Re: Physics chalange in signal processing

[sloopools]

Hi Doug!

 

I am sorry, maybe I went too straight forward without explaing well.

By modulating, I realy mean to modulate it in terms of signal ad processing, meaning, to describe the amlitude and frequency for each time point. This would be the main goal but of course impossible or very hard to do it, because my original signal O is never stationary and many more variables can also for noise...

 

My idea was to try to make it simple and get a hint from you about where and how to look for an answer. I dont even know what kind of matematical problem I am dealing with...

 

So, to be realistic and simple, lets say I have a signal that is represented by the Fourrier Transform (FFT) ploted in Black (see figure I have attached on original post). I want to change that signal in terms of amplitude and frequencies shifts (everything that is necessary) in order to generate a different signal witch FFT is given by te RED line. In short words, I want to be able to artificially generate my red line FFT signal tweeking my data.

 

Let me try to brainstorm here for a second:

I read that in order to make a frequency shift to a signal, we can multiply the raw signal by a cousin, like this:
raw_data_signal.*cos(2*pi*1000*t) --> This would cause a w=1000 Hz shift of frequency.

Then if do some adjustments in the amplitude of the raw signal in some frequency bands, I might be able to get a very similar curve as my red FFT one. Would you agree?

Is this the typically way that people do it or am I beeing naive?

Keywords?

 

Sloopools

On 11 February 2013 17:29, Doug Stewart-4 [via Octave] <[hidden email]> wrote:

On Mon, Feb 11, 2013 at 10:38 AM, sloopools <[hidden email]> wrote:

Hey guys!
I have a chalange for you guys:

*Scenario 1:* /This is a scenario of Frequency_Shift - Same_Amplitude/
Lets say you have an ambulance car passing by you and you are standing still
in a sidewalk. The sirenes sound as the car is approximating to you is
different from the sound of the same sirenes as the car moves away from you.
This fenomena is obviously because of the sound propagation velocity that is
added or subtracted from the car velocity as it passwer by you.
Now if you record the sound of that sirene, you would see a higher speed
*signal A *(higher frequencies) as the car approximates and a lower speed
*signal B *(lower freqs) as the car distances from you. So far, no news.
If you have an original recording of the sirenes without the ambulance
moving, lets call it *signal O*, and compare it with signal A, you would see
that signal A has higher frequencies for the whole spectra and Signal B
would have lower frequencies than signal O for the whole spectra as well.
Moreover, it the ambulance moves at constant speed, the shift of frequencies
between O-A and 0-B should be the same. Note that the although the
frequencies of those sounds are different, the power of those sounds A and B
would be the same and equal to signal 0.

*Scenario 2:* /This is a scenario of Constant_Frequency -
Amplitude_Decrease/I have a sound that propagates trough a wall. This means
that some frequencies will pass more easily than others. Lets say that lower
frequencies can propagate easier than higher freqs, so the sound after going
trough that wall will have less AMPLITUDE (power) in the higher frequencies.

/*Now the big question:*/ /Scenario 1 + scenario 2/
I have a signal that travels at a certain speed (like in scenario 1) and I
am measuring it after going trough a wall (like in scenario 2). I have the
original signal (signal O) for comparison purposes.
I am trying to quantify the captured signal after going trough that wall.
I see a decrease of amplitude(power) for certain frequency bands that are
due to that wall, but I also see a frequency shift because the source of
noise moves at considerable speed.

How can I modulate this problem?
To put it simple, how can I artificially generate the sound wave after
crossing that wall, tweeking the velocity(shifting the frequency bands
linearly) and amplitude.

I am not an expert on sound or signal processing. Most of the work I do goes
in Matlab.
I have plotted my original signal FFT (in black) and the recorded signal (in
red). Frequency goes in the xx and amplitude (db) in the yy axes.
Looks like tis wave hit a truck and all the frequencies shifted a little bit
to lower freqs and alsoe there was some power diminuishing...

<http://octave.1599824.n4.nabble.com/file/n4649745/zscore_raw_data.jpg>

Note that the peak on the 20Hz was lowered to 15 Hz and also lowered in
amplitude.
the frequncies up to 10 Hz (inclusively the minima at 10 Hz) didnt suffer
any frequency shift BUT AMAZINGLY has more POWER. I think this is because as
there is a shift of the 20Hz curve and that the power of that frequency band
must go somewere...

Eitherway, not so much for the details, what I really want is to know how to
modulate these signals?
What is the relation between them?
Multiplication in power, change if fase, shift if frequency.

By the way, do you think is ok to make an FFT of and FFT, meaning, to do an
FFT of those curves (that are acctually already the FFT signals of my sound)
what is the meaning of the FFT of and FFT?

I am available to answer your questions... and I will be fast :)
Thank you for now

"Eitherway, not so much for the details, what I really want is to know how to

modulate these signals?" 

What do you mean by   modulate ?????

Do you mean alter them or to model them???? 

--

DAS

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