It is easy enough to measure the frequency of a clean sinusoid, assuming that we have seen enough of the signal for its frequency to be determinable. For more complex signals the whole concept of frequency becomes more complex. We previously saw two distinct meanings, the spectrum and the instantaneous frequency. The concept of spectrum extends the single frequency of the sinusoid to a simultaneous combination of many frequencies for a general signal; as we saw in Section 4.5 the power spectral