[opendtv] Re: MPEG2 system clock recovery
- From: Ron Economos <k6mpg@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Fri, 23 Jun 2006 22:38:00 -0700
If the bitstream is being "pushed" over some channel
(the situation for OTA ATSC, satellite and cable),
you must recover the PCR clock if you want the decoder
to run properly 24/7. Otherwise you are guaranteed
to experience underflows or overflows with the elementary
streams after a period of time determined by the clock error.
This leads to stuttering video and audio dropouts.
I don't know of any decoder manufacturer that doesn't do
PCR recovery. It's fundamental to any (MPEG-2, VC-1,
H.264) decoder system. For IPTV (or any network delivered
stream), you have to de-jitter the PCR somehow. This can
be accomplished with higher layer network packet timestamps.
You can find very inexpensive VCXO chips tailored for this
application. For example:
http://www.icst.com/pdf/mk3720.pdf
http://www.pericom.com/pdf/datasheets/PI6CX100-27.pdf
As for PC's as decoders, I'll make an inverse Will Rogers
comment. I've never met one that I liked (or thought worked
properly).
Ron
Steve Wilson wrote:
>The MPEG2 system spec 13818-1 (H.222), Appendix D, describes a process
>whereby the decoder time clock get synchronized with the encoder time
>clock. This is done by comparing SCR values in the transport stream
>with the decoders local clock and taking appropriate action. There were
>many papers written on various ways to do this quite a few years ago.
>I am told some (hardware) decoder implementations don't do this
>anymore. I was wondering if anyone could shed some light on this?
>Presumably the PTS time stamps provide enough information to accurately
>decode and present the content....isn't this all PC decoders use? Are
>modern day STB designs foregoing this "SCR clock sync" at the input to
>the decoder? Is there any resulting inaccuracy in the output? Maybe
>the when the spec was put in place, system designers thought it was
>needed. Then as technology and people got better/smarter, we figured
>out how to eliminate that step? Anyone have first hand knowledge they
>can share?? or comment?
>
>Also, I dont think this step applies in MPEG over IP.....
>
>
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