In order to convey speech on the RF, either in analog or digital form, the transmitted information must be propagated on the radio link. It must be placed on the carrier. A carrier in this respect is a single radio frequency. The process of combining the audio and the radio signals is known as modulation. The resultant waveform is known as a modulated waveform. Modulation is a form of change process where we change the input information into a suitable format for the transmission medium. We also changed the information by demodulating the signal at the receiving end.

Three normal forms of modulation are used:

Amplitude Shift Keying (ASK)

In ASK (see Figure 21-24), the radio wave is modulated by shifting on the amplitude. The frequency is left constant, but the amplitude is shifted high if the data is a 0 and low if the data is a 1. Normally, we see two amplitude shifts represent a single bit.

Input Data 0 0

Normal (Unmodulated Carrier)

Modulated Carrier


Figure 21-24: ASK

Frequency Shift Keying (FSK)

The alternative to ASK is FSK. In the case of FSK (see Figure 21-25), applying the data onto the radio wave modulates the carrier by changing the frequency. The amplitude is kept constant, and the frequency is changed. Normally, a single frequency shift represents a bit of data.

Input Data 0 0

Normal (Unmodulated Carrier)

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