I know sometimes speakers/amps will pick up a radio signal. I'm confused as to how the signal is demodulated accidentally. Are the signals picked up exclusively AM, or do FM signals get picked up as well?
A common way is via rectification. If you rectify an RF signal, you will generally get the envelope information, which can be audio. Certainly this can happen for AM signals, but it can happen for FM signals as well, if there is some filtering going on in the rectification path.I know sometimes speakers/amps will pick up a radio signal. I'm confused as to how the signal is demodulated accidentally. Are the signals picked up exclusively AM, or do FM signals get picked up as well?
and just to expand on Berkeman's rectification commentsThanks!
I'm quite new to the subject. So still trying to gain an intuitive understanding...
We have no idea of your background.I'm quite new to the subject. So still trying to gain an intuitive understanding...
I know what a rectifier does in terms of converting to one polarity, but I don't yet understand it's relevance to this application.We have no idea of your background.
Have you ever studied radio or electronics at all? Ever build a crystal set as a kid ?
Do you know what the guys meant by "rectification" ?
My understanding (possibly incorrect) is that the audio signal is just summed onto the carrier frequency (or is that not the case?)
I think mathematically Amplitude Modulation is a multiplication not addition...
If so, why does the RF frequency need to be filtered, wouldn't it be inaudible anyway?
Remember, 100 years ago the intent was to recover voice or operate an electromagnet for telegraphy.
In that crystal set the inductance of the headphones would not pass RF , XL = 2πfL
in communication radios the detected output is handed to audio frequency amplifiers for delivery to a loud speaker.
So removing the RF allows simpler audio amplifiers. Still done today.
And why is the rectification necessary (why do we have to remove everything below 0 volts)?
There are other methods, see http://web.mit.edu/6.02/www/s2012/handouts/14.pdf
but the answer to your question is it's the easiest way to demodulate.
in the early days of radio it was easy to find crystals or metal oxides that performed rectification , So that's what they did.
In grade school i had a crystal set.
in high school (early 1960's ) when i was taught radio basics we used vacuum tube rectifiers .
Think of it like this -
Look at left waveform in post #5.
What is the short term average of the last few RF cycles ? It's zero because the average of a symmetric wave is zero.
Now look at the middle waveform in post #5.
What is the short term average of the last few rectified RF cycles? It's a positive value that varies at the rate of the signal we wished to convey. How very convenient !
Mathematically, a short term averager is a low pass filter.
Amplitude modulation basics
When an amplitude modulated signal is created, the amplitude of the signal is varied in line with the variations in intensity of the sound wave. In this way the overall amplitude or envelope of the carrier is modulated to carry the audio signal. Here the envelope of the carrier can be seen to change in line with the modulating signal.
I looked up Tremolo in Webster and found it refers to either pitch or loudness.So is tremolo not a good analogy in that case, if the carrier frequency isn't changing in amplitude?
Pitch is incorrect, tremolo and vibrato are often used interchangeably but they're not the same thing. Pitch(vibrato) would be frequency modulation and volume (tremolo) would be amplitude modulation. That's why I thought it would be applicable(?)The Fourier transfer term for carrier doesn't change amplitude
I looked up Tremolo in Webster and found it refers to either pitch or loudness.
So i left that one lone.
in that case you have it right.Pitch is incorrect, tremolo and vibrato are often used interchangeably but they're not the same thing. Pitch(vibrato) would be frequency modulation and volume (tremolo) would be amplitude modulation. That's why I thought it would be applicable(?)
is it hard to learn ? Excel was nothing but frustration and led to serial mouse-icide.Matlab seems to be software of choice at the moment for communications work, at least in academia. One can generate a signal, perform any kind of modulation, characterize a channel, dump statistics, and plot resulrs all in a few dozen lines of code.
Easy to learn and powerful, that's not the drawback. Matlab is proprietary, costs few hundred $ for non-students, more for some of the plugin packages, which often causes problems I find as soon as one wants to ship code around to others outside your own shop. Also, its not quite a proper programming language, so as soon as a problem with some complexity and size comes along, requiring many pieces to cooperate, or requires high, C type compute performance, or something that requires longer term maintenance, Matlab becomes troublesome. For these reasons, the last serious team project I worked used Matlab to prototype all the technically challenging algorithms, and then converted the most successful Matlab algorithms to C++.is it hard to learn ? Excel was nothing but frustration and led to serial mouse-icide.