[opendtv] Re: Finally anamorphically compressed 480i

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Tue, 25 Jan 2011 07:50:20 -0500

At 7:12 PM -0600 1/23/11, Manfredi, Albert E wrote:
But it shouldn't! How hard would it be to take that pillarboxed 1080i60 material and transcode it to an identically pillarboxed, anamorphically squeezed, 704 X 480i60 stream? It doesn't seem like it should be difficult or ugly. 

One more time

 This is a messy conversion that involves deinterlacing the 480i
 source, upscaling to 1440 x 1080 then re-interlacing. Then this
 must be undone at the station...
Interlace is a crude form of compression that cannot easily be undone. When you deinterlace the 480i you lose important information and create some false information that interferes with the digital entropy encoding process. The scaling up to 1440 x 1080 cannot create all the information in the source that would be there if the source were acquired at 1080i or even better an oversampled progressive format. When you reinterlace at 1080i you are concatenating another analog compression process; this is then digitally compressed with less than ideal efficiency because of the use of interlace.
When the 1080i source gets to the station it is decompressed to baseband and deinterlaced again so it can be down converted to 480i, then reinterlaced and compressed again.
Now how many concatenation steps did the source go through? And the deinterlacing and reinterlacing steps all lose useful information and replace it with false information. 

This is what I meant by messy.

Can you explain a better way to do it?

Regards
Craig



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