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I have been lusting over an MR78 for some time... but nobody around me knows how to restore them!
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Does anyone else like FM?
- Thread starter Mark1
- Start date
- Thread Starter
- #282
Originally I wanted to visualize how changes in loudness vs frequency of the music signal "looked" different in the modulated carrier. I can visualize it now after reading some poster's replies. But if at any given point in time, changing frequency and changing loudness are both just a change in frequency of the carrier how is a tuner/decoder to know if the tone is changing or the loudness is changing?
I probably have a basic misunderstanding of EM waves and what amplitude looks like in a signal in general. I imagine varying amplitude as it is usually depicted- varying distance of the crest to trough a visualization aide fiction, and that in reality this is fixed by the frequency and constant speed of the wave, and the amplitude is really just the strength of the electric and magnetic field.-analogous to a light of a given color at a varying brightness .
Maybe I should instead ask what does the music signal look like at the other end after demodulation by the tuner when amplitude/loudness changes? The frequency doesn't deviate then with loudness/amplitude, right?
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Dimitri
Senior Member
naahhh....they don't want to ask for the hundreds of dollars it would involve and brand themselves as "expensive" and/or might not have all the equipment - on bench / between the ears - necessary .
(aka , why mess around with a tuner, when you can be busy changing caps)
ok, I threw enough people under the bus, I'll stop now
wadude
Active Member
Since the ‘80s, AM has largely transformed into a medium for people that were bored by the music they grew up with and couldn’t find new music they wanted to explore.Does anyone like *AM*
That should be the question
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Fringe FM stations and AM have attracted conspiracy theorists.
wadude
Active Member
A vacuum filled with vacuity.
- Thread Starter
- #287
Let me simplify my last reply question and there's no obligation to answer
.But if both changing amplitude and changing frequency of a signal are represented by a changing frequency (or changing amplitude), how is a device to know at any given point in time if it's the signal's pitch or its loudness is changing?
- Thread Starter
- #288
It seems like now you're saying broadcasting in stereo should be quieter than mono even though it's louder when I switch to stereoWell...I guess depending on the number of pints, and considering "over there" beer is served warm
but if we are talking about the same thing, it's more like L+R -0.9dB
unless overmodulating, a station transmitting in Stereo will be about 1 db less loud than a station not going through an FM multiplexer ( which is probably what fueled the overprocessing "make it louder" disease )
View attachment 422676
. You electronics experts like to confuse us non-experts .
In an audio "signal" or sound in general:
Loudness is is how much the amplitude changes.
Frequency is a result of how quickly the amplitude changes.
Consider a guitar string. You pluck it, the string vibrates (amplitude). And wiggles at some rate (frequency)
You pluck another string. Maybe the string vibrates the same amount (amplitude) but either faster or slower (the frequency).
Over time, the amplitude decreases (loudness) but the frequency you hear remains the same.
There is no separate "frequency" data in the audio signal, only changes in amplitude, over time.
Here is 30 milliseconds of audio, representing change in voltage (amplitude) over that time. There are very many frequencies in that snippet. The faster the amplitude goes up and down, the higher the frequency. You can see some slow variations superimposed with much faster (and usually smaller) ups and downs. The data is read off a CD.
That voltage can drive a Voltage Controlled Oscillator to create the Carrier Wave the FM station broadcasts.
Some "98 Rock" station will transmit a carrier wave (sine) at 98MHz, modulated just like the waveform above, to increase or decreast the frequency by up to 75kHz (in the USA)
There is no "amplitude" change in the FM Signal. The Amplitude Change of the audio signal is traded for Frequenccy Change in the FM signal.
Your radio recovers the Audio Waveform from the varying frequency of the FM transmission.
The changing audio voltage drives the speaker. Faster/slower = frequency, more or less movement in and out= amplitude.
It's magic!
FM Stereo is even more magic, but basically the same idea - the amplitude of a wave is encoded as frequency change of a fixed amplitude carrier wave.
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Dimitri
Senior Member
In a past life 98.5MHz was my favorite frequency. I was using it syncronously and bidirectionally.
Assuming a max deviation of 75KHz ( or 0.075 in MHz ) which technically is as loud as it can get a station with 98.5 carrier will deviate ( "go") from 98.5 +0.075 to 98.5 -0.075 .
To use exact numbers, the carrier frequency will change from 98.425 to 98.575 and it will be LOUD.
If it's being modulated with a 1KHz tone, it will be doing this 98.425 to 98.575 exactly 1000 times per second.
If the audio tone frequency is decreased to exactly 100Hz , the carrier will be changing back and forth exactly 100 times per second from 98.425 to 98.575
If the audio tone level is decreased by half, we'll call the deviation half, so instead of a plus/minus 75KHz swing , we now have a 37.5KHz swing.
So the carrier frequency will change from 98.5 plus / minus 0.0375 MHz it will be half as loud than before but for a 1KHz tone, the carrier will still be changing from 98.4625 to 98.5375 MHz exactly 1000 times per second.
So yeah..there is frequency and frequency and the more the word frequency is used, the more confusing it sounds
F.M- Thread Starter
- #291
Thanks for the detailed explanation. The part I bolded is where I get hung up.-no amplitude change of the FM signal so how does the radio extract the audio signal amplitude or loudness when all it was fed is varying frequency? If the amplitude of the signal is not varied how does the radio discriminate in real time between the signal. frequency varying because of pitch or because of loudness?
August West
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I have fond memories of FM. In the early 1970's KSAN (San Francisco) used to broadcast live shows from a recording studio...Pacific High Recorders and then the Record Plant. I taped a bunch of these shows. Examples: Commander Cody New Riders John Cippolina in various bands Dr. Hook B.B. King and the list goes way on. And plenty of live concerts . The big one was all the Grateful Dead concerts they did especially NYE shows. There will always be a fondness for FM and those years in general.
Music is variable air pressure.
A microphone encodes the variable air pressure as variable voltage.
The variable voltage can be encoded as a variable frequency with a Voltage-Controlled Oscillator and transmitted with an antenna.
Another antenna can detect the variable frequency and an FM Demodulator can convert that back to a variable voltage.
The variable voltage can be used to drive a speaker.
The speaker causes variable air pressure.
Your ear detects the variable air pressure and you hear music.
- Thread Starter
- #294
The bolded part is what I don't get. If a radio input signal varied by amplitude, then it can know the loudness and pitch in real time, but if all the radio is fed is a FM variable frequency at a fixed amplitude it seems it could only determine the pitch of the audio signal ..I know you are saying frequency is interchangeable with amplitude, but I can't seem to picture it. Thanks for the explanations. I'm probably driving you crazy at this point
.I sort of understand what you're saying using Dimitri's post above that the carrier frequency deviates and the FM demodulator converts that to a variable voltage representing amplitude/loudness of the audio signal, and how often the carrier frequency deviates represent the frequency/pitch of the audio signal. It's just ahrd for me to visual how the radio does this in real time for some reason. But I guess frequency demodulations is just a reversal of modulation.
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Dimitri
Senior Member
The radio discriminates by using a discriminator of all things! Not being facetious, that's what it's called.
Foster Seeley Discriminator: FM Detector Demodulator » Electronics Notes
The Foster Seeley FM discriminator or detector was widely used in radios using discrete components find out all about it.
"Not crazy, my mom had me tested"
- Thread Starter
- #296
So this FM carrier frequency deviation is decoded by the tuner demodulator as a variable voltage representing audio signal amplitude/loudness, and how often the deviation represents the audio tone/pitch?In a past life 98.5MHz was my favorite frequency. I was using it syncronously and bidirectionally.
Assuming a max deviation of 75KHz ( or 0.075 in MHz ) which technically is as loud as it can get a station with 98.5 carrier will deviate ( "go") from 98.5 +0.075 to 98.5 -0.075 .
To use exact numbers, the carrier frequency will change from 98.425 to 98.575 and it will be LOUD.
If it's being modulated with a 1KHz tone, it will be doing this 98.425 to 98.575 exactly 1000 times per second.
If the audio tone frequency is decreased to exactly 100Hz , the carrier will be changing back and forth exactly 100 times per second from 98.425 to 98.575
If the audio tone level is decreased by half, we'll call the deviation half, so instead of a plus/minus 75KHz swing , we now have a 37.5KHz swing.
So the carrier frequency will change from 98.5 plus / minus 0.0375 MHz it will be half as loud than before but for a 1KHz tone, the carrier will still be changing from 98.4625 to 98.5375 MHz exactly 1000 times per second.
So yeah..there is frequency and frequency and the more the word frequency is used, the more confusing it sounds
Dimitri
Senior Member
Yes!
- Thread Starter
- #298
Well what confused me was visual diagrams showing for each cycle of the audio signal amplitude, the carrier signal being deviated multiple times (cycles) I guess since carrier is higher frequency. But if I think of the carrier signal deviating one cycle per audio signal amplitude cycle (even though this isn't actually correct because the audio wavelength is much longer than the carrier) then it makes sense.
.But bottom-line the carrier frequency deviates higher then lower a certain amount representing audio signal loudness/amplitude for a certain duration representing audio signal pitch/frequency.
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Here is a waveform representing one second of music voltage:
The FM carrier wave for "98 Rock" would oscillate around 98 million times in that second.
As the audio voltage increases, the FM carrier frequency increases.
As the audio voltage decreases, the FM carrier frequency decreases.
The change in carrier frequency represents the change in the audio voltage. That's all.
(No way I can draw that)
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So, this is utterly unvetted (i.e., it's the first YT video that popped up on google
), but it looked OK at a quick glance, and it might be helpful for those asking about how a frequency modulated carrier contains both frequency and amplitude information. By Berserkerus - Own work, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=5071748
