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Calculation of snr

 
 
Kennedy McEwen
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      05-28-2008
In article <(E-Mail Removed)>, Floyd L. Davidson
<(E-Mail Removed)> writes
>Kennedy McEwen <(E-Mail Removed)> wrote:
>>In article <(E-Mail Removed)>, Floyd L. Davidson
>><(E-Mail Removed)> writes


>>>You were referring to the design of a system to measure
>>>noise in the light.
>>>

>>No, I was explaining how to measure the signal to noise ratio that the
>>camera is capable of measuring.

>
>Cameras don't measure noise ratios,


Cameras measure light, and they do so with a certain signal to noise
ratio.

> which is why describing
>a system to measure noise in the light is a strange way to
>discuss the signal to noise ratio of data processing in a
>camera.
>

The camera is just a "black box": it takes an image in, in the form of
spatially modulated light, and produces data in a form that can be
output as an image, in the form of spatially modulated light. Nobody
cares what the SNR is at some point buried in the middle of that, they
care about the effect that the camera has on the image, the light, and
so it makes perfect sense to measure the performance of the camera in
terms of light power. In fact, for the user who is only interested in
how well a particular image is represented by a particular camera it is
probably the ONLY meaningful representation of SNR.

"The noise in the light" is something completely different and doesn't
even need to be measured, since it is simply shot noise on the total
light amplitude.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
 
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Kennedy McEwen
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      05-28-2008
In article <MxZ_j.10328$(E-Mail Removed)> , David J
Taylor <(E-Mail Removed)-this-bit.nor-this-bit.co.uk>
writes
>
>I didn't realise that physicists even
>knew about dB - I thought it was just engineers! <G>
>

That's a bit naive, David, of course physicists know, and use, dB. Since
physicists are more concerned about the underlying principles of the
universe around us, they are probably more aware of the definition and
correct use of the decibel than engineers, who use it mainly as a tool,
are.

Why do screwdrivers have long shafts? So its easier to open tins of
paint with them. A bit like the dB!
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
 
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David J Taylor
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      05-28-2008
Floyd L. Davidson wrote:
> "David J Taylor"

[]
>> all-black signal has exactly the same levels as an all-white signal
>> (e.g. measuring sync voltage) while having greatly different power
>> levels. It is the voltage level which is important to the
>> television engineer.

>
> If you think the all-black signal has exactly the same
> levels as an all-white signal, what point is there in
> talking to you.


By level I mean: the sync, black and peak white voltages. That is what is
measured in television. Not the power. Not the average voltage. Not the
rms voltage.

> It simply isn't true. It *is* true that the sync pulse
> does not change level, but the level of the video as a
> whole certainly does! (And we might note that the
> output power of the typical TV broadcast transmitter
> changes as a result, because it is AM modulated. That
> would not happen if the signal level did not change.)


Please read again what I wrote. The power levels in an all-black signal
and an all-white signal are different, even though the defining voltage
levels are the same (when specified in dBV). Therefore I believe it is
misleading to talk about dBV as being "power" related, when it is actually
a voltage ratio measurement.

No-one is suggesting that a formula other than 20 log (V2/V1) be used,
defining the voltage ratio in power terms.

Cheers,
David


 
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David J Taylor
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      05-28-2008
Kennedy McEwen wrote:
> In article <MxZ_j.10328$(E-Mail Removed)> , David J
> Taylor <(E-Mail Removed)-this-bit.nor-this-bit.co.uk>
> writes
>>
>> I didn't realise that physicists even
>> knew about dB - I thought it was just engineers! <G>
>>

> That's a bit naive, David, of course physicists know, and use, dB.
> Since physicists are more concerned about the underlying principles
> of the universe around us, they are probably more aware of the
> definition and correct use of the decibel than engineers, who use it
> mainly as a tool, are.
>
> Why do screwdrivers have long shafts? So its easier to open tins of
> paint with them. A bit like the dB!


DId you miss the smiley?

Cheers,
David


 
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David J Taylor
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      05-28-2008
Floyd L. Davidson wrote:
> "David J Taylor"
> <(E-Mail Removed)-this-bit.nor-this-bit.co.uk> wrote:
>> Floyd L. Davidson wrote:
>>> "David J Taylor"

>> []
>>>> all-black signal has exactly the same levels as an all-white signal
>>>> (e.g. measuring sync voltage) while having greatly different power
>>>> levels. It is the voltage level which is important to the
>>>> television engineer.
>>>
>>> If you think the all-black signal has exactly the same
>>> levels as an all-white signal, what point is there in
>>> talking to you.

>>
>> By level I mean: the sync, black and peak white voltages. That is
>> what is measured in television. Not the power. Not the average
>> voltage. Not the rms voltage.

>
> Have you ever aligned a TV camera? Or a TV transmitter?


Yes, designed, built, tested and aligned both camera and transmitter.
Both valve and transistor transmitters.

> What is measured is the signal power. That is by definition
> if one uses dBv values.


No, in the camera chain you measure the voltage level - in the (analog)
transmitter RF side the sync or peak-white output power, according to
whether you have negative or positive modulation. In our practice, you
would be more likely to quote the signal level in volts rather than in
dBV.

> (And it should also be noted that not all "video"
> measured with dBv is composite sync NTSC either!)


NTSC seems to use even more archaic IRE units!

[]
> But when you use your own definitions, it makes what you
> say worthless to everyone else. dBv is a power, not a voltage.


My definition is that dBV is a ratio of the signal voltage to a standard
1V video signal (in the case of TV), expressed in decibels. Even FS-1037C
says in Appendix A:

dBv: "dB relative to 1 V (volt) peak-to-peak"

Cheers,
David


 
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Ilya Zakharevich
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      05-28-2008
[A complimentary Cc of this posting was NOT [per weedlist] sent to
Floyd L. Davidson
<(E-Mail Removed)>], who wrote in article <(E-Mail Removed)>:
> Rest assured that anyone who thinks common usage
> contradicts the formal definition is mistaken.


LOL!

> I assume you want to know about the SNR of the image
> files produced by your camera...


I do not think this sentence indicates any understanding of the
problems which arise here. Example: you change gamma of your images.
What, in your definitions, happens to S/N ratio? How to measure S/N
ratio of gamma=0.45 JPEG files?

[IMO, the only reasonable approach is to ignore the way the
information is stored on DASD, and translate the stored values back
to linear scale; only AFTER this one should calculate the properties
of the image.]

Yours,
Ilya
 
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Ilya Zakharevich
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      05-28-2008
[A complimentary Cc of this posting was sent to
Marc Wossner
<(E-Mail Removed)>], who wrote in article <(E-Mail Removed)>:
> Now the context is even clearer to me than in Ilyas post.
> But if db should be avoided in this context, how should snr be defined
> to be comparable?


Just use Plain Old Numbers. E.g.: S/N ratio of correctly exposed 18%
gray on Velvia 50 is about 20:1 (averaged over 6.2um squares; data
according to Roger).

If you want log-scale, you can represent the same number in steps (=log2):
this S/N ratio is "slightly better than 4 steps". (These are the only
usages I know which won't cause any confusion.)

Hope this helps,
Ilya
 
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Ilya Zakharevich
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      05-28-2008
[A complimentary Cc of this posting was sent to
John O'Flaherty
<(E-Mail Removed)>], who wrote in article <(E-Mail Removed)>:
> I think you are conflating the power in light intensity
> (I=Power/Asurf) with signal power. Once the intensity is mapped into a
> signal, signal power has its usual meaning. You can create signals of,
> for examples, stock prices or annual rainfall, and can investigate the
> power spectra of the resulting signals, without a power interpretation
> for the original data. And once you have a voltage signal, it doesn't
> matter what it originally represented, its power is proportional to
> voltage squared, referred to a standard 1 ohm impedance.


I'm afraid you use terminology outside of their usual bounds. There
is no such thing as "signal power", "power of a voltage signal" etc.

> >> dB = 10 * log ( Signal_voltage ^ 2 / Noise_voltage ^ 2 )
> >> dB = 2 * 10 * log ( Signal_voltage / Noise_voltage )


> >These squares/twos are as much misplaced/wrong as "S" in "RMS power".


> The squares, when applied to statistical analysis of signals, are
> correct.


There is no "correct" thing in statistics. For example, let me
restate what you say in more "truthful" form:

The math of the L2-norm is much less tricky than math of other norms
(in spaces of functions), since it is a Hilbert norm (as opposed to
more general Banach norms). Therefore, when taught to beginners,
L2-norms are prefered as the first topic to expose.

Some people are exposed only to the 101-part of the topic, so may
think that it is all there is...

In reality, the norm to use is dictated by the "physical meaning" of
the function. E.g., in context of the signal of a photo-sensor, the
meaningful norm is the L1-norm, since each measurement already
represents energy.

> For example, in a textbook*, the definition of rms SNR for a
> decompressed image vs. an original image is given as the square root
> of the _square_ of the row-column double sum of the pixel errors,
> divided by the number of pixels.


As you can guess, the extremely low quality of US textbooks is a
recurring topic at some dinner tables...

Hope this helps,
Ilya
 
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ASAAR
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      05-28-2008
On Wed, 28 May 2008 22:23:44 +0000 (UTC), Ilya Zakharevich wrote:

> As you can guess, the extremely low quality of US textbooks is a
> recurring topic at some dinner tables...


I'm sure that someone here will try to make you eat our words!




 
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Kennedy McEwen
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      05-29-2008
In article <(E-Mail Removed)>, Floyd L. Davidson
<(E-Mail Removed)> writes
>Kennedy McEwen <(E-Mail Removed)> wrote:
>>In article <(E-Mail Removed)>, Floyd L. Davidson
>><(E-Mail Removed)> writes
>>>Kennedy McEwen <(E-Mail Removed)> wrote:
>>>>In article <(E-Mail Removed)>, Floyd L. Davidson
>>>><(E-Mail Removed)> writes

>>
>>>>>You were referring to the design of a system to measure
>>>>>noise in the light.
>>>>>
>>>>No, I was explaining how to measure the signal to noise ratio that the
>>>>camera is capable of measuring.
>>>
>>>Cameras don't measure noise ratios,

>>
>>Cameras measure light, and they do so with a certain signal to noise
>>ratio.

>
>Exactly.

So, in measuring light, you should have no problem with some performance
elements of a camera being defined in terms of the ratio of maximum
light power detected without saturation to minimum light power
detectable, and expressing that performance in decibels, the ratio
between the two.

>The *don't* measure noise ratios.
>

Whatever they measure, two or more of those measurements can always be
expressed as a ratio - signal and noise being just the two examples of
relevance in this thread.

>>The camera is just a "black box": it takes an image in, in the form of
>>spatially modulated light, and produces data in a form that can be
>>output as an image, in the form of spatially modulated light. Nobody
>>cares what the SNR is at some point buried in the middle of that, they

>
>Some photographers may not care about that SNR. The
>people who design cameras do, and discerning
>photographers do.
>

More deliberate misquoting and general dishonesty from Floyd! The whole
thread is ABOUT SNR - read the subject line! However, nobody is
interested in the SNR of some component buried in the middle of the
black box, what they are interested in is the SNR of the black box in
terms of its input and/or output. Since the output also requires
knowledge of the SNR of the display medium, in this case the output SNR
is irrelevant. Consequently the ONLY measure of relevance is input
referred, and the input to a camera is LIGHT!

>>care about the effect that the camera has on the image, the light, and
>>so it makes perfect sense to measure the performance of the camera in
>>terms of light power.

>
>And that does not include measuring noise ratios of the
>light.
>

Only you seem to think so - it is certainly relevant to know what the
ratio of maximum unsaturated light level that the camera can work with
relative to its minimum detectable light under whatever circumstances it
is operated.

>>In fact, for the user who is only interested in
>>how well a particular image is represented by a particular camera it is
>>probably the ONLY meaningful representation of SNR.

>
>If someone is researching which camera to buy, they want
>to know the effects that show up from different
>situations. They may not label them as "SNR of the
>sensor", "SNR of the read amplifier", and "SNR of the
>digital output format", but that is exactly what they do
>want to see comparable results for, and they do tend to
>decide which model camera to purchase based on the
>results they see.
>

Precisely, which is why the RATIO of the incident light power is the
important parameter, not the absolute light power or the SNR at some
irrelevant point in the processing chain.

>>"The noise in the light" is something completely different and doesn't
>>even need to be measured, since it is simply shot noise on the total
>>light amplitude.

>
>That was my point.


No it wasn't - you think that what is being described is "the noise in
the light", however that is not the case. What is being described is
the SNR of the camera referred to its input, light. The noise in the
light is something completely different from what is being measured.

>Your system to measure it is not
>germane to this discussion.
>

Only to you.

As I mentioned in a previous post in this thread, every scanner
manufacturer who specifies the capability of their product uses the same
"input referred" definition, and users have been happily comparing
scanners for years based on such numbers and measurements from
independent 3rd parties.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's ****ed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
 
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