Stuff about FM radio
8/12/20 9:23 AM
Questions I had and Answers to those questions

How does radio condense an entire spectrum of audible sound and bottleneck it through a single extremely specific frequency?
The answer is modulation. There isn’t really anything special about radio that does this, in fact it's prevalent in pretty much any electronic audio processing process, especially in the realm of electronic music production and sound design within that. What can be thought of as a base signal (which is called the carrier signal) that oscillates at a desired broadcasting frequency is used, well, as a base. Then another signal (called the modulating signal, yknow, since it does the modulating) is combined with that to produce a new waveform that, when played back, produces the desired sound to be broadcast.

The new waveform produced by the modulated signal won"t really be "regular" like the base signal (which is just a pure sine wave of the desired frequency), but it will have some kind of consistency that can be calculated and mapped compared to the exactness of the carrier signal. This deviation from the main frequency allows for more sound dynamics to be sent through the airwaves, so the higher quality sound you want the more bandwidth you will have to take up, since your modulated signal could be deviating a lot from the more regular sine wave.

See Notes for more specifics about the details and common conventions for FM radio in the US.

What is the range of frequencies for broadcasting?
It depends on the country, but all specifics for information about American standards are available in the Notes section.

Who determines what frequencies are used for what? I.e. who is in charge of making sure that there isn’t a bunch of signals randomly interfering with each other?


Difference between AM and FM:
AM stands for amplitude modulation
FM stands for frequency modulation

Frequency modulation can be used for re-shaping the waveform of the sounds you want to send through a given radio frequency. These waveforms are not going to be regular when compared to the sine wave of the base frequency, but there exists a margin of deviation as to how far off the waveform deviates in frequency from the sine wave of that given frequency.

***(The following information is based heavily on Rohde Schwarz’s YouTube video Understanding Frequency Modulation. Sauce.)***

Let us be more precise. In FM, the carrier frequency (f_c) changes based on the instantaneous level of the modulating signal. The current distance (in Hz) of the carrier from the normal center frequency (f_0) is the deviation (∂f). The maximum deviation (∂f_max) is a function of the transmitter design and configuration (which is mostly arbitrary and will depend on the exact application you are using these signals for).

The deviation ratio (DR) is the ration between the max frequency deviation to the maximum modulating frequency. So,
DR = (max. freq. dev.)/(max. mod. freq.)

For example, music has a high maximum modulating frequency, so a high deviation is needed for good audio quality. Human speech on the other hand has a lower max. mod. freq. so a lower deviation can be used and have it still come out audible.

FM signals are categorized as wideband if DR ≥ 1 and narrowband if DR < 1.

Wideband FM usually has a maximum deviation of about 75 kHz and a DR of about 5. It is used for playing music over commercial frequencies, like the ones your car picks up. As a result, the more bandwidth a signal has the higher the deviation can be, so the more fidelity the audio has. But, it takes up more space on the frequency continuum, so the intervals between them are larger and there are fewer available frequencies to use. Specifically, in the United States FM stations range from 87.9 – 107.9 MHz and each used frequency is spaced 200 kHz apart, so that would translate to about 100 kHz of breathing room for the bandwidth on each side of the normal center frequency (f_0).

You can see this for yourself when tuning in the radio as you normally would. FM broadcast stations are spaced 200 kHz apart, so that results in how the frequencies are divvied up, as in 97.1, 97.3, 97.5, 97.7, and so on. If you’ve ever tried to tune in to a radio frequency with an even decimal place, such as 97.6, it will either be largely static or just nearly-inaudible remnants of one or both of the neighboring main frequencies. These are known as guard bands, so that each designated frequency has enough room to breathe.

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