[Elphel-support] the discussion on the Heptaclops camera

[Biel Bestué de Luna]

let me see if I've got shot noise correctly, the only way to have less shot
noise would be to have more photons hit the photo-sites in order to excite
more electrons, so bigger photo-sites would reduce shot noise isn't it?
(that would also mean, bigger sensors in order to have the same final
resolution)

would it be significantly more costly to use a ADC of 13 bits (if they
exist which of course, i don't have any clue if they do) my older camera
the panasonic HVX-200 advertised that it had "a 14 bit chip" to handle the
colour, it is a 3CCD camera so I guess they are measuring their counts
significantly different, but this means that at least there is a ADC of 14
bits with significantly more counts that clearly the Elphel would need, why
not use then a 13 bits? if you where to use a 13 bits ADC chip wouldn't you
be able to skip half the count values and still do the camera's current
scheme of the 150 values? but you could also gather more values if you want
more info for your post production needs. is it possible?

you said: "It is good to have more quantization level than the number of
S+N levels you can discriminate." what do you mean by "S+N levels"? shot
noise or Signal to noise ratio?

about the article of linux devices:

"Such conversion significantly reduces the number of output values, and
therefore the number of bits required to encode them, without sacrificing
much of the pixel values." (sic)

So you're recoding the 150colours of the camera to the 8bit 256colours with
a non-linear gamma
isn't it? apart of the higher data amount, what would be the other problem
using more counts of the ADC? what relation there is between the amount of
ADC counts used currently and shot noise? and if more ADC counts were used
to encode the colour how would shot noise would be affected? (all of this
using the current hardware of course)

Andrey you're giving me a huge amount of very important and relevant info
those days! thank you very much :D


2012/10/30 support-list <support-list at support.elphel.com>

> Hi Biel, have you read the article -
> http://www3.elphel.com/linuxdevices/articles/AT9913651997.html? And maybe
> also about the shot noise - http://en.wikipedia.org/wiki/Shot_noise ?
>
> ---- On Tue, 30 Oct 2012 09:25:24 -0700 Biel Bestué de Luna  wrote ----
>
> >Andrey, about only having 150 gray levels, why does the camera only
> output 150 gray levels when the image that comes out of the sensor has 4096
> output values? maybe this is because this amount of sampled information
> contain a lot of noise?
>
> You know, when they have polls to measure public opinion on something -
> they have "margin of error"?  The totla group of electrons "polled" in a
> pixel is limited to the "full well capacity" - in our case ~8500. ADC has
> just a little less precision - can count by groups of two persons (4096
> levels). So even if you had ideal 13-bit ADC (and zero noise preamplifier)
> that could count each single person (electron) and you can get measurement
> where "each vote counts", the difference between 8000 and 8001 is
> meaningful for democracy, but not a statistically meaningful difference if
> you try to measure public opinion. So depending on the "margin of error"
> you are willing to accept, there could be certain number of polling results
> that are different (by at least that "margin of error) from each other, and
> theer will be much less than 8500 or even 4096 outcomes from such public
> opinion measurements. And yes, you can use just 8 bits to represent such
> results.
>
> >and, if 8 bits allow 256 shades of gray, does that meant that, while 8
> bits per pixel allows it the camera can't reach the 256 shades of gray? if
> so, wouldn't it mean having a lot of banding?
>
> When the quantisation step is safely smaller than the noise, there will be
> no banding because even the sensor looks at perfectly evenly illuminated
> surface, you will not get the same output code, because pixel-to-pixel
> variations (from the same noise) will be larger than the output code levels
>
>
> >what factors need to change in order to have a camera that at least has
> the full 256 colours from 8 bits from the sensor?
>
> It is good to have more quantization level than the number of S+N levels
> you can discriminate. Quantization noise add to the total noise, just when
> it is smaller than the sesnor noise, it's contribution is smaller in the
> total.
>
> > maybe a sensor with higher ADC would do the job?
>
> ADC is trying to measure number of electrons, so there is a natural limit
> to the ADC resolution needed.
>
> >about shot noise, what factors make shot noise optimal for gamma 0.5, can
> this factors be changed for other gamma values? like 0.46 (1/2.2)?
>
> Why do you need 0.46? Shot noise (predominant source of noise when you
> have good ADC and analog circuitry) is Math.sqrt() or Math.pow(x, 0.5). It
> is about preserving sensor information. When you want different contrast on
> the result video - that would be a separate story - you can apply it in
> post-processing, in the video editor - not in the camera.
>
> Andrey
>
>
>
>
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