[opendtv] Re: Some results - 1080p @ 60 is Next?

  • From: Mark Schubin <tvmark@xxxxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Fri, 04 May 2007 13:51:25 -0400

Ah!

Interlace has motion artifacts and vertical-temporal resolution artifacts even without motion. In NHK's subjective testing with still images, they found 731 lines total progressive to offer subjectively similar resolution to 1125-lines total interlaced. At the time, 1125 was said to be 1052 active, which would correspond to about 684 active lines progressively. Many other studies have found significant resolution loss for interlaced systems.

Against that, all that 1080i offers is 1920-pixel resolution instead of 1280, but, if you calculate the maximum possible theoretical contrast ratio (ignoring diffraction) offered by a 1920-pixel system at 1280 pixels, it is just 41%. All of the contrast above 41% comes from lower resolutions. At even f/4, red-light diffraction will allow only about 60% of what's left at 1280 lines, or a contrast ratio of a maximum of about 24%. Throw in real-world lenses and filtering, and there's very little area under the MTF curve between 1280 pixels per line and 1920 pixels per line. Perceived sharpness is proportional to the square of the area under that curve.

If, however, the IMAGER were 1280x720, the MTF at 1280 pixels would be zero. That's why it's important to acquire at 1920 pixels (or more).

TTFN,
Mark


John Shutt wrote:
Mark,

No, not misreading, I posed my question backwards. I'm having a bad day I suppose.

I meant why was 720p50 was preferred over 1080i25 even in the uncompressed domain.

John

----- Original Message ----- From: "Mark Schubin" <tvmark@xxxxxxxxxxxxx>

You seem to be misreading again. There is no point I can see where the third bar (1080i) was higher than the second (720p). At very high data rates, the first bar (1080p) did better than the second, but the third was never better than the second.

TTFN,
Mark


John Shutt wrote:
Mark,

Do you have an explanation as to why uncompressed 1080p50, downconverted to uncompressed 1080i25, then upconverted back to 1080p50 again at the display was preferred over 1080p50>720p50>1080p50?

Was there perhaps a Nyquist filtering added at the 1080i conversion stage that killed resolution for the subsequent upconvert?

John




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