Thread: Writing audio to file

Hello,

I've been researching some audio and trying a few implementations. I've hit a wall I can't solve, and wanted to see if anybody had any ideas. I'm generating samples off a sine wave, formatting the wave file, and everything is going great. When I actually write the information to a file, it seems that every so often the information gets corrupted. "Every so often" meaning consistently and in the same places every time.

I was able to narrow it down to the fwrite calls by checking every short that was generated, and each that was written. When it's writing, all of the values are correct. When I open the file and read it to double check, there are tons of samples that have different values than it calculated. Here's a picture:



The sine wave is in there, it's just got what looks like random noise. The audio does play, it just sounds like it looks like it sounds. Here is the relevant code:

Code:

typedef struct
{
    CHUNK_HEADER header;
    //uint8_t* byteArray; // for 8 bit sound
    short* shortArray; // for 16 bit sound
} DATA_CHUNK;

format.header.dwChunkSize = 16;
    format.wFormatTag = 1;
    format.wChannels = 1;
    format.dwSamplesPerSec = 44100;
    format.dwBitsPerSample = 16;
    format.wBlockAlign = format.wChannels * (format.dwBitsPerSample / 8);
    format.dwAvgBytesPerSec = format.wBlockAlign * format.dwSamplesPerSec;


    data.shortArray = (short*)malloc((seconds * format.dwSamplesPerSec) * sizeof(short));
    memset(data.shortArray, '0', seconds * format.dwSamplesPerSec * sizeof(short));
    data.header.dwChunkSize = seconds * format.dwAvgBytesPerSec;
    populate_samples(data.shortArray, &format, seconds, check_bytes);
    main_header.dwFileLength = 4 + 16 + (format.dwAvgBytesPerSec * seconds);


    FILE *wav_file = fopen(FILENAME, "w+");
    FILE *check_file;
    fwrite(&main_header, 1, sizeof(main_header), wav_file);
    fwrite(&format, 1, sizeof(format), wav_file);
    fwrite(&data.header, 1, 8, wav_file);
    fwrite(data.shortArray, 2, (format.dwSamplesPerSec * seconds), wav_file);
    fclose(wav_file);

I'm stumped. Any ideas?

Have you packed the structs? The following should work for both MSVC and gcc (unless your gcc is old). Put it before the structs.

Code:

#pragma pack(1)

For better portability, you should avoid packing and write each member of the structs individually.

One pretty obvious problem is that you're not opening the file in binary mode. On Windows text mode is the default with the system CRT and unlike some other systems, actually matters.

OH, yeah, that fixed it. For anybody interested, the only change I had to make was changing this line:

Code:

FILE*wav_file = fopen(FILENAME, "w+");

To this:

Code:

FILE*wav_file = fopen(FILENAME, "wb+");

And you get this!



Thanks for the responses, I appreciate the help.