QuickTime Audio - Retrieving the correct audio format bit depth value

Q:  I'm importing AIFF files containing 24-bit audio ( k24BitFormat ). However, when I examine the sound media SoundDescription, sampleSize always says 16. How can I find the actual audio bit depth without having to check the dataFormat field?

A: The value reflected in the sampleSize field is a remnant of the old Sound Manager where the native resolution was 16-bit integer samples through the processing chain, and all formats reported a sample size of either 8-bit or 16-bit.

In order to find the true sample size, you'll need to retrieve the AudioStreamBasicDescription from the SoundDescriptionHandle by calling QTSoundDescriptionGetProperty using the kQTSoundDescriptionPropertyID_AudioStreamBasicDescription property.

Listing 1  Retrieving the ASBD from a SoundDescriptionHandle snippet.

 SoundDescriptionHandle hSoundDescription = (SoundDescriptionHandle)NewHandle(0);

    AudioStreamBasicDescription asbd = {0};

    OSStatus err;

    ...

    GetMediaSampleDescription(aSoundMedia, index, (SoundDescriptionHandle)hSoundDescription);

    if (err = GetMoviesError()) goto bail;

    err = QTSoundDescriptionGetProperty(hSoundDescription, kQTPropertyClass_SoundDescription,

                  kQTSoundDescriptionPropertyID_AudioStreamBasicDescription, sizeof(asbd), &asbd, NULL);

    ...

bail:

    If (hSoundDescription) DisposeHandle((Handle) hSoundDescription);

In the returned AudioStreamBasicDescription, check the mBitsPerChannel field. This is the number of bits of sample data for each channel in a frame of data.

If mBitsPerChannel is zero, you're dealing with a compressed format. The mFormatID field of the AudioStreamBasicDescription indicates the general kind of data in the stream.

Some compressed formats have the ability to indicate the precision of the original source data. Apple Lossless for example ( mFormatID field == kAudioFormatAppleLossless ) performs true lossless compression and reports the precision of the original source data though flags set in the mFormatFlags field of the AudioStreamBasicDescription.

The kAppleLosslessFormatFlags_xxx flags are located in CoreAudioTypes.h

kAppleLosslessFormatFlag_16BitSourceData - This flag is set for Apple Lossless data that was sourced

from 16 bit native endian signed integer data.

kAppleLosslessFormatFlag_20BitSourceData - This flag is set for Apple Lossless data that was sourced

from 20 bit native endian signed integer data aligned high in 24 bits.

kAppleLosslessFormatFlag_24BitSourceData - This flag is set for Apple Lossless data that was sourced

from 24 bit native endian signed integer data.

kAppleLosslessFormatFlag_32BitSourceData - This flag is set for Apple Lossless data that was sourced

from 32 bit native endian signed integer data.

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