[opendtv] Re: Mediaflo Technology
- From: Mark Schubin <tvmark@xxxxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 14 Nov 2006 21:22:27 -0500
Craig Birkmaier wrote:
The same camera with the same lens will have the same MTF, whether it
is putting out 1920 x 1080 or 1280 x 720. It will have more contrast
at 1280 than at 1920, but that contrast should be the same whether
the camera is in 720 mode or 1080 mode (not counting some minor
losses in scaling).
I think I understand. You get a contrast benefit with the 720P output,
due to resampling. But this benefit is there in both 720P and 1080i/p
modes.
I think you are saying the MTF of the 720P signal is improved because
of the oversampling, bringing it up to the same or similar level as
the 1080i/p signal.
Is this correct?
Not exactly. If you have a spreadsheet program, make a column with 0 to
180 degrees (or 0 to pi, if your program does trig functions in
radians). Put in as many steps as you like. Then, in the next column,
calculate the sine of the value in the first column, and divide the
result by the value in the first column (for 0, just enter 1). Now
graph the second column. You will have a slope that represents the MTF
due to SINC-function filtering.
Note that nowhere in the above paragraph have I said anything about
resolution. You simply have a curve going from 1 at zero to 0 at 180
degrees (or pi radians). At 90 degrees (or 0.5 pi radians), it will
have a value of roughly 0.64.
The 180-degree point is at the limit of whatever number of samples you
have in the imager. If there are 1920 samples, then you have zero
output at 1920, but 0.64 (64% of maximum contrast) output at 960. If
there are 1280 samples, you have zero output at 1280, but 0.64 at 640.
1280 is 2/3 of 1920. At 120 degrees or 2/3*pi radians, the output is
0.41. So a 1280-sample camera would have zero output at 1280, but a
1920-sample camera would have 0.41 output at 1280. That's the benefit
of oversampling. A 3840-sample camera would have 0.83 output at 1280.
The more you oversample, the less restrictive your filtering has to be.
TTFN,
Mark
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