1.6 Signal energy vs. signal power

The idea of the "size" of a signal is crucial to many applications. It is nice to know how much electricity can beused in a defibrillator without ill effects, for instance. It is also nice to know if the signal driving a set of headpones isenough to create a sound. While both of these examples deal with electric signals, they are clearly very different signalswith very different tolerances. For this reason, it is convenient to quantify this idea of "size". This leads to theideas of signal energy and signal power.

Signal energy

Since we often think of signal as a function of varying amplitude through time, it seems to reason that a goodmeasurement of the strength of a signal would be the area under the curve. However, this area may have a negative part.This negative part does not have less strength than a positive signal of the same size (reversing your grip on the paper clipin the socket is not going to make you any more lively). This suggests either squaring the signal or taking its absolutevalue, then finding the area under that curve. It turns out that what we call the energy of a signal is the area under the squared signal.

Signal power

Our definition of energy seems reasonable, and it is. However, what if the signal does not decay? In this case wehave infinite energy for any such signal. Does this mean that a sixty hertz sine wave feeding into your headphones is asstrong as the sixty hertz sine wave coming out of your outlet? Obviously not. This is what leads us to the idea of signal power .

Power is a time average of energy (energy per unit time). This is useful when the energy of the signal goes to infinity.

• Compute $\frac{\mathrm{Energy}}{T}$
• Then look at $\lim_{T\to }T\to$∞ Energy T

$P_f$ is often called the mean-square value of $f$ . $\sqrt{P_f}$ is then called the root mean squared ( RMS ) value of $f$ .

Energy vs. power

• "Energy signals" have finite energy.
• "Power signals" have finite and non-zero power.