[opendtv] Re: Public M/H information

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Fri, 19 Dec 2008 07:24:08 -0500

At 11:19 PM -0500 12/18/08, Tom Barry wrote:
Interesting. I'm not sure 240p will be enough even for mobile. It has to compete with what I can get already as part of my Sprint PCS Vision plan and is I think already less than the rez of an iphone.

Supposedly at a viewing distance of 3 screen heights the sweet spot for human vision is about 1080p.

This is only true for a screen that is VERY LARGE. In tests of preferred viewing distance the RAI found that most human observers would not choose to sit 3 picture heights from a screen until that screen was about 117" diagonal.

I guess I view my 2" high phone screen (sideways) at maybe 8 screen heights viewing distance so it would peak out at about a resolution of 1080p * 3/8, or about 405 lines even for that very small screen. I'm guessing even smart phones will soon have an average resolution of above 240p.

This is the correct form of analysis. 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. In a few pathological cases, a stimulus as small as 40CPD may be observed. A classic example is a red blinking light against a black night sky in the distance when you are driving; the temporal component is probably important in the detection of such a small stimulus. Compare this to the same blinking light against the normal blue daylight sky.

So what is critical is the delivered CPD at the desired or typical viewing distance. This is the main reason that 720P is more than adequate for most DTV displays - i.e. people typically sit 9-10 feet from the display so it takes a very large display to see the extra detail in a 1080P display.

I would also note that we are now moving into the era of display oversampling. This not only hides the raster, but it also makes content that is NOT Nyquist filtered look less aliased. Thus we see many computer displays moving to 110 DPI or greater and small displays such as smartphones with even higher pixel densities. For example, my iPhone has a 3" X 2" display that is 480 x 360, or 160 DPI.

That being said, video is Nyquist filtered and typically not a sharp as other sources. A 240P video clip looks fine on my 360P display, IF it is not falling apart with compression artifacts. YouTube is 240P but their default player scales to 480 x 360. Hard to argue that people will watch THIS.

But maybe a free advertising supported mobile simulcast of the network HD broadcast could still be viable. ??

Why a simulcast. Yes for sports, but hardly anything else. Virtually all prime time programming can be viewed on demand or downloaded. Prime time for mobile applications is between 7AM and 6PM.

The time and channel business model of broadcasting is already dying. Extending this to the new mobile service is not going to change that, but it could cause the new service to be still born. If this service is to be successful it is going to take some INNOVATION...

AND MUCH MORE attention to locally produced content and the needs of local advertisers.


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