[MPlayer-users] Request info / Bug report : How SMPlayer reads "sequence header" & "sequence extension" from mpg ?

[Alain-Pierre CHERTIER]

Hello everybody,

I'm looking for method(s)/object(s)/function(s) involved by MPlayer to
retrieve informations
from an MPEG file, especially how it will read information fields from
"sequence header" (start_code 0x00001B3)
and "sequence extension" (start_code 0x000001B5), because I obtain a 6X
speed factor when trying to read
an MPEG video file which was encoded as 5 frames per second as MPlayer is
thinking of a 30 fps video stream file...

I think it is because MPlayer (or 3rd-party library functions used by
MPlayer ?) don't use properly mixed info between
"sequence header" and "sequence extension" which are well present in the
MPEG file and have reliable detailed
information about the video stream according to MPEG standard...

Here below are valid informations in the MPEG file that don't seem to be
taken into account properly by MPlayer:

inside "sequence_header":

  frame_rate_code = 5 (=> "frame_rate_value = 30 fps", if ignoring or
miss-interpreting
                                             extension data information in
the "sequence extension"...)

inside "sequence_extension":

  frame_rate_extension_n = 0
  frame_rate_extension_d = 5

as specified in the MPEG standard, the formula to calculate the effective
"frame_rate" shall be

  frame_rate = frame_rate_value * (frame_rate_extension_n + 1) /
(frame_rate_extension_d + 1)

  with "frame_rate_value" being hard-coded and deriving from
"frame_rate_code", as specified by the MPEG standard.

With implementing the right standard formula, frame_rate shall be calculated
as :

  30 * (0 + 1) / (5 + 1) = 30 * 1 / 6 = 5 fps       yyyeeeeeesssssssssss !!!

I recompiled MPlayer (on a Linux platform) with "-g" gcc compile flag,
hopping to debug/run it step by step with "ddd",
but I didn't find what object/function is effectively reading or not reading
the "sequence extension" data
to correct 1st level information data read from the "sequence header".

As more information to you, please notice that the interesting "MediaInfo"
tool seems to have (same) problem
in its version 0.7.26 about mixing "sequence header" and "sequence
extension" information, not (yet) applying
the right standard MPEG formula, deciding by mistake to take
"frame_rate_extension_n / frame_rate_extension_d"
ratio from "sequence extension" as frame rate ( ! ) and ignoring
frame_rate_code (frame_rate_value) from "sequence header"
because "frame_rate_extension_n" is zero ! So, MediaInfo displays an invalid
"0 fps" frame rate as a result from
"0 / 5" division ! I sent an email about that to the french developper
maintaining the MediaInfo tool
(suggesting to him what modification to apply to source code to obtain the
right frame rate value,
as I tested/validated this modification by recompiling my own standard copy
of MediaInfo source code).
He replied to me, acknowledging for suggested modification.

Hopping anybody can have a look at official source code of MPlayer and post
an (approved) update,
many thanks in advance, it may allow me to play "slow" videos which are ok
against MPEG stream packet
format, but unfortunately, almost all multimedia players are not ok with
such "slow" encoded video streams !?
VLC is hanging with such vids, only Windows Media Player from Microsoft
seems to sometimes accept decoding & displaying
of such vids at the right 5 fps frame rate display speed, but surprisingly
announcing a 30 fps frame rate (!?) in the info window,
like SMPlayer/MPlayer...

It shall be noticed that other data information fields shall have to be also
examined against being defined
partly in "sequence header" and partly in "sequence extension":

a good example (with potential "difficulty") is the "bit rate" which is
defined as a 30bit value by the MPEG standard,
with 18 lowest bits named as "bit_rate_value" ( ! ) in the "sequence header"
and higher 12 bits "bit_rate_extension"
in the "sequence extension"...

'hope my post can help to build a new version of MPlayer, because for now, I
have to add the "-fps 5" option to MPlayer
to properly decode/render our slow 5 fps mpeg stream, but we would have a
transparent solution/way to play such streams
without to have to specify the "-fps ?" option because we don't know the
right fps before pre-decoding input files...

Kind regards & many thanks in advance.

Alain-Pierre
<apclinux at gmail.com>