Autospectrum computation with vDSP_zaspec doesn't work - swift

Question

I try to compute the autospectrum of a recorded signal, I use the function vDSP_zaspec. Normally the output array is real.
class func autoSpectrum (input: [Float])->[Float] {
var real = [Float](input)
var imaginary = [Float](repeating: 0.0, count : input.count)
var Output:[Float] = [Float](repeating:0 , count: input.count)
let length = vDSP_Length(real.count/2)
real.withUnsafeMutableBufferPointer {realBP in
imaginary.withUnsafeMutableBufferPointer {imaginaryBP in
var splitComplex = DSPSplitComplex(realp: realBP.baseAddress!, imagp: imaginaryBP.baseAddress!)
vDSP_zaspec(&splitComplex, &Output, vDSP_Length(input.count))
}
}
let value = vDSP.rootMeanSquare(Output)
Output = vDSP.divide(Output, value)
return Output
}
I did a test with a 500 Hz sin wave and this is what the Output array looks like :
The chart is far from the expected result...The result looks like the absolute value of the recorded audio file...
If someone could help me, it will be great !

Answer 1

vDSP_zaspec returns the sum of each real part squared and the corresponding imaginary part squared. The Apple documentation describes it as "Compute the element-wise sum of the squares of the real and imaginary parts of a complex vector".
The following pieces of code calculate the same results:
var real: [Float] = [-2, 7, -3]
var imaginary: [Float] = [4, -1, -4]
var vDSPresult: [Float] = [0, 0, 0]
var scalarResult: [Float] = [0, 0, 0]
real.withUnsafeMutableBufferPointer {realBP in
imaginary.withUnsafeMutableBufferPointer {imaginaryBP in
var splitComplex = DSPSplitComplex(realp: realBP.baseAddress!,
imagp: imaginaryBP.baseAddress!)
vDSP_zaspec(&splitComplex,
&vDSPresult,
vDSP_Length(vDSPresult.count))
}
}
print(vDSPresult)
for i in 0 ..< scalarResult.count {
scalarResult[i] = pow(real[i], 2) + pow(imaginary[i], 2)
}
print(scalarResult)