[opendtv] Re: tolerance Latency , Jitters & packet loss issues for MPEG-2 TS over IP network

On Sun, 2005-02-27 at 18:31 -0500, Manfredi, Albert E wrote:
> John McClenny wrote:
> 
> > I think that the IPTV people using FEC are FECing at
> > the UDP/IP packet level, not at the video stream level.
> > So a discarded packet gets reconstructed.  I have not
> > direct experience with this, but this is what I have
> > been told.  Not my preferred mthod of handling this
> > problem.
> 
> Kon Wilms wrote:
> 
> > Right, cause the IP layer will discard a corrupted
> > packet (or it just won't arrive). Hence you are not
> > really doing forward error correction, but rather
> > erasure correction. With the entire packet missing you
> > can't do your standard bit error reconstruction, you
> > have to do block error reconstruction. Reed-solomon is
> > also useless for this application. Also to do it
> > correctly, one has to randomize the processed block
> > and parity blocks (you don't want your parity blocks in
> > sequence, it defeats the purpose) - the impact of this
> > is latency in the decoder buffer...
> 
> Yes but ...
> 
> What you guys are describing is not a limitation of IPTV
> per se. It is a limitation of using Ethernet framing at
> the Link Layer, for the purpose of transmitting one-way
> streams of packets, and expecting a vanishingly low
> error rate end-to-end.

See, you didn't read my first comment then :-)

"FEC on IP is rather pointless unless you want to start modifying the IP
stack drivers."

I'll say personally that block ECC has saved my a** a number of times
when doing demos where the ATSC signal had severe multipath or signal
loss. Even more so on small payloads.

Block erasure correction also works well with high error rates in
encapsulated ip over mpeg2 ts.

Cheers
Kon


 
 
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