Programmer Guide/Command Reference/EVAL/hcomb

This function implements a methods to detect harmonics in a spectrum and to create a harmonic comb filter. It was especially implemented for the script application MulAc.

Usage

hcomb(mflag, oflag, a, imin, imax, ot, ok, oo, nget, rrms, nrms, nset)

mflag

selects the method used to detect the test tone ft

mflag=0	peak picking
mflag=1	harmonic grid

oflag

selects the format and content of the result (see Result); the values 0 and 1 are supported

a

a vector containing an amplitude spectrum (linear, with constant df); in the description below n is the length of a.

imin, imax

index range for test tone (ft) detection; 0 <= imin < imax < n

ot

test tone order (f0 = ft / ot); 0 < ot

ok, oo

order factor and offset for the components of the comp fc[i];

fc[i] = f0 * (oo + ok*i); with: i=1,2,...

nget

the number of points (+/-) to detect the peak of a harmonic component fc[i]; 3 <= nget

rrms

defines the energy ratio threshold for harmonic components; 0 <= rrms < 1

nrms

defines the bandwidth (+/- nrms points) used to compute the energy ratio; 2 <= nrms

(component-rms / testband-rms >= rrms)

nset

defines the comb bandwidth (+/- nset points); if nset equals 0, a variable comb bandwidth is used to remove the detected components; this argument is only used if oflag equals 1

Description

1. The function extracts the frequency ft of the test tone from the selected test frequency band (arguments: mflag, imin, imax and rr).
2. The argument ot is used to compute the fundamental frequency f0 of the harmonic comb.
3. For each frequency band of the harmonic comb is tested if there is a spectral component (peak) or not. The frequency, amplitude and order of the detected harmonic components are saved.

Result

• oflag=0: The result r is a (M+1)x3 matrix, with M is the number of detected harmonics

r[0,0]	fundamental frequency f0 (weighted average of component frequencies)
r[0,1], r[0,2]	always 0
r[i,0]	frequency of component i (i=1..M)
r[i,1]	amplitude of component i (i=1..M)
r[i,2]	order of component i (i=1..M)

• oflag=1: The result r is a vector with n elements containing the amplitude spectrum of the comb filter. Inside each detected band the amplitudes are set to 1, outside the amplitudes are set to 0.

Note: All frequencies computed and returned by this function are relative frequencies, computed for a frequency resolution df=1.

See also

optmm, cvphase, script application MulAc

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