How is the amplitude component of an audio signal encoded in an FM signal?

On Dec 14, 9:19 pm, snipped-for-privacy@hughes.net wrote: > In the given explanations of basic FM that I have read, I can > understand how a single audio tone modulates the carrier frequency as > the carrier shifts frequency as it follows the audio wave form. How > are dynamics in amplitude of the audio signal addressed? In the > typical examples, there doesn't seem that there would be any > difference if the amplitude of the audio signal where to be increased > or decreased.

More audio amplitude into the modulator translates to larger frequency changes. Higher frequency audio yields faster frequency changes. Amplitude variations on the modulated carrier are intentionally limited (clipped) in the reciever. One broadcast modulation monitor used essentially a monostable triggered on zero crossing after the carrier was heterodyned down to 700kHz. It had extremely linear demod characteristics.

BTW, your VCR uses FM modulation to encode the 'Y' (luminance) signal for recording onto the tape. That 'Y' FM carrier is also the bias for recording the down converted chroma information. Video on an FM carrier was first done by Ray Dolby at Ampex in the early 1950's. Yes, THAT Ray Dolby.

GG

The frequency of the tone only tells the FM transmitter how fast it should move back and forth around the center frequency.

The amplitude of that tone tells the FM transmitter how much that deviation should be.

So a low level of modulating signal at 1KHz will have the FM transmitter move above and below its center frequency at a 1KHz rate, but it will only move a tiny amount either way.

A large level of modulating signal at 1KHz will have the FM transmitter move above and below its center frequency at a 1KHz rate, but it will move a far larger amount either way.

A real FM transmitter has the modulating signal applied to the oscillator in the transmitter. Take out the coupling capacitors, and you can move the transmitter's frequency by applying a DC voltage to that modulation input point. Vary the DC voltage. A small voltage will shift the transmitter a tiny bit, a large voltage will shift the transmitter frequency more. But this will be static, it will be like you've adjusted the tuning control of the transmitter. But, vary the DC voltage rapidly with your hand, and the trasnmitter's frequency will follow. How rapidly you move the level control will determine how fast the frequency moves about. How much you turn the control will determine how far the frequency moves.

Michael

_____________

Amplitude dynamics of the program audio are conveyed by the amount of deviation of the FM carrier from its center frequency. Higher amounts of deviation produce greater audio output on an FM receiver.

Here's the way I explain it on a "gut level" basis:

Imagine that you had a transmitter that consisted of nothing but a pure carrier frequency, which was controlled by a big knob on the front. There is no level control, only frequency. You attach a lever to that knob, so that when the lever wiggles back and forth the carrier frequency changes.

Now connect that lever to a diaphragm so that sound waves can do the wiggling. Loud sounds cause bigger lever excursions (and hence bigger frequency excursions) than soft sounds. The frequency of sound vibration (pitch) controls how rapidly the lever makes each full excursion in carrier frequency.

The equivalent AM transmitter would be one that has a fixed carrier frequency and the lever and knob control the amplitude.

Best regards,

Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, FREE Signal Generator Science with your sound card!

Then you don't really understand FM at all, and are probably confusing yourself with overly-complicated mental models.

Let's ignore stereo for the moment -- you can ask about it in a separate post if you must, but first understand how mono FM works.

The FM transmitter has some sort of a "line in" input, and it accepts an audio-frequency signal. Within the range of voltages that the transmitter accepts, there will be a 1:1 correspondence between the instantaneous voltage at the input and the frequency out. So if the FM carrier is at 92.3MHz, for example, +1V on line in will cause the transmitter to transmit at 92.4 (i.e. 100kHz up), where -1V on line in will cause the transmitter to transmit at 92.2 (i.e. 100kHz down).

For basic FM, THAT'S IT. To reiterate:

Frequency out = carrier + some constant * volts in.

At the receiver, there's a circuit that monitors the deviation of the received frequency from the expected, and the circuit's job is to perform the following:

volts out = some other constant * (frequency in - carrier).

For basic FM, THAT'S IT.

Now, "real" broadcast FM has a bunch of folderol and whoop-de-doos attached to it, to make sure that the signal out isn't of too wide a bandwidth, that the average frequency stays insanely close to the carrier, that there's stereo, possibly that there's elevator music encoded on the carrier, and other things that have accreted onto the standard over the years.

But once you understand basic FM, the rest should come a lot easier.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

Much thanks to all. Sometimes I need to get my head around stuff like this but while reading the posts there was a bit of illumination happening and the explanation is all there -- I just need to ponder what has been said, draw a few pictures, and create a place in my head to put it. The text book illustrations were leading me to the wrong conclusions. Thanks again for the better explanations.

Check this out:

I don't know if it helps, but it sure is K3WL!!!!! ;-)

If they were changing the frequency of the modulating signal more slowly, the sidebands wouldn't be so dense - - - it just hit me, this is frequency modulation where the modulating signal is itself frequency modulated.

OK, this should have been lesson 19. Sorry. ;-)

Cheers! Rich