Amplitude modulation -- misnamed?
in [DSP]
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From: Jerry Avins on 2 May 2010 12:14 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. �����������������������������������������������������������������������
From: Fred Marshall on 2 May 2010 20:52 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
From: dvsarwate on 2 May 2010 22:45 On May 2, 1:07 am, spop...(a)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)
From: Randy Yates on 2 May 2010 22:51 dvsarwate <dvsarwate(a)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(a)ieee.org % 'Mission (A World Record)', http://www.digitalsignallabs.com % *A New World Record*, ELO
From: glen herrmannsfeldt on 2 May 2010 22:14
Fred Marshall <fmarshallx(a)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? I asked about this some time ago in this group. If you do 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 |