How does an amplifier inadvertently demodulate a radio signal?

[musichascolors]

Sorry, here's the demo



"I converted into mp3 and then ran it through a software program that rectified it. Then I lowpassed it so frequencies above 500 hz were removed (the carrier frequency is 558)"

Ok so i'll do another low pass at 225 hz (or 200 to be safe)

there won't be much audio in that case but yeah.

Also your comment is really helpful, thanks!

 

[jim hardy]

That doesn't look rectified...

 

[musichascolors]

Well I don't think it's rectified in the audio example

 

[AlexCaledin]

During WW2 the landlord of a British public house called to police to report hearing "someone speaking German up my chimney". Turned out they weren't far from a BBC transmitter used to broadcast messages to Europe, including some in German. As I heard it the chimney was metal lined and somehow a joint partially rectified the signal causing the transmissions to become audible.

More likely, the strong RF current created a magnetic force which's proportional to the square of current, thus having the audio frequency and turning that chimney into a sort of speaker...
As for a joint, it could make RF sparks also creating quadratic force.

 

[Averagesupernova]

More likely, the strong RF current created a magnetic force which's proportional to the square of current, thus having the audio frequency and turning that chimney into a sort of speaker...
As for a joint, it could make RF sparks also creating quadratic force.

You won't hear it until it is detected which was likely done by the joint in the chimney. If all it takes is strong RF then every conductor at every transmitter site would be blasting with some sort of audio.

 

[jim hardy]

early rectifiers were copper oxide... my guess would be a joint in the roof flashing somewhere

 

[Baluncore]

To demodulate a signal requires a non-linear electrical junction such as a semiconductor.

There are many natural semiconductors available in an amplifier's environment. For example; zinc oxide, copper oxide and carborundum grinding grit. Any signal connector contaminated with a semiconductor particle can play a part in the demodulation.

 

[Baluncore]

During WW2 the landlord of a British public house called to police to report hearing "someone speaking German up my chimney". Turned out they weren't far from a BBC transmitter used to broadcast messages to Europe, including some in German. As I heard it the chimney was metal lined and somehow a joint partially rectified the signal causing the transmissions to become audible.

Which reminded me of another semiconductor, discovered in, and named after Germany.
Germanium was concentrated by plants that then become coal. The coal is compressed over time then, when the coal is burned, the germanium vapour condensed on the walls of the refinery or power station flue. One of the original sources of germanium for semiconductors was from the chimney stacks of German furnaces. So a coal fire below a metal lined chimney would be quite capable of condensing the semiconductor crystal needed to demodulate the local signal.

 

[sophiecentaur]

So is tremolo not a good analogy in that case, if the carrier frequency isn't changing in amplitude?

Tremolo is a fair way to describe FM as the carrier frequency is varied by the modulating signal. When the frequency deviation is high the carrier can actually be seen to sweep from side to side when you look with a spectrum analyser . That would be, for instance if you swept the carrier frequency from +10kHZ to -10kHz relative to the steady value (of 100kHz) at a rate of, say 50Hz. But sometimes the frequency deviation is low and the carrier deviation might only be +/-1kHz with a modulation frequency of 2kHz. Then the spectrum of the modulated signal really becomes unrecognisable to the eye. Nonetheless, the system works and a suitable FM demodulator can dig the modulating signal out of the received signal. The Maths of FM is a nightmare compared with AM.
On the whole, though, analogies are not very fruitful and you need to use Maths in order to get a good grip of signalling processes. It is a good thing that AM, at least, can be done in pictures!

 

[jim hardy]

On the whole, though, analogies are not very fruitful and you need to use Maths in order to get a good grip of signalling processes. It is a good thing that AM, at least, can be done in pictures!

What latched in FM for me was tinkering with Phase Locked Loops (PLL's).
LM567 gives a DC output proportional to how far off center frequency it must operate to maintain phase lock.
So it'll 'de-frequency-modulate' with ease.
I once built a PLL gizmo that modulated two ultrasonic carriers , 22 and 28 khz, with telephone "TouchTone" DTMF audio tones and added it to the plant PA system to identify which of the PA stations was active.

This is a great learner's textbook on that fifty year old technology.
https://ia801608.us.archive.org/30/...s_5800304/1972_Signetics_PLL_Applications.pdf

old jim

 

[mheslep]

To demodulate a signal requires a non-linear electrical junction such as a semiconductor...

I think you mean mixing a signal (frequency shifting) requires a nonlinear element. Demodulation can be done in some cases with a simple linear filter.

 

[sophiecentaur]

So it'll 'de-frequency-modulate' with ease.

There are so many ways of demodulating an FM signal. If you off tune an AM receiver, you can 'slope detect' an fm signal by the AM detector producing an output which is proportional to the carrier frequency over the sloping response of the edge of the IF filter. That is an actual FM demodulator and there are a lot of such designs. Then there is the phase detector - which is what your PLL feedback signal is actually doing. They all have their advantages and disadvantages and the arrival of Integrated Circuits (tarrraaa!) caused a flurry of designs.

 

[Baluncore]

Demodulation can be done in some cases with a simple linear filter.

How do you demodulate AM or FM with a linear filter?

 

[sophiecentaur]

How do you demodulate AM or FM with a linear filter?

You will always need an amplitude detector of some sort as well as the linear filter. So you still need a non-linear element.

 

[jim hardy]

I learned FM limiter-discriminator in high school
i think it was Foster-Seely with vacuum tube diodes
http://www.radio-electronics.com/in...n/foster-seeley-fm-detector-discriminator.php
but have now forgot the details
Never forget aligning them though !

 

[mheslep]

How do you demodulate AM or FM with a linear filter?

You will always need an amplitude detector of some sort as well as the linear filter. So you still need a non-linear element.

Yes, sorry, a detector is required.

 

[musichascolors]

Tremolo is a fair way to describe FM as the carrier frequency is varied by the modulating signal.

Musically speaking, tremolo is when the amplitude is modulated, vibrato is when the pitch is modulated, that's why I thought they might be good analogies.[QUOTE="sophiecentaur, post: 5470557, member: 199289"On the whole, though, analogies are not very fruitful and you need to use Maths in order to get a good grip of signalling processes. [/QUOTE]

I guess it depends on what one considers a good grip. I feel that I understand things better when I actually make them work and can observe them. Going to play around some more with tremolo/rectifier plugins when i get the chance. (Though the tremolo plugins I've found only go up to about 7 hz or so, so it makes it a bit challenging.

 

[sophiecentaur]

I guess it depends on what one considers a good grip.

I think that's an easy one. A 'good grip' means the ability to predict, accurately, what will happen in any particular circumstance. Very hard to do without Maths in pretty well any form of modulation except basic AM.
(But the point is well made about the difference between tremolo and vibrato. I think that true tremolo is probably only obtainable on an electronic processor / instrument.)

 

[jim hardy]

Musically speaking, tremolo is when the amplitude is modulated, vibrato is when the pitch is modulated,

Tremble vs Vibrate ? I'm having a hard time tying one word to pitch and the other to intensity, Is there a memory peg you musical folks use?

 

[sophiecentaur]

Vibrating your finger on a violin finger board will vary the tension and length of the string. That will be FM.
Tremolo is hard to produce on wind or string without pulling the frequency too. Perhaps a vibrating shutter on the air supply to a pipe organ?

 

[musichascolors]

Well guitar tremolo can be done by just picking the string very quickly up and down. Think Dick Dale.



Or it's done through an amplifier (electronics) in this case Buffalo Springfield



(Both are pretty "slow" obviously)Anyway... haha

There is a program called MondoMod that let's one do high speed FM/AM to audio signals. So i'll demo that and should help with my goals of intuitive understanding.

 

[jim hardy]

vibrato:
tremolo:

sorta hard for me to tell the difference

but from his descriptions they're two distinct muscular movements

i'll stick to a phonetic memory aid
i can visualize Robert Preston 'The Music Man' singing
Tremolo - "That starts with T and that stands for Tallness of carrier"
Vibrato - "Second letter I and that stands for pItch of carrier."

 

[musichascolors]

Yeah, they sound similar, which is why they're often used interchangeably, for example, Fender amplifiers often have tremolo switches that are called Vibrato.

But the Monomod software I'm going to try actually labels tremolo as AM and Vibrato as FM, so I'm feeling optimistic.

 

[sophiecentaur]

Yeah, they sound similar, which is why they're often used interchangeably, for example, Fender amplifiers often have tremolo switches that are called Vibrato.

But the Monomod software I'm going to try actually labels tremolo as AM and Vibrato as FM, so I'm feeling optimistic.

None of the above examples of "tremolo" are free of some frequency mod at the same time.
A tremolo arm / 'whammy' bar on a guitar stretch the strings so no well justified 'A.M. Only' example for a tremolo.

 

[Baluncore]

What happens when we add two similar amplitude sinewaves that have very close frequencies? We hear a beat frequency between them that is actually an AM tone = tremolo. Is our ear unable to separate the two individual frequencies because they are too close?

It is easy to believe that the beat frequency we hear must be a non-linear product, but that is certainly not the case in a linear environment. We are actually hearing the variation in amplitude of the sum. The difference frequency or detected envelope would be too low in frequency to be heard by our ear. We could feel the difference frequency alone as a physical push–pull, but not hear it.

A human, tuning a piano, compares the frequencies of two notes by listening to the tremolo of their linear sum. When those combined notes have no audible tremolo, it suggests to me that the two strings, (or oscillators), have “coupled” and so are then harmonically phase locked and being pulled to the same harmonic frequency.

 

[sophiecentaur]

We hear a beat frequency between them that is actually an AM tone = tremolo

I think that introducing the AM idea is adding complication. It doesn't strictly involve normal 'AM' because AM would have a carrier and two sidebands. It would be more like suppressed carrier AM and the carrier, if it were there, would be half way between the two tones and would demodulate as a tone of half the beat frequency, I think. The SCDSB signal looks like a 'row of beads' with peaks at twice the frequency of the peaks on the equivalent DSBAM signal.
I'm sure I could find a reference somewhere.

 

[Merlin3189]

Do you remember wow and flutter? I think they are FM.

 

[sophiecentaur]

Do you remember wow and flutter? I think they are FM.

Yep. The tape or disc speed variation - slow or fast - would change the frequency.
Btw, is that the lovely Magnus Pike on your avatar?

 

[jim hardy]

Is our ear unable to separate the two individual frequencies because they are too close?

The ear is a remarkable instrument in itself and is aided by quite the pattern recognizing computer.

http://www.cochlea.eu/en/cochlea/function
The cochlea is capable of exceptional sound analysis, in terms of both frequency and intensity.
The human cochlea allows the perception of sounds between 20 Hz and 20 000 Hz (nearly 10 octaves), with a resolution of 1/230 octave (from 3 Hz at 1000 Hz).
At 1000 Hz, the cochlea encodes acoustic pressures between 0 dB SPL (2 x 10-5 Pa) and 120 dB SPL (20 Pa).

Somebody who listens a lot can name a violinist just by listening. I've not heard of an electric computer so capable.

 

[Baluncore]

I think that introducing the AM idea is adding complication.

I think that complication is necessary to understand the situation.

When the separation of the two signals is less than the bandwidth of the analyser, there can be no sidebands. Our ear is unable to differentiate the two frequencies because they both stimulate the same hair cells in our “spectrum analysing” cochlea. We then hear the constructive–destructive interference between the two as their phases pass slowly. That sounds to us like AM.

Where the frequencies are not audibly different, where they interfere, or where the oscillators actually lock, is a fascinating corner or audiology.