1.5 Energy and power

From physics we've learned that energy is work and power is work per time unit. Energy was measured in Joule (J) and work in Watts(W).In signal processing energy and power are defined more loosely without any necessary physical units, because the signals may represent verydifferent physical entities. We can say that energy and power are a measure of the signal's "size".

Signal energy

Analog signals

Since we often think of a 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. 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 thearea under the squared signal, see

Discrete signals

For time discrete signals the "area under the squared signal" makes no sense, so we will have to use another energy definiton.We define energy as the sum of the squared magnitude of the samples. Mathematically

Signal power

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

Analog signals

For analog signals we define power as energy per time interval .

Discrete signals

For time discrete signals we define power as energy per sample.

Matlab files

energy_area.m power_sine.m

• Introduction
• Discrete time signals
• Analog signals
• Discrete vs Analog signals
• Frequency definitions and periodicity
• Exercises