[FFmpeg-user] Glossary: Nyquist

Anatoly anatoly at kazanfieldhockey.ru
Tue Oct 6 04:29:45 EEST 2020

On Sun, 4 Oct 2020 22:52:19 -0400
"Mark Filipak (ffmpeg)" <markfilipak at bog.us> wrote:

> Correction:
> "if your sampling frequency exceeds Nyquist" was "if your sampling
> frequency exceeds Nyquist/2" (which was an inadvertent mistake).
> Sorry.
> On 10/04/2020 09:44 PM, Anatoly wrote:
> > On Sun, 4 Oct 2020 10:37:41 -0400  
> -snip-
> > Are you watched "Part 2" ?
> > https://www.youtube.com/watch?v=ht4Mv2wIRyQ
> >   
> >> I don't think spacial image resolution is
> >> related to frequency at all.  
> > Then watch mentioned above video at 8:07
> > He didnt explain this, but those plots actually is so called
> > frequency responces (attenuation vs frequency).
> > What do you see on upper plot there?  
> What the upper graph shows is the frequency response of the serial
> bitstream from the photosite array through the analog-to-digital
> converter.
Are you watched video with sound OFF? Author explicitly says that upper
plot is for optical system and lower is for electronics.
> Nyquist does indeed apply to that, 
So then what is "optical Nyquist" author talks about?
 Pictures are not made of sine waves. Coded
> pictures are DCT encoded but Nyquist has nothing to do with that part
> because DCT is done entirely in the digital domain, after sampling,
> and time is not a factor in the digital domain.
Then space (pixel numbers) is factor. Splitting continuous image into
discrete pixels is sampling.
> The sampling system limits resolution, yes. And that limit is Fo/2,
> yes. But pictures don't have a Nyquist frequency, no matter what
> number of samples/line or lines/frame. And pictures are not made of
> sine waves. You are getting lost in the camera design.
Hmm, btw I've worked on repairing and adjusting cameras at least for
five years but seems that it is not enough...
> > X axis is "spatial frequency / line pairs" as distance between two
> > pairs of lines of picture of alternating black and white lines.  
> (line pairs)/(picture height) -- I assume "line pairs" means the
> horizontal distance between vertical lines
No, pairs is literally - pairs - two lines together. This is because
only alternating lines can produce frequency. Lines of same colour
produce DC. Take a look at test pictures that used to check resolution.
> -- is a pretty odd
> frequency scale, and calling that a spacial frequency is pretty
> bogus. 
There'no need to assume or interpret, just take literally what author
wrote and says, or say that you disagree with author of that matherial.
>The bottom line is this: Nyquist applies to serial
> analog-to-digital conversion frequency, not to the ultimate
> resolution. It says that if your sampling frequency exceeds Nyquist,
not sampling frequency, but signal frequency. That's why there is
> you're going to get aliasing.
yes, kind of some interference, but at electronics part, frequences over
Nyquist may be only a noise, at optics side to me thats does not
harm moving pictire and I prefer extra sharpness over presence of slight
interference. It may harm still pictures yet.
> That's the kernel of the presentation
No, kernel is that optics and electronics apply multiplying slopes to
frequency response of camera, thus reducing camera resolution.
Let's say 960 pairs of alternating black-white stripes (1920 pixels) may
still be reproduced, but they will be far less contrast than 480 pairs.
> and you are misinterpreting what is being presented.
You free to understand it the way you want,
But note that I call nothong "bogus" or "odd" in that material and
I've used same terms as an author even before you bring up this video.
> In other words, the Nyquist frequency is a function of each
> particular camera and the frequency at which that camera does
> analog-to-digital conversion.
> The premise that to get 720x480, frames should be sampled at 1440x960
> is bogus. I've read that so many times that I put it into the
> glossary without really thinking about it. But it's wrong.
> Oh, and did I say that pictures are not made of sine waves?
Yes, many times. I've seen great pictures that was made many years
ago, just of oil paint and canvas.
And let me stop discuss it for now because it greatly reduces S/N ratio
of this list.

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