decodeAudioData

Different results of the channel data between audiocontext.decodeAudioData and octave reference (expectation)

Case

We have a client app built with Google Polymer 3 implementing Audiocontext API to decode audio data. From our analyzing, the decoder returns different result of channel data to our reference/expectation. We use octave software as the reference.

On the folder audiofile, there is a sample wav file to be experimented.

Run

• Open the file index.html on chrome browser
• Select the audiofile/h.wav file
• See the log result of channel data, will be like :

0: 0.738193154335022
1: 0.17993010580539703
2: 0.143886536359787
3: -0.49501433968544006
4: -0.581611156463623
5: -0.07922420650720596
6: 0.25858914852142334
7: 0.42484045028686523
8: 0.12840349972248077
9: -0.15169838070869446
...

Compare

• Install first octave software :

sudo apt-get install octave

• Run the octave :

octave-cli

• Load the audio data

> audioDump

• Look at the fs and first 10 sample points of the audio data on the octave console

fs =  48000
ans =

   1.000000
  -0.059232
   0.408143
  -0.503169
  -0.554996
  -0.336158
   0.122070
   0.389484
   0.336810
   0.057144

Expectation

The data we load into the browser should be identical or scaled to octave result

Result

Both results are different and not identical nor scaled

How the client app work?

• Read the audio file as ArrayBuffer using FileReader web API
• Decode the ArrayBuffer using AudioContext::decodeAudioData API
• Print out the channel data using getChannelData()

Problem Solving

1. We have to know the audio file sample rate before create audiocontext instance
2. Once we know the audio file sample rate, set it on audiocontext initialization :

var audioCtx = new (window.AudioContext || window.webkitAudioContext)({
  sampleRate: <audio_file_sample_rate>
});

3. Now, we can decode the audio data through Audiocontext and get the same result as octave reference

Note : Currently, the sampleRate of an AudioContext can only be set manually in Firefox and Chrome 74+