mel.m

function varargout = mel(options)
% librosa.mel Design Mel filter bank
%
%  This function matches the mel function from Librosa (tested for
%  version 0.10.1). Parameter defaults are identical to the Librosa
%  function.
%
%  FB = librosa.mel(SampleRate=fs) returns a frequency-domain
%  Mel filter bank, filterBank. fs is the input sample rate, in Hz.
%
%  FB = librosa.mel(FFTLength=NFFT) specifies the FFT length.
%
%  FB = librosa.mel(NumBands=NUMBANDS) specifies the number of bands in
%  the filter bank.
%
%  FB = librosa.mel(Fmin=FMIN) specifies the lowest frequency (in Hz).
% 
%  FB = librosa.mel(Fmax=FMAX) specifies the highest frequency (in Hz).
% 
%  FB = librosa.mel(Normalization=NORM) specifies the type of filter bank
%  normalization.
%
%  FB = librosa.mel(HTK=FLAG) specifies what type of Mel scaling is used.
%  If FLAG is true, HTK scaling is used. If FLAG is false, Slaney scaling
%  is used. The default is Slaney-scaling.
%
%  FB = librosa.mel(GenerateMATLABCode=true) generates and opens an
%  untitled file containing code that implements the code of librosa.mel
%  using the MATLAB function designAuditoryFilterBank.
%
%  % Example:
%  % Design an auditory filter bank and use it to compute a mel
%  % spectrogram.
%
%  [audioIn,fs] = audioread("Counting-16-44p1-mono-15secs.wav");
%       
%  % Compute spectrogram
%  win = hann(1024,"periodic");
%  [~,F,T,S] = spectrogram(audioIn,win,512,1024,fs,"onesided");
%
%  % Design auditory filter bank
%  filterBank = librosa.mel(SampleRate=fs,FFTLength=1024, ...
%                     NumBands=16,Normalization="None",HTK=true);
%
%  % Visualize filter bank
%  plot(F,filterBank.')
%  grid on
%  title("Mel Filter Bank")
%  xlabel("Frequency (Hz)")
%      
%  % Compute mel spectrogram
%  SMel = filterBank*S;

%  Copyright 2022-2023 The MathWorks, Inc.

%% Parse function parameters
arguments
    options.FFTLength (1,1) double {mustBeNumeric,mustBePositive,mustBeInteger} = 2048
    options.SampleRate (1,1) double {mustBeNumeric,mustBePositive} = 22050
    options.NumBands (1,1) double {mustBeNumeric,mustBePositive,mustBeInteger} = 128
    options.Fmin (1,1) double {mustBeNumeric,mustBeNonnegative} = 0
    options.Fmax (1,1) double {mustBeNumeric,mustBePositive}
    options.HTK (1,1) logical {mustBeNumericOrLogical} = false
    options.Normalization = 'Slaney'
    options.GenerateMATLABCode (1,1) logical {mustBeNumericOrLogical} = false
end

if ~isfield(options,'Fmax')
    options.Fmax = options.SampleRate/2;
else
    options.Fmax = min(options.SampleRate/2,options.Fmax);
end

switch options.Normalization
    case 'None'
        options.Normalization = 'none';
        customNorm = false;
    case 'Slaney'
        options.Normalization = 'bandwidth';
        customNorm = false;
    otherwise
        options.Normalization = 'none';
        customNorm = true;
        mustBeNumeric(options.Normalization)
end
if options.HTK
    melStyle = "oshaughnessy";
else
    melStyle = "slaney";
end

if options.GenerateMATLABCode
    strWriter = StringWriter;
else
    strWriter = librosa.utils.StringWriter;
end
strWriter.addcr('%s\n%% Construct filter bank.','%%');

filterBank = designAuditoryFilterBank(options.SampleRate, ...
    FrequencyScale="mel", ...
    FFTLength=options.FFTLength, ...
    NumBands=options.NumBands, ...
    FrequencyRange=[options.Fmin options.Fmax], ...
    Normalization=options.Normalization, ...
    OneSided=true, ...
    FilterBankDesignDomain="linear", ...
    MelStyle=melStyle);

strWriter.addcr('filterBank = designAuditoryFilterBank(%f,...',options.SampleRate);
strWriter.addcr('FrequencyScale="mel",...');
strWriter.addcr('FFTLength=%d,...',options.FFTLength);
strWriter.addcr('NumBands=%d,...',options.NumBands);
strWriter.addcr('FrequencyRange=[%f %f],...',options.Fmin,options.Fmax);
strWriter.addcr('Normalization=''%s'',...',options.Normalization);
strWriter.addcr('OneSided=true,...');
strWriter.addcr('MelStyle=''%s'',...',melStyle);
strWriter.addcr('FilterBankDesignDomain="linear");');

if customNorm
    n = options.Normalization;
    strWriter.addcr('%s\n%% Normalize filter bank.','%%');
    if ~isequal(n,-Inf)
        val = vecnorm(filterBank,n,2);
        strWriter.addcr('n = vecnorm(filterBank,%f,2);',options.Normalization);
        filterBank = filterBank./val;
        strWriter.addcr('filterBank = filterBank./n;');
    end
end

varargout{1} = filterBank;
varargout{2} = strWriter.char;

if options.GenerateMATLABCode
    footer = sprintf('%% _Generated by MATLAB (R) and Audio Toolbox on %s_',string(datetime("now")));
    strWriter.addcr('\n%s\n%s','%%',footer);
    matlab.internal.liveeditor.openAsLiveCode(strWriter.char)
end
end