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Why sampling is done first and then quantization??

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sheraz.pervaiz

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sampling quantization reversing order

To convert an analog signal into digital signal two steps are required
1. Sampling
2. Quantization

We can perform these operations in any order to get a digital output.But why it is common to do sampling first?

Looking forward for reply.
Thanx in advance.
 

why sampling is done

Hi
Sampling is done first so that each sample can be represented by 8 digits corressponding to 128 levels of quantization.
 

sheraz.pervaiz,

As a practical matter, sampling is done first to ensure the the A/D converter sees a constant analog input during its conversion process. This is particularly important with a successive approximation type A/D conversion, where gross errors can occur if the analog input changes during the conversion.
Regards,
Kral
 
to convert analog to digital ,the analog voltage must be represented by bits.Representing each and every iinput in terms of bits is impossible due to time delay in conversion .So sampling will give some samples at regular intervals of time which can be represented in digital form .
 

how can you quantization first ? :)
 

In Quantization we represent an infinite precision voltage level by a specific number of bits which can easily be stored in digital system. Quantization before sampling doesn't make any sense as in sampling u throw away indermediate samples and u must have samples which have infinite precision analog voltage levels so that they can be quantized to a given number of bits.
 

quantization is assigning a common value to a set of values in the vicinity of a particular value.
i dont think that quantization can be done prior to sampling.
 

It's a bit like first having to look at the voltage on a voltmeter before you can write it down. You cannot write it down before you have determined what the value is.
 

To convert an analog signal into digital signal two steps are required
1. Sampling
2. Quantization

We can perform these operations in any order to get a digital output.
Click to expand...

How can we quantize before we have a sample? Even if we could, why would we need sampling once quantization is done?

M
 

how can we quantize before sampling...sampling has to be first...or else there is nothing to quantize....

we have to have values of the signals at discrete times so that the ADC would be able to see a constant value and make its output into digital bits
 

sampling can get the value, then quantize them. It is more easy
 

sampling is a the process you take some instant value of the specified analog signal,and quantization is the process you covert this instant value into a digital number.so,the reason is clear.
 

In general the process of converting an analog signal which is a continuous time signal to digital signal involves discritising the time frame of the analog signal and then converting these discrete-time continuous amplitude signal to discrete amplitude discrete time signal. The first part of the process is done using sampling where as the second objective is acieved using quantization. And finally this discrete amplitude discrete time signal is coded using a binary sequece of certain bits. Logically the whole process must be performed in this sequece and in practice it is done so.
However, the first two parts can be performed in either order. If quantisation is done first, it will take a very large number of quantisation levels to represent the same analog signal and in turn need a high resolution a/d convertor to conver the signal to binary sequence.
 

if we do quantization first, during quantization procedure,

the input signal may vary and induce error.

best regards




sheraz.pervaiz said:
To convert an analog signal into digital signal two steps are required
1. Sampling
2. Quantization

We can perform these operations in any order to get a digital output.But why it is common to do sampling first?

Looking forward for reply.
Thanx in advance.
Click to expand...
 

You have to do sampling first, because remember that sampling has the time reference, that is the sampling period.

Quantization is just mapping a set ot measured voltages, to a set of predefined discrete voltajes, that are in turn mapped to a set of binary digital values by ADC.

So in order to do this, you have to meassure the analog voltages first, that is done by Sampling&Hold, and then map these values to a set of discrete values by quantization.

You can't reverse the order.
 

There are adc´s that can be made without sample and hold circuit (sigma-delta, flash)...
 

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