[opendtv] Re: Food for thought

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Wed, 21 Feb 2007 07:41:36 -0500

At 8:47 PM -0500 2/20/07, Manfredi, Albert E wrote:

"Each HD channel is likely to need bandwidth equivalent to that required
by three or four standard definition channels even when using more
advanced MPEG-4 technology."

They quote 19.5 Mb/s as their max rate.


Here you go again drawing unfounded conclusions from the thin air.

Once again I must note that SD and HD are NOT fixed points along a scale. There are many variables, INCLUDING the level of pre-filtering that is used to keep the encoders from choking.

There was NOTHING in the BBC report that allows one to draw the conclusion you made.

Yes, using the SAME codec, a good HD encoding MAY take up the space of four SD encodings. But I can just as easily make the claim that a GOOD SD encoding can take up the space of ONE GOOD HD encoding.

The reality is that an encoder may produce peak bits rate for BOTH SD and HD content that will exceed 1.5 Mbps. When we are multiplexing multiple programs into 19.5 Mbps something has to give. What the BBC was really saying is that consumers will accept the level of degradation of SD video when four programs are multiplexed into one 19.5 Mbps pipe, but for HD they expect better quality, which will require the entire 19.5 Mbps to maintain that level of quality.

Bert continues down this illogial path:
In short, H.264 when transmitted in practical transmissions might not
give you the theoretical best compression people talk about. As we
discussed in the past, more than once. If one HD channel, to look good,
takes up the space occupied by 3 or 4 SD channels, that is just what
H.262 requires. In fact, some stations transmit 3 SDs *and* and HD
channel, with H.262.

My guess is that you can degrade the HD channel more gracefully with
H.264, or you can tell the users to just sit tight while the station
syncs up.

My guess is that you just like to hear yourself type Bert. Stations that multiplex 3 SD and one HD signal into a 19.3 Mbps pipe are degrading ALL of these programs substantially. I discussed this with respect to the WCJB-2 SD feed of the CW network in my post last night.

What difference would using H.264 make versus MPEG-2? The quality of all signals would improve, as each program would have more bits to deal with peak bit rate requirements and the level of pre-filtering could be reduced. How much more? The reported evidence is in the range of 15% - 50% depending on the nature of the source; 25% is probably a safe "average."


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