Re: FFT and changing frequency and vectorizing for loop

[Sergei Steshenko]

--- On Sat, 4/2/11, Rick T <address@hidden> wrote:

From: Rick T <address@hidden>
Subject: Re: FFT and changing frequency and vectorizing for loop
To: "Sergei Steshenko" <address@hidden>
Cc: address@hidden
Date: Saturday, April 2, 2011, 7:00 PM

It works it's just slow due to the for loop here's an example of the output 
which is correcthttp://dl.dropbox.com/u/6576402/questions/q1.png




On Sat, Apr 2, 2011 at 2:41 PM, Sergei Steshenko <address@hidden> wrote:





--- On Sat, 4/2/11, Rick T <address@hidden> wrote:



From: Rick T <address@hidden>

Subject: FFT and changing frequency and vectorizing for loop

To: address@hidden

Date: Saturday, April 2, 2011, 3:02 PM



FFT and changing frequency and vectorizing for loop

Greetings All

I can increase and decrease the frequency of thesignal using the combination of 
fft and a series expansion for loop in the code below



but if the signal/array is to large it becomes extremelyslow (an array that's 
1x44100 takes about 2 mins to complete) I'm sure it has to do with the for loop 
butI'm not exactly sure how to vectorize them to improve performance








Any recommendations

tia sal22 

%create signalclear all, clc,clf,ticx= linspace(0,2*pi,44100)'; 

%Used in exporting to ycalc audio file make sure in sync with above



freq_orig=1;freq_new=4vertoff=0;vertoffConj=0;vertoffInv=0;vertoffInvConj=0;phaseshift=(0)*pi/180
 ; %can use mod to limit to 180 degrees





y=sin(freq_orig*(x)); [size_r,size_c]=size(y);

N=size_r; %to test make 50T=2*pi;dt=T/N;t=linspace(0,T-dt,N)';phase = 0;



f0 = 1/T; % Exactly, one period 

y=(y/max(abs(y))*.8)/2; %make the max amplitude hereC = fft(y)/N; % No 
semicolon to display output 

A = real(C); 



B = imag(C)*-1; %I needed to multiply by -1 to get the correct sign

% Single-Sided (f >= 0) An = [A(1); 2*A(2:round(N/2)); A(round(N/2)+1)]; 
%needed to put the ' to get vaules in rows 



Bn = [B(1); 2*B(2:round(N/2)); B(round(N/2)+1)]; %needed to put the ' to get 
vaules in ro

pmax=N/2;ycalc=zeros(N,1); %preallocating space for ycalcw=0;for p=2:pmax



                %        %%1 step) re-create signal using equation          
ycalc=ycalc+An(p)*cos(freq_new*(p-1).*t-phaseshift)+Bn(p)*sin(freq_new*(p-1).*t-phaseshift)+(vertoff/pmax);



         w=w+(360/(pmax-1)); %used to create phaseshift         phaseshift=w;   
     end;fprintf('\n- Completed in %4.4fsec or %4.4fmins\n',toc,toc/60);





subplot(2,1,1), plot(y),title('Orginal Signal'); 
subplot(2,1,2),plot(ycalc),title('FFT new signal'); 



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I don't think you are doing a right thing - not WRT loop/vectorization,

but WRT final result.



If I understand correctly your code, you multiply by 4 each individual

frequency. If so, your signal will appear in your output 4 times in a row.



Try a simple example with, say, just 16 samples.



There is a package for 'octave' doing polyphase resampling:



http://octave.sourceforge.net/signal/function/resample.html

.



Regards,

  Sergei.






-- 

No, your example doesn't prove it works. I.e. the image shows one original
'sin' period, and four 'sin' periods in the "upsampled" signal.

Take as input signal, for example, a triangle pulse - you'll see
four triangle pulsed after your "upsampling". So, if you want to upsample
a song this way, you'll hear it four times.

What you did is simple replication in time domain - can be done quickly
using just several lines of code without any FFT in the first place.

Regards,
  Sergei.

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