Is resolution strictly defined in volts?

[Femme_physics]

I didn't know that resolutions are defined in volts until today. I never wrote units to them. I read in wiki that...

Resolution
Fig. 1. An 8-level ADC coding scheme.The resolution of the converter indicates the number of discrete values it can produce over the range of analog values. The values are usually stored electronically in binary form, so the resolution is usually expressed in bits. In consequence, the number of discrete values available, or "levels", is a power of two. For example, an ADC with a resolution of 8 bits can encode an analog input to one in 256 different levels, since 28 = 256. The values can represent the ranges from 0 to 255 (i.e. unsigned integer) or from −128 to 127 (i.e. signed integer), depending on the application.

Resolution can also be defined electrically, and expressed in volts. The minimum change in voltage required to guarantee a change in the output code level is called the least significant bit (LSB) voltage.

So it can be expressed as both? I'm confused. Must I write units to resolution, or can I interchange them between bits and volts?

 

[Studiot]

It's the same thing by a scale factor.

Say your 150mm engineer's pocket ruler has markings at 1mm intervals so you can read to the nearest 1mm.
You have available measurement values of 0, 1, 2, 3...148,149,150mm

We say you can 'resolve' the distance measured to 1mm.

So say you measure 127mm.


Now this distance is 157mm on a map at 1:2500 scale so represents 317.5 metres on the ground.

The difference you can resolve on the ground with your ruler is 1mm x 2500 ie 2.5 metres.

So you apply your 8 bit ADC which has 256 values available to some voltage V which means you scale the voltage into lengths of V/256.

So if you were measuring 10 volts your minimum resolution would be 10/256 volts in voltage measure or 1 bit in bit measure.

Does this help?

 

[I like Serena]

It depends how you use it in a sentence.


If you say the resolution of the signal is 256, then the unit is "dimensionless".

If you say the resolution is 8 bits, then the unit used is "bit", but is signifies 256 levels that are "dimensionless".

If you say the resolution is determined by a least-significant-bit voltage of 20 mV, then the unit is volt, although that's not really the unit of the resolution.
Volt here is the unit of the least-significant-bit voltage.


If you say the resolution is 20 mV, then that certainly does not mean that you have 20 levels.
It would be shorthand for saying that the least-significant-bit voltage is 20 mV.

Engineers are usually a bit sloppy in how they write things down.
However, the key is that you understand what was intended.

 

[M Quack]

It depends how you use it in a sentence.
If you say the resolution of the signal is 256, then the unit is "dimensionless".

If you say the resolution is 8 bits, then the unit used is "bit", but is signifies 256 levels that are "dimensionless".

If you say the resolution is determined by a least-significant-bit voltage of 20 mV, then the unit is volt, although that's not really the unit of the resolution.
Volt here is the unit of the least-significant-bit voltage.

If you say the resolution is 20 mV, then that certainly does not mean that you have 20 levels.
It would be shorthand for saying that the least-significant-bit voltage is 20 mV.

Engineers are usually a bit sloppy in how they write things down.
However, the key is that you understand what was intended.

The key certainly is that you (or whoever reads what you write) understands what is meant.

To a physicist, the only meaningful resolution would be 20 mV.

When you refer to 8 bits or 256 levels, a physicist would consider that the dynamic range (the resolution always being 1 bit (the least significant), or 1 level.

In case of doubt give a complete description, specifying number of bits (or dynamic range in decades) and the LSB resolution in mV (or nA or whatever other analog input you use).

 

[Studiot]

To a physicist, the only meaningful resolution would be 20 mV.

When you refer to 8 bits or 256 levels, a physicist would consider that the dynamic range (the resolution always being 1 bit (the least significant), or 1 level.

Careful.

Say you had a signal from a microphone, digitised to 8 bits and passed it through a pre-amplifier then to an amplifier and then to a loudspeaker.

Your signal now would never have greater resolution than 8 bits at any stage in the chain, however the voltage resolution would alter dramatically at each stage.

 

[I like Serena]

Hmm, I just looked up what the word "resolution" actually means and what its unit should be.

It has a lot of meanings, but as I understand it it's not a quantity that you measure, but it's a concept.
The relevant dictionary meaning seems to be:

5. the act or process of resolving or separating into constituent or elementary parts.

Typical usage in a sentence is that the resolution is "described" or "indicated" by something.
So the resolution of a signal is "indicated" by the number of levels that can be distinguished, or by the number of bits.
The resolution of a monitor is "described" by the number of horizontal and vertical pixels.


Here's a couple of usages on wikipedia (bolding mine):

Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged.

In digital audio, bit depth describes the number of bits of information recorded for each sample. Bit depth directly corresponds to the resolution of each sample in a set of digital audio data. Common examples of bit depth include CD quality audio, which is recorded at 16 bits, and DVD-Audio, which can support up to 24-bit audio.

The display resolution of a digital television or display device is the number of distinct pixels in each dimension that can be displayed.

 

[M Quack]

Hmm, the digitizer that converts the microphone voltage into an 8-bit digital signal has a defined resolution of x mV. I think so far we agree.

Then you are skipping a step, as the digital signal has to be converted back into an analog voltage that then passes pre and power amp.

What happens then is a bit more complicated. Assume that both amps are absolutely perfect. Then the signal quality will be conserved (it can't get better, no matter how expensive you amps are), and in the end you still have an audio signal with the dynamic range of the original 8-Bit encoding. The actual voltage levels will vary of course.

Now assume that you have a really crappy amp with loads of noise. For the sake of argument, let the DAC put out 8 bits with 20 mV smallest steps, i.e 256*20mV = 5.12V max.
Now let the preamp have a noise level of 100mV (for sake of argument, not even physicists built that bad amps). The resolution (in mV) cannot be better than that. Then the dynamic range will be 5.12V/100 mV = 51 which is closer to 6 bits.

Can you see my point?

See section on measurement resolution:

http://en.wikipedia.org/wiki/Resolution

Note that audio resolution is not included in the section for measurement resolution. For digital audio, my guess is that there is a convention for the maximum voltage, so that the resolution ends up as this reference value divided by 2^(number of bits). For analog audio, Dezibels (dB) are used, which define a dynamic range relative to a 0 dB reference signal.

 

[Studiot]

Yes that's what I've been saying.

There is a scale factor at work in there.

So for two monitors, one 5 times as big as the other, but with the same pixel resolutionthe pixels of one will be five times bigger than those of the other.

 

[Studiot]

Then you are skipping a step, as the digital signal has to be converted back into an analog voltage that then passes pre and power amp.

Not necessarily.

The signal, these days can be digital all the way to a digital loudspeaker, driven with 8 bit PWM.

For the rest, whilst true, you are introducing extraneous material.

 

[Femme_physics]

Sorry, I was not specific enough... I will be more specific: In A to D converters. Is it always Volts in this case?

 

[M Quack]

For a physical instrument, the resolution is defined as the smallest separation of two signals it can distinguish=resolve. As such, the resolution is an absolute value in mV, nA, micrometers, millidegrees, millikelvins, or whatever units you happen to be measuring.

The dynamic range is the ratio of the largest to the smallest signal that can be measured.

If the largest signal is fixed, then the resolution can be calculated from the dynamic range. If the absolute value is pretty much meaningless because the signal is scaled back and forth (as in audio), then only the dynamic range is of interest.

These definitions are commonly used in the world of physics. It appears that in electronics (and in particular audio) these definitions are used more freely.

 

[I like Serena]

Sorry, I was not specific enough... I will be more specific: In A to D converters. Is it always Volts in this case?

No, for A to D converters the resolution can be specified in the number of levels, bits, or volts.
In each case something different is meant.

 

[Studiot]

For a physical instrument

You still haven't taken the scale factor into account.

Look again at my map example in post #2 .

The instrument in this case is the scale rule.

 

[M Quack]

Yes that's what I've been saying.

There is a scale factor at work in there.

So for two monitors, one 5 times as big as the other, but with the same pixel resolutionthe pixels of one will be five times bigger than those of the other.

I would call that pixel count, not (pixel) resolution, but you are correct that in common use it *is* called resolution (what I call resolution is listed as dot pitch or pixel density). This makes sense if you change the settings on your video card while using the same monitor. Displaying a different number of pixels on the same area then changes the resolution.

For printers, on the other hand, the resolution is defined in dpi (dots per inch). If you double the size of the print, keeping the same resolution, then you quadruple the number of pixels.

I guess the message to take home is to be as precise as possible in your writing.

BTW, the current that goes through the loudspeakers is still analog. By remaining digital as long as possible you avoid signal degradation and loss of dynamic range. But you know that :-)

 

[Femme_physics]

No, for A to D converters the resolution can be specified in the number of levels, bits, or volts.
In each case something different is meant.

In this specific case?

https://www.physicsforums.com/showthread.php?t=594561

 

[I like Serena]

You calculated a "resolution" in volts in that thread.

That resolution is the voltage between two successive values, or the least-significant-bit voltage.
Its unit is volt.

It matches M Quack's definition (post #11).

 

[M Quack]

I would say the resolution is 6 V/255 = 24 mV

 

[Studiot]

Bear in mind that for any A/D converter the step size is compared against an internal reference so is fixed.
In particular it does not depend upon the input voltage.
Each step will be in exactly the same place (at the same voltage) regardless of the input voltage.

So saying that the max input voltage or full scale input voltage equals 2000 mV and there are 1024 (8 bit) steps or that the resolution is 2000/1024 mV is one and the same thing.

 

[I like Serena]

Can't help responding, nitpicker that I am.
Sorry.

1024 is 10 bits (not 8 bits).

If there are 1024 levels, then there are 1023 steps.

It still depends on the specification of the chip what the stepsize is.
It could both be 2000/1024 mV or 2000/1023 mV (or even something else).
I, for one, certainly wouldn't make an assumption which one it is.
Only when I see it on an oscilloscope (or something like that) will I believe which one it is (for a specific chip).

 

[Studiot]

10 bits yes what was I thinking? Thanks.

However there is only one possible interpretation of the process that takes an input voltage and replaces it with a unique digital code with one of 1024 possible values.