When we spoke about the mean value of the signal, x, we spoke as if we knew its value. In fact, we do not know the value of x . If we have taken one image, we have only one sample of the signal, which we will call x^. If we take another image, we can call that sample x2, and a third, x3. If we take n samples, the nth sample we take is called x„.
To find the mean value of the signal, x, we simply add the samples together and divide by the number of samples:
We can express the same operation of finding the mean value of a set of samples using summation notation:
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