NES DMC sample to WAVE converter

dmdecode.c

// dmdecode v0.00
// extremely simple CLI tool for converting
// Nintendo DMC (1-to-7-bit mono DPCM) samples to 8-bit PCM WAVE
//
// C99 (or newer) required!
//
// cs127
// https://cs127.github.io
// 2025-07-02


#define INITIAL_VALUE 0x40


#include <stddef.h>
#include <stdint.h>
#include <stdio.h>


#define eprintf(fmt, ...) fprintf(stderr, fmt, __VA_ARGS__)
#define fgoto(fp, n) fseek(fp, n, SEEK_SET)
#define fskip(fp, n) fseek(fp, n, SEEK_CUR)


void fputu16(uint16_t v, FILE* fp)
{
	fputc(v >> 0 & 0xFF, fp);
	fputc(v >> 8 & 0xFF, fp);
}

void fputu32(uint32_t v, FILE* fp)
{
	fputc(v >>  0 & 0xFF, fp);
	fputc(v >>  8 & 0xFF, fp);
	fputc(v >> 16 & 0xFF, fp);
	fputc(v >> 24 & 0xFF, fp);
}


int main(int argc, char** argv)
{
	register int inv;
	register uint8_t outv = INITIAL_VALUE;
	register size_t i;
	int ret = 0;
	long int riff_start, data_start, file_end;
	FILE* in = NULL, * out = NULL;

	if (argc != 3)
	{
		eprintf("usage: %s INFILE OUTFILE\n", argv[0]);
		ret = 1;
		goto end;
	}

	in = fopen(argv[1], "rb");
	if (!in)
	{
		eprintf("could not open file `%s' for reading.\n", argv[1]);
		ret = 3;
		goto end;
	}

	out = fopen(argv[2], "wb");
	if (!out)
	{
		eprintf("could not open file `%s' for writing.\n", argv[2]);
		ret = 4;
		goto end;
	}

	// main RIFF file header

	fputs("RIFF", out);
	fskip(out, 4); // chunk size (written later)

	riff_start = ftell(out);

	fputs("WAVE", out); // file type

	// fmt chunk

	fputs("fmt ", out);
	fputu32(16, out); // chunk size

	fputu16(1, out); // format (1 = integer PCM)
	fputu16(1, out); // channel count (1 = mono)
	fputu32(33144, out); // sample rate, i.e. sampling points per second
	fputu32(33144, out); // bytes per second
	fputu16(1, out); // bytes per sampling point
	fputu16(8, out); // bit depth

	// data chunk

	fputs("data", out);
	fskip(out, 4); // chunk size (written later)

	data_start = ftell(out);

	while ((inv = fgetc(in)) != EOF)
	{
		for (i = 0; i < 8; i++)
		{
			if (inv & (1 << i))
			{
				if (outv < 0x7F) outv++;
			}
			else
			{
				if (outv > 0x00) outv--;
			}

			fputc(outv * 2, out);
		}
	}

	file_end = ftell(out);

	// go back and write the chunk sizes

	fgoto(out, riff_start - 4);
	fputu32(file_end - riff_start, out);

	fgoto(out, data_start - 4);
	fputu32(file_end - data_start, out);

	if (ferror(out))
	{
		eprintf("could not successfully write to `%s'.\n", argv[2]);
		ret = 6;
		goto end;
	}
end:
	if (in) fclose(in);
	if (out) fclose(out);

	return ret;
}