dvsarwate <dvsarwate@gmail.com> wrote: (snip)> 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).Yes, the term is supposed to be intensity, the (more or less) time averaged square of the signal. Somehow I was thinking about that when reading magnitude in the OP. In the case of vectors, magnitude is the square root of the square (dot product with itself). I don't know that it would be used for the square root of the intensity.> Adding two equal power coherent sinusoids > quadruples the power because the amplitudes > add and the power is proportional to the square > of the amplitude.Anywhere from 0 to 4, depending on phase.> Adding two equal power > noncoherent sinusoids (in this context, think > orthogonal signals) only doubles the power.So LED output is intensity modulated, not amplitude modulated?> 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)You will have to change a lot of optics books. -- glen
Amplitude modulation -- misnamed?
Started by ●May 2, 2010
Reply by ●May 3, 20102010-05-03
Reply by ●May 3, 20102010-05-03
On 5/2/2010 10:14 PM, glen herrmannsfeldt wrote:> Fred Marshall<fmarshallx@remove_the_xacm.org> wrote:...>> 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.Synchronous demodulation works, but you need to generate a replica of the carrier to implement it, usually with a PLL. (A narrow band-pass filter around the carrier frequency followed by high gain is the original way.) Once that carrier surrogate is available, it can be directly added to the signal, so a balanced modulator isn't really needed. The choices are then synchronous- or enhanced-carrier demodulation. ... Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
Reply by ●May 3, 20102010-05-03
On 5/3/2010 12:09 AM, glen herrmannsfeldt wrote: ...> So LED output is intensity modulated, not amplitude modulated?Strictly, yes. Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
Reply by ●May 3, 20102010-05-03
On 3 Mai, 04:45, dvsarwate <dvsarw...@gmail.com> wrote:> 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)Your students might be interested in a similar game we played here a few years ago: http://groups.google.no/group/comp.dsp/browse_frm/thread/46174ccbd583b5bb/34a749d251ddfc0e?hl=no&lnk=gst&q=dictionary#34a749d251ddfc0e Rune
Reply by ●May 3, 20102010-05-03
Jerry Avins wrote:> On 5/2/2010 10:14 PM, glen herrmannsfeldt wrote: >> Fred Marshall<fmarshallx@remove_the_xacm.org> wrote: > > ... > >>> 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. > > Synchronous demodulation works, but you need to generate a replica of > the carrier to implement it, usually with a PLL. (A narrow band-pass > filter around the carrier frequency followed by high gain is the > original way.) Once that carrier surrogate is available, it can be > directly added to the signal, so a balanced modulator isn't really > needed. The choices are then synchronous- or enhanced-carrier > demodulation. > > ... > > JerryYep. I was trying to refer to the underlying signals and not the mechanics of doing it all that much. But, indeed, a synchronous demodulator, or close facsimile thereof, is what would be used for DSB-SC. And, it's the modulators that are difficult of the 4 quadrant multiplier variety, not the demodulators. All you need for a demodulator is a clocked rectifier sort of circuit - I don't recall what they were like but that would be the idea. Well, I guess nothing but a switch or two and an inverter. Then a lowpass of suitable bandwidth and delay. Just to shake your cage: the 4 quadrant multiplier used in this mode as a modulator is a linear system with respect to the modulating signal. :-) no troll intended really ...... I believe the motivation behind suppressed carrier control system signalling was because: 1) "dc" signalling was subject to drift and offsets and "zero" is an important signal value. 2) some of the sensors may have rather had "ac" pickoffs - or maybe they were developed this way to accomodate #1 ? 3) signals naturally had signs associated with them as in Up/Down, right/left, etc. so unipolar signalling oro magnitude wasn't what was needed. Same today. [The "4 quadrant multipliers" I used were really modulators that expected a sinusoidal input for one of the inputs - so in that sense I believe they were "tuned". The specs on the output were really tight with respect to phase, distortion and gain WRT the "dc" input. They were electromagnetic devices called "magnetic modulators".) glen: I'm not up to speed on current optical signalling methods but first thoughts would be: - if you really want continuous "analog" modulation then you're stuck with having either a bias so that phase is unimportant or you will have distortion - or you will have to have a synchronous demodulator. Just like any other standard AM situation. - most things are digital now so 1 or 0? is usually all we care about - and that can be magnitude if amplitude modulation is being used. Then, if SNR is good enough multi-level, multi-bit PAM, etc. I don't know that +/- has much importance in such a scheme. Fred
Reply by ●May 3, 20102010-05-03
On 5/3/2010 10:17 AM, Fred Marshall wrote:> Jerry Avins wrote: >> On 5/2/2010 10:14 PM, glen herrmannsfeldt wrote: >>> Fred Marshall<fmarshallx@remove_the_xacm.org> wrote: >> >> ... >> >>>> 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. >> >> Synchronous demodulation works, but you need to generate a replica of >> the carrier to implement it, usually with a PLL. (A narrow band-pass >> filter around the carrier frequency followed by high gain is the >> original way.) Once that carrier surrogate is available, it can be >> directly added to the signal, so a balanced modulator isn't really >> needed. The choices are then synchronous- or enhanced-carrier >> demodulation. >> >> ... >> >> Jerry > > Yep. I was trying to refer to the underlying signals and not the > mechanics of doing it all that much. But, indeed, a synchronous > demodulator, or close facsimile thereof, is what would be used for > DSB-SC. And, it's the modulators that are difficult of the 4 quadrant > multiplier variety, not the demodulators. All you need for a demodulator > is a clocked rectifier sort of circuit - I don't recall what they were > like but that would be the idea. Well, I guess nothing but a switch or > two and an inverter. Then a lowpass of suitable bandwidth and delay.A diode ring driven hard does it, provided the diodes are well matched.> Just to shake your cage: the 4 quadrant multiplier used in this mode as > a modulator is a linear system with respect to the modulating signal. > :-) no troll intended really ......Sure. Even a plate-modulated class-C amplifier is linear in that respect.> I believe the motivation behind suppressed carrier control system > signalling was because: > 1) "dc" signalling was subject to drift and offsets and "zero" is an > important signal value. > 2) some of the sensors may have rather had "ac" pickoffs - or maybe they > were developed this way to accomodate #1 ? > 3) signals naturally had signs associated with them as in Up/Down, > right/left, etc. so unipolar signalling oro magnitude wasn't what was > needed. Same today.DC amplifiers are notoriously cranky and drift-prone, especially when built with vacuum tubes. I think the near universal use of AC servos owed as much to amplifier considerations as it did to sensor and actuator design. despite good transistor DC amplifiers, though, we still use AC sensors and synchronously demodulate them. LVDTs come to mind, and we often go out of our way to excite strain-gauge bridges with AC. Instead of brushes to instrument rotating machines, rotary transformers are used with carrier systems. Back in the old days, implementing a lead-lag network to stabilize a 400-Hz-carrier servo was an interesting exercise.> [The "4 quadrant multipliers" I used were really modulators that > expected a sinusoidal input for one of the inputs - so in that sense I > believe they were "tuned". The specs on the output were really tight > with respect to phase, distortion and gain WRT the "dc" input. They were > electromagnetic devices called "magnetic modulators".)That's new to me. Diode modulators work best when the carrier was strong enough so that it might as well have been a square wave. The carrier switches the polarity, washing out any diode drop that the signal might see.> glen: I'm not up to speed on current optical signalling methods but > first thoughts would be: > - if you really want continuous "analog" modulation then you're stuck > with having either a bias so that phase is unimportant or you will have > distortion - or you will have to have a synchronous demodulator. Just > like any other standard AM situation. > - most things are digital now so 1 or 0? is usually all we care about - > and that can be magnitude if amplitude modulation is being used. Then, > if SNR is good enough multi-level, multi-bit PAM, etc. I don't know that > +/- has much importance in such a scheme. > > FredJerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
Reply by ●May 3, 20102010-05-03
On 3.5.10 5:49 , Jerry Avins wrote:> >> [The "4 quadrant multipliers" I used were really modulators that >> expected a sinusoidal input for one of the inputs - so in that sense I >> believe they were "tuned". The specs on the output were really tight >> with respect to phase, distortion and gain WRT the "dc" input. They were >> electromagnetic devices called "magnetic modulators".)For balanced modulators there were special beam-deflection tubes, a thing like a cross between an amplifier tube and a CRT, e.g. the RCA 7360.> That's new to me. Diode modulators work best when the carrier was strong > enough so that it might as well have been a square wave. The carrier > switches the polarity, washing out any diode drop that the signal might> see. A more modern way of this approcah is to use CMOS analog switches with the selection inputs driven by hard-limited carrier. -- Tauno Voipio tauno voipio (at) iki fi
Reply by ●May 3, 20102010-05-03
On 5/3/2010 2:07 PM, Tauno Voipio wrote:> On 3.5.10 5:49 , Jerry Avins wrote: >> >>> [The "4 quadrant multipliers" I used were really modulators that >>> expected a sinusoidal input for one of the inputs - so in that sense I >>> believe they were "tuned". The specs on the output were really tight >>> with respect to phase, distortion and gain WRT the "dc" input. They were >>> electromagnetic devices called "magnetic modulators".) > > For balanced modulators there were special beam-deflection tubes, > a thing like a cross between an amplifier tube and a CRT, e.g. > the RCA 7360. > >> That's new to me. Diode modulators work best when the carrier was strong >> enough so that it might as well have been a square wave. The carrier >> switches the polarity, washing out any diode drop that the signal might > > see. > > A more modern way of this approcah is to use CMOS analog switches > with the selection inputs driven by hard-limited carrier.I've done that too. my "hard-limited carrier" was the square wave output of a CD4046. Jerry -- "I view the progress of science as ... the slow erosion of the tendency to dichotomize." --Barbara Smuts, U. Mich. �����������������������������������������������������������������������
Reply by ●May 3, 20102010-05-03
On May 3, 4:44�am, Jerry Avins <j...@ieee.org> wrote:> On 5/3/2010 12:09 AM, glen herrmannsfeldt wrote: > > � �... > > > So LED output is intensity modulated, not amplitude modulated? > > Strictly, yes. > > Jerry > -- > "I view the progress of science as ... the slow erosion of the tendency > � to dichotomize." --Barbara Smuts, U. Mich. > �����������������������������������������������������������������������Actually you are modulating the number of photons emitted by the LED per unit time. Each photon has energy E=h*frequency. So in the case of where your LED is narrowband, then all of the photons have pretty much the same energy. And in the case of a laser diode, then each photon's wavefunctions are also in phase with each other's wavefunctions. Clay
Reply by ●May 3, 20102010-05-03
On May 2, 10:45�pm, dvsarwate <dvsarw...@gmail.com> wrote:> On May 2, 1:07�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)Try explaining to physics students that carrying a large heavy rock horizontally a distance involves no work! Clay p.s. the optics books most def. use the term "incoherent"
