A modem is capable of transforming a sequence of zeros and ones into a signal (sound).
Suppose we want to transmit bits 011011.

There are many ways to modulate the signal to represent 0s and 1.

Amplitude modulation: we change the amplitude (strength) of the signal.

Amplitude Modulation Example

frequency modulation: This time, we change the frequency (the beat speed) of the signal.

Frequency modulation example

phase modulation: this time, we're going to shift the signal in time.

Phase modulation example

There are tips that can make you even better!

For example, in frequency modulation, let's just use two different frequencies to encode 0 and 1. We could have used four different frequencies to encode 00, 01, 10, and 11:

Example of frequency modulation with 4 states

To transmit our 6 bits, we take twice as little time as with the previous frequency modulation!

We can also apply this method in amplitude and phase modulation.

Vocabulary (for those who want to know more): Below, we use four different frequencies to encode 00, 01, 10 and 11. The modulation is said to be quadrivalent. In the beginning, we had used only two frequencies: it was a modulation bivalent.

The valence is the number of different states that can receive a signal at any given time. These states can be represented by different frequencies, different amplitudes and different phases.

In 1 second, we can modulate multiple states. The number of different states per second is calledbaud (measurement of signaling speed).
1 baud = 1 valence state per second; 9600 baud = 9600 valence states per second.

Bauds are not necessarily equal to bits/second !

In our example above, if we do our 3 modulations in 1 second, we will be at 3 baud.

But in three modulations, we transmit 6 bits: we are at 6 bits per second.
We can do even better: it is possible to do amplitude, frequency and phase modulations at the same time!

For example, with frequency and amplitude modulation at the same time:

Example of frequency and amplitude modulation

Modems (by standards) use one or more of these modulations.
It's what allows you to get the transmission speeds of current modems.