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

  • From: "Alan Roberts" <roberts.mugswell@xxxxxxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Sat, 11 Dec 2004 17:06:55 -0000

The Nyquist limit is exactly 50% of the sampling frequency, you have to
filter to lower that this to get smooth motion. Typically, digital systems
have analogue filters to prevent frequencies above 0.45 of sampling
frequencies getting into the system. The ITU recommended filters for 601 and
709 sampling are both flat to 42%, then dive steeply to -12dB at 50%,
allowing some aliases to creep in. Conventional wisdom is that this is a
good idea, I don't go along with it fully because those aliases cause havoc
in compressors, even at low levels.

So, you need 20 pixels to show 18 lines or 9 cycles. The filtering refuses
to allow such frequencies to start/stop suddenly, so you get confusion at
the ends of a block of such frequencies, but it all looks fine once the edge
effects have passed. It's interesting to work out the actual pixel pattern
for such frequencies, and to see what happens when it all moves, I've done
so and the results are fascinating, it explains why aliases are such a bad

----- Original Message ----- 
From: "Tom Barry" <trbarry@xxxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Saturday, December 11, 2004 4:32 PM
Subject: [opendtv] Re: Why Europe should choose 720P for HDTV

> Mark -
> How much do you have to filter so that an image can move smoothly across
> or down a fixed pixel display 1/2 pixel at a time without aperture
> effect artifacts.  Is it just to the Nyquist limit, or more?

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