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This is the same as Figure 32 except that noise has been added at 1.221 kilohertz. This shows as a thick band of colour around the horizontal axis on the time-domain graph on the left, and as a blue spike at 1.221 kilohertz on the frequency-domain graph on the right. It is clear that the noise gets mixed up with the signal in a way that is hard to see in the time domain, but it emerges as a clear, isolated vertical line in the frequency domain.

 4.7  Signals and noise