[FFmpeg-user] Glossary: Nyquist

Mark Filipak (ffmpeg) markfilipak at bog.us
Sat Oct 3 03:47:57 EEST 2020


On 10/02/2020 02:15 PM, Eduardo Alarc├│n wrote:
> El vie., 2 oct. 2020 a las 7:34, Anatoly (<anatoly at kazanfieldhockey.ru>)
> escribi├│:
> 
>> On Thu, 1 Oct 2020 20:25:30 -0400
>> "Mark Filipak (ffmpeg)" <markfilipak at bog.us> wrote:
>>> When sampling an analog voltage, resolution is the ability to resolve
>>> voltage value within a certain period of time (i.e. within a given
>>> channel bandwidth). When sampling a visual field of view however,
>>> resolution is the ability to resolve stationary edges that vary
>>> spacially, going from light to dark or dark to light. It's the same
>>> gaussian energy transfer issue (i.e. that transferring energy
>>> requires time) with the same signal-to-noise issues and the same
>>> handy half-power shorthand, but it applies to ... wait for it ...
>>> human eyes! Human eyes resolve edges only so good, even totally black
>>> abutting totally white. There is nothing you can do about that, and
>>> staring at the edge doesn't bring it into higher resolution. However,
>>> if the image source itself has fuzzy edges because it was sampled at
>>> lower than Nyquist, then the result in our brains is a double
>>> gaussian, the first from the CCD and the second from our eyes. It's
>>> that double gaussian that is avoided by spacially sampling at higher
>>> than 2x the display resolution.
>>
> I think this is wrong, Nyquist theorem or principles apply to sampling of a
> signal, nothing to do with eyes or brain, ...

Not correct. It's not biological, but it does apply to biology, specifically, to the eyes.

Okay, 2 thought experiments:
1 - Imagine a film scanner sampling a film frame line by line. Isn't the scanner making a signal 
that the sampler uses to make samples? If you think that Nyquist applies only to signals, then, 
there's your signal.
2 - What about a CCD array that makes all the samples at one time? Doesn't that expand the signal to 
2 dimensions?

>... it describes the minimum sampling rate ...

Nyquist has nothing to do with rate. If Wikipedia says otherwise, then Wikipedia is wrong. Rate only 
applies to broadcast media like television. Rate determines bandwidth needed (which may be more than 
what's allowed for channels), but bandwidth is meaningless in a film scanner or a camera because 
they are not broadcast.

What Wikipedia may be referring to is the bandwidth needed for digital TV. That really has nothing 
to do with Nyquist. But then, Wikipedia isn't written by experts, is it? I can see how it would 
mislead you.

By the way, I've given up trying to make an illustration of 2-dimensional Nyquist sampling. It's too 
hard.

-- 
What if you woke up and found yourself in a police state?
African-Americans wake up in a police state every day.


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