[opendtv] Re: Finally anamorphically compressed 480i

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Sat, 29 Jan 2011 11:25:32 -0500

At 7:58 AM -0500 1/28/11, John Shutt wrote:
Craig, the scaling is done in frame mode, because all MPEG-2 compression is frame based. There is no field encoding in MPEG-2, which is why interlaced source material compresses less efficiently than progressive source material. You end up trying to compress frames with interfield motion within them, making it harder for the encoder to find matching macroblocks from frame to frame.
As a result, what gets spit out of the scaler is progressively scanned, which is then broken up into interlace if necessary for the final baseband output, or kept progressive if that is the target output. 

Not exactly...
Look at the H.262 specs and you will find all kinds of tools for encoding native interlaced source. 

For example:

4.1.2   Coding interlaced video
Each frame of interlaced video consists of two fields which are separated by one field-period. The specification allows either the frame to be encoded as picture or the two fields to be encoded as two pictures. Frame encoding or field encoding can be adaptively selected on a frame-by-frame basis. Frame encoding is typically preferred when the video scene contains significant detail with limited motion. Field encoding, in which the second field can be predicted from the first, works better when there is fast movement.
The major problem with this with MPEG-2 is that the most common macroblock structure can deal with the luminance signals from each field independently, but the color difference signals from each field are combined so you typically get less color resolution and smearing when using the 4:2:0 coding format which is specified by ATSC.
4:2:0 works great with progressive source, as everything in the frame is from the same time slice.
The sad fact is that several key Japanese companies saw the handwriting on the wall when ISO began work on MPEG-2 (Note that MPEG-1 only had frame based tools). They wanted to entrench (and profit from) Interlace and other legacy tools that were either already in the public domain or about to be. So they created tons of new IP to encode interlace and dominated the ISO MPEG process to get all of this stuff into h.262. To make matters worse, they did this again with h.264, which SHOULD NOT have incorporated any tools for interlace.
There is much to be said about deinterlacing BEFORE entropy coding, which would have been the best way to handle the DTV transition, as the deinterlacing could have been done with high quality professional gear at the source of the emissions (broadcasters, networks cable head ends). 

Alas, Interlace was entrenched for several more decades...

Regards
Craig


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