Re: Sanity check on AudioEndianAtom usage

On Feb 21, 2006, at 1:47 PM, Timothy J. Wood wrote:

We have some audio compression code using SoundConverter which we are feeding input via float values from CoreAudio input. On PPC everything worked fine, but under x86, apparently we need to tell the SC the endianness of the data (i.e., it assumes big endian).

After a bit of googling, I ended up writing the code attached below. It seems to work (tells the SC it can _not_ byte swap the input host-endian float stream when 'decompress'ing it), but I had a few questions:

1) should I put a #pragma around my 'atoms' definition to pack it tightly?

Not necessary.

2) does the SoundConverterSetInfo call copy the memory passed in? right now I'm passing bytes from the stack...

It copies, so stack-based is ok.

3) any other obvious problems? :)

To be totally correct, you're supposed to pass in the format atom as well, so the codec knows whether this is data intended for it. Here's how to do it using a Handle:

{

// we need to add the extension that says "it's little endian".

AudioFormatAtom formatData = {0};

AudioEndianAtom endianData = {0};

AudioTerminatorAtom terminatorData = {0};

Handle extension = NULL;

formatData.size = EndianU32_NtoB(sizeof(AudioFormatAtom));

formatData.atomType = EndianU32_NtoB(kAudioFormatAtomType);

formatData.format = EndianU32_NtoB(dataFormat); // i.e. kFloat32Format, kFloat64Format, k24BitFormat

err = PtrToHand(&formatData, &extension, sizeof(AudioFormatAtom));

if (err)

goto bail;

endianData.size = EndianU32_NtoB(sizeof(AudioEndianAtom));

endianData.atomType = EndianU32_NtoB(kAudioEndianAtomType);

endianData.littleEndian = EndianU16_NtoB(isLittleEndian);

err = PtrAndHand(&endianData, extension, sizeof(AudioEndianAtom));

if (err)

goto bail;

terminatorData.size = EndianU32_NtoB(sizeof(AudioTerminatorAtom));

terminatorData.atomType = EndianU32_NtoB(kAudioTerminatorAtomType);

err = PtrAndHand(&terminatorData, extension, sizeof(AudioTerminatorAtom));

if (err)

goto bail;

SoundConverterSetInfo(_soundConverter, siDecompressionParams, *extension);

DisposeHandle(extension);

}

HTH

-Brad Ford

QuickTime Engineering

-tim

#ifdef __LITTLE_ENDIAN__
short isLittleEndian = true;
#else
short isLittleEndian = false;
#endif

// Atoms are always big-endian
struct {
AudioEndianAtom endian;
AudioTerminatorAtom terminator;
} atoms;
memset(&atoms, 0, sizeof(atoms));
atoms.endian.size = CFSwapInt32HostToBig(sizeof(atoms.endian));
atoms.endian.atomType = CFSwapInt32HostToBig(kAudioEndianAtomType);
atoms.endian.littleEndian = CFSwapInt16HostToBig(isLittleEndian);
atoms.terminator.size = CFSwapInt32HostToBig(sizeof(atoms.terminator));
atoms.terminator.atomType = CFSwapInt32HostToBig(kAudioTerminatorAtomType);

SoundConverterSetInfo(_soundConverter, siDecompressionParams, &atoms);

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References:
	>Sanity check on AudioEndianAtom usage (From: "Timothy J. Wood" <email@hidden>)