[opendtv] Re: Video compression artifacts and MPEG noise reduction

  • From: John Willkie <johnwillkie@xxxxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx, opendtv@xxxxxxxxxxxxx
  • Date: Thu, 19 Jun 2008 18:29:20 -0700 (GMT-07:00)

I've been told that the hits bits are just passed on, and that the customer 
base and the proposition -- is that you only need to decide which programs to 
pass on, and no need for processing.

The people that I know that use Pathfire also just pass on the bits.  By the 
way, I don't Pathfire can offer HD programming.  Yet.

John Willkie

-----Original Message-----
>From: Craig Birkmaier <craig@xxxxxxxxx>
>Sent: Jun 19, 2008 3:41 AM
>To: opendtv@xxxxxxxxxxxxx
>Subject: [opendtv] Re: Video compression artifacts and MPEG noise  reduction
>
>At 10:51 PM -0700 6/18/08, John Willkie wrote:
>>yeah, the box, fool, is practically useless for that, since, uh, 
>>NOBODY takes a compressed MPEG-2 stream back to baseband, then uses 
>>this filter, then recompresses it. 
>>
>>And a "prefilter" would imply that it goes before any compression, 
>>not in the middle of a decompression/compression daisy chain.  Kind 
>>of hard at the beginning of a chain to know the noise that would be 
>>encountered in the middle of the chain.
>>
>>Indeed, taking a compressed MPEG-2 stream down to baseband would 
>>actually impose noise JUST BY ITSELF since MPEG-2 compression is 
>>lossy and asymmetrical.  So, you'd be lucky if the "prefilter" was 
>>able to remove the noise caused by the decompression to baseband.
>>
>>The market for this device, Bert, is most likely YOU.  Let us know 
>>how you fare with it.
>>
>
>Actually John, there are many situations in which a broadcaster or 
>cable head end might be trying to clean up a compressed feed then 
>re-encoding it for emission.
>
>Many cable systems utilize the HITS (Headend In The Sky) service.
>
>http://www.hits.com/
>
>This service (I believe it is operated by Comcast) compresses a wide 
>range of cable networks and distributes them to cable head ends via 
>satellite.
>
>In most cases these feeds are not decompressed, however they may be 
>groomed by devices like the Cherry Picker to cram more stuff into a 
>cable multiplex.
>
>Because of the high cost of satellite bandwidth, many syndicated 
>programs are now distributed to broadcast facilities via satellite as 
>compressed MPEG-2 bitstreams. And then there is the age old practice 
>of capturing the broadcast signals from other stations in a market to 
>source clips for use in newscasts (with permission of course!). So it 
>is becoming far more common for the feeds coming into a station to 
>arrive as compressed MPEG-2 bitstreams.
>
>The MNR mode of the Algolith device is specifically designed to deal 
>with quantization noise, also know as mosquito noise. This type of 
>distortion is much more predictable than the random/Gaussian noise 
>found in uncompressed sources. It typically occurs at the boundaries 
>of high frequency details, so a clever algorithm can look for these 
>details then process the distortions around them.
>
>You are correct that MPEG-2 creates "noise," but the noise is 
>correlated to image detail, not random, and thus may be reversed to 
>some extent if the damage is not too severe.
>
>And yes there may be a consumer market for devices that undo the 
>damage of video compression. Some of this was incorporated into h.264 
>decoders (the deblocking filter tools).  And h.264 dropped the 8 x 8 
>DCT quantizer entirel, replacing it with a scalar quantized that is 
>less complex and more accurate:
>
>
>http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/76/27384/01218192.pdf?temp=x
>
>Summary: This paper presents an overview of the transform and 
>quantization designs in H.264. Unlike the popular 8/spl times/8 
>discrete cosine transform used in previous standards, the 4/spl 
>times/4 transforms in H.264 can be computed exactly in integer 
>arithmetic, thus avoiding inverse transform mismatch problems. The 
>new transforms can also be computed without multiplications, just 
>additions and shifts, in 16-bit arithmetic, thus minimizing 
>computational complexity, especially for low-end processors. By using 
>short tables, the new quantization formulas use multiplications but 
>avoid divisions
>
>So h.264 provides a significant improvement in arithmetic accuracy 
>and the visibility of artifacts is reduced by acting upon smaller 4 x 
>4 regions of the image.
>
>So my educated guess is that products like the Algolith device will 
>not find their way into TV receivers. The shift to improved 
>compression techniques will make this unnecessary. And we can hope 
>that broadcasters and other program distributors have learned that 
>excessive compression is
>
>BAD!
>
>Regards
>Craig
> 
> 
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