create mel filter band

melFilterBand.js

// http://d.hatena.ne.jp/aidiary/20120225/1330179868
// port python to JavaScript.

function melFilterBand(sampleRate, nfft, numChannels) {
    var fmax = sampleRate >>> 1,
        melmax = hz2mel(fmax),
        nmax = nfft / 2,
        df = sampleRate / nfft,
        dmel = melmax / (numChannels + 1),
        melcenters = [],
        fcenters = [],
        indexcenter = [],
        indexstart = [],
        indexstop = [],
        filterbank = [],
        i, j, n, m, increment, decrement, triWindow;

    function hz2mel(f) {
        return 1127.01048 * Math.log(f / 700.0 + 1.0);
    }

    function mel2hz(m) {
        return 700.0 * (Math.exp(m / 1127.01048) - 1.0);
    }

    for(i = 0, n = numChannels; i < n; ++i)  melcenters[i] = (i + 1) * dmel;

    for(i = 0; i < n; ++i) fcenters[i] = mel2hz(melcenters[i]);

    for(i = 0; i < n; ++i) indexcenter[i] = Math.round(fcenters[i] / df);

    indexstart[0] = 0;
    for(i = 0; i < n - 1; ++i) indexstart.push(indexcenter[i]);

    for(i = 1; i < n; ++i) indexstop.push(indexcenter[i]);
    indexstop.push(nmax);

    for(i = 0; i < n; ++i) {
        increment = 1 / (indexcenter[i] - indexstart[i]);
        triWindow = [];
        for(j = indexstart[i], m = indexcenter[i]; j < m; ++j) {
            triWindow.push((j - indexstart[i]) * increment);
        }
        decrement = 1 / (indexstop[i] - indexcenter[i]);
        for(j = indexcenter[i], m = indexstop[i]; j < m; ++j) {
            triWindow.push(1 - (j - indexcenter[i]) * decrement);
        }
        filterbank[i] = triWindow;
    }

    return {
        filterband: filterband,
        indexcenter: indexcenter
    };
}