[ExtractStream] Possible end of ZSS development...

[Jim Hauxwell]

Hi,

Just reading your mail, and I noticed a point in which you mention
re-ordering the IPB frames. Why on earth do you want to do this? They are
in the correct order already (from what I have gathered).
If you are making this assumption based on the fact that the first GOP of a
standard mpeg produced stream runs

IPBBPBBPBBPBBPBBPBBIBBPBBPBBPBBPBBP

^
notice here - do you see that the sequence gets into a
familiar pattern?

and you assume that Tivo is

IBBPBBPBBPBBPBBP....

so must need re-ordering then this is not correct.

The MPEG2 specification says that B frames are reconstructed against the
previous and next I or P frame. In the case of the Tivo stream, if you
number the frames the actual order would be

1xx423756....

Without looking into it at some great length I couldn't say 100% that this
is correct in the case of the Tivo, but as a standard Tivo stream decodes on
a reference MPEG2 decoder (without errors) then I can say with some
certainty that the stream is a valid ES.

Jim

-----Original Message-----
From: Roger Merchberger [mailto:zmerch7@y...]
Sent: 18 April 2002 01:12
To: ExtractStream@yahoogroups.com
Subject: [ExtractStream] Possible end of ZSS development...


Holymygoodness! Don't let that scare you... I'm not leaving the
scene or moving to Antarctica or anything, but I think I've finally
gone beyond the "know just enough to be dangerous" stage & am ready
to totally redefine the structure of the splitstream/zss code.

I've been making changes to zss 0.56 (no, it's a hack so it's not on
the webpage) to get it to figure out the GOP frame timestamps and I
think I've [eek] succeeded [first time I've diddled with bitshifting
in C!] -- except -- with Edmond's help, I've figured out that the
MPEG video frame breaks & the Tivo video frame breaks are nowhere
near a 1:1 ratio - including [but not limited to] the GOP headers.
:-(

Splitstream (and of course zss) is set up to: 1) slurp in a chunk 2)
spit out A & V 3) throw away the rest, 4) rinse & repeat. Getting the
Video to split on GOP headers, the possible reorganizing of I, P & B
frames to a different order (if necessary) and other things would be
just too hard to try to hack into the existing code as it sits... not
to mention a *lot* of the error messages that Joe had in splitstream
seem to be never used, or only come out when the stream's fubar'd so
bad that there's no point to continuing...

So I'm thinking of stripping zss right down to it's skivvies &
[mostly] starting from scratch. This will slow down development a
little bit, as it will take some time to get a new code base in
place, but it will enable us all to learn more about the streams, and
hopefully get closer sooner to a fully self-synchronizing MPEG
splitter.

I may have to 're-add' a little bit of the chunk-handling code, but
it seems like some of it is throwing out the baby with the bathwater;
so we'll see.

I have made a few more changes 'splainin' this on my webpage, and I
started a Glossary: section - The usual questions, suggestions,
whatever just email me & I'll do my best to get it in. For those
folks just tuning in:

http://www.30below.com/~zmerch/tivo/index.cfm

Hard to believe that I've gotten over 2150 hits to the page in under
3 months!

Laterz,
Roger "Merch" Merchberger


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