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# TI-84 Plus Fast Fourier Transform

TI-84 Plus Fast Fourier Transform

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Introduction

The program FFT1 performs the fast Fourier transform of discrete data points named in List 1 (small x, signal at time points) to  List 2 (big X, frequency), using the formula:

X_k = ∑( x_n * e^(-i*2*π*k*m)/n from m = 0 to n – 1)

For the set of n signals.

The program IFFT1, the Inverse Fast Fourier Transform, reverses the process (big X to small x, List 2 to List 1).

x_k = 1/n * ∑( x_n * e^(i*2*π*k*m)/n from m = 0 to n – 1)

TI-84 Plus Program FFT1

“FFT VERSION 1”
“EWS 2018-12-09”
Input “SMALL X LIST:”,L₁
“SETUP”
dim(L₁)→N
L₁→L₂
a+bi
“LOOP”
For(M,0,N-1)
0→T
For(K,0,N-1)
T+L₁(K+1)*e^(-­2*π*i*K*M/N)→T
End
T→L₂(M+1)
End
Pause L₂

TI-84 Plus Program IFFT1

“IFFT VERSION 1”
“EWS 2018-12-09”
Input “BIG X LIST:”,L₂
“SETUP”
dim(L₂)→N
L₂→L₁
a+bi
“LOOP”
For(M,0,N-1)
0→T
For(K,0,N-1)
T+L₂(K+1)*e^(2*π*i*K*M/N)→T
End
T/N→T
T→L₁(M+1)
End
Pause L₁

Example

FFT Example:
L1 = {2i, 1+i, 3}
Result (Fix 4):   {4.0000+3.0000*i, -1.1340+3.2321*i, -2.8660-0.2321*i}

Inverse FFT Example:
L2 = {0.5, -0.7i, 0.9+0.3i}
Result (Fix 4): {0.4667-0.1333*i, 0.3053-0.1931*i, -0.2720+0.3265*i}

Trigonometric Version

For calculators that do not handle complex numbers, here are the sample code that can handle FFT and IFFT.  The techinque here is to use a pair of lists, one for the real parts, the other for imaginary parts.  Example:  {2, 3+3i, -2i} get split into {2, 3, 0} and {0, 3, -2}.

The following identities and calculation are used:

e^(i*θ) = cos θ + i * sin θ

(a + b*i)*e^(-i*θ)
=  (a + b*i) * (cos θ – i * sin θ)
=  a * cos θ +  i * b * cos θ – i * a * sin θ + b * sin θ
= (a * cos θ + b * sin θ) + i * (b * cos θ – a * sin θ)

(a + b*i)*e^(i*θ)
=  (a + b*i) * (cos θ + i * sin θ)
=  a * cos θ +  i * b * cos θ + i * a * sin θ – b * sin θ
= (a * cos θ – b * sin θ) + i * (b * cos θ + a * sin θ)

where θ = 2*π*m*k/n

TI-84 Plus Program FFT1

“FFT VERSION 2”
“EWS 2018-12-10”
Disp “SMALL X”,”L₁ + I*L₂”
Input “REAL: “,L₁
Input “IMAG: “,L₂

“SETUP”
dim(L₁)→N
L₁→L₃
L₂→L₄
“LOOP”
For(M,0,N-1)
0→S
0→T
For(K,0,N-1)
2*π*M*K/N→θ
S+L₁(K+1)*cos(θ)+L₂(K+1)*sin(θ)→S
T+L₂(K+1)*cos(θ)-L₁(K+1)*sin(θ)→T
End
S→L₃(M+1)
T→L₄(M+1)
End
Disp “REAL = L₃”,”IMAG = L₄”
TI-84 Plus Program IFFT1
“IFFT VERSION 2”
“EWS 2018-12-10”
Disp “BIG X”,”L₃ + I*L₄”
Input “REAL: “,L₃
Input “IMAG: “,L₄
“SETUP”
dim(L₃)→N
L₃→L₁
L₄→L₂
“LOOP”
For(M,0,N-1)
0→S
0→T
For(K,0,N-1)
2*π*M*K/N→θ
S+L₃(K+1)*cos(θ)-L₄(K+1)*sin(θ)→S
T+L₄(K+1)*cos(θ)+L₃(K+1)*sin(θ)→T
End
S/N→S
T/N→T
S→L₁(M+1)
T→L₂(M+1)
End
Disp “REAL = L₁”,”IMAG = L₂”
Source:
“An Interactive Guide To The Fourier Transform”  Better Explained.  Retrieved December 9, 2018.  https://betterexplained.com/articles/an-interactive-guide-to-the-fourier-transform/
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