Discussion in 'General Electronics Discussion' started by revv, Aug 3, 2016.

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1. ### revv

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1
Jun 16, 2012
I was watching this video

And at one point in the video (6m 34seconds in) they show a person speaking into a microphone and they say that the sound waves then travel on the stronger radio waves created by the alternator.

I seem to have trouble understanding this, what exactly is happening? How can the sound waves "travel" on the stronger radio waves created by the alternator?

Last edited: Aug 3, 2016
2. ### dorke

2,342
665
Jun 20, 2015
This is called modulation.
In this specific form shown in the video it is Amplitude Modulation (AM).

The two waves are "combined by multiplication" so that the final wave is no longer a fixed amplitude signal
i.e sinusoidal carrier.
The amplitude of the carrier wave(produced by the alternator) will vary according to the sound wave that modulates it.

Hope that helps.

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3. ### duke37

5,364
769
Jan 9, 2011
To generate a high frequency wave was very difficult and various designs of alternator were developed. To get a frequency of say 200kHz, the alternator had to have many poles and run as if the devil was after it. To send information, they used Morse code initially by switching the alternator on and off. After a while they developed a system of varying the strength of the signal so being able to send sound from a microphone.

This variation is known as amplitude modulation (AM) and the system has been used extensively with later transmitters. There is still one working alternator transmitter which is fired up once a year.

Look up 'alternator transmitter' in Wikipedia for a description of the Alexanderson transmitter

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4. ### revv

23
1
Jun 16, 2012
thank you guys so much! you rock!

5. ### revv

23
1
Jun 16, 2012
Im still not sure I understand how the sound wave actually "travels" on the carrier wave, how exactly does it do that?

I tried researching but I still don't quite understand.

6. ### Externet

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Aug 24, 2009
revv :
Tonight, shine a flashlight to a far, far target in your neighborhood. A friend there, can tell you if it is on.

Turn it off and on six times and your friend will tell you 'six'

The light propagated over the air and carried a signal from you to a far, far distance with the information you provided (six)
Same as radio. A oscillator of the proper frequency fed to an antenna can travel over the air to a receiver. You can superimpose, add, modulate that radio signal with 'six' times on/off or with a microphone circuitry.

The radio wave = light beam = the carrier. Travels far, very far.
The data or on/off = voice = modulation. Can be recovered at the receiving end = demodulated = listening or counting on/off

7. ### davennModerator

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Sep 5, 2009
you are describing a CW ( Morse ) transmission not an audio modulated transmission

8. ### duke37

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Jan 9, 2011
To make things a little more complicated, as @dorke said, the carrier and audio are multiplied together. The result is the carrier plus an upper sideband and a lower sideband. The sound information is in the sidebands and it is possible use only one of these and dispense with the carrier, saving a lot of transmitter power.

There is no sound wave transmitted, only the amplitude modulated carrier wave.

This morning I have been listening to the SDR receiver at the secret nuclear bunker near Crewe. This will send the received demodulated signal over the net and shows a waterfall display so that you can see the carrier and sidebands as they are transmitted. I do not know where you are but you should be able to access this site. Google 'hack green sdr'. You may not see a AM signal there, most amateurs have gone over to SSB. The sunday morning AM net has now finished (0845 BST).

9. ### davennModerator

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Sep 5, 2009

dorke wasn't correct, that should be mixed not multiplied

10. ### duke37

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Jan 9, 2011
If I remember right, sin A * sin B = sin (A+B) + sin (A -B)
In other words this form of mixing is done by multiplying.

Audio mixing in a mixer is done by adding.

11. ### davennModerator

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Sep 5, 2009
hmmmm not sure ?? ..... I will have to go do some reading
as I don't see any multiplication of the carrier freq and the modulation freq occurring

12. ### Externet

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Aug 24, 2009
Yes, tried to simplify. Perhaps putting colored filters in the light beam would be better.

13. ### BobK

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Jan 5, 2010
From the AARL handbook:

"The mixer, on the other hand, multiplies the instantaneous voltages of the two signals together to produce the output at each point in time"

This is as opposed to an audio mixer which adds the two inputs.

One way to do AM mixing is the have a variable gain amplifier controlled by the modulating signal and pass the carrier signal through it. This effectively multiplies the signals together since the carrier amplitude out is proportional to the modulation input in. (Of course a further complication is two quandrant vs four quadrant multiplication)

Bob

14. ### davennModerator

13,782
1,934
Sep 5, 2009
that still doesn't satisfy me

this really seems to be a really bad use of the word multiply ... think about it

I have a 2 kHz audio signal and a 2 MHz carrier if you multiply that you get a 4 MHz carrier ????
or even worse, 2000 x 4,000,000

we know that isn't what is happening

rather than sidebands peaking at around ± 2kHz of the 2MHz carrier

15. ### duke37

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Jan 9, 2011
But it is not the frequencies that you multiply, it is the instantaneous voltages of the two inputs to generate a new waveform.
The traditional high level anode modulation varies the output voltage due to the change in HT voltage.

Interestingly,a similar process occurs during demodulation where a non linear circuit mixes the sidebands with the carrier.