Radio:

FM, probably the only radio you have actually listened to. Probably the only radio you can still listen to as AM is becoming more and more defunct. But what does FM stand for, why is it head and shoulders better for audio quality than AM and how did that make it THE musical medium? 

FM: Freqy Movement?

FM stands for frequency modulation. If we look back at AM, amplitude modulation, it communicated audio by changing the signal strength of a consistent wavelength carrier wave. That wavelength is what defines the frequency. So what happens if we modulate or change the frequency of the carrier signal? We get FM.

Let’s talk about the information signal. The range of human hearing is from 20 Hz to 20,000 Hz. FM can replicate between 50 Hz to 15,000 Hz for listening as there are technical applications for the higher frequency signals. This loss of range on the top end is relatively negligible, therefore FM is considered “high-fidelity”. The Department of Defense manual lays out the frequency breakdown clearly.

FM radio is broadcast in stereo, meaning two channels of information. In practice, they generate three signals prior to applying the modulation:

• • the L + R (left + right) signal in the range of 50 to 15,000 Hz.

• • a 19 kHz pilot carrier.

• • the L-R signal centered on a 38 kHz pilot carrier (which is suppressed) that ranges from 23 to 53 kHz .

The receiver uses this additional information encoded in the information signal to replicate the stereo transmission of the music. FM is also more immune to outside noise than AM. For something to affect the signal, it would have to modulate the frequency. This is much more difficult compared to simply interfering with the signal strength of an AM broadcast. 

Fidelity: The Feature That Propelled Music on FM and its Growth

FM was invented in the 1930s credited to Edwin Armstrong. The first commercial FM stations were simply a simulcast of their AM sister stations; a facet of radio regulation used today to get a Low Powered FM license at less cost by using it to simulcast what your AM signal is pushing.

Now FM’s ability to broadcast high fidelity stereo audio made it immediately more useful for enjoying music. But the one challenge every new medium faces is traction with the consuming public. AM took off as it was the first broadcast medium and was helped along by government usage like in the case of FDR’s Fireside Chats. FM didn’t quite have that sort of boost and had to crawl into its own after many years of being the “alternative” radio band.  

In the 70’s FM was being used for Album Oriented Rock formats, due to the small enthusiast audience that listened for longer cuts compared to the King of AM, the Top 40 format. The major audience switch from AM to FM happened in the late 70s, during which those Top 40 stations made the jump and it was all downhill for AM after that. AM has been relegated to sports and talk formats as AM is most suited to transmit spoken word. But what’s all this talk about format and how does that impact a station’s decision making? We’ll cover that next.

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