# mic ?

Discussion in 'General Electronics Chat' started by Mathematics!, Nov 29, 2009.

1. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
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Ok I am curious does an electric mic modulate a carrier wave by varying the frequency of the wave or the height of the wave.

I.E does the changing electrical current down a mic when you talk into it modulate the carrier by FM or AM.

Like if I took a mic and hooked it between the LC tank circuit would it modulate the LC ossilating at 1/2*pi sqrt(LC) with AM or FM?

Does the changing current of the mic vary the height of the voltage or the frequency of the wave?

This question has been bugging me for quite sometime if I have a ossilationing circuit with a carrier wave can I just modulate it using just a mic? If so what modulation scheme will it be FM or AM.

And how if you have already created a circuit that ossilates with a given carrier wave frequency. What do you do to modulate it with FM with a mic and after that what do you have to build to make it modulate AM with the mic on the same carrier frequency?

Note I am not changing the ossilating tank circuit just want to know how you change the modulation of the mic from FM to AM and vesa-versa?

Assuming I have built the ossilating/tank/tuning circuit --- amplifying circuit ---antenna
but somewhere in between the tank circuit and the amplifying circuit I need to modulate with my electric mic the carrier with FM then take out this circuit and replace it with modulating the carrier using AM with my electric mic.

I don't think this should be to hard of a circuit maybe I am wrong.

Thanks for any help

Last edited: Nov 29, 2009
2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
Most modern microphones will not work in the ways you are speculating about - http://www.soundonsound.com/sos/1995_articles/jun95/microphones.html

Historically, the first voice radio transmission was made with a carbon microphone. The principle there is of a varying resistance, caused by the changing conduction that occurs when sound energy is coupled in mechanically and causing the carbon granules to alter electrical resistance through the mass.

The system used a custom made 50 KHz alternator feeding an antenna (they called it an aerial - it would have been a Marconi type, most likely) through the microphone. The microphone had to be water cooled, and managed a brute-force AM modulation.

For more modern equipment, the microphone output is run through an appropriate preamp, and then modulates the oscillator signal by means of a mixer circuit. Try this link for some info - http://www.electronics-radio.com/articles/radio/receivers/rf-mixer/rf-mixing-basics.php

3. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Ok , no

What I thought is
forgetting for the moment amplifing a mic signal when you talk into the elctric mic isn't this signal the exact voice wave that you want to modulate on to the fix carrier sine wave. (and won't just adding the carrier wave and voice wave produce FM modulation or AM ????)

For instance if your carrier was rock 102 MHz (assume you have a tank/tuning circuit with resonanting frequency 102Mhz )

if you some how amplify the mic's signal and send it into the tuning/tank circuit will it modulate the carrier by AM or FM modulation by default?

And how would you change the mic output to do AM or FM visa-versa?

What I thought is that the microphone when spoken into already creates the equivalent voice wave that you would just need to add to the carrier wave and send to the antenna leaving out ofcourse amplifying/filtering ,...etc? I would think the mic produces the voice signal and the carrier wave just needs to have this signal add to it for transmision out the antenna?
Ofcourse amplification/filtering would need to be done in theory unless you had an extremely sensitive receiver.

I am still debating on what adding the 2 carrier and output mic signal would produce I would think it would vary the frequency as well as amplitude? confused on how you get the mic signal to just vary the amplituded or frequency but not both? Maybe I am overlooking something?

curious was it like condensor mic --- alternator --- antenna?
So the alternator was the equivalent of the tank circuit creating 50khz carrier wave. And the mic was the equivalent of an AM modulator/amplifier all in one? So then the mic was simple adding it's wave to the alternators 50khz wave and sending it out the antenna that's it?

Thanks for any clarity.

Last edited: Nov 29, 2009
4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
Not by default - you need to amplify and buffer the mictophone output and use a mixer to do the modulation.

The mixer does the modulation. The microphone output is not changed for AM vs. FM.

Mixers are not simple in operation, nor is their output a single frequency or variation in amplitude. You might need to spend some time reading about theory.

I may not have been clear - it was a carbon mic. But that was the hookup.

Just brute force AM. The microphone changed its resistivity according to the words shouted into it. That varied the amplitude of the alternator output reaching the antenna.

5. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
was the alternator gas powered or something because if somebody had to physically crank the alternator I would think it would be hard to crank a steady 50khz wave frequency?

Must have been hooked up so that the alternator supplied an fixed amount of ac voltage/current at 50khz and the mic was between the antenna and alternator and whenever you talked into the mic it would vary the resistance of the mic which would modulate the data as AM and send it out the anttena.

The more mic resistance the more voltage dropped and the shorter the amplitude , the less resistance the the least voltage drop across the mic the high the amplituted of the wave.

If this is it then the mic was just like a varible resistor to sound?

If I had one of these mic then I should be able to modulate the carrier just by setting a mic after the tank circuit and then amplifying the signal and sending it to antenna.

Correct me if I am wrong?

Also is the carbon mics the only type of mics that vary sound with resistance. (i.e the change in resistance of carbon granules )

What I read from you your mic link I would believe capacitor mic's would vary not the Amplitude since their is a fixed charge and the capacitance changes (from the moving of the capacitor plate) but this would vary the voltage since Q is fixed C = Q/V if capacitance varies then voltage varies? but this ofcourse doesn't imply that the frequency of the mic out put doesn't change. They both could change frequency and voltage. (so it's like AM and FM modulation altogether ummm)

Anyway I am trying to figure out if they make a mic like the carbon mic that modulates AM but modulates FM ?

Also can you still buy carbon mics and in your carbon mic rf example
How much power/voltage could a typical carbon mic be made to withstand?
I would think alot if it was AM broadcasting?

Eitherway I am curious about what the reciever was?
how they demodulated/tuned/amplified it out to a speaker was it just a diode or something for the demodulation???

6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
Perhaps steam power.

Think a bit - a variable resistance only applies to electricity. Sound is a mechanical phenomenon.

You really need some theory - the output of a microphone is a voltage that varies in a manner related to the sound waves striking the active element. Sound is composed of frequency elements (pitch) and amplitude elements (intensity). Some physics as well as electronics will help clear this up.

I've been too lazy to run down the material. This might have taken place before the De Forest Audion, so the receiver might have been a tuned circuit and detector diode.

Note: I felt guilty and ran down Fessenden's broadcast in December of 1906 - http://www.hammondmuseumofradio.org/fessenden-2006-recreation.html

Last edited: Nov 29, 2009
7. ### Audioguru New Member

Dec 20, 2007
9,411
896
An electret mic (not an electric mic) feeds a preamp that modulates the base of a colpitts oscillator transistor not its tuned circuit. But you could modulate the capacitance of a varactor diode and feed it to the tuned circuit to produce FM.

Whether the modulation is AM or is FM depends on how the modulation is done. Many simple FM transmitters produce both AM and FM at the same time.

8. ### BMorse Senior Member

Sep 26, 2009
2,675
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can you explain how a Frequency Modulated Transmitter can transmit Amplitude Modulated signals at the same time, I always though Frequency modulation was different than Amplitude modulation, that is why we needed separate AM or FM bands on radio receivers.... if indeed this was true, wouldn't the device be called something other than an FM transmitter?? I have seen and heard of devices that can take an AM signal and convert it to FM or vise-versa.

9. ### Audioguru New Member

Dec 20, 2007
9,411
896
Most simple FM transmitters modulate the amplitude of the oscillator producing AM in the FM frequency band.
But modulating the amplitude of the oscillator changes the capacitance of the transistor changing its frequency producing FM in addition to the AM. FM radios ignor the AM.

AM is old and was set at low frequencies. FM is newer and better and was set at high frequencies.

Airplane communications use AM at frequencies higher than the FM broadcast band. They use AM so that other airplanes can break-in if there is an emergency. With FM it is difficult to break-in because a break-in is interference and FM rejects interference.

10. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
Ok , first of all I thought the transistor in the colpitts oscillator and many others are just to make the LC tank circuit not damp out from wire resistance ,...etc etc (i.e it is for the cancelation of the small resistance in every LC circuit )

If you move the base of the transistor with the preamp electret mic signal how do you know it is modulating by changing the waves frequency???

Also what transistor are we talking about PNP or NPN ???

If the transmitter is varying both the amplitude and frequency then I would believe that the AM signal would never be on a constant channel since the wave frequency would be changing so I don't understand how you would ever beable to pick this up on an AM radio?

Second since both are varying the FM receiver should have times when the voice is really loud and times when it is really low ...etc since the amplitude is changing? ( voltage increase means louder sound )

Maybe this all gets down into the way it is demodulated. ( maybe the demodulation circuit keeps the volume fix in some way by fixing the amplitude of the wave or maybe the amplifier reaches some cutoff or something dun curious though)

Either way I am shakey on how to build demodulator circuits wikipedia is great for ossilator circuits but I have found not to much in how you demodulate and modulate stuff. It only tells you the method but doesn't explain or talk you thru how to design one. Is their any good sites that show you how to build modulator/demodulator circuits and why they are built the way they are?

To answer the second question quotes, is that AM is just varying the amplitude with a fix frequency , FM is varying the frequency with fix amplitude. You could use AM with an FM carrier wave in the mhz instead of the regular khz AM band. However then you would have to demodulate the AM signal from that carrier wave. You could also create an FM signal on an AM carrier wave in the khz but you then have to make the receiver demodulate the carrier waves FM signal. Normal all radios are created to demodulate by AM in the khz range on a radio and demodulate FM on the mhz range. In theory you could do it if you knew how to create the tuner/demodulator circuits for the reciever which is why I am wanting to learn how.

I can create the ossilator for any frequency I want I know how to amplify things but I am shaky on how to demodulate/modulate stuff on my random fix carrier frequency that I can produce using colpitts oscillator or variants???

FM transmitter only means it's reciever is a FM demodulator weather the FM varies the amplitude a little doesn't affect that it still is only concerned with the frequency to extract the voice it should still be called an FM transmitter/reciever pair.

I guess you could do the same thing with AM if the demodulator is not concerned with the changing frequency?

This all boils down into understanding how to make modulation/demodulation circuits any place I can go to learn these?

Last edited: Nov 30, 2009
11. ### Audioguru New Member

Dec 20, 2007
9,411
896
An AM radio works at only around 1MHz and will not pickup a signal at 100MHz which is the FM broadcast band.

The volume of an FM receiver depends on how much the carrier frequency is deviated, not on the signal strength (amplitude).

If an audio signal modulates the base of a colpitts oscillator then the transistor is turned on a little more and turned off a little which causes amplitude variations of its carrier output producing AM. When the transistor turns on a little and turns off a little then its capacitance changes which changes the frequency producing FM.

12. ### Mathematics! Thread Starter Senior Member

Jul 21, 2008
1,022
4
So then making a transmitter in this way will vary the amplitude and the frequency of the carrier. Correct?

And if their is an FM reciever it won't be effected by the changing amplitude in any way because it is extracting the voice data by frequency demodulating and ingores the changing amplitude.
Similarly I would think an AM transmitter that varied both amplitude and frequency could still be used with any AM reciever the demodulator circuit would ignore the changing frequency and just worry about the amplitude to extract it data.

Ofcourse you would have to use suitable carrier waves AM in the khz , FM in the mhz but I would think if you used these simple fm transmitters and just changed the carrier to khz am station you should beable to switch the fm transmitter into an am transmitter. Correct me if I am wrong?

For example this simple transmitter
http://www.reconnsworld.com/transmit/fm_trasm.gif

modulates the transistor base so it is producing am and fm modulation.
I would think if I just dropped the carrier frequency down to a AM station I could turn my radio to that AM station and here my voice.

If this is true then I would think it would be easier just to make transmitters that modulate both ways and then all you would have to do is change the carrier and you could go from am to fm.

Maybe I am overlooking something?