fault. I’ve also tried using 32-bit signed integers instead of 64-bit floats, but the result is the same.

I’m not sure what I’m missing. Is it possible the Apple Lossless codec doesn’t support 12-channel encodings?

Answer 1

I don't know for certain if the spec at MacOSForge is definitive, but according to http://alac.macosforge.org/trac/browser/trunk/ReadMe.txt

3. From one to eight channels are supported. Channel orders for the supported formats are described as:
            Num Chan        Order
            1               mono
            2               stereo (Left, Right)
            3               MPEG 3.0 B (Center, Left, Right)
            4               MPEG 4.0 B (Center, Left, Right, Center Surround)
            5               MPEG 5.0 D (Center, Left, Right, Left Surround, Right Surround)
            6               MPEG 5.1 D (Center, Left, Right, Left Surround, Right Surround, Low Frequency Effects)
            7               Apple AAC 6.1 (Center, Left, Right, Left Surround, Right Surround, Center Surround, Low Frequency Effects)
            8               MPEG 7.1 B (Center, Left Center, Right Center, Left, Right, Left Surround, Right Surround,  Low Frequency Effects)