[FFmpeg-user] Glossary: Nyquist

Mark Filipak (ffmpeg) markfilipak at bog.us
Sat Oct 3 18:05:03 EEST 2020

On 10/03/2020 06:41 AM, Anatoly wrote:
> On Fri, 2 Oct 2020 20:47:57 -0400
> "Mark Filipak (ffmpeg)" <markfilipak at bog.us> wrote:
>> By the way, I've given up trying to make an illustration of
>> 2-dimensional Nyquist sampling. It's too hard.
> I think is's easy. Just slale dows to every one dimesion to tart from.
> Lets draw XY plot of one line of our picture of alternating black-white
> stripes
> Voltage     ^
>    -or-      |
> Light       |
> intencity   | b    w    b    w
>              |     ___       ___
>              |    /   \     /   \
>              |___/     \___/     \_
>              |_______________________> Time -or- position
>              --|----|----|----|--- samples
>                _    _    _    _
>               / \__/ \__/ \__/ \_  sampling freq -or- distance.
> Here we are digitizing 4 pixels. Does not matter how they are separated
> one from another - temporarily (analogue video signal) or spatialy
> (laying on CCD silicone surface). Nyquist criteria says that to
> digitize (somehow) 4 pixels we need to take 4 samples. Note that
> our "signal" frequency (again, temporal or spatial) is 1/2 of sampling
> frequency. That is it.

Where's the twice the display resolution in your diagram?

My understanding of Nyquist is limited. I think that it's based on the information density present 
in a signal having amplitude S, that transitions from S to S+d(S) (not black to white) and that it 
therefore defines a minimal slope (hence, the connection to bandwidth). I, myself, question that 
bandwidth is an adequate metric and whether 'information' is adequately characterized, but science 
only 'sees' what it can measure, eh? I'll stick with a definition based on energy density (which, in 
the listening and the seeing, has a gaussian profile and is based on physics).

> Maybe it's a fun to discuss such a things, but I think here is not
> right place to do it, beacuse it has no straight relation to ffmpeg
> usage.

If not ffmpeg.org, then where? doom9.org? -- no organization there, a glossary would get lost. Or 
Wikipedia? Ha!

The audience is here. ...Perhaps Wikipedia some day.

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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