Option Explicit
Const fftIn = 128
Const Pi = 3.1415926
Public Function FFT0(AR() As Double, AI() As Double, N As Long, ni As Long)
Dim i As Long, j As Long, k As Long, L As Long, M As Long
Dim IP As Long, LE As Long
Dim L1 As Long, N1 As Long, N2 As Long
Dim SN As Double, TR As Double, TI As Double, WR As Double, WI As Double
Dim UR As Double, UI As Double, US As Double
M = NTOM(N)
N2 = N / 2
N1 = N - 1
SN = ni
j = 1
For i = 1 To N1
If i < j Then
TR = AR(j - 1)
AR(j - 1) = AR(i - 1)
AR(i - 1) = TR
TI = AI(j - 1)
AI(j - 1) = AI(i - 1)
AI(i - 1) = TI
End If
k = N2
While (k < j)
j = j - k
k = k / 2
Wend
j = j + k
Next i
For L = 1 To M
LE = 2 ^ L
L1 = LE / 2
UR = 1#
UI = 0#
WR = Cos(Pi / L1)
WI = SN * Sin(Pi / L1)
For j = 1 To L1
For i = j To N Step LE
IP = i + L1
TR = AR(IP - 1) * UR - AI(IP - 1) * UI
TI = AI(IP - 1) * UR + AR(IP - 1) * UI
AR(IP - 1) = AR(i - 1) - TR
AI(IP - 1) = AI(i - 1) - TI
AR(i - 1) = AR(i - 1) + TR
AI(i - 1) = AI(i - 1) + TI
Next i
US = UR
UR = US * WR - UI * WI
UI = UI * WR + US * WI
Next j
Next L
If SN <> -1 Then
For i = 1 To N
AR(i - 1) = AR(i - 1) / N
AI(i - 1) = AI(i - 1) / N
Next i
End If
End Function
Private Function NTOM(N As Long) As Long
Dim ND As Single
ND = N
NTOM = 0
While (ND > 1)
ND = ND / 2
NTOM = NTOM + 1
Wend
End Function
Private Sub Form_Load()
Dim i As Integer
Dim xr(128) As Double
Dim xi(128) As Double
Dim IaIn(128) As Double
For i = 0 To 128
IaIn(i) = Sin(i) + 0.5 * Sin(10 * i)
xr(i) = 100 * IaIn(i)
xi(i) = 0
Next
Call FFT0(xr(), xi(), 128, 1)
picI_FFT.Scale (0, 100)-(fftIn - 1, -10)
picI_FFT.DrawWidth = 2
For i = 0 To fftIn - 1
picI_FFT.Line (i, Abs(xr(i)))-(i + 1, Abs(xr(i + 1))), vbRed
Next i
End Sub
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