Sound can be encoded onto an electromagnetic wave for transmission and reproduction using a process called modulation. Modulation allows the sound signal, which is in the form of varying air pressure waves (acoustic signal), to be converted into an electrical signal that can be transmitted through different mediums, such as wires or radio waves.
Here's a simplified explanation of the modulation process:
Analog-to-Digital Conversion: In modern systems, the original sound signal is first converted from analog to digital format. This involves sampling the sound wave at regular intervals and measuring the amplitude of the wave at each sample point. These measurements are then converted into a digital representation, typically using binary values (0s and 1s), creating a series of discrete samples that represent the sound wave.
Modulation: The digital representation of the sound signal is then modulated onto an electromagnetic carrier wave. The carrier wave is typically a high-frequency sinusoidal wave that serves as a carrier for the sound information. The two most common types of modulation used are amplitude modulation (AM) and frequency modulation (FM).
Amplitude Modulation (AM): In AM, the amplitude of the carrier wave is varied in proportion to the amplitude of the sound signal. The varying amplitude of the carrier wave corresponds to the variations in air pressure of the sound wave. The modulated wave now contains the original carrier wave with the sound information embedded in its amplitude variations.
Frequency Modulation (FM): In FM, instead of varying the amplitude, the frequency of the carrier wave is modulated in proportion to the sound signal. The changing frequency of the carrier wave represents the varying air pressure of the sound wave. The modulated wave now carries the original carrier wave with the sound information encoded in its frequency variations.
Transmission: The modulated electromagnetic wave, which now carries the encoded sound signal, can be transmitted through different mediums depending on the application. For example, if it's a radio transmission, the electromagnetic wave is transmitted wirelessly through the air. If it's a wired transmission, such as in telephone lines or audio cables, the modulated wave is sent through the conductive medium.
Demodulation: At the receiving end, the process is reversed through demodulation. The modulated wave is passed through a demodulator, which extracts the original sound signal by reversing the modulation process. The demodulator separates the carrier wave from the variations (amplitude or frequency) imposed on it and produces an electrical signal that represents the original sound wave.
Digital-to-Analog Conversion: If the sound signal was digitized in the first step, it may be converted back to an analog signal to drive a speaker or produce audible sound. This process is called digital-to-analog conversion (DAC). The digital samples are reconstructed into a continuous analog signal that can be amplified and sent to a speaker to reproduce the original sound wave.
By encoding the sound signal onto an electromagnetic wave using modulation and then decoding it at the receiving end, the original sound can be faithfully transmitted and reproduced, allowing us to hear sound from distant sources or through various devices like radios, telephones, or speakers.