MPEG-2 Frame Aligner - Revision history

CityK: MPEG2 Frame Aligner moved to MPEG-2 Frame Aligner: name

2008-01-06T22:05:27Z

MPEG2 Frame Aligner moved to MPEG-2 Frame Aligner: name

← Older revision

Revision as of 22:05, 6 January 2008

CityK: fixed links

2008-01-06T22:05:09Z

fixed links

← Older revision		Revision as of 22:05, 6 January 2008
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	== Introduction ==		== Introduction ==

-	Most hardware does not provides aligned ~~MPEG2~~ data streams, also the linux-dvb API's DVR device can get out of sync if the ~~MPEG2~~ TS data stream is interrupted, e.g. due to DMA buffer overflows or if the demod delivers partial packets in case of uncorrectable block errors.	+	Most hardware does not provides aligned MPEG-2 data streams, also the linux-dvb API's DVR device can get out of sync if the MPEG-2 TS data stream is interrupted, e.g. due to DMA buffer overflows or if the demod delivers partial packets in case of uncorrectable block errors.

-	~~MPEG2~~ TS packets can be 188 (without FEC data) or 204 bytes long (including 16 FEC bytes).	+	MPEG-2 TS packets can be 188 (without FEC data) or 204 bytes long (including 16 FEC bytes).

	== Algorithm ==		== Algorithm ==

-	The ~~MPEG2~~ Frame Aligner can get implemented as simple state machine, whenever we are in an "out-of-sync"-state we check all followig bytes until we encounter a ~~MPEG2~~ TS Sync Byte (0x47). From then on we check every 188 or 204 bytes if the next 0x47-sync-byte is present. If so the packet is passed to the [[PID_Filter_-_Demultiplexer\|PID filter]]	+	The MPEG-2 Frame Aligner can get implemented as simple state machine, whenever we are in an "out-of-sync"-state we check all followig bytes until we encounter a MPEG-2 TS Sync Byte (0x47). From then on we check every 188 or 204 bytes if the next 0x47-sync-byte is present. If so the packet is passed to the [[PID_Filter_-_Demultiplexer\|PID filter]]

	== Sample Code ==		== Sample Code ==

Guidod: +cat

2005-04-02T22:33:48Z

+cat

← Older revision		Revision as of 22:33, 2 April 2005
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	}		}
	</tt>		</tt>
		+
		+	[[Category:Technology]]

Holger at 20:38, 1 November 2004

2004-11-01T20:38:22Z

New page

== Introduction ==

Most hardware does not provides aligned MPEG2 data streams, also the linux-dvb API's DVR device can get out of sync if the MPEG2 TS data stream is interrupted, e.g. due to DMA buffer overflows or if the demod delivers partial packets in case of uncorrectable block errors.

MPEG2 TS packets can be 188 (without FEC data) or 204 bytes long (including 16 FEC bytes).

== Algorithm ==

The MPEG2 Frame Aligner can get implemented as simple state machine, whenever we are in an "out-of-sync"-state we check all followig bytes until we encounter a MPEG2 TS Sync Byte (0x47). From then on we check every 188 or 204 bytes if the next 0x47-sync-byte is present. If so the packet is passed to the [[PID_Filter_-_Demultiplexer|PID filter]]

== Sample Code ==

The following sample code works for 188-byte packets. This code is taken from the LinuxDVB kernel driver (originally written by Florian Schirmer) and thus GPL'd.

A more generic and robust version based on a state machine should not be too hard to implement and would be able to synchronize both 188- and 204-byte-packets. Please let us know when you feel in the mood to implement+test this, we'll be happy to help whereever possible.

<tt>
struct framealigner {
char tsbuf[188];
int tsbufp;
};

void framealigner_process_ts_data (struct framealigner *fa,
const char *buf,
unsigned int count)
{
unsigned int p = 0, i, j;

if ((i = fa->tsbufp)) {
if (count < (j=188-i)) {
memcpy(&fa->tsbuf[i], buf, count);
fa->tsbufp += count;
return;
}
memcpy(&fa->tsbuf[i], buf, j);
pidfilter_ts_packet(fa->tsbuf);
fa->tsbufp = 0;
p += j;
}

while (p < count) {
if (buf[p] == 0x47) {
if (count-p >= 188) {
pidfilter_process_ts_packet(buf+p);
p += 188;
} else {
i = count - p;
memcpy(fa->tsbuf, buf+p, i);
fa->tsbufp = i;
return;
}
} else
p++;
}
}
</tt>