9.2 WHY MODULATION?

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Suppose we want to transmit two voice channels from one place to another. If we combine the two voice signals and transmit them on the medium, it is impossible to separate the voice conversations at the receiving end. This is because both voice channels occupy the same frequency band, 300Hz to about 4kHz. A better way of transmitting the two voice channels is to put them in different frequency bands and then send them, translating the voice channels into different frequency bands.

Low-frequency signals have poor radiation capability and so low-frequency signals such as voice signals are translated into high frequencies. We need to superimpose the voice signal onto a high-frequency signal to transmit over large distances. This high-frequency signal is called the carrier, and the modulation is called carrier modulation.

When different voice signals are modulated to different frequencies, we can transmit all these modulated signals together. There will be no interference.

If radio is used as the transmission medium, the radio signal has to be sent through an antenna. The size of the antenna decreases as the frequency of the signal goes up. If the voice is transmitted without superimposing it on a high-frequency carrier, the antenna size should be 5,000 (yes, five thousand) meters!

For these reasons, modulation is an important transformation of the signal that is used in every communication system.

To summarize, modulation allows:

Transmitting signals over large distances, because low-frequency signals have poor radiation characteristics.
It is possible to combine a number of baseband signals and send them through the medium, provided different carrier frequencies are used for different baseband signals.
Small antennas can be used if radio is the transmission medium.

Modulation allows transmission of signals over a large distance, because low-frequency signals have poor radiation characteristics. It is also possible to combine a number of baseband signals and send them through the medium.

Note

In transmission systems using radio as the medium, the higher the frequency of operation, the smaller the antenna size. So, using high-frequency carrier reduces the antenna size considerably.

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