# Amplitude modulation -- misnamed?

Started by May 2, 2010
```Shouldn't amplitude modulation really be called "Magnitude
Modulation"?

After all, in FM, the amplitude is still being modulated, but
the magnitude is constant.

Or maybe I have too much time on my hands.

S.
```
```On 2 Mai, 08:07, spop...@speedymail.org (Steve Pope) wrote:

> Or maybe I have too much time on my hands.

...maybe...

Rune
```
```Steve Pope <spope33@speedymail.org> wrote:
> Shouldn't amplitude modulation really be called "Magnitude
> Modulation"?

> After all, in FM, the amplitude is still being modulated, but
> the magnitude is constant.

When I learned it, in the expression A*sin(wt+b)

A is the amplitude, w the angular frequency and b the phase offset.

There are also expressions like "for the coherent case, add
amplitude, for the incoherent case add magnitude."

The magnitude, then, is proportional to A**2.

So it seems to me that for radio, where the signal is coherent,
it is amplitude modulation, but for optical communication, such
as in optical fibers, it should be magnitude modulation.
(Even for diode laser source, two sources aren't usually coherent
with each other, and the coherence length and coherence time
aren't all that long.)

-- glen
```
```On 2.5.10 9:07 , Steve Pope wrote:
> Shouldn't amplitude modulation really be called "Magnitude
> Modulation"?
>
> After all, in FM, the amplitude is still being modulated, but
> the magnitude is constant.

Would you please explain. The creation of sidebands does
not change the amplitude of the combination of carrier
and all sidebands in FM.

AM for sinusoidal modulation:

f(t) = A*(1 + M*cos(omega_m*t))*cos(omega_c*t)

FM for sinusoidal modulation:

f(t) = A*cos(omega_c + d*cos(omega_m*t))

Asterisks (*) are for plain old multiplication.

The amplitude of the FM signal is constant A.

--

Tauno Voipio (MSEE, OH2UG)
tauno voipio (at) iki fi
```
```"Steve Pope" <spope33@speedymail.org> wrote in message
news:hrj4qa\$j1e\$1@blue.rahul.net...
> Shouldn't amplitude modulation really be called "Magnitude
> Modulation"?
> After all, in FM, the amplitude is still being modulated, but
> the magnitude is constant.

Both signals, the modulating signal and the transmitted signal
are broadly cisoidal in nature and have an amplitude, or magnitude,
depending on how you choose to call it.

You seem to want to restrict the use of amplitude to refer to
the modualting signal and the use of  magnitude to refer to
the transmiited signal, when either term can legitimately be
used for either signal.

```
```On 5/2/2010 2:07 AM, Steve Pope wrote:
> Shouldn't amplitude modulation really be called "Magnitude
> Modulation"?
>
> After all, in FM, the amplitude is still being modulated, but
> the magnitude is constant.
>
> Or maybe I have too much time on my hands.

Doesn't "modulate" mean "vary"? In AM, the amplitude; magnitude; size,
whatever is varied. In FM, it (whatever you call it) remains constant.

Jerry
--
"I view the progress of science as ... the slow erosion of the tendency
to dichotomize." --Barbara Smuts, U. Mich.
&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;
```
```Jerry Avins wrote:
> On 5/2/2010 2:07 AM, Steve Pope wrote:
>> Shouldn't amplitude modulation really be called "Magnitude
>> Modulation"?
>>
>> After all, in FM, the amplitude is still being modulated, but
>> the magnitude is constant.
>>
>> Or maybe I have too much time on my hands.
>
> Doesn't "modulate" mean "vary"? In AM, the amplitude; magnitude; size,
> whatever is varied. In FM, it (whatever you call it) remains constant.
>
> Jerry

Having done some suppressed carrier control systems stuff, I was rather
drawn to amplitude (with sign) and magnitude (sign independent).  I
think that provides a useful framework here:

In suppressed carrier, as long as the modulating signal is always
positive then the carrier phase doesn't "flip".  This would be the same
as <=100% modulation in AM.  The results would be indistinguishable ..
and the carrier would not actually be supressed either.  It's only
supressed when the average modulating signal is zero.

Then, if the AM modulation is greater than 100% (in some sense yet to be
defined) then it depends on how it's handled doesn't it?

- If the modulation is with amplitude and sign then the carrier phase
has to be able to flip 180 degrees.  We don't call this "AM" any more
though...rather DSB-SC. There's a lot of history with useful AM
modulators that can't do this - so thus a fair bit of language built up
around AM.

- If the modulation is with amplitude but not sign (i.e. magnitude) then
the carrier phase doesn't know when to flip (even if it can in the
implementation) and an underlying modulating signal that has zero
crossings (say a sinusoid) is grossly distorted at what would have
otherwise been the zero-crossings when the magnitude "signal" is created.

From Tauno Voipio's post:
f(t) = A*(1 + M*cos(omega_m*t))*cos(omega_c*t)
where A<=1 is a condition for distortionless modulation in plain old AM
and combined with the "1" in there as a bias, holds up the carrier to
avoid making it "magnitude modulation" in a normal AM modulator.

So, I would NOT call it magnitude modulation under any circumstances
that I can think of.

And, FM has nothing do to with the original question, even though it was
mentioned as an example.

Fred

```
```Fred Marshall <fmarshallx@remove_the_xacm.org> wrote:
> Jerry Avins wrote:
>> On 5/2/2010 2:07 AM, Steve Pope wrote:
>>> Shouldn't amplitude modulation really be called "Magnitude
>>> Modulation"?

>>> After all, in FM, the amplitude is still being modulated, but
>>> the magnitude is constant.

>>> Or maybe I have too much time on my hands.

>> Doesn't "modulate" mean "vary"? In AM, the amplitude; magnitude; size,
>> whatever is varied. In FM, it (whatever you call it) remains constant.

> Having done some suppressed carrier control systems stuff, I was rather
> drawn to amplitude (with sign) and magnitude (sign independent).  I
> think that provides a useful framework here:

> In suppressed carrier, as long as the modulating signal is always
> positive then the carrier phase doesn't "flip".  This would be the same
> as <=100% modulation in AM.  The results would be indistinguishable ..
> and the carrier would not actually be supressed either.  It's only
> supressed when the average modulating signal is zero.

It seems that the usual uses for DSB-SC do use both signs, though.

> Then, if the AM modulation is greater than 100% (in some sense yet to be
> defined) then it depends on how it's handled doesn't it?

synchronous modulation, or use a four quadrant multiplier as the
mixer, then you get both signs.  I don't know in so much detail
how they build modulators or receivers now to know.

> - If the modulation is with amplitude and sign then the carrier phase
> has to be able to flip 180 degrees.  We don't call this "AM" any more
> though...rather DSB-SC. There's a lot of history with useful AM
> modulators that can't do this - so thus a fair bit of language built up
> around AM.

How many demodulators can do it?  Is there any point to building
the modulators?

> - If the modulation is with amplitude but not sign (i.e. magnitude) then
> the carrier phase doesn't know when to flip (even if it can in the
> implementation) and an underlying modulating signal that has zero
> crossings (say a sinusoid) is grossly distorted at what would have
> otherwise been the zero-crossings when the magnitude "signal" is created.

As I noted earlier, this is most likely in the case of optical
modulation.  Current modulating and LED or Laser diode, for
example.  That seems closer to magnitude, since you normally can't
detect the phase of an optical signal.  A four quadrant multiplier
would be difficult to build, for example.  (There are a lot of
optical tricks that I don't know about, though.)

> From Tauno Voipio's post:
> f(t) = A*(1 + M*cos(omega_m*t))*cos(omega_c*t)
> where A<=1 is a condition for distortionless modulation in plain old AM
> and combined with the "1" in there as a bias, holds up the carrier to
> avoid making it "magnitude modulation" in a normal AM modulator.

> So, I would NOT call it magnitude modulation under any circumstances
> that I can think of.

-- glen
```
```On May 2, 1:07&#4294967295;am, spop...@speedymail.org (Steve Pope) wondered:
> Shouldn't amplitude modulation really be called "Magnitude
> Modulation"?

You say to-mato, I say tom-ato....

To me, if x(t) = A sin(wt + b) where A > 0, then the
magnitude of x(t) is a time-varying function having
value |A sin(wt_0 + b)| at time t = t_0 while the amplitude
is A and is fixed for all time.  So, if A varies slowly with
respect to the carrier frequency w, the result is called
amplitude modulation, and not magnitude modulation.

I don't agree with Glen Herrmansfeldt's expressions

> "for the coherent case, add amplitude, for the
> incoherent case add magnitude."

either.  Usually, it is the squared amplitudes that
get added (square-law detectors, remember).
Adding two equal power coherent sinusoids
quadruples the power because the amplitudes
add and the power is proportional to the square
of the amplitude.  Adding two equal power
noncoherent sinusoids (in this context, think
orthogonal signals) only doubles the power.

--Dilip Sarwate

P.S.  My students have requested that the word
incoherent not be used in such contexts, only
noncoherent.  They want to reserve the word
incoherent to describe my lectures (and postings
to comp.dsp)
```
```dvsarwate <dvsarwate@gmail.com> writes:

> P.S.  My students have requested that the word
> incoherent not be used in such contexts, only
> noncoherent.  They want to reserve the word
> incoherent to describe my lectures (and postings
> to comp.dsp)

Dilip, then they're just out-of-phase with reason. :)
--
Randy Yates                      % "How's life on earth?
Digital Signal Labs              %  ... What is it worth?"
mailto://yates@ieee.org          % 'Mission (A World Record)',
http://www.digitalsignallabs.com % *A New World Record*, ELO
```