[opendtv] Re: Europe now debates i vs p
- From: Craig Birkmaier <craig@xxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Mon, 13 Sep 2004 10:43:04 -0400
At 9:16 AM -0500 9/13/04, Doug McDonald wrote:
>The signal above 25 MHZ is NOT aliasing. It is real signal.
It may be real, however, it did not come from any commercial HD
camera that may be in use today. Existing cameras typically begin to
roll of the high end frequency response at about 20 MHz, and there is
NOTHING left at 25MHz. A film-to-tape transfer could do better, since
the source is oversampled, however, it is very unusual for film
content to contain such high frequencies because of the optical
filtering that most cinematographers use.
>I think it fair to say that the real signal has content to
>30 MHz. And why should it not? These are the sharpest and
>clearest pictures I have ever seen on a TV ... the static
>logos and score blocks are extremely sharp on the screen.
>Both ABC and Fox apparently take pride in having the sharpest HDTV
>pictures around, and they most certainly do, at least on my TV.
What you most likely are seeing at 25MHz is some form of compression
artifact. The input source would be filtered to roll-off at lower
frequencies (note that the encoder may do further spatial filtering
to roll-off details when it is stressed. Compression artifacts can
create very high frequency transitions, including the worst case, a
black to white transition at a block boundary.
The main reasons that the ABC and Fox pictures look so good can be
attributed to the following factors:
1. The absence of interlaced acquisition which can adversely impact
compression efficiency, while allowing for small area artifacts that
must be encoded.
2. Improved filling of the wanted frequency spectra with good
information (the MTF argument).
3. Reduced stress on the encoder, which is working with coherent
frames at a lower spatial resolution.
Finally, there may be some advantage in the fact that Doug is looking
directly at the 720P signal that is broadcast, rather than having it
re-sampled for local display.
Regards
Craig
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