SPECTRUM - fft spectrum
SPECTRUM X TYP C LFFT SP MAMI MAMX MPV
|X||signal vector (nX samples)||no default|
|TYP||selects the type of output spectrum||def.=LOGARITHMIC|
|C||spectrum scale factor (linear; def=1) or offset (dB, def=0)|
|LFFT||fft length for spectrum computation||def.= next 2M >= nX|
|SP||starting point for phase locking (in samples)|
|MAMI,MAMX||minimum and maximum amplitude for phase masking|
|MPV||value for masked phase||def.=0|
|Y||complex or amplitude spectrum (depends on TYP)|
|P||phase spectrum (if TYP not equal COMPLEX)|
|LFFT||fft length used for spectrum computation (may differ from input LFFT)|
This function computes the complex spectrum or the amplitude and phase spectrum of the input signal by using the fft. The output format is selected by the input TYP.
If SP is connected, phase locking is performed and the value of SP is used as analysis starting point (in samples). The phase locking algorithm converts the fft phase of the current frame to a phase relative to the specified starting point (e.g. the begin of the 1st frame)
The inputs MAMI and MAMX can be used to set the phase values for amplitudes outside the specified (linear!) amplitude range to the constant value supplied with input MPV (default=0). This 'masking' method was implemented to compute the phases for 'relevant components' only and is applied only if one or both of the inputs MAMI and MAMX are connected.
PI is the value of output P and aI is the amplitude of fft-bin I
Note: it is also possible to supply a minimum amplitude MAMI only
The phase algorithms are still in development and we are working on more efficient methods for phase locking, unwrapping and masking.
To get comparable amplitudes for different frame lengths (= NX, length of vector X), the signal is scaled by the factor 1/NX. If NX is not a power of 2, the fft-length is set to the next power of 2 greater than NX and the signal is zero padded.
The value of C can be used to shift/scale the amplitudes into a user defined range (e.g. to get absolute sound pressure values).
|value of TYP
|complex spectrum:Y2I = C.reI, Y2I+1 = C.imI
|power spectrum:YI = C.aI2
|phase spectrum: if input SP is not connected and if input SP is connectedphase values are scaled to the range –p to +p (using a module operation)
|amplitude spectrum:YI = C.aI
|logarithmic amplitude spectrum in dB:YI = 20.log10(aI) + C
|real part of complex spectrum:YI = C.reI
|imaginary part of complex spectrum:YI = C.imI
with: N is the length of the fft, I is the index of the fft-bin (I=0..N/2+1)