Help on MPEG2 header ISO/IEC standards

[Konran]

Hello,

while doing some programming with MPEG analysis I got stuck with the MPEG2 headers.
During debugging and dumping some data of a PS stream (PAL 25fps, 720x576) I get stuck with some data bytes according to the spec sheet ISO/IEC 13818-1 (2000) that I'm working with. Here are some pieces of the header data part in question:

1: 00 00 01 e0 => video stream

2: 07 dc 31 00 03 de 91 11.00 03 8a 31

3: 00 00 01 b3 2d 02 40 33.24 9f 23 69 10 11 11 12 ... => sequence header

The problem is at line 2: After the 16 bit length field PES_packet_length of 0x7dc I get 16 bits of flags (0x3100) and a length byte for the options following PES_header_data_length which is 0x03. The data alignment flag is 0, so I have to use a standard start code for the next sequence after PES_header_data_length + the value in there. When I do this I point after the 11 in line 1 and get 0x00038a31 and that is of course wrong. It should continue with line 3 at 0x000001b3. I'm 4 Bytes in offset less than that.

How's that? The flags say that the scrambling value is '11' which is user defined, no off. Is there some header scrambling in place?

At another place in the same stream I've found a '00' scrambling value, an alignment flag of '1' so that I have to jump to a one byte start code after the header ... which is also garbage (a 0x46 that matches no stream in this file) and I have '01' as PTS_DTS_flag which is undefined to ISO of 2000.

Are there major updates to the specification that say different things as of my interpretation? Or am I totally wrong with my assumptions?

I hope someone can help

Cheers,
Konran

[blutach]

See here and especially here for data on MPEG2 headers (these may not show if you are not a member of mpucoder.com - you are a member automatically if you have a premium / professional version of muxman; if you are not a member, it is very worthwhile becoming one).

ISO/IEC 13818-1 is the correct reference on this!

In any event, on first view, the top bit of byte 6 should be set.

The extension

<TABLE border=1><TBODY><TR><TH colSpan=8><CENTER>byte 6</CENTER></TH><TH colSpan=8><CENTER>byte 7</CENTER></TH><TH colSpan=8><CENTER>byte 8</CENTER></TH></TR><TR><TH>7</TH><TH>6</TH><TH>5</TH><TH>4</TH><TH>3</TH><TH>2</TH><TH>1</TH><TH>0</TH><TH>7</TH><TH>6</TH><TH>5</TH><TH>4</TH><TH>3</TH><TH>2</TH><TH>1</TH><TH>0</TH><TH>7</TH><TH>6</TH><TH>5</TH><TH>4</TH><TH>3</TH><TH>2</TH><TH>1</TH><TH>0</TH></TR><TR><TH bgColor=#9999ff colSpan=2>10</TH><TD bgColor=#66ff66 colSpan=2>PES scrambling control</TD><TD bgColor=#66ff66>PES priority</TD><TD bgColor=#66ff66>data alignment indicator</TD><TD bgColor=#66ff66>copyright</TD><TD bgColor=#66ff66>original or copy</TD><TD bgColor=#66ff66 colSpan=2>PTS DTS flags</TD><TD bgColor=#66ff66>ESCR flag</TD><TD bgColor=#66ff66>ES rate flag</TD><TD bgColor=#66ff66>DSM trick mode flag</TD><TD bgColor=#66ff66>additional copy info flag</TD><TD bgColor=#66ff66>PES CRC flag</TD><TD bgColor=#66ff66>PES extension flag</TD><TD bgColor=#66ff66 colSpan=8>PES header data length</TD></TR></TBODY></TABLE>The bit fields of the basic extension:

PES scrambling control -- 00 = not scrambled, others are user defined.
PES priority -- provides 2 priority levels, 0 and 1.
data alignment indicator -- if set to 1 indicates that the PES packet header is immediately followed by the video start code or audio syncword.
copyright -- 1 = packet contains copyrighted material. original or copy -- 1 = original, 0 = copy.

Regards

[Konran]

Thanks Les ... I hope getting fast access to mpucoder after donation is done.
Well indeed I had a copy of the ISO/IEC 13818-1, but
a) I did not understand everything
b) it is covered what it has to be in the headers but not always, what has not
I really thought that all the fields that are marked as "fixed" must have the bit values specified. So how should a file being treated that does not match those things? Throw away?

In the meantime I've found that my example file must come from a garbled muxer. Examining another one did it much better and I can see the fields you copy-pasted properly.

It seems in fact that a video block e.g. a picture can be splitted across multiple PES packets so that the data after the pack header can be somewhere in the middle of e.g. a slice. Right?

Regards, Konran

[blutach]

a) I did not understand everything

You are not Robinson Crusoe there

In the meantime I've found that my example file must come from a garbled muxer.

I suspected as much. Use muxman in the future.

It seems in fact that a video block e.g. a picture can be splitted across multiple PES packets ...

Yes, of course. For example, an I-frame is always spead over several packets. But each packet always has a header (continuation packs only have the Pack ID and Video Stream header details).

Regards

[Konran]

You are not Robinson Crusoe there

Ohh fine ... it's not too bad beeing accompanied

I suspected as much. Use muxman in the future.

I probably will ... but the file I was talking about is not from my stock. In the meantime thanks to your given short summary of headers I've found that the suspicious block part is a MPEG1 pack header. This was neither covered in ISO/IEC 13818-1 nor in ISO/IEC 13818-2. Could you also provide a link to ISO/IEC 11172-2 (and part 1 if any)? That would be great!

But each packet always has a header (continuation packs only have the Pack ID and Video Stream header details).

Thanks, that helped a lot!

Regards

[blutach]

Go to Donald Graft's library page and click on the MPEG-1 stuff.

http://neuron2.net/library/library.html

But, these are the direct links for you (they are Word documents):

http://neuron2.net/library/mpeg1/MPGSYS.DOC

http://neuron2.net/library/mpeg1/MPGVIDEO.DOC

BTW: Don and mpucoder are the gurus on this. If you are not a member of Doom9, now's the time to join as they tend to hang out there.

Regards

[Konran]

Great, thanks. U made me the lucky man