Basic Radio: Down in the Weeds...

[Jinx]

I follow the basic idea laid out here, but can someone provide a good analogy on how a song becomes a radio wave? So many references like this one gloss over the details. When they say: “The DJ’s voice is modulated onto that carrier wave by varying the amplitude…”, they lose me! Is there something analogous I can picture?

Is the amplitude modulation no different than how one might pitcure the Richter scale or an EKG recording wave patterns on a scrolling sheet of paper? Except, for radio, the paper would be the carrier frequency? Would an AM signal be as periodic as pictured at this link, or would it vary in-step with a song’s peaks and valleys? Could I see the wave if the radio were connected to an oscilloscope?

Last, what does the carrier frequency look like when the airwaves go silent because the DJ fails to start the next song? Do I simply see an unmodulated, nice even sine wave? What would the amplitude be? …Or, is it simply a flat line when broadcasting silence?

Just struggling to conceptualize this beyond what all the common references say.

[Napier]

Modulating an AM wave would be like this. Imagine you have a big sheet of styrofoam as a microphone. It vibrates a little with the sound - you can imagine if there were a very loud roaring noise near it, you could feel the styrofoam vibrating.
OK, now you have some electronic oscillator that keeps cycling smoothly back and forth in voltage, like a crank pushing a piston back and forth in space. But, it’s much faster, maybe a million times a second.
Now, you get an amplifier with a tiny little gain control knob on the front, so tiny you can attach it to your styrofoam sheet, and as the sheet vibrates, it turns the knob back and forth, making the amplifier more or less powerful.
Now you put your oscillator in the amplifier input, and on the amplifier output you place an antenna. The oscillator signal is much bigger, and it grows and shrinks a few percent as the styrofoam moves. When the announcer is not saying anything, the styrofoam stays still, and the 1 MHz signal going to the antenna has a constant strength, but when there is some sound, the signal gets a bit stronger and weaker with the sound.

[Tripler]

@Jinx

Is the amplitude modulation no different than how one might pitcure the Richter scale or an EKG recording wave patterns on a scrolling sheet of paper? Except, for radio, the paper would be the carrier frequency? Would an AM signal be as periodic as pictured at this link, or would it vary in-step with a song’s peaks and valleys? Could I see the wave if the radio were connected to an oscilloscope?

The amplitude modulation is pretty comparable to a Richter scale, or, beating a drum (think vibrations in a medium). The AM carrier wave is way too fast to vary in step with any song; sing a quick tune to yourself and draw a sine wave according to what you think the volume of your voice is. That’s the “unmodulated signal” or the information you’re carrying. Now take a pencil, and scribble scribble scribble underneath that information wave. Like, tiny little space between lines. That’s the carrier wave. The vibrations of power in the carrier wave is electronically converted into vibrations in the electrical signal going to the speaker. If you connected an oscilloscope to the speaker side of the demodulator, you would see the information wave. Your Richter paper/EKG scale isn’t the best analogy, because your “carrier wave”–the paper–doesn’t fluctuate, it simply exists, so it’s a frequency of like. . . infinity.

*Last, what does the carrier frequency look like when the airwaves go silent because the DJ fails to start the next song? Do I simply see an unmodulated, nice even sine wave? *

What would the amplitude be? …Or, is it simply a flat line when broadcasting silence?

Just struggling to conceptualize this beyond what all the common references say.

Yep. On your oscilloscope, you’d see a nice, clean sine wave because there’s no information (i.e. no hills or valleys) to fluctuate the power. The amplitude would be whatever power the station was broadcasting at, but it wouldn’t fluctuate because there’s no information.

I guess another example would be at a pond–a nice steady glass-like surface carries no information (no ripples or waves in the surface) and thus it’s quiet. You drop a stone in the end, and it’ll make those waves that carry in the hills and valleys of the water, and when they hit the shore (basically demodulate), you’ll hear the splish-splash on the shoreline.

Make sense?

Tripler
And on preview, I see Napier types faster than me.

[Apex_Rogers]

An AM radio signal looks pretty much like the picture in your link. The signal is contained in the envelope described by the peaks of the sinusoidal carrier wave. I think your analogy of the EKG machine or seismograph is actually pretty close to the mark. You can imagine how the needle shifts up and down on the machine as the paper travels across and apply this to following the path of the carrier wave and shifting up and down according to the signal you are transmitting. Of course, you need to realize that the signal is band-limited to frequencies a few orders of magnitude lower than that of the carrier wave, so the ups and downs are captured in the values of the peaks of the wave rather than the instantaneous values of the carrier wave.

If you were to look at a radio signal in an oscilloscope, you would see something very similar to the diagram, where the period of the signal is fixed and only the amplitude changes. This is true for Amplitude Modulation (AM), but isn’t so for Frequency Modulation (FM).

Take a look at the diagram on this Wikipedia page: Amplitude modulation - Wikipedia

[Crafter_Man]

AM is a little bit easier to describe.

Disclaimer: my description below is not 100% accurate, and I’m being loose & sloppy with terminology in an effort to explain the concepts better.

First of all, you need to understand that a sound wave is nothing more than an air pressure signal that varies over time. You can easily convert the pressure signal into a voltage signal using a microphone. This is what music looks like after a microphone converts it to a time-varying voltage.

In the video, the voltage is simply a representation of the air pressure signal. We can call this a “music wave.” An audio amplifier & speaker can convert the voltage signal back into an air pressure signal.

Now you ask… how do I transmit this over the radio?

Simple.

Using a frequency generator, first generate a sinusoidal electromagnetic wave of constant frequency and constant amplitude. The frequency of the wave should be between 520,000 Hz and 1,610,000 Hz (AM band). Something like this.

Notice how the amplitude is constant over time? Kinda boring, huh?

So here’s what you do next… vary the sine wave’s amplitude so that it follows the music wave. The frequency generator should have an input for doing this. So you simply take the output of the microphone and connect it to the frequency generators “amplitude control” input jack.

Look at this diagram. See the green sine wave? It has a constant amplitude over time. That’s the signal out of the frequency generator *before *you starting screwing around with its amplitude.

The red signal is the music wave; it is the voltage at the output of the microphone.

The blue signal is the signal out of the frequency generator *after *you connect the microphone into the frequency generator’s “amplitude control” input jack. The amplitude of the signal now “follows” the music. Pretty cool, huh?

Take the blue signal and broadcast it to the world using an RF amplifier and antenna.

On the receiving end - an AM radio - you need to tune to the same frequency. The radio uses a filter to “extract” ONLY the amplitude. Thus the output of the filter is the red music signal. This signal is fed to an audio amplifier and speaker.

[Derleth]

@Jinx

I follow the basic idea laid out here, but can someone provide a good analogy on how a song becomes a radio wave? So many references like this one gloss over the details. When they say: “The DJ’s voice is modulated onto that carrier wave by varying the amplitude…”, they lose me! Is there something analogous I can picture?

See below for a reasonably good image, but I think I can help you out with the words right here: “Modulated” means “modified.” The thing that is being modified is the “carrier wave,” which is a sine wave of a given frequency and amplitude. The carrier wave carries no information on its own besides the statement “someone’s broadcasting a carrier wave on this frequency with this amplitude.”

To use a carrier wave to carry information, you have to modify some aspect of it in a way that encodes the information. This can be as simple as on-off keying, when you just turn the carrier on and off to carry, for example, Morse code. The very earliest radios operated on this principle; voice and music only came later.

Is the amplitude modulation no different than how one might pitcure the Richter scale or an EKG recording wave patterns on a scrolling sheet of paper? Except, for radio, the paper would be the carrier frequency? Would an AM signal be as periodic as pictured at this link, or would it vary in-step with a song’s peaks and valleys? Could I see the wave if the radio were connected to an oscilloscope?

The best way I can explain it is via this picture. Note that it shows both AM (amplitude modulation) and FM (frequency modulation) in addition to the original signal waveform. This image is in the time domain, meaning that as you go left or right you are moving forwards and backwards in time. It’s just like an oscilloscope, for example, or an EKG.

Last, what does the carrier frequency look like when the airwaves go silent because the DJ fails to start the next song? Do I simply see an unmodulated, nice even sine wave?

Bingo. A perfect sine wave carries no information at all beyond “I exist at this frequency and amplitude.”

What would the amplitude be?

It depends on the station and what the FCC has allowed them to put out. ERP (effective radiated power) is subject to strict regulation because if it gets too large, the signal causes interference with other stations.

…Or, is it simply a flat line when broadcasting silence?

No, never, unless the station is off the air completely, or you’ve already decoded the signal and aren’t looking at the carrier wave at all anymore.

[Jinx]

Thanks all for the improved examples and links. I will have to read this a few times to sink in, but I am already starting to see the light…or, is that, hear the AM???