[linux-dvb] Re: Clock synchronization

[Toby Jaffey <toby@earth.li>]

On Sat, Mar 02, 2002 at 12:58:21PM +0100, mocm@metzlerbros.de wrote:
>  > In the cards with a hardware decoder that makes sense. The decoder would
>  > run at the speed dictated by the 27mhz VCXO. But, in the cheaper cards
>  > which only do a demux then rely on software decoding, how does the
>  > software decoder adjust it's speed accordingly? Is PCR recovery done by
>  > the software decoder?

>  > Is it just forced to repeat/skip frames to make up for the time
>  > difference?

>  > Or does the provided software decoder take a clock pulse from the DVB
>  > card's VCXO to run by?

> What provided software decoder?

Sorry, that was unclear. I was referring to the software MPEG decoder
provided by the DVB card manufacturers.

> There are a couple of MPEG2 decoders for Linux, which have different
> synching schemes. Most don't handle transport streams yet and you have
> to use some TS to PS/PES converter. And they don't necessarily need an
> external clock from the DVB card or even an SCR/PCR to get synching
> right. Even the DVB hardware doesn't need SCR/PCR information for
> that. 

Yes, I agree. It's entirely possible to play just a PES stream without
the PCR stream. But, as the 27mhz clock the decoder uses won't exactly
match the broadcaster's clock the software decoder will be forced to
skip frames (if the broadcaster is faster) or repeat frames (if the
broadcaster is slower).

What I was wondering is whether this problem has been dealt with by the
Windows software MPEG decoder by running it at a speed dictated by the
27mhz VCXO.

-- 
 ()=  | Toby Jaffey : pkl.net/~trj
//\   | "The world is complex; sendmail.cf reflects this."                   
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