📡 Radio From Scratch

Amplitude Modulation

In this article we’ll look at the simplest, and earliest form of signal modulation.

This article builds on:

What is modulation?

Simply put, modulation is a way of embedding information within another wave, so that data can be recovered from the combined wave when broadcast.

Why modulate at all? There are benefits to being able to change both the frequency, and modulation or encoding technique, when broadcasting a signal. This will be clearer in a moment.

When modulating a signal, we have two signals of interest. First is the “carrier wave”, which is the original signal that will be modified in order to carry our data. For radio stations, an example carrier wave would be the 850 kHZ wave for the 850 AM station.

Then we have our “data”, or information which we want to transmit. For broadcast radio, this would be the voice or music information.

How does AM modulation work?

The carrier wave used for AM broadcast radio is of a much higher frequency of the signal we wish to transmit. In the US, AM broadcast frequencies range from 520 kHZ to 173 kHZ. A benefit to this is that the wavelength is much shorter. This enables us to reasonably construct antennae to receive it. An antenna for unmodulated voice would need to at least pick up waves at 155 hz. Our wavelength calculator shows that this wavelength would be 2 million meters when traveling as an electromagnetic wave, so we’d want a 1 million meter antenna. Expensive.

AM works by taking the carrier wave, and adjusting its amplitude or height by the value in the signal.

Lets visualize this for a 155hz tone, at the high end of the male adult speaking range.

Here is one full period of our signal tone. Singing this tone would look like several repetitions of this wave.

Lets imagine we’re broadcasting at 20 Khz. This is lower than AM radio is used in the US, but it makes for clearer visualization. The carrier wave would look like: (zoomed to the same scale as our voice above).

Depending on your screen size, you might not even be able to see the wave. It is a smooth wave just like our voice, but the period is much smaller since it is a higher frequency wave.

Essentially we multiply the values in the carrier by the value of the data at the corresponding point in time. That would look like this:

Now we have the data embedded in the carrier wave. Unfortunately, we’ve lost some information. We cannot tell when the signal was positive, and when it was negative. Rather than just multiply the values directly, we instead multiply by (1 + data value). This makes the multiplier always positive, so we can more easily recover the data.

And thats all it takes to modulate AM signals. Notice how the top surface of the combined wave looks just like our original data.

Next up we’ll look at implementing an AM demodulation pipeline in GNU radio.

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