This blog post explains the principle behind how sounds from tens of kilometers away are transmitted so vividly through the radio, and examines the development of information transmission technology through this process.
Turn on the radio and tune the frequency, and voices flow out. For generations who grew up with radios, TVs, and telephones, this seems utterly natural. Yet just a century ago, hearing someone’s voice from afar was unimaginable. Voices from people talking tens of kilometers away come through as vividly as if they were speaking right in the room. How is this possible?
Sound is a wave that uses air as its medium. That is, sound is transmitted as vibrations originating from a sound source spread through the air. When these air vibrations strike our eardrums, our brains interpret them appropriately to extract information about the sound. How we perceive sound depends on how the air vibrates. For example, we perceive a high-frequency sound as high-pitched and a low-frequency sound as low-pitched; waves of various frequencies combine to form a single sound. Two sounds we perceive as different are waves created by different combinations of frequencies. Conversely, two air waves created by the same combination of frequencies sound the same.
Another characteristic of sound is its transmission speed. Sound travels at different speeds depending on the medium; in air, it moves at approximately 340 m/s. This naturally limits how far sound can travel. If a machine could perfectly mimic the air vibrations produced when a person speaks, a listener would be unable to distinguish whether the sound came from a machine or a human voice. Utilizing this principle, if the vibration pattern of a person’s voice in Region A is transmitted to Region B, enabling a machine to replicate the human speech identically, the sound could be heard in Region B.
However, it is impossible to transmit sound information from one region to another while preserving its original waveform. A simple example: Z’s voice spoken here is barely audible 100 meters away. This is because the wave’s intensity gradually diminishes with distance, so the original air vibration pattern is no longer maintained at the 100-meter point. Furthermore, the phenomenon where sound intensity decreases exponentially with distance practically makes direct conversation with someone at a great distance impossible.
Therefore, to transmit sound information obtained in Region A to Region B, the sound information must be processed into a form suitable for transmission. The process of processing sound information into a form that is easy to transmit over long distances is called modulation. Modulation carries information by varying one of the amplitude, frequency, or phase of a carrier wave, which is the simplest form of wave, such as a sine wave. While various modulation methods exist, commonly used ones include AM (Amplitude Modulation) and FM (Frequency Modulation). AM transmits sound signals by varying amplitude, while FM transmits sound information by varying frequency. Each method differs in noise immunity and transmission distance, and they are utilized for various purposes.
Conversely, at location B, the received signal must undergo a reverse processing step to restore the original sound; this process is called demodulation. Demodulation is the process of removing the carrier wave from the radio wave, extracting only the altered portion to obtain the desired information. In summary, sound is modulated and transmitted at location A, and after receiving the signal at location B, demodulation allows the sound from location A to be heard at location B. This is the fundamental principle of radio operation. When a broadcasting station modulates an audio signal and transmits it via radio waves, a radio receiver picks up these waves, demodulates them, and restores the audio signal, enabling the listener to hear the radio broadcast.
Using carrier waves at multiple frequencies also allows the simultaneous transmission of several different audio signals. By modulating carrier waves at different frequencies, multiple audio signals can be transmitted at once without mixing. Radio broadcasting involves transmitting sound information by loading it onto the carrier wave at the frequency assigned to each broadcaster. The frequency we tune to when listening to the radio is the carrier wave’s frequency. A radio receives all frequency components, then reduces the other frequency components while amplifying only the target frequency component, thereby extracting only the sound information corresponding to the specific broadcast.
This technology is not limited to voice signals alone. Similar principles apply to TV broadcasting and modern digital communications. While digital signals use more complex modulation methods, the fundamental process of transmitting and restoring information remains the same. This enables us to communicate in real-time with people far away, as we do today. As information transmission technology via radio waves advanced, it led to the emergence of various communication devices beyond just radios, significantly transforming our lives.
