Understanding radio broadcasting

From the Swadhin Bangla Betar Kendra to today's FM favorites, radio has occupied an important space in our national history. FM radio is our unfaltering companion during a jam-clogged city commute or a jukebox that plays our desired tracks. We all enjoy this marvelous gift of science, but not all of us know how it actually works.
In its most basic form, radio broadcast takes place in two ways: Amplitude Modulation (AM) and Frequency Modulation (FM). Historically, AM was invented first. In the backdrop of this development, communication was limited to one-to-one transfer of information over the telephone channel. AM enabled data transmission from one user to multiple listeners. It involves multiplying the signal frequency with a generated high frequency electromagnetic wave, called a carrier. The result is a modulated wave whose amplitude depends on the amplitude of the input signal. Reception of this AM wave can be done with a simple diode detector and low pass filter. Ease of use of this technology culminated in its rapid adoption by news and mass-media services, after first successful commercial demonstration back in 1906. Acronyms like MW (Medium Wave) or SW (Short Wave) refer to AM modes of propagation and span in frequency from 535 kHz to 1705 kHz.
However, some disadvantages of the AM soon became apparent. First, it was vastly affected by transmission channel noise, and second, it could not transmit high fidelity audio. AM radio has a limited audio range from 200Hz to 5 kHz, which limits its usefulness to speech mainly. To address these limitations, Edwin Armstrong proposed the concept of FM in 1935. Unfortunately, this genius engineer had to fight Radio Corporation of America during his later years regarding patent rights. Years of legal battle left him mentally tattered and culminated in a tragic suicide. He was later posthumously recognized as the originator of the FM radio, though.
A FM transmitter is similar to an AM, with the exception that here the frequency of the carrier wave is varied according to the instantaneous amplitude of the input signal. An FM receiver circuitry is quite complex, but can be accomplished in a number of different ways. The most popular scheme is the Phase-Locked Loop (PLL). In this method, a voltage-controlled frequency oscillator tracks the input FM wave frequency through continuous comparison of phase differences.
The advent of FM elevated radio broadcasting to a whole new level. Noise and voltage spikes due to lightning could now be eliminated completely at the receiver by amplitude limiting the output signal. Furthermore, Wide Band FM (WBFM) system can deliver broad dynamic music with a range from 30Hz to 15 kHz. Carrier of an FM can swivel from 88 MHz to 108 MHz in the VHF region.
Even with apparent drawbacks, AM modulation is still used worldwide by news channels and long-distance voice broadcasters. Short wavelength of an AM wave reflects back from the ionosphere and covers a longer area. Besides, it requires lesser bandwidth compared to FM, giving rise to higher spectral data efficiency. However, in the urban metropolitan context, FM has the especial ability to overcome obstancles. A high frequency wave undergoes lesser absorption when passing through obstacles such as building walls. FM is therefore better suited for city recipients.
So the next time we tune into Bangladesh Betar in MW or Radio Foorti in FM, we should not forget the sacrifices of unsung heroes like Armstrong that made this amazing technology possible.

The writer is a Senior Year Undergrad Student, Dept. of EEE, BUET.