BLURSPEC is the jMatern module of jLab.

 BLURSPEC  Returns the blurred and aliased spectrum given the autocovariance.
 
    BLURSPEC is used to rapidly compute blurred, aliased, and other 
    modified versions of a spectrum from a known autocovariance.
 
    Performing these calculations in the time domain, making use of 
    Fourier relationships between modified versions of the autocovariance
    and spectrum, is much faster than working in the frequency domain.
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    Blurred and aliased spectra
 
    [F,S]=BLURSPEC(R) inverse Fourier transforms a given *one-sided*
    autocovariance R to obtain a spectrum S that incorporates the effects
    of aliasing, as well as blurring by the default taper, the 'boxcar'.
 
    Here F is an array of frequencies and S is the one-sided Fourier 
    spectrum.  If R is length N, the output arrays will be length (N/2+1) 
    if N is even and (N+1)/2 if N is odd.
 
    [F,SPP,SNN]=BLURSPEC(R) for complex-valued R returns SPP and SNN, the 
    positive and negative rotary spectra, respectively.  
 
    BLURSPEC(DT,R) optionally uses the sample interval DT in computing the 
    frequency array F, and in setting the spectral levels.
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    Tapered and aliased spectra
 
    BLURSPEC(R,'tapered',TAPER) incorporates the spectral smoothing due to 
    the use of data TAPER, rather than the boxcar taper, in addition to the
    effects of aliasing.  TAPER must be the same length as R.
 
    Note that BLURSPEC(R,'tapered',[]) simply returns the blurred spectrum,
    as an empty taper is taken to indicate the periodogram.
 
    BLURSPEC(R,'window',WIN) uses the pre-computed window that is to 
    multiply the autocovariance function WIN.  This is the half of the 
    sequence obtained by convolving the taper with itself.  This version
    is primarily used for speed in an internal call from MATERNFIT.
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    Aliased-only spectrum
 
    BLURSPEC(R,'aliased') computes an approximation to *aliased* spectrum,
    without blurring.  This approximation will be accurate to the extent
    that R has decayed to zero by the end of its duration.
 
    This is much faster than explicitly summing over aliased frequencies.
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    Differenced spectra
 
    BLURSPEC(R,'difference') returns the blurred and aliased spectrum of 
    the first forward difference of the process with autocovariance R.  In 
    this case the output will be length N-1.
 
    BLURSPEC(R,'seconddifference') returns the blurred and aliases spectrum
    of the second forward difference of the process with autocovariance R.  
    The output will be length N-2.
 
    These options can be combined with the 'taper' and 'aliased' options 
    described above. 
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    'blurspec --t' runs some tests.
 
    Usage: [f,S]=blurspec(R);
           [f,Spp,Snn]=blurspec(dt,R);        
           [f,Spp,Snn]=blurspec(dt,R,'tapered',TAPER);        
           [f,Spp,Snn]=blurspec(dt,R,'aliased');        
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    This is part of JLAB --- type 'help jlab' for more information
    (C) 2015--2020 J.M. Lilly and A.M. Sykulski 
                                  --- type 'help jlab_license' for details