HTML Web Audio API - audio visualisation of left & right channels

Question

I'm newbie in Web audio API. I need to visualize left&right channels signal strength of playing audio like below image.

evt.inputBuffer.getChannelData(0) will return whole channel information, How will I get left and right channel datas, where should I change my code? Please help.

This is my code:

<style>
   #meter1, #meter2 {
   width: 0%;
   height: 15px;
   margin: 2px 0;
   background: green;
   }
</style>

<div style="width: 250px;">
   <div id="meter1"></div>
</div>
<div style="width: 250px;">
   <div id="meter2"></div>
</div>
<script>
   // Create the audio context - AudioContext is a little container where all our sound will live. 
   //It provides access to the Web Audio API, which in turn gives us access to some very powerful functions.
   // Browser support
   window.AudioContext = window.AudioContext || window.webkitAudioContext;
   // Initialize audio context
   var audioContext = new AudioContext();

   var soundUrl = "sample.mp3";
   var soundBuffer; 

   var audio = new Audio(soundUrl);
   var processor = audioContext.createScriptProcessor(2048, 1, 1), meter1 = document.getElementById('meter1'), meter2 = document.getElementById('meter2'), source, splitter;
   audio.crossOrigin = 'anonymous'

   audio.addEventListener('canplaythrough', function(){
   //create a new MediaElementAudioSourceNode object, given an existing HTML <audio> or <video> element, the audio from which can then be played and manipulated.    
   source = audioContext.createMediaElementSource(audio)
   splitter = audioContext.createChannelSplitter(2);
   source.connect(splitter);
   source.connect(processor)
   source.connect(audioContext.destination)
   processor.connect(audioContext.destination)

   audio.play()
   console.log(source);
   }, false);


   // loop through PCM data and calculate average
   // volume for a given 2048 sample buffer
   processor.onaudioprocess = function(evt){
   //console.log(evt.inputBuffer);
   var input = evt.inputBuffer.getChannelData(0)
     , len = input.length   
     , total = i = 0
     , rms
   while ( i < len ) total += Math.abs( input[i++] )
   rms = Math.sqrt( total / len )
   meter1.style.width = ( rms * 100 ) + '%'
   }

</script>

Answer 1

Use createScriptProcessor(2048, 2, 2) to have two input channels and two output channels. Then, within onaudioprocess , the second input channel data will be in evt.inputBuffer.getChannelData(1) .

Note that:

• after creating audioContext , you may want to store (or to check) inChannels = audioContext.destination.channelCount and outChannels = audioContext.destination.channelCount
• a value of 2048 for processor.bufferSize is good for data visualization, but consider that 16384 would be better for audio quality