[opendtv] Re: (S) DTV adaptors available for less than 50 euros in France
- From: "Donald Koeleman" <donald.koeleman@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Wed, 14 Sep 2005 22:25:10 +0200
Bert seems to have a valid point. At last year's IBC I asked one
well-established iptv set-top box supplier if they were planning on comming
up with a HD version of their AVC set-top. One of the primary reasons they
wouldn't was that to reach an acceptable datarate for iptv the channel
changing (and initial acquisition) time would be unacceptably long, due to
the long gop length required.
With linespeeds on deployed adsl networks being increased in many market
with the introduction of adsl2+ this may no longer be as much of an issue,
but still it sides with Bert's analysis.
According to various reports (aka the IBC rumourmill) on the Alcatel/MS
system being implemented at Swisscom, channelchanging time is one of the
problems that still needs to be fixed befor launch of the service. So,
advanced compression does seem to rely on increased goplength.
Donald
----- Original Message -----
From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Wednesday, September 14, 2005 5:27 PM
Subject: [opendtv] Re: (S) DTV adaptors available for less than 50 euros in
France
> > AVC achieves higher compression than MPEG-2 in large
> > part by allowing the I frames to be spread further
> > apart. So a disruption in the signal could result in
> > more waiting time before the picture recovers.
> >
> > However, there will probably be limits set on the
> > spacing between I frames, for this reason and for
> > channel surfing too, which in turn will result in less
> > than optimal compression from AVC.
Craig Birkmaier wrote:
> Where do you come up with this stuff Bert?
By figuring out how this stuff works.
> I frame spacing is variable in both MPEG-2 and AVC. To
> the best of my knowledge the improvements in efficiency
> with AVC have NOTHING to do with increased I frame
> spacing. They have everything to do with improved tools
> for motion compensated prediction. improvements in the
> frequency domain transform used to code differences from
> the predictions, improved handling of quantization,
> especially as it relates to gradients, and improve error
> masking.
We have been though this already, Craig. The improvements
offered by AVC, some of which you listed here, can either
be used to create more perfect pictures from the same
bandwidth as MPEG-2, or if you degrade the AVC image to
where it matches MPEG-2 quality, you will achieve that
equivalent quality with *lower bandwidth*.
MPEG-2, just like AVC, depends on wide spacing of I frames
as one of its bandwidth compression techniques. That's why
MPEG and AVC are different from M-JPEG. Probably *the*
most important difference, in fact.
The better the motion vectors, the further apart the I
frames can be spaced, for a given level of moving image
quality, and the greater the compression factor becomes.
Isn't that the whole point?
So if you are expecting to see lower bandwidth with AVC
streams than with equal quality MPEG-2, you will see
increased spacing between I frames as one of the techniques
used to achieve the lower bandwidth. If you don't see an
increase in I frame spacing, then you also won't see as
much of a bandwidth reduction.
Bert
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