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/*
* audio.c:
* Attempt to extract audio frames embedded in buffers.
*
* We only try MPEG at the moment. Ogg Vorbis is a bit harder because we'd need
* to find the stream-start packet as well-- in practice, we probably would be
* able to, but it's a bit of a hassle.
*
* 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: audio.c,v 1.3 2002/06/10 21:25:48 chris Exp $";
#include <string.h>
#include "driftnet.h"
#include "mpeghdr.h"
/* The minimum chunk of MPEG data, in frames, which we accept. MPEG layer 2/3
* frames have 1152 samples, so there are something like 40 frames/s at
* 44.1kHz. If there are several MPEG streams available, driftnet may chop
* between them in chunks of this size. */
#define MIN_MPEG_EXTENT 100
/* find_mpeg_stream:
* Try to find some MPEG data in a stream. The game here is that we look for
* an MPEG audio header and see whether it's followed by a bunch more MPEG
* audio headers. If there's as much as MIN_MPEG_EXTEND data, then we give
* it back to the application and move our pointer on. */
unsigned char *find_mpeg_stream(const unsigned char *data, const size_t len, unsigned char **mpegdata, size_t *mpeglen) {
unsigned char *stream_start, *p;
struct mpeg_audio_hdr H;
*mpegdata = NULL;
if (len < 4) return (unsigned char*)data;
/*printf("find_mpeg_stream\n"); */
p = (unsigned char*)data;
while (p < data + len - 4) {
int nframes;
unsigned char *q;
/* Look for something which might be a frame header. */
stream_start = memchr(p, 0xff, len - 4 - (p - data));
if (!stream_start)
return (unsigned char*)(data + len - 4);
if ((stream_start[1] & 0xe0) != 0xe0) {
/*printf(" not followed by e0\n");*/
p = stream_start + 1;
continue;
}
/* OK, found something which might be a header.... */
if (!mpeg_hdr_parse(stream_start, &H)) {
p = stream_start + 1;
continue;
}
/* See how many frames we get. */
nframes = 0;
q = stream_start;
do {
int delta;
++nframes;
delta = mpeg_hdr_nextframe_offset(&H);
if (delta == 0)
return q + 1;
q += delta;
} while (nframes < MIN_MPEG_EXTENT && q < data + len - 4 && mpeg_hdr_parse(q, &H));
if (nframes >= MIN_MPEG_EXTENT) {
/* got some data. */
/* printf("stream_start = %p, q = %p, len = %d\n", stream_start, q, q - stream_start);*/
*mpegdata = stream_start;
*mpeglen = q - stream_start;
return q;
} else
return stream_start;
}
return p;
}
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