| Communications Toolbox | |
Modulation Terminology
Modulation is a process by which a carrier signal is altered according to information in a message signal. The carrier frequency, denoted Fc, is the frequency of the carrier signal. The sampling rate is the rate at which the message signal is sampled during the simulation.
The frequency of the carrier signal is usually much greater than the highest frequency of the input message signal. The Nyquist sampling theorem requires that the simulation sampling rate Fs be greater than two times the highest frequency of the modulated signal, in order for the demodulator to recover the message correctly. The sampling rate Fs of a modulated digital signal is greater than or equal to the sampling rate Fd of the original message signal before modulation.
The table below lists the requirements in terms of the input arguments for this toolbox's modulation and demodulation functions. Note that the situations are not mutually exclusive.
| Situation |
Requirement |
| Passband simulation |
2*(highest frequency of modulated signal) < Fs |
| Digital signals |
Fd |
| Passband simulation, digital signals |
Fd < Fc |
| | Modulation Features of the Toolbox | Representing Analog Signals | |
Fs