Get logarithmic byteFrequencyData from Audio

Question

I asked a question similar to this earlier, but it did not solve my issue and was explained poorly. This time I've made illustrations to hopefully explain better.

I have a simple frequency spectrum analyser for my audio player. The frequencies are stored in an array that gets updated on each requestAnimationFrame, the array looks like this:

fbc_array = new Uint8Array(analyser.frequencyBinCount);
analyser.getByteFrequencyData(fbc_array);

Read more about getByteFrequencyData here.

So this works fine however I would like the frequencies to be evenly spaced throughout the spectrum. Right now it's displaying linear frequencies:

As you can see, the dominating frequency range here is the Treble (High end), and the most dominated frequency range is the bass range (low end). I want my analyser presented with evenly distributed frequency ranges like this:

Here you see the frequencies evenly spaced across the analyser. Is this possible?

The code I used for generating the analyser looks like this:

// These variables are dynamically changed, ignore them.
var canbars = 737
var canmultiplier = 8
var canspace = 1

// The analyser
var canvas, ctx, source, context, analyser, fbc_array, bars, bar_x,
    bar_width, bar_height;

function audioAnalyserFrame() {
    'use strict';
    var i;
    canvas.width = $('analyser-').width();
    canvas.height = $('analyser-').height();
    ctx.imageSmoothingEnabled = false;
    fbc_array = new Uint8Array(analyser.frequencyBinCount);
    analyser.getByteFrequencyData(fbc_array);
    ctx.clearRect(0, 0, canvas.width, canvas.height); // Clear the canvas
    ctx.fillStyle = "white"; // Color of the bars
    bars = canbars;
    for (i = 0; i < bars; i += canmultiplier) {
        bar_x = i * canspace;
        bar_width = 2;
        bar_height = -3 - (fbc_array[i] / 2);
        ctx.fillRect(bar_x, canvas.height, bar_width, bar_height);
    }
    window.requestAnimationFrame(audioAnalyserFrame);
}

function audioAnalyserInitialize() {
    'use strict';
    var analyserElement = document.getElementById('analyzer');

    if (analyserElement !== null && audioViewIsCurrent() === true) {
        if (analyserInitialized === false) {
            context = new AudioContext();
            source = context.createMediaElementSource(audioSource);
        } else {
            analyser.disconnect();
        }
        analyser = context.createAnalyser();
        canvas = analyserElement;
        ctx = canvas.getContext('2d');
        source.connect(analyser);
        analyser.connect(context.destination);
        if (analyserInitialized === false) {
            audioAnalyserFrame();
        }
        analyserInitialized = true;
        analyser.smoothingTimeConstant = 0.7;
    }
}

Take note that I am skipping 8 bars (See canmultiplier at the top) in the for loop (If I don't, the other half of the analyser gets rendered outside the canvas because it's too big.) I don't know if this is also what could be causing the inconsistent frequency ranges.

Answer 1

If I understood you correctly, I think this will work for you, although is far from perfect.

What you are doing in your for loop is to sample the array, once every 8 elements. What I would do is do the sampling in a logarithmic way.

An example:

//Given a range, transforms a value from linear scale to log scale.
var toLog = function(value, min, max){
    var exp = (value-min) / (max-min);
  return min * Math.pow(max/min, exp);
}

//This would be the frequency array in a linear scale
var arr = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20];

//In this case i'm using a range from 1 to 20, you would use the size of your array. I'm incrementing 'i' by one each time, but you could also change that
for (var i = 1; i < 20; i += 1) {
  //I'm starting at 1 because 0 and logarithms dont get along
  var logindex = toLog(i,1,19); //the index we want to sample

  //As the logindex will probably be decimal, we need to interpolate (in this case linear interpolation)
  var low = Math.floor(logindex);
  var high = Math.ceil(logindex);
  var lv = arr[low];
  var hv = arr[high];
  var w = (logindex-low)/(high-low);
  var v = lv + (hv-lv)*w; //the interpolated value of the original array in the logindex index.
    document.write(v + "<br/>");  //In your case you should draw the bar here or save it in an array for later.
}

I hope I explained myself well. Here you have a working demo that has some boundary bugs but it works as I think you need.