[opendtv] Re: Public M/H information

  • From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
  • To: <opendtv@xxxxxxxxxxxxx>
  • Date: Fri, 19 Dec 2008 18:07:38 -0500

Craig Birkmaier wrote:

> Human visual acuity is typically measured in cycles per
> degree of viewing field. 22 CPD is considered to be a sharp
> image. The spread of human visual acuity across the population
> puts the upper end for normal vision in the range of 22 to 30
> CPD.

That's fine, but what does it mean?

Since each visible cycle is represented by a dark and a light line, or
two pixels at least, you're saying that human visual acuity goes up to
about one arcminute, or 60 individual lines per degree. This is what
I've read elsewhere, for someone with 20/20 vision. A lower limit of
visual acuity sometimes quoted is 1.6 arcminutes.

If you use the 1 armin rule, and you limit the vertical pixel count to
240, then you'll need to be a little over 14 picture heights away from
the screen, to see a smooth image.

If you use the more relaxed 1.6 arcmin rule, then you can be 9 picture
heights from the screen.

Either way, it looks like 240 pixels in the vertical dimension is not
overly generous, for vehicle applications. How far are you from a laptop
screen, for example, under normal conditions? Isn't it more like 3
picture heights at most?

I expect the answer is that people in moving vehicles won't expect the
very best images, just as I don't, when I watch the "Retro Television
Network" on our ABC affililate's -3 subchannel.

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