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rude man submitted a new PF Insights post
Modulation vs. Beating Confusion
Continue reading the Original PF Insights Post.
Modulation vs. Beating Confusion
Continue reading the Original PF Insights Post.
Averagesupernova said:-
I think that the word beat was originally used interchangeably with mixing. A BFO used in a SSB or CW (morse) receiver is in fact mixed with the IF in order to generate an audio signal. It is NOT linear. The ear is in fact non-linear but we don't beat a couple of MHz signals together in our ear to get an audible signal. The non-linear process has to occur in the radio, not the ear.
I agree with the idea. But I could understand someone including beating in their definition of modulation, basically using modulation as a catch all term for any signal "mixing".meBigGuy said:I disagree. I am in total support of rude man. Everything I am saying is in his paper.
When you linearly add two sine waves, as described in the OP paper, no new frequencies are created, so there is no way to create the equivalent of a IF frequency. The beats experienced during the tuning of a piano in no way represent a prior art with regard to a superhet architecture.
The summed signals only have the appearance of a modulated signal. They were not created by modulation. They could be created by a true modulator that started with an (x+y)/2 carrier, but in that case the x and y frequencies would be newly created by the modulator.
Remember, the disagreement here is with regard to this sentence in an article discussing the history of the superheterodyne receiver:
"in it the author remarked that the modulation (or mixing) principle was really nothing new, being already known to piano tuners who traditionally used a tuning fork to beat against the piano string’s vibrations."
The summed time domain waveform ONLY APPEARS as a modulated signal. Its method of creation is of no value in a superhet architecture since no new frequencies are created in the frequency domain. That is a key point, and cannot be ignored. In any truly modulated or mixed signal, new frequencies are actually created.
The beating of linearly summed sinewaves is in no way (either practically or mathematically) similar in principle to mixing or modulating to produce true new frequencies.
Fell free to write up the terms as you want, such that one could consider piano tuning beating in any way similar to superhet mixing or true modulation.
Just because the beating signal looks like a signal created by modulation does not mean the process to create it in any way involved modulation.
may have a little more validity than would appear. When was it determined that it is the non-linearity of our ears that create the perception of a new signal with signals that are simply summed together and listened to? Was this knowledge responsible for the idea of superhet? Who was the first person to understand that non-linearity is required?in it the author remarked that the modulation (or mixing) principle was really nothing new, being already known to piano tuners who traditionally used a tuning fork to beat against the piano string’s vibrations.
Averagesupernova said:When was it determined that it is the non-linearity of our ears that create the perception of a new signal with signals that are simply summed together and listened to?
You can claim to be rapidly and changing the volume of a tone with a volume control but this is not all that is happening. You ARE generating new frequencies at the rate you are moving the volume control. The same thing when you walk through the room in your example. But in the walk through example it is happening in the ear.meBigGuy said:That ear thing may be a real phenomenon, but is not the cause of the beat we hear. The beat we hear is the same as what we hear when we walk through a reflective room with a 1KHz tone playing. It is caused by actual increases and decreases in amplitude (wavelength of 1KHz = 1 foot). THERE IS NO NEW FREQUENCY. (well, not exactly, because dopplar from moving effectively changes the single tone to 2 tones)
Except the human each which is not linear.There is no non linearity of any kind involved (needed?) in the beating we hear when we sum two tones. PERIOD! It is detectable by a fully linear system.
Are you claiming that I have said the following? Because I have not.I repeat from my previous post: You can create the two tones (x and y) by modulating an (x+y)/2 carrier with an (x-y)/2 signal. That action will produce two new tones, x, and y. If piano tuners were doing that then I would agree.
Saying two tones in ANY way represents a modulated signal is the same as saying 1 tone represents an SSB signal. Is whistling a precursor to ssb modulation? After all, the signals happen to look the same, just as in the piano tuner case.
Averagesupernova said:You can claim to be rapidly and changing the volume of a tone with a volume control but this is not all that is happening. You ARE generating new frequencies at the rate you are moving the volume control. The same thing when you walk through the room in your example. But in the walk through example it is happening in the ear.
You can't actually say that since no one I know of has ears that hear in a linear manner. I am not saying a person would not hear any effect at all if hearing was linear but it likely would not be perceived as the same thing it is now.meBigGuy said:There is nothing non linear happening in the ear that is required to hear beating. If you used a perfectly linear microphone you would see the same thing. The envelope varies between 0 and 2A. That is what you hear. You can see it in a scope.
Where have I said that if you look at the spectrum we would see more than 2 signals after said 2 signals have been summed?If you look at a spectrum, there are no new frequencies created by the summation. The two sine waves are the frequencies created by the modulation of a (x+y)/2 carrier. When you look at them in the time domain they look exactly like a modulated (x+y)/2 carrier. Those two frequencies are ALL THAT EXISTS. There is no energy at any other frequency, and the envelope effect is detectable with a linear microphone.
Why would I not like that paragraph? That is exactly what I would expect.Just the fact that you sum the two frequencies creates the appearance of the results of modulation of a (x+y)/2 carrier. But, there is no spectral energy at (x+y)/2 (unless you want to venture into instantaneous frequency land)
In a room, when you move a microphone through it (forget the ear), you see peaks and valleys cause by summing of different phases. If you move through those at some rate v, then dopplar creates the equivalent of two tones (since there a different relative velocities to the reflective sources). Now, don't tell me dopplar is modulation, because it creates the appearance of 2 tones in a microphone, and they also appear in the spectrum analysis of the microphone output.
You are not going to like this next paragraph at first. Your comments about a volume control are an interesting phenomenon. That is 1 frequency varying in amplitude. What does it look like in a spectrum analyzer. It appears as two sine waves (sidebands of the modulation). The original sine wave is the equivalent of the (x+y)/2 carrier in the original example, and the volume control is the (x-y)/2 modulating signal.
This is really simple if you abandon preconceptions. Look at the trig identity and think about what it means:
The left side is 2 sine waves summed, which are EXACTLY IDENTICAL to the right side product, which represents an (x+y)/2 carrier modulated by an (x-y)/2 signal. You can think of your volume control being varied at an (x-y)/2 rate as the modulator (which it actually is). The hard part is "what happened to the (x+y)/2 carrier when I modulated it"
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/beat.html (replace the ear with a linear microphone and the effect is the same)
Again, you can't say that until someone has linear hearing. Good luck with that.meBigGuy said:Forget the meaning of beat. I agree that it gets thrown around in a way that confuses the issues. The question is whether the wah-wah effect of two sine waves is in anyway related to hetrodyning.
Yes, that seems to be the problem here. I thought I made it pretty clear what kind of "beats" I was referring to, but I also acknowledge that the term "beat" can include mixing. A clear example was already mentioned, to wit, the BFO, which of course is a mixing operation.Jeff Rosenbury said:This seems to be a semantic argument. It has far more to do with how we define terms than any actual disagreement.
rude man said:Yes, that seems to be the problem here. I thought I made it pretty clear what kind of "beats" I was referring to, but I also acknowledge that the term "beat" can include mixing. A clear example was already mentioned, to wit, the BFO, which of course is a mixing operation.
I do disagree totally with whoever thinks mixing is done in the ear to any audible extent. The lowest audible sound would have to be at the sum frequency, i.e. at twice the t-f frequency, which it clearly isn't; or it would have to be a very high harmonic of the difference frequency, which still would be at a very low frequency, near the lower end of audibility, which again is not at all what the tuner hears. So please, folks, forget about nonlinear ear response!
nsaspook said:Maybe I'm wrong here but in the BFO example I used no new RF frequencies are generated, only an audio envelope caused by the summing of the two RF frequencies. The detector diode functions as a envelope detector in this circuit. With the RF L1 there is no path for an audio frequency signal to exist or to induce voltage into the diode to audio transformer path.
meBigGuy said:Do most receivers do this at IF just before the detector? That makes the oscillator pretty simple. I agree that it is linear. You use to have to use them to insert a carrier for SSB (like the SX-43 I used to have).
Baluncore said:The BFO generates a replacement for a suppressed or missing carrier. The frequency of the BFO is on one shoulder of the IF channel. The IF signal is usually multiplied by the BFO, but in a simple system the BFO is linearly added to the IF signal prior to a diode detector. A capacitor to ground forms the LPF needed to remove the IFs from the audio difference frequency before the audio amplifier input. The diode in that situation is not just an envelope detector, it is also a non-linear mixer.
A similar technique was/is used to transmit a 1kHz master reference signal over frequency multiplexed telephone lines. The 1kHz was not transmitted directly but as the sum of two harmonics, of 2kHz and a 3kHz tones on one telephone line. At the receiver, the one signal with both harmonics was fed through a diode into a 1kHz filter. The output difference frequency of 1kHz generated by the non-linear diode, was independent of the phone line frequency multiplexers.
I think you have an important point on what functions are called vs what that do.Baluncore said:The BFO generates a replacement for a suppressed or missing carrier. The frequency of the BFO is on one shoulder of the IF channel. The IF signal is usually multiplied by the BFO, but in a simple system the BFO is linearly added to the IF signal prior to a diode detector. A capacitor to ground forms the LPF needed to remove the IFs from the audio difference frequency before the audio amplifier input. The diode in that situation is not just an envelope detector, it is also a non-linear mixer.
nsaspook said:Is it (the simple diode detector circuit) also a non-linear mixer when it demodulates the exact same signal (as on a spectrum analyzer) that's received as modulated AM when it's normally called and explained as an Envelope detector?
Averagesupernova said:Yes it is. You cannot demodulate without a carrier. Envelope detector is a bit of a misnomer. The audio signal follows exactly what the envelope is but the carrier is required. Think of it this way. A detector used in this manner is a device that intentionally generates intermodulation distortion. Several signals come into the detector (sidebands and carrier) and they intermodulate to form the audio.
IMD is also distinct from intentional modulation (such as a frequency mixer in superheterodyne receivers) where signals to be modulated are presented to an intentional nonlinear element (multiplied). See non-linearmixers such as mixer diodes and even single-transistor oscillator-mixer circuits. However, while the intermodulation products of the received signal with the local oscillator signal are intended, superheterodyne mixers can, at the same time, also produce unwanted intermodulation effects from strong signals near in frequency to the desired signal that fall within the passband of the receiver.
Averagesupernova said:Not sure what your last post is getting at nsaspook. I find you contradict yourself.