[opendtv] Re: Optimizing the system
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Wed, 22 Dec 2004 09:51:21 -0500
At 10:53 AM -0500 12/21/04, John Shutt wrote:
>Perhaps. I've seen that effect mostly with LCD projectors, and it is truly
>annoying.
Yup. I agree that this is NOT the only problem, but it may be a
contributing factor.
>
>However, That effect does not surround person's head as it moves across the
>screen. But a person's head is usually covered (except in my case) with
>hair, and the fine hair moving against a more or less stationary background
>results in macroblocks with much detail and little correlation from frame to
>frame, and so usually are the first to be DCT truncated by the encoder,
>causing the cloud of bees.
The "official" term for this MPEG artifact is "Mosquito Noise,"
however, I have heard the swarm or bees analogy many times. I wrote
extensively about these issues a decade ago, as the MPEG-2 standard
was being finalized. I have produced a number of examples (still
images demonstrating this problem, that have been published in
Videography and Digital Television magazines.
An explanation of the EXACT CAUSE of this problem is in order. The
problem is most acute when there is a high contrast transition within
a DCT block. The DCT transform will produce a number of coefficient
that are VERY CRITICAL, because of the contrast within the block; it
will also produce a number of coefficients that are associated with
the areas near the actual transition. The problem comes when we start
to quantize the coefficients. As soon as we start to make the samples
more "alike" we distort the samples with the highest contrast levels,
followed by those will less contrast. If you look at something like a
block that contains an edge of text, it will quickly become a buzz of
distorted samples that contain information from the contrast
extremes. This usually manifests itself as the buzz around the high
contrast detail.
Other less demanding scenes (like hair) may still exhibit the same
problem when they are quantized too much. It's just another case
where excessive quantization can cause MPEG-2 to introduce artifacts
into the imagery. There are two potential solutions:
1. Provide an adequate channel for MPEG-2 content, so that excessive
quantization is not necessary.
2. Use better compression tools that are not subject to this
limitation of the DCT transform.
Here there is some good news: The MPEG-4 AVC codec does a much better
job in this area, as it does not use the DCT as the basis for block
coding, and its replacement, allows for better control over the
allocation of bits to areas of the image that are challenging. The
nominal block size is 4 X 4 rather than 8X8, which further
"localizes" the energy within a block. The transform is similar to
the DCT but integer based, and there are special prediction modes for
different kinds of energy (dominant H, dominant V, gradients, etc.
And there are deblocking filters, which can be used when the encoder
is forced to use excessive quantization.
>
>If you have access to an MPEG encoder, you can make the effect more and more
>pronounced as you encode the same scene over and over with decreasing
>bitrate. HD or SD, the only difference is the relative size of the
>macroblocks to overall picture size, and the relative size of the 'bees.'
>As I said, the effect is constant on MSNBC's ticker at the bottom of the
>screen when viewed from DirecTV. The edge of each letter and number is
>surrounded by a similar cloud of truncated macroblocks.
Yup.
Actually, Mosquito Noise is the FIRST MPEG-2 artifact to become
apparent (other than the lousy 4:2:0 encoding of the color difference
signals for interlaced source, which sucks even when there is little
or NO QUANTIZATION. When the quantization becomes too sever, the
entire DCT block takes the DC value, and we begin to see blocking
artifacts.
>
>However, I suppose the only real way to tell for sure is to meet Tom at
>***The Technology Retreat*** and view material together to discuss what is
>being seen. ;^)
Tom promises a very good demo. I assume and hope that there will be
some comparisons specifically of the issues described here.
Regards
Craig
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