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Very good Fred.On Sun, 07 Nov 2010 04:22:16 +0000, Jasen Betts wrote:
show me a signal with negative amplitude.
a sin x at x>pi<2*pi
--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
(Richard Feynman)
Multiplying by any m(t) is a linear operation, since it satisfiesIn comp.dsp Vladimir Vassilevsky<nospam@nowhere.com> wrote:
(snip regarding rectiification, and I wrote)
You need some non-linear operation, which may or may not be
considered rectification.
No nonlinear operations. Just multiply the AC by a synchronous reference.
Multiplying by other than a constant is a non-linear operation.
RMS or peak? How much time are we talking about?Is there anyway to get an amplitude of an ac signal averaged over time
w/o some kind of rectification?
That is true for time-invariant systems. Look at my previous post and,On Sat, 06 Nov 2010 09:52:02 -0700, Fred Marshall
fmarshall_xremove_the_xs@xacm.org> wrote:
On 11/5/2010 5:26 PM, glen herrmannsfeldt wrote:
In comp.dsp Fred Marshall<fmarshall_xremove_the_xs@xacm.org> wrote:
(snip)
Multiplying by other than a constant or a stable periodic waveform is a
non-linear operation. Multiplying by a constant or a sinusoid or a sum
of sinusoids is a linear operation.... believe it or not.
Hmmm. As I understand it (longer ago than I remember) they used
to make rectifiers out of synchronous motor driven commutators.
That is, multiply by a square wave (sum of sines) of the appropriate
frequency. The result doesn't seem like a linear function of
the input anymore.
-- glen
Well, Jerry and I argued about this at length here some time ago. The
key is in the tests for linearity. It rather surprised me too and it
didn't "seem like a linear system" but it worked out.
In all these things we're talking about a 2-port situation and asking
whether the 2-port system is linear or not.
So, if the 2-port system is a 4-quadrant multiplier with a stable
sinusoidal input (which is not one of the "ports") then the result of a
signal on the "input" port is a sinusoid of amplitude proportional to
that input. And, if a different input is applied, same thing. And, if
the sum of those two inputs is applied, the output is the sum of the two
independent outputs.
etc.
Now, I must say that I was pulling from memory about the composite
sinusoidal modulating function .. but I think it still holds with that
being used instead of a simple sinusoid...
I think this depends on what you mean by "linear". Most of
the time, IMHO, we mean "no new frequencies produced in the
output". Multiplying two sinusoids violates this, because
you end up with sum and difference frequencies.
. cos wt .------------
. | .
Think about amplifiers. We talk about "nonlinear
distortion" when we get new frequency components (THD, IMD,
slew limiting, etc), while "linear distortion" (which is a
rarely used term) means only changes in amplitudes or phases
of the original frequency components.
Best regards,
Bob Masta
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If your signal x(t) is described by x(t) = Aˇ(cos wt + fi), then A isIs there anyway to get an amplitude of an ac signal averaged over time
w/o some kind of rectification?
This doesn't include the trivial, i.e., dc offsets or partial cycles.
The rectification could be analog, digital or software.
Bret Cahill
You need a precision rectifier..
couple of op-amps and maybe 1 diode ..
http://sound.westhost.com/appnotes/an001.htm
Poorly written OP. The goal is getting the amplitude _without_
rectification. Rectification" is to be very broadly construed and
includes anything that results in the final "signal" being positive at
all times.
Maybe peak to peak wil work if you know the wave form.
Bret Cahill
Sorry!On 11/05/2010 01:00 PM, Bret Cahill wrote:
Is there anyway to get an amplitude of an ac signal averaged over time
w/o some kind of rectification?
This doesn't include the trivial, i.e., dc offsets or partial cycles.
The rectification could be analog, digital or software.
Don't feed the troll
p-p IM.On Fri, 5 Nov 2010 13:00:54 -0700 (PDT) Bret Cahill
Bret_E_Cahill@yahoo.com> wrote in Message id:
e6a620ac-ec32-4ae8-8f56-6f948ed1f142@x7g2000prj.googlegroups.com>:
Is there anyway to get an amplitude of an ac signal averaged over time
w/o some kind of rectification?
RMS or peak? How much time are we talking about?
^^^^
Dirk,On Nov 7, 11:25 am, Fred Abse<excretatau...@invalid.invalid> wrote:
On Sun, 07 Nov 2010 04:22:16 +0000, Jasen Betts wrote:
show me a signal with negative amplitude.
a sin x at x>pi<2*pi
--
I searched all of the posts here to see if anyone caught that.
Posters are using the terms "amplitude" and "magnitude" as if they are
the same.
Dirk
In physics, at least in optics and QM, amplitude is signed.Posters are using the terms "amplitude" and "magnitude" as if
they are the same.
Good point. I've probably done that! So you got me thinking....
It occurs to me that there's another:
"value of a function" or "value of a signal" which, obviously can be
negative.
This goes along with:
"magnitude of a signal or function" (which I don't think implies any
time frame).... as in "abs(value of a function)"
"amplitude of a wave" as in "a sin x"
or, amplitude of noise perhaps...
where we most often use "a" as a positive number in physics but don't
*have to* in mathematics, eh? In the latter case it could be said to
have a negative amplitude but that would be most unusual wouldn't it?
Or maybe it's just implied that 0=<a ??
(snip)I'm not sure how "amplitude" really applies otherwise:
What's the "amplitude" of a*sin(wt) + b*cos(pi*w*t) ????
The term "time ave." in the header implies the quantity is over someshow me a signal with negative amplitude.
a sin x at x>pi<2*pi
--
I searched all of the posts here to see if anyone caught that.
Posters are using the terms "amplitude" and "magnitude" as if they are
the same.
Dirk
Dirk,
Good point. I've probably done that! So you got me thinking....
It occurs to me that there's another:
"value of a function" or "value of a signal" which, obviously can be
negative.
This goes along with:
"magnitude of a signal or function" (which I don't think implies any
time frame).... as in "abs(value of a function)"
"amplitude of a wave" as in "a sin x"
or, amplitude of noise perhaps...
where we most often use "a" as a positive number in physics but don't
*have to* in mathematics, eh? In the latter case it could be said to
have a negative amplitude but that would be most unusual wouldn't it?
Or maybe it's just implied that 0=<a ??
I'm not sure how "amplitude" really applies otherwise:
What's the "amplitude" of a*sin(wt) + b*cos(pi*w*t) ????
Then things like "rms value" or "rms whatever" usually imply a time
frame over which the sum or integral is taken - often a single period
but could be just some arbitrary time frame.
Temperature is a signal. Fourier analysis _originated_ in heatIs there anyway to get an amplitude of an ac signal averaged over time
w/o some kind of rectification?
Run it through a resistor an measure heat output.
sqrt(a˛+b˛)On 11/7/2010 10:01 AM, Dirk Bell wrote:
On Nov 7, 11:25 am, Fred Abse<excretatau...@invalid.invalid> wrote:
On Sun, 07 Nov 2010 04:22:16 +0000, Jasen Betts wrote:
show me a signal with negative amplitude.
a sin x at x>pi<2*pi
--
I searched all of the posts here to see if anyone caught that.
Posters are using the terms "amplitude" and "magnitude" as if they are
the same.
Dirk
Dirk,
Good point. I've probably done that! So you got me thinking....
It occurs to me that there's another:
"value of a function" or "value of a signal" which, obviously can be
negative.
This goes along with:
"magnitude of a signal or function" (which I don't think implies any
time frame).... as in "abs(value of a function)"
"amplitude of a wave" as in "a sin x"
or, amplitude of noise perhaps...
where we most often use "a" as a positive number in physics but don't
*have to* in mathematics, eh? In the latter case it could be said to
have a negative amplitude but that would be most unusual wouldn't it?
Or maybe it's just implied that 0=<a ??
I'm not sure how "amplitude" really applies otherwise:
What's the "amplitude" of a*sin(wt) + b*cos(pi*w*t) ????
Then things like "rms value" or "rms whatever" usually imply a time
frame over which the sum or integral is taken - often a single period
but could be just some arbitrary time frame.
Fred
so brett wants a low pass filter?On Sun, 07 Nov 2010 04:22:16 +0000, Jasen Betts wrote:
a sin x at x>pi<2*pi
In what circumstances do you want a negative result?Is there anyway to get an amplitude of an ac signal averaged over time
w/o some kind of rectification?
Run it through a resistor an measure heat output.
Temperature is a signal. Fourier analysis _originated_ in heat
transfer.
Your resistor is taking an AC signal and putting out a positive only
signal.
What is that if not rectification?
To be sure the quantity representing the signal changes from voltageIs there anyway to get an amplitude of an ac signal averaged over time
w/o some kind of rectification?
Run it through a resistor an measure heat output.
Temperature is a signal. Fourier analysis _originated_ in heat
transfer.
Your resistor is taking an AC signal and putting out a positive only
signal.
What is that if not rectification?
Sign isn't of interest here. The goal is to determine if there is anyIn what circumstances do you want a negative result?
Huh? A Fourier transform produces real and imaginaryAs pointed out above amplitude is always positive. A Fourier
transform is always positive so that might be one way to determine the
magnitude of the signal over time.
If you only take the magnitude, to get the "always positive"It might be hard to argue that a FFT is rectification. When you
rectify by diode or resistor you lose some information. You no longer
know when the unrectified signal was positive or negative.
Not true for the FFT.
If the only information displayed in the FFT is an amplitude and aAs pointed out above amplitude is always positive. A Fourier
transform is always positive so that might be one way to determine the
magnitude of the signal over time.
Huh? A Fourier transform produces real and imaginary
arrays, any component of which may be negative.
In that case there seems to be no way to end run rectification.It is
common to display the *magnitude* at each frequency bin, but
that involves a separate step (square root of
sum-of-squares), which I assume you would call
rectification.
A phase angle for each frequency would allow recovery the sign of theIt might be hard to argue that a FFT is rectification. When you
rectify by diode or resistor you lose some information. You no longer
know when the unrectified signal was positive or negative.
Not true for the FFT.
If you only take the magnitude, to get the "always positive"
result you mention, then that of course has no information
about the original polarity.
Yes.You can also compute the phase
at each frequency bin, as a separate operation on the same
real and imaginary arrays. Is that what you are talking
about?
The magnitude of a randomly fluctuating signal over a period of time.Why the concern over whether a method is a form of
"rectification" or not? What are you really trying to
measure?
Don't give up your day job, Cahill. An angle of 180 degrees is *negative*. IfAs pointed out above amplitude is always positive. A Fourier
transform is always positive so that might be one way to determine the
magnitude of the signal over time.
Huh? A Fourier transform produces real and imaginary
arrays, any component of which may be negative.
If the only information displayed in the FFT is an amplitude and a
phase angle for each frequency, and if amplitude is always positive as
suggested above, then amplitudes are always positive in an FFT.
You haven't explained what's wrong with "rectification", andWhy the concern over whether a method is a form of
"rectification" or not? =A0What are you really trying to
measure?
The magnitude of a randomly fluctuating signal over a period of time.
This doesn't include the trivial case where the time the signal is
positive is integrated along with the time the signal is negative.
Peak to peak will only work with a known waveform so it looks more and
more like it must involve taking absolute values or sqrts of sum of
squares at some point which would ordinarily be considered
"rectification" even in a relatively narrow sense of the word.
On Wed, 10 Nov 2010 08:43:04 -0800 (PST), Bret Cahill
Bret_E_Cahill@yahoo.com> wrote:
Why the concern over whether a method is a form of
"rectification" or not? =A0What are you really trying to
measure?
The magnitude of a randomly fluctuating signal over a period of time.
This doesn't include the trivial case where the time the signal is
positive is integrated along with the time the signal is negative.
Peak to peak will only work with a known waveform so it looks more and
more like it must involve taking absolute values or sqrts of sum of
squares at some point which would ordinarily be considered
"rectification" even in a relatively narrow sense of the word.
You haven't explained what's wrong with "rectification", and
especially true RMS for this purpose.