3. An Introduction to Pulse Modulation
- An Introduction to Pulse Modulation
Pulse Modulation is a technique that uses a carrier wave made not of a smooth sine wave, but of a periodic sequence of rectangular pulses. The characteristics of these pulses are then altered to carry the message information.
Pulse Modulation is divided into two main categories:
- Analog Pulse Modulation: Where a property of the pulse (like its height or width) is varied continuously to match the analog message signal.
- Digital Modulation: Where the analog signal is first converted into a digital form (1s and 0s) and then used to modulate the carrier.
Exploring Analog Pulse Modulation
In analog pulse modulation, one of three characteristics of the pulse is varied:
- Pulse Amplitude Modulation (PAM): The amplitude (height) of each pulse in the carrier wave is varied in proportion to the instantaneous amplitude of the message signal.
- Pulse Width Modulation (PWM): The duration (width) of each pulse is varied according to the message signal’s amplitude. The height of the pulses remains constant.
- Pulse Position Modulation (PPM): The position (timing) of each pulse is shifted relative to a reference point, based on the amplitude of the message signal. The height and width of the pulses remain constant.
Comparison of Analog Pulse Techniques
The table below summarizes the key differences between these three techniques.
| PAM | PWM | PPM |
| Amplitude is varied | Width is varied | Position is varied |
| Bandwidth depends on the width of the pulse | Bandwidth depends on the rise time of the pulse | Bandwidth depends on the rise time of the pulse |
| Instantaneous transmitter power varies with the amplitude of the pulses | Instantaneous transmitter power varies with the amplitude and width of the pulses | Instantaneous transmitter power remains constant with the width of the pulses |
| System complexity is high | System complexity is low | System complexity is low |
| Noise interference is high | Noise interference is low | Noise interference is low |
| It is similar to amplitude modulation | It is similar to frequency modulation | It is similar to phase modulation |
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While varying the physical properties of pulses is useful, converting a signal into a digital code of 1s and 0s opens up even more possibilities. This is achieved through a process called Pulse Code Modulation.
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