Modulation Techniques

What is Modulation?

Modulation is the process of making some message signal suitable for transmission.
There are 3 signals in the modulation process:
1. The message signal (also called modulating signal): this is signal contains data that will be transmitted and has a low frequency. E.g. a voice signal
  - Denoted $m(t)=A_m \cos(2\pi f_mt)$
2. The carrier signal: a high frequency signal which will be modified in accordance to the message signal
  - Denoted $c(t)=A_c\cos(2\pi f_c t)$
3. The modulated signal: a signal that is the result of modifying the carrier signal in accordance to the message signal.
  - Denoted $\phi(t)$
Consider this example: a voice signal is to be transmitted with amplitude modulation.
- The voice signal is the message/modulating signal
- The carrier signal is a sinusoidal wave with a frequency in the range of MHz.
- The modulated signal will have the same frequency as the carrier signal, but its amplitude will vary according to the message signal (hence “amplitude modulation”). For example,
  - When there are loud voices in the voice signal, this will cause the modulating signal to have a larger amplitude.
  - When there are quieter voices in the voice signal, the modulated signal will have a smaller amplitude.
  - This changing size of amplitude in the modulated signal is how information about the voice signal is embedded.
There are 4 categories of modulation, but in this section, we’ll focus on the two main types:
- Analog and digital modulation

There are many reasons why modulation is used, but the main one is for efficiency.
Consider a voice signal that will be transmitted. A person’s voice waves are typically at a frequency of 100 Hz to 300 Hz.
- $f=300 \text{ Hz}, \ c=3\times10^8 \text{m/s} \implies \lambda=1\times10^6 \text{ m}$
An antenna would then have to be 250 km long!

$$ l=\frac{\lambda}{4}=\frac{1\times10^6 \text{ m}}{4}=250,000 \text{ m} $$

It is much more efficient to transmit the data with a high frequency wave (the carrier signal) to make the antenna size more reasonable.