## Signals and Noise

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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