AM, The Original Speech Transmission Mode

Here’s a question: when did you last listen to an AM radio station? If your answer is “recently”, chances are you are in the minority.

You might ask: why should you listen to AM? And you’d have a point, after all FM, digital, online, and satellite stations offer much higher quality audio, stereo, and meta information, and can now be received almost anywhere. Even digital receivers are pretty cheap now, and it’s by no means uncommon for them to not even feature the AM broadcast band at all. Certainly this has driven an exodus of listeners to the extent that AM radio has been in slow decline for decades, indeed it’s disappearing completely in some European countries.

Amplitude Modulation

On your radio receiver “AM” will probably refer to the broadcast band between about 550KHz and 1600kHz depending on where you are in the world, also known as “Medium Wave”. But AM itself is not related to a particular frequency, instead it refers to “Amplitude Modulation”, and was the original modulation mode for conveying speech and music. You’ll also find it on the shortwave (HF) broadcast bands and in CB radio. With a modified sideband structure it provided the modulation scheme for analogue television.

The simplest form of AM transmitter takes the output of an audio power amplifier and mixes it with that of an RF power amplifier. This is normally done by applying the audio to the power supply of the RF amplifier and is referred to as series modulation. The textbooks describe the effect of AM modulation in the time domain with the supposition of a pair of waveforms to form a modulation envelope, but the story is better told in the frequency domain. There an AM signal has a central constant carrier wave with symmetric sidebands containing the audio information. The overall bandwidth of the transmission is thus twice the bandwidth of the audio.

Looking at the two graphs depicting the AM signal, we can start to appreciate some of the system’s flaws.

In the time domain, the varying amplitude of the signal means that it is vulnerable to interference from  any other RF sources within its bandwidth whose amplitude also varies. Lightning strikes, switching transients, noise from faulty switch-mode power supplies and more can all be heard when they occur in the background of an AM transmission. It’s not all bad, if you live on a farm you might have found it useful to always know when your electric fence is working from the tick… tick… tick… on your radio.

Meanwhile, in the frequency domain, the width of those sidebands is proportional to the bandwidth of the audio being modulated. For a higher quality audio source with a 15kHz bandwidth for example, the total AM bandwidth would be 30kHz. The demands of the spectrum regulators to fit as many broadcast stations as possible into the available bandwidth mean that depending where in the world you live each channel will be allocated only 9 or 10 kHz, so the practical audio bandwidth is reduced to around 5kHz. Thus as well as being vulnerable to interference, the AM broadcast also sounds poor, especially for music, compared to its FM competitor.

Cheap Plastic

The final nail in the coffin for the quality of AM broadcasts comes from the typical modern AM receiver. In AM broadcasting’s early years, a radio receiver was a premium  product, comparable perhaps to a modern 4K TV set. The radio manufacturers of the 1920s and 1930s were in fierce competition to deliver technical innovations to improve their product’s sound quality in all areas, and the best radios had extremely high quality amplifiers and speaker cabinets. This was the golden age of AM broadcasting, when AM stations had more bandwidth in a less crowded band, and were a primary entertainment medium.

By contrast a typical plastic portable AM radio of today is built at as low a cost as possible with very little attention to the quality of the sound it delivers. Even with a bandwidth-restricted modern broadcast station it is obvious just how poor quality its sound can be if you compare a cheap plastic portable with a high-end 1930s radio or a modern high-end AM receiver like the Tivoli Model One. If most people’s experience with AM is low-bandwidth audio accompanied by electrical noise and played back on poor quality electronics through a cheap speaker, is there any wonder it’s taken up by ever fewer people?

Golden-Eared Hams

When AM is treated with some care and attention in both transmitter and receiver though, it is capable of delivering very good results. Surprisingly there is a small area where you will find AM enthusiasts hard at work refining their art, though you won’t find them on your broadcast radio. The golden age of AM in amateur radio is long past, but there is a dedicated band of AM diehards for whom the quality of the received audio is more important than achieving distance at all costs. Some of them maintain huge “boat anchor” transmitters from many decades ago, while others are at the cutting edge with DSP-based AM generation at low-level, or class D modulators driving class E RF amplifiers. They do this in spite of considerable institutional opposition from their national amateur radio lobby groups, or maybe in some cases even because of it.

Whether or not we still have AM broadcasts in the coming decades will depend on whether or not a viable niche can be found for them. Sometimes, as in the case of the British long-wave BBC Radio 4 on 198kHz, it has even attained a cult status, sparked public demonstrations when its demise is predicted, and become a National Treasure. But that is the exception, the overall picture is one of decline, which leaves one wondering whether it is not the medium that is at fault but its regulation. The AM broadcast bands are still run according to rules and expectations with their roots in a different century, when a medium-wave frequency was a valuable asset and so-called “clear channel” high-powered stations covered entire continents. Perhaps if we recognised that those days have passed, allowed a slightly higher bandwidth at the expense of a lower channel count and created a constellation of low-power local stations with better quality sound, it might have a future. Otherwise if its current decline continues it can’t be long before we’ll be faced with a young relative approaching with a dusty transistor radio and asking “What does this do?” And as we start to explain to the child it’ll dawn on us that with no broadcasts left to receive we’ll no longer be able get them into electronics the way so many of us did, with a crystal set.

Farmer listening to crystal radio image: public domain via Wikimedia Commons.