Encoding a wav file using G711 encoding - c

Question

I am trying to encode a PCM uncompressed Wav file using A law encoding.
I have written a function which takes in the 16 bit PCM data and returns 8 bit encoded data..After encoding, my file does not play properly..I feel that there is something I am not doing correctly to handle the files.I have separated the header information of the file and written the same header to output file.
// Code for compressing data is below
short inbuff;
unsigned char outbuff;
while (!feof(inp))
{
fread(inbuff, 2 , BUFSIZE, inp);
for (i=0; i < BUFSIZE; ++i)
{
temp_16 = inbuff[i];
temp_8 = Lin2Alaw(temp_16);
outbuff[i] = temp_8;
}
fwrite(outbuff, 1 , (BUFSIZE), out);
}

Answer 1

You are writing the data with the same header, which means that any audio program will think the data inside the WAV file is still PCM. Check the file format for WAV and change it accordingly.
Mainly you need to change audio format at 0x0014-0x0015 to a-law and other values also to mark the proper bytes per second, block size etc.
Easiest way to make sure they're correct might be to convert the file with an audio editor and then checking for the differences in the values.

Answer 2

How did your code even compile when you are not using arrays? Even so, your use of feof isn't good, please see Why is “while ( !feof (file) )” always wrong?
#include <stdio.h>
#define BUFSIZE 512
int main(void) {
short inbuff[BUFSIZE]; // must be an array
unsigned char outbuff[BUFSIZE]; // must be an array
size_t bread, i;
unsigned char temp_8;
short temp_16;
FILE *inp, *out;
// ... open the file
// ... transcribe the header
// rewrite the data
while ((bread = fread(inbuff, 2 , BUFSIZE, inp)) > 0)
{
for (i=0; i < bread; ++i) // only the data actually read
{
temp_16 = inbuff[i];
temp_8 = Lin2Alaw(temp_16);
outbuff[i] = temp_8;
}
fwrite(outbuff, 1 , bread, out); // only the data actually read
}
// ... finish off and close the file
return 0;
}
I notice too you are using signed short for the 16-bit data - should that be unsigned short?

Answer 3

See the format of wave file is at
http://www.topherlee.com/software/pcm-tut-wavformat.html
Now check all bytes of header and make sure all information about bit rate,sample rate etc are correct.
If your code for compressing is correct then issue should be with header file only.