[opendtv] Re: Chroma Formats and Why Europe should choose 720P for HDTV

  • From: "John Shutt" <shuttj@xxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Mon, 20 Dec 2004 09:19:26 -0500


Our digital channel has two subschannels, one is the PBS HD service, one is 
a simulcast of our analog channel that was, until recently, mandatory.  We 
receive the PBS HD service from PBS pre-encoded as an ATSC bitstream.  We 
had to reduce the bitrate of the 1080i video to make room for our SD 
subchannel.  When we first did so, the macroblocking during almost any 
motion was very annoying.

So we transcoded the 1080i video to 720p, and at the same encoding bitrate 
there was much less macroblocking.  Therefore, I would have to see side by 
side comparisons of 720p and 480p material at the same bitrates before I 
would believe that 480p and 720p would macroblock similarly in challenging 
scenes.  In my experience, I would predict that the 480p would macroblock 
much less using a real time encoder.  Perhaps using off-line encoding, a 
good compressionist could tweak the 720p adequately for theatrical material.

Also, at least to my aging engineer eyes, I prefer sharp 480p to soft 720p 
or 1080i.  I look at the SDI output of our studio cameras, where the SDI 
digital video is created from the RGB of the camera, and the results are as 
pleasing to me as softened and bit starved broadcast 1080i.  But then, 
uncompressed SDI 4:2:2 480i video has a bitrate of 270 Mbps!


----- Original Message ----- 
From: "Bob Miller" <bob@xxxxxxxxxx>

> What is the trade off if we just compressed 720P more and pre filtered
> it some so that the bit rate would be the same as a 480P for broadcast
> anyway? You would lose no more in difficult frames than you do with 480P
> but with simpler scenes you would have true 720P. That is let the codec
> do the choosing instead of pre choosing 480P.

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