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 idea. ----- 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? > ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.