Re: Why is everything on E4 always out if lipsync ?


"Chas Gill" <Chas.Gill@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:dg8s2h$2eu$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> "Paul Schofield" <paul_AT_sonifex_DOT_co_DOT_uk> wrote in message
> news:8u-dnepGrMJGcrreRVnysw@xxxxxxxxxxxx
> >> >>> Why is everything on E4 always out if lipsync ?
> >> >>>
> >> >>> And when are they going to fix it. C4 isn't much better.
> >> >>>
> >> >>
> >
> >> I have noticed that when I watch satellite tv, sometimes when the lips
> >> are
> >> out of sync I just switch the box off, then back on and it fixes it.
> >
> > This means it's likely to be an STB buffering problem and is not the
> > responsibility (and in fact beyond control) of the broadcaster. It may
> > well
> > be that the stat muxing and variable compression systems are not handled
> > very well in some STB's so some channels will exhibit the problem more
> > than
> > others.
> >
> > FWIW we used to have a netgem iplayer with a VCR routed through the
netgem
> > scart sockets. After playing a video there was always a lip sync problem
> > of
> > about 2-3 seconds! Entering standby mode solved this.
> >
> > --
> > Paul Schofield
>
> Before I go off on another rant (I may be demonstrating my ignorance here)
> could somebody point me to a good internet resource that explains exactly
> how DTV works, i.e. why is it necessary to code audio & video separately.
> (I presume that they are separate bitstreams otherwise how could they get
> out of sync. - and if this is the case, what are the problems that
conspire
> to cause unsync.) I have a £200 Canopus ADVC110 which does an excellent
job
> of digitising analogue video for later editing on my PC - it can be left
to
> run for hours at a time and the resulting digitised file has perfect sync.
> As an amateur with an engineering background, but not in TV systems, I am
> eager to learn why this can't be achieved with broadcast systems, with
their
> huge resources and budgets and, if it truly can't be achieved
consistently,
> why the hell have "they" bothered to introduce DTV before somebody has
> figured out how to make it work properly?
>
> Chas
>
>

Can't help you with the technical sites, however...

I'm pretty sure that it's that audio and video are DECODED separately that
causes the problems. The audio is coded as distinct frames so that the
system needs to decode a frame (1152 samples of audio?) and then queue this
up to be sent to the audio DAC, the time to decode a frame is more or less
the same and relatively small. The video is made of all sorts of info that
depends on the complexity of the signal and the resultant bitrate that is
required. Some of this stuff is whole frame, some is predictive, some is
just changed data, and then add error correction data to the mix. The time
to decode a specific video frame is extremely variable and can be relatively
large. This means the both video and audio data has to be buffered and the
playout to the various DACs has to be synchronised to avoid lip-sync faults.
It's clear that this doesn't always work, and it's difficult to pin down the
problem. I'd guess it could be a number of things - that the lock to the
stream may be faulty so that losing a couple of frames of video due to
errors in the stream causes the video to step and so losing sync to the
audio (switching from & to the channel should solve this); it could be that
some channels have very long decode times so causing one of the buffers to
run out of data; it could be that the STB processor cannot keep up so we
lose sync that way; or even, as in the VCR case above, there is a bug in the
calculation to determine the sync or buffer pointers are set or cleared
erroneously
to again lose lip-sync.

On the closed system you describe everything starts at the correct, known
point; there should be no imperfections in the data stream; lack of MIPS in
the processor; or shortage of memory for buffering to cause loss of sync.


HTH

--
Paul Schofield



.

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