[opendtv] Re: Panasonic's AG-HVX200
- From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
- To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 28 Apr 2005 14:20:39 -0400
Tom Barry wrote:
> But mtf must be more than just a multiplier factor.
> If I take a sinusoidal signal and multiply it by .5
> then the amplitude is only half as much but no
> information has yet been lost, barring quantization.
> I could multiply it by 2 later and get it back. It
> is only when errors (random info) are introduced
> that information goes away. But I'm not sure how
> this figures in gain or mtf and I'm not sure how
> that part is supposed to be measured.
>
> I realize there are many sources of errors, A/D
> noise, quantization noise, various optical,
> electronic, and digital filters etc. and each
> contributes in complex ways to lowered mtf (either
> case). But I still can't quite visualize what is
> being measured when "contrast" is lowered.
I don't know if you're more comfortable with audio,
Tom.
Audio analogy: the gain-bandwidth curve. An amp might
provide 100 watts of output at 1 KHz. Now you crank
up the frequency, and you might find that for an
input signal at 30 KHz, that same amp can only manage
50 watts. Crank the frequency up further, and pretty
soon the output will drop to 0.
At 1 KHz, where the amp gives outputs of 0 to 100
watts, the amp can reproduce 20 dB of "contrast."
That is, the difference between softest and loudest
sounds it can reproduce is 10*log(100) or 20 dB. But
at 30 KHz, the difference is only 10*log(50), or 17
dB.
The higher the frequency, the less contrast that amp
can reproduce. So someone can tell you his audio
gear reproduces 100 KHz, but at what output level?
Perhaps only 5 watts by the time you're up to 100
KHz, or 7 dB of "contrast," compared with 20 dB of
contrast at 1 KHz. The amp with the flattest
bandwidth will sound better (sharper) than one where
the bandwidth curve droops at higher frequencies.
(Beyond 20 KHz or so, people don't care. Dogs might.)
With a lens, film, sensor, etc., it's the same thing,
but in two dimensions. If you measure the maximum
lines/mm the medium can resolve, the next question is
"but at what contrast level?" Just like the max
frequency the amp can reproduce, "but at what output
level?" In images, this number will vary over the
area of the image.
If your lens, say, can "resolve" 90 pairs/mm, just
how much contrast are you getting between the black
and white lines? If, in the 90 lines/mm chart, the
black and white lines are both mostly gray, then even
though in principle the image detail is there, the
picture will not look very sharp because the small
detail is washed out.
So, the MTF curve is to images what the
gain-bandwidth curve is to audio. Contrast vs
frequency.
I'm not sure how the error questions crept into this,
but it seems to me that once the contrast has been
lost by the medium, it can't be recreated accurately
anymore. Just pushing the gain after the fact will
amplify noise along with signal.
Bert
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