[opendtv] Re: Mediaflo Technology

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Sat, 11 Nov 2006 08:25:28 -0400

Looks like Tom caught you here Mark.

You can't compare apples with oranges. We are talking about Nyquist limited images in both cases - i.e. video.

As we have discussed MANY TIMES in the past, the energy in these images is spread out over many pixels. For that "peaked" black line we are talking about at least three, and probably five or more. The peak will move through the digital grid just as it moves through the shadow mask of the analog TV. You are correct that the analog signal allows for improved edge positioning, but modern codecs are moving to quarter pel positioning in the encoder to deal with these issues as well. Sadly, we rarely watch analog TV on a B&W monitor, where we can take real advantage of the detail possible with an analog CRT.

By the way, a "digital display" has the ADVANTAGE in that it can also display non-Nyquist limited images for locally generated graphics.

Even more important, you have left out the all important optical filter in the acquisition device that would capture the image. This filter will assure that the energy of that edge will be spread out to allow that peak detail to move through the sample grid without aliasing.

Bottom line, it's all about filtering and the benefits of oversampling ahead of the encoder.


And one more thing - protecting the integrity of samples. MPEG quantization robs detail from an image, just like distortion and noise degrade an analog transmission.

At 4:41 PM -0500 11/10/06, Tom Barry wrote:
Mark -

I think in order for nyquist to apply properly (such that you can calc with it) you would have to have the lines fading back to white and back in a sin wave, not with sharp edges which already imply higher frequencies.

And the Nyquist limit for that for, say, at 720 pixels would have to be LESS than 720, say 718 lines or 359 complete cycles. But I believe a Fourier transform of these 359 (sinusoid) line pairs using 720 samples would indeed capture all the information, regardless of phase. So you would not have the blinking effect as something scrolled very slowly, or the confusion of motion compensation due to aliasing. And if you upscaled the subsequent sampled (but nyquist limited) image you could still get an exact representation of the original. But, again, this assumes the magnitude of the signal AT the Nyquist frequency and all higher frequencies was originally zero.

Unfortunately I'm not sure the convenient discrete cosine transform used in the various MPEG's has this property. And turning things into a 8x8 block based transform further limits the resolution to 7/8 of the nyquist frequency, much worse than my 359/360 in my example above.

Of course all this is only an approximation since all the sampling theory assumes we are talking about a periodic waveform, which in turn probably implies the real original image (at much larger resolution) really contains only integer multiples of the sampling frequency. And that isn't generally true in any real image anyway.

This means I'm still not so sure about the difference between digital and analog in this regard. It's hard to get a grip on some of this stuff.

- Tom

Mark Schubin wrote:
Consider a pair of vertical lines, one black and one white, each the same width as a digital pixel. An ideal digital system would just barely be able to reproduce them (they'd be at the Nyquist limit). An analog system of the same Nyquist bandwidth would also reproduce them, though sinusoidally instead of as black and white stripes.

Now shift the lines one-half pixel horizontally. The analog system has no trouble moving the sine wave by a 90-degree phase shift. The digital system, however, is now all gray. Each pixel is getting both a black value and a white value.

Thus, Ron's 440 pixels per line actually compares favorably to a digital system with 880 (which is more than Rec. 601's 720).


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