[MEncoder-users] MPEG-2 muxer

[Nico Sabbi]

Nicolas Hesler wrote:

> Nico Sabbi wrote:
>
>> Nicolas Hesler wrote:
>>
>>>  So when Mencoder muxes a  file, it tries to ensure sync.
>>> Do demuxers (like MPEG StreamClip) then retain that sync or it is 
>>> the same result as going to two separate files directly from Mencoder?
>>>
>> I don't know what that program is
>>
> I know.  I was a little unclear.
>
> What I know:
>
> 1. mencoder will mux and retain sync as much as possible because it is
> aware of timestamps.
> 2. going directly to 2 separate files will potentially create sync
> problems because it is not using timestamps.
>
> Basically, what I want to know is:
>
> - if I take the muxed file that mencoder produced and then demux it,
> will I have the same result as when you go directly to 2 separate
> files?


of course yes

> or will the 2 demuxed files produced from the Mencoder file have
> potentially  better sync than going directly to 2 separate files?
>

potentially worse, not better. It seems that users don't realize that
mpeg files produced by mencoder may have slight temporal discontinuities,
e.g. few consecutive timestamps (without considering b-frames) may be:

0.40, 0.80, 1.20,   2.0, 2.40, 2.80,   3.60, 4.00  and so on
                      ^^ gap here        ^^^

gaps are introduced when mencoder inserts 0-length frames to keep sync
(similar to N-frames in avi).
Demuxing the file you will lose those gaps, so remuxing the elementary 
stream
files will result in a mux containing (possibly) different timestamps 
than in the
original file (and surely won't contain the gaps supposed to keep the 
streams in sync)