How to write 24 bit pcm samples in sndfile?

Question

I have file which is opened with SF_FORMAT_WAV|SF_FORMAT_FLOAT but have samples in 24 bit format. Sndfile docs says that the data type used by the calling program and the data format of the file do not need to be the same so using sf_write_int() i can write 32 bit integer samples and library converts to float on the fly. But what about 24 bit. Do i pack it into int and then write using sf_write_int() or something else.

Answer 1

It should be possible to just scale it by 256 to make it 32-bit and use sf_write_int . If you were expecting libsndfile to do any dithering or something for you this would be a bad idea, but as far as I can tell that's not in its job description.

Answer 2

You have to define the format of your output file to be SF_FORMAT_WAV|SF_FORMAT_PCM_24; . When you do that, whatever your internal data are ( float , int16 ...), your output file will be written with the requested format.

Here is an example where I read a file (a simple 0.5 second sine at FS of 44100 Hz designed with Audacity) with floating point 32 bit data. My data is also float internally. Then I write a 24 bit PCM file.

#include <stdio.h>
#include <stdlib.h>
#include "sndfile.h"

#define DATA_TO_CONVERT     22050

int main(void)
{
  char *inFileName, *outFileName;
  SNDFILE *inFile, *outFile;
  SF_INFO inFileInfo, outFileInfo;

  float dataBuffer[DATA_TO_CONVERT];

  // Input file 32 bit float
  inFileName = "sine_32.wav";
  inFile = sf_open(inFileName, SFM_READ, &inFileInfo);

  // Output file 24 bit
  outFileName = "sine_24.wav";
  outFileInfo.frames        = inFileInfo.frames;
  outFileInfo.samplerate    = inFileInfo.samplerate;
  outFileInfo.channels      = inFileInfo.channels;
  outFileInfo.format        = SF_FORMAT_WAV|SF_FORMAT_PCM_24; // Wanted output format
  outFile = sf_open(outFileName, SFM_WRITE, &outFileInfo);

  sf_read_float(inFile, dataBuffer, DATA_TO_CONVERT);
  sf_write_float(outFile, dataBuffer, DATA_TO_CONVERT);
  sf_close(inFile);
  sf_close(outFile);

  // Check Output file
  sf_open(outFileName, SFM_READ, &outFileInfo);
  printf("Output File format : 0x%x\n", outFileInfo.format);

  return 0;
}

The console output is:

Output File format : 0x10003

Which means SF_FORMAT_WAV|SF_FORMAT_PCM_24 .