How is the amplitude component of an audio signal encoded in

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.

 

[Michael Black]

(wrnchbndr@hughes.net) writes:

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.

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

 

On Dec 14, 9:19 pm, wrnchb...@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

 

[Richard Fry]

How are dynamics in amplitude of the audio signal addressed?
_____________

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.

 

[Bob Masta]

On Fri, 14 Dec 2007 21:19:03 -0800 (PST), wrnchbndr@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.

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
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

 

[Tim Wescott]

On Fri, 14 Dec 2007 21:19:03 -0800, wrnchbndr 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.

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.

 

[Rich Grise]

On Fri, 14 Dec 2007 21:19:03 -0800, wrnchbndr 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.

Check this out:
http://www.bitscope.com/software/dso/replay/11.gif

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