[opendtv] Re: (S) DTV adaptors available for less than 50 euros in France
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 16 Sep 2005 16:11:22 -0400
Craig Birkmaier wrote:
> > B frames are interpolated between either I and P
> > or P and P frames. So having accurate motion
> > vectors affects B frames too, although only
> > secondarily.
>
> Huh?
> B frames are NOT interpolated. They are no different
> than P frames EXCEPT that the motion vectors are
> referenced to two frames - before (I) and after (P)
> - rather than only to the I frame that occurs before
> the P frames.
Close, Craig.
Try this:
----------------------------------
http://bmrc.berkeley.edu/frame/research/mpeg/mpeg_overview.html
What do B-pictures buy you?
A. Since bi-directional macroblock predictions are an
average of two macroblock areas, noise is reduced at low
bit rates (like a 3-D filter, if you will).
....
B pictures are interpolative in two ways: 1. predictions
in the bi-directional macroblocks are an average from
block areas of two pictures 2. B pictures "fill in" like
a digital spackle the immediate 3-D video signal without
contributing to the overall signal quality beyond that
immediate point in time. In other words, a B picture,
regardless of its internal make-up of macroblock types,
has a life limited only to itself. As mentioned before,
B picture energy does not propagate into other frames. In
a sense, bits spent on B pictures are wasted.
---------------------------------
> The actual improvements in efficiency have been measured
> in the range of 25% to 50%, depending on the source
> material and tools used. GOP size has almost nothng to
> do with it.
This makes no sense. The largest frame is the I frame.
So obviously the goal is to transmit I frames as seldom
as possible. You don't want to waste an I frame to achieve
the required frame rate unless there's good reason to.
GOP length is directly related to frequency of I frames.
What about this is ambiguous?
Bert
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