Hotword Detection on SUSI MagicMirror with Snowboy

Magic Mirror in the story “Snow White and the Seven Dwarfs” had one cool feature. The Queen in the story could call Mirror just by saying “Mirror” and then ask it questions. MagicMirror project helps you develop a Mirror quite close to the one in the fable but how cool it would be to have the same feature? Hotword Detection on SUSI MagicMirror Module helps us achieve that.

The hotword detection on SUSI MagicMirror Module was accomplished with the help of Snowboy Hotword Detection Library. Snowboy is a cross platform hotword detection library. We are using the same library for Android, iOS as well as in MagicMirror Module (nodejs).

Snowboy can be added to a Javascript/Typescript project with Node Package Manager (npm) by:

$ npm install --save snowboy

For detecting hotword, we need to record audio continuously from the Microphone. To accomplish the task of recording, we have another npm package node-record-lpcm16. It used SoX binary to record audio. First we need to install SoX using

Linux (Debian based distributions)

$ sudo apt-get install sox libsox-fmt-all

Then, you can install node-record-lpcm16 package using npm using

$ npm install node-record-lpcm16

Then, we need to import it in the needed file using

import * as record from "node-record-lpcm16";

You may then create a new microphone stream using,

const mic = record.start({
   threshold: 0,
   sampleRate: 16000,
   verbose: true,

The mic constant here is a NodeJS Readable Stream. So, we can read the incoming data from the Microphone and process it.

We can now process this stream using Detector class of Snowboy. We declare a child class extending Snowboy Hotword Decoder to suit our needs.

import { Detector, Models } from "snowboy";

export class HotwordDetector extends Detector {
  1 constructor(models: Models) {
           resource: `${process.env.CWD}/resources/common.res`,
           models: models,
           audioGain: 2.0,

   // other methods

First, we create a Snowboy Detector by calling the parent constructor with resource file as common.res and a Snowboy model as argument. Snowboy model is a file which tells the detector which Hotword to listen for. Currently, the module supports hotword Susi but it can be extended to support other hotwords like Mirror too. You can train the hotword for SUSI for your voice and get the latest model file at . You may then replace the susi.pmdl file in resources folder with our own susi.pmdl file for a better experience.

Now, we need to delegate the callback methods of Detector class to know about the current state of detector and take an action on its basis. This is done in the setUp() method.

private setUp(): void {
   this.on("silence", () => {
      // handle silent state

   this.on("sound", () => {
      // handle sound detected state

   this.on("error", (error) => {
      // handle error

   this.on("hotword", (index, hotword) => {
      // hotword detected 

If you go into the implementation of Detector class of Snowboy, it extends from NodeJS.WritableStream. So, we can pipe our microphone input read stream to Detector class and it handles all the states. This can be done using

mic.pipe(detector as any);

So, now all the input from Microphone will be processed by Snowboy detector class and we can know when the user has spoken the word “SUSI”. We can start speech recognition and do other changes in User Interface based on the different states.

After this, we can simply say “Susi” followed by our query to ask SUSI on the MagicMirror. A video implementation of the same can be seen here: 


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GSoC Student Developer at FOSSASIA