[opendtv] Re: Mediaflo Technology

  • From: Tom Barry <trbarry@xxxxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Fri, 10 Nov 2006 16:41:09 -0500

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).

TTFN,
Mark



Tom Barry wrote:

I'm not sure I understand that. I thought a properly sampled signal with only Nyquist frequencies could accurately represent the input signal even in the face of shifting the image left or right a fractional pixel. Is that different in analog?

Or is that not what you are referring to?

- Tom

Allen Le Roy Limberg wrote:

Comparing resolution of analog and digital systems on a pixel per line count
ignores the important fact that edge position resolution is not
pixel-bounded in the analog transmission. No quantum shifts as in a digital
sampled-data transmission.

Al
----- Original Message ----- From: "Craig Birkmaier" <craig@xxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Friday, November 10, 2006 11:22 AM
Subject: [opendtv] Re: Mediaflo Technology



At 11:02 AM -0500 11/9/06, Mark Aitken wrote:

In-line response.

Craig Birkmaier wrote:

14 live streaming national interest
5 live streaming local interest

50 national and 15 local services that will deliver ~20 minutes of
content per day to receiver cache.


What does local mean?



I presume that this is content that is localized to the specific
transmitter - for now it looks like Mediaflo will have one
transmitter per market served; later they claim they will build out
SFNs around markets. There is more detail about the service
opportunities in the resources on the website, however, it is
important to note that MediaFlo is NOT the operator of the system,
per se. They are offering the service to existing system operators in
each market as an adjunct to a cellular service. The operator in each
market will have a strong say in what is carried, and the ability to
partner with local content providers within that market to fill the
"local" channels.


NTSC is 'effectively' 320 x 240? Really?


You could argue that there is more horizontal resolution, but not
vertical resolution The cable guys started out with 1/2 D1 MPEG-2
encoding, which was 352 samples per line.  I looked at several sites
about the horizontal resolution of NTSC and the consensus sees to be
about 330 dots per screen width when transmission losses are taken
into consideration.

There are two factors at work here:

1. Transmission bandwidth - Ron did the math to show what is
"possible " if you can receive an NTSC signal perfectly. About 440
pixels per line. The 240 pixels vertically does not take into account
any positive benefit from "the interlace factor," which could add a
bit more vertical detail, however, with Mediaflo, the signal must be
de-interlaced for encoding as 320 x 240 progressive frames, thus it
is unlikely that any additional detail will make it through the
conversion process.

2. NTSC displays - the typical NTSC receiver has no more than 330
dots/stripes per screen width. With higher quality tubes (usually
with S-video inputs) the number of dots/stripes may increase to
approximately 450-500.

Bottom line, you are wasting bandwidth if you are inputting more than
480 samples per line into an NTSC encoder. We can use the full 704
samples per line (ATSC) if we  are encoding for DTV, but even here
this may be overkill if we don't provide enough channel bandwidth
(bit rate) to assure that the detail makes it through the encoder.

MediaFlo says that the input source for their network will be
primarily from satellite, and they expect MPEG-2 encoding at 704 x
480 or 720 x 480. If this stuff is not overly compressed, there
should be enough detail left to produce a much higher quality 320 x
240 progressive stream than would be possible if one started with
NTSC source (analog or encoded using MPEG-2).

This all brings back memories from the early '90s when we noted that
a properly encoded 480P signal (i.e. 854 x 480) would provide a very
high quality viewing experience comparable to HDTV on many consumer
displays.

How quick we forget how poor NTSC quality actually is...

Regards
Craig


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--
Tom Barry                       trbarry@xxxxxxxxxxx     
Find my resume and video filters at www.trbarry.com


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