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mpeghdr.c
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mpeghdr.c
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/*
* mpeghdr.c:
* Parse MPEG audio headers.
*
* Copyright (c) 2002 Chris Lightfoot. All rights reserved.
* Email: chris@ex-parrot.com; WWW: http://www.ex-parrot.com/~chris/
*
*/
static const char rcsid[] = "$Id: mpeghdr.c,v 1.4 2002/06/10 21:25:48 chris Exp $";
#include <stdio.h>
#include "mpeghdr.h"
/* bits:
* Get bits a through b < a of val. */
//#define bits(v, a, b) (((v) & ((4 << ((a) - (b))) - 1)) >> (b))
#define bits(v, a, b) (((v) & (((2 << ((a) - (b))) - 1) << (b))) >> (b))
/* bitrate_tbl:
* Table of bit rates. Column index: 0 = v1 l1; 1 = v1 l2; 2 = v1 l3;
* 3 = v2 l1; 4 = v2 l2, l3. */
static int bitrate_tbl[16][5] = {
{ 0, 0, 0, 0, 0}, /* free */
{ 32, 32, 32, 32, 8},
{ 64, 48, 40, 48, 16},
{ 96, 56, 48, 56, 24},
{128, 64, 56, 64, 32},
{160, 80, 64, 80, 40},
{192, 96, 80, 96, 48},
{224, 112, 96, 112, 56},
{256, 128, 112, 128, 64},
{288, 160, 128, 144, 80},
{320, 192, 160, 160, 96},
{352, 224, 192, 176, 112},
{384, 256, 224, 192, 128},
{416, 320, 256, 224, 144},
{448, 384, 320, 256, 160},
{ -1, -1, -1, -1, -1} /* bad */
};
/* samplerate_tbl:
* Table of sample rates. Column index: v1 = 0, v2 = 1, v2.5 = 2. */
static int samplerate_tbl[4][3] = {
{44100, 22050, 11025},
{48000, 24000, 12000},
{32000, 16000, 8000},
{ -1, -1, -1} /* reserved */
};
/* mpeg_hdr_parse:
* Parse an MPEG header into a struct mpeg_audio_hdr. */
int mpeg_hdr_parse(const uint8_t *data, struct mpeg_audio_hdr *h) {
struct mpeg_audio_hdr hz = {0};
uint32_t hh;
int i;
*h = hz;
if (data[0] != 0xff || (data[1] & 0xe0) != 0xe0)
return 0;
hh = (data[1] << 16) | (data[2] << 8) | data[3];
/*fprintf(stderr, "%08x\n", hh);*/
/* Version. */
switch (bits(hh, 20, 19)) {
case 0: h->version = m_vers_2_5; break;
case 2: h->version = m_vers_2; break;
case 3: h->version = m_vers_1; break;
default: return 0;
}
/* Layer. */
switch (bits(hh, 18, 17)) {
case 1: h->layer = m_layer_3; break;
case 2: h->layer = m_layer_2; break;
case 3: h->layer = m_layer_1; break;
default: return 0;
}
/* Possible CRC. */
h->has_crc = bits(hh, 16, 16);
if (h->has_crc)
h->crc = (data[5] << 8) | (data[6]);
/* Bit rate. */
i = (h->version == m_vers_1) ? 0 : 3;
i += h->layer - 1;
if (i == 5) i = 4;
h->bitrate = bitrate_tbl[bits(hh, 15, 12)][i];
if (h->bitrate == -1)
return 0;
/* Sample rate. */
i = h->version - 1;
h->samplerate = samplerate_tbl[bits(hh, 11, 10)][i];
if (h->samplerate == - 1)
return 0;
/* Is frame padded? */
h->padded = bits(hh, 9, 9);
/* Channel mode. */
h->channels = bits(hh, 7, 6);
/* Mode extension. */
h->mode_extn = bits(hh, 5, 4);
h->copyr = bits(hh, 3, 3);
h->original = bits(hh, 2, 2);
h->emph = bits(hh, 1, 0);
return 1;
}
/* mpeg_hdr_nextframe_offset:
* Return the offset of the next MPEG header frame.
* XXX this doesn't work with (at least) L3 mono frames-- need to fix it. */
int mpeg_hdr_nextframe_offset(const struct mpeg_audio_hdr *h) {
int off = 0;
/* if (h->has_crc) off += 2; */
if (h->layer == m_layer_1)
off += ((12 * h->bitrate * 1000) / h->samplerate + h->padded) * 4;
else
off += (144 * h->bitrate * 1000) / h->samplerate + h->padded;
return off;
}
/* mpeg_hdr_print:
* Print out useful information about an MPEG audio header. */
static char *vers_tbl[] = {"unknown", "1", "2", "2.5"};
static char *channel_tbl[] = {"stereo", "joint stereo", "dual stereo", "mono"};
void mpeg_hdr_print(FILE *fp, const struct mpeg_audio_hdr *h) {
fprintf(fp, "version %s layer %d\n", vers_tbl[h->version], h->layer);
if (h->has_crc)
fprintf(fp, " has CRC %04x\n", h->crc);
fprintf(fp, "bit rate %d kbps\nsample rate %dHz\n", h->bitrate, h->samplerate);
fprintf(fp, "%s padded\n", h->padded ? "is" : "is not");
fprintf(fp, "%s\n", channel_tbl[h->channels]);
if (h->channels == m_chan_joint)
fprintf(fp, "mode extension %d\n", h->mode_extn);
fprintf(fp, "%s copyright\n%s original\n", h->copyr ? "is" : "is not", h->original ? "is" : "is not");
if (h->emph)
fprintf(fp, "emphasis %d\n", h->emph);
}
#if 0
int main(int argc, char *argv[]) {
char buf[8192], *p;
struct mpeg_audio_hdr h;
read(0, buf, sizeof buf);
for (p = buf; p < buf + sizeof buf; ) {
if (!mpeg_hdr_parse(p, &h)) {
fprintf(stderr, "no MPEG audio header\n");
return -1;
}
mpeg_hdr_print(stdout, &h);
printf("offset to next frame = %d\n", mpeg_hdr_nextframe_offset(&h));
p += mpeg_hdr_nextframe_offset(&h);
}
}
#endif