Radio 5/28/21

Science with Richard Bleil

As a child, I always wanted to know how things work. Recognizing this curiosity, my mother bought me the complete twenty volume set of the Young People’s Science Encyclopedia by Encyclopedia Britannica which I read, cover to cover. Today, as the oldest young man you’ll ever meet, the way radios work is still a great source of fascination to me.

I’m sitting in a sub shop looking for underwater sea vessels as I have a stereo installed a few doors down. I don’t know why; I have no idea where I could even park a submarine, but on the drive here I was listening to the radio since my stereo actually works just barely well enough to do that, and wanted to write a bit about how radios work.

Driving along, the antenna (which is broken off and being replaced but still draws in stations today) is collecting the signals of every radio station within reception range. Yes, including country stations. The question becomes, then, how does it sort them out. That’s actually the role of the stereo unit itself; sorting out the stations.

Stations broadcast not one, but actually two waves simultaneously. What can be considered to be the “data wave”, the oscillations that correspond with the music itself, is carried on the “carrier wave”. The carrier is a giant ship that carries fighter and bomber aircraft, but that’s not important right now. Think about the carrier wave as a large ocean wave, but one that is not smooth. It’s carrying additional waves on top of it. It’s easy to see the big wave, but the smaller waves, ripples and interferences on top of it just looks like so much chaos and noise if you’re not looking closely.

The carrier waves are the waves that the stereo sorts out into individual stations. Each station is assigned a very specific carrier wave by the FCC so the stations in a given radio market don’t overlap. The stereo sorts them based on one of two ways. The waves are either “Frequency Modulated” (FM) or “Amplitude Modulated” (AM). “Frequency” is the number of waves that reach the antenna per second, so “Frequency Modulated” tells us how many waves, measured in mega hertz (mHz), will reach the antenna per second. Growing up, one of my favorite radio stations (that was bought out and no longer exists) was FM104.5 WDJX! Since frequency modulated carrier waves are measured in mHz (or one million waves per second), to listen to this station the radio had to be tuned to only listen to carrier waves with a frequency of 104.5 million waves per second. Amplitude modulated waves are based on the length of the wave, which is mathematically related to frequency. AM stations are measured in kilo hertz (kHz), so to listen to, oh, say 93.9 WFAKE, the stereo has to be tuned to listen to waves with amplitudes corresponding to the frequency of 93.9 thousand waves per second. It seems a little confusing to be talking about frequency when I was just talking about amplitude (amplitude is wavelength), but frequency times wavelength (amplitude) is equal to the constant c, or the speed of light, so if you know frequency you can precisely find amplitude and vice versa. This is why the two appear to be used interchangeably.

Old fashioned radio waves, then, had a second wave built onto the carrier wave, kind of modifying it so it’s jagged instead of smooth. This wave had (note the past tense) frequencies in the range of 15-17 kHz. Notice that this is much smaller than the carrier wave frequencies (especially in FM stations), and unlike the carrier waves these waves are in constant states of fluctuation. That’s because this is the frequency our ears can detect and interpret as sound. If you think about an opera singer, the higher pitched notes being sung are higher frequency, and the lower is notes are lower frequency. This is an extremely small range of frequencies compared to those of the carrier waves.

I wrote the above paragraph in past tense because today there’s a new twist. This is “analog” data, meaning the sound waves can have any value, from the minimum to the maximum allowed values. It’s these varying frequencies that are detected as differences in pitch, but modern stereos are often digital rather than analog. That means there is a digital signal added to the analog signal on the carrier wave. I’m certain this digital wave is simply beyond the range we can hear, although I’m not sure if it’s higher frequency or lower (I suspect higher as that would allow for more data to be sent). In other words, if the digital data is sent at a frequency of, say, 50 kHz, we simply would not be able to hear it, but a computer like the one in modern stereos could easily sort it out and interpret it. This analog data is “digital” because it only has two possible values, on or off. If “on” is, for example, 52 kHz, and “off” is, say, 57 kHz, there is no signal at 55 kHz. It’s not continuous, it’s either one or the other. This is how computers work, and the data sent by this digital signal tells the computer in the stereo, at any given instance, the frequency of sound to send to each speaker to make up the music to which we are listening. This digital signal can contain additional information as well, which is why many car stereos, like the one I’m having installed, can also display data like the song name and the artist name playing on the radio.

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