[opendtv] Re: 20060117 Mark's (Almost) Monday Memo

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Tue, 24 Jan 2006 08:49:21 -0500

At 5:23 PM -0500 1/21/06, Albert Manfredi wrote:
>I believe you are referring to the Nyquist limit here. That
>does apply to images, of course. But Shannon's Law refers
>to the limiting bit rate that can be transferred in a given
>channel width, as a function of signal to noise ratio. The
>analogous limit in image compression might be stated as
>"given the pixel count, frequency content, color content, of
>a given image, what is the smallest file size that can be
>achieved, in theory (i.e. not restricted to any existing
>algorithm), with no loss of image information?

Sorry for the confusion. I was thinking about Shannon's sampling 
theorem, which does involve the Nyquist limit.

This raises an interesting point. Shannon's Law does overlap with 
video compression theory, as various forms of lossless entropy coding 
are used to deal with the huge number of bits that are delivered when 
compressing video.

As you correctly assert, there are well known limits on how much you 
can crunch any bitstream without introducing errors.  Lossless video 
compression may be an important tool if the application demands a 
lossless coding system,. say for acquisition or post production 
special effects work. But this is largely irrelevant to emission 
coding; most digital acquisition systems use lossy coding today; HDV 
uses MPEG-2 long GOP coding.

Modern video compression techniques rely upon the ability to 
selectively lose some information, based on the "theory" that the 
human visual system cannot process all of the information in motion 
imaging streams, and that small errors will be difficult to perceive.

In this context, Shannon's Law has little relevance to the 
discussion, but his sampling theorem is highly relevant, as it 
provides the basis for  determining how accurate the initial sampling 
is, and how much entropy there is in any given image sequence, that 
can be eliminated.

You can UNSUBSCRIBE from the OpenDTV list in two ways:

- Using the UNSUBSCRIBE command in your user configuration settings at 

- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word 
unsubscribe in the subject line.

Other related posts: