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Voice Activity Detection

Voice activity detection (VAD) is a process of identifying speech part in audio stream. For speech recognizer, both "detecting speech part" and "rejecting noise part" are essential. Accuracy and low-latency VAD is critical and a key technique for practical ASR applications.

Julius has multiple VAD detection modules. When multiple VAD detectors are enabled, only the speech frames that qualifies all the detectors will be processed.

  • Level and zero cross threshold detector
  • WebRTC detector
  • Static GMM detector
  • Decoder-based detector

Note that level / zero cross detector and WebRTC detector are parallel but others are cascaded: both level / zero cross detector and WebRTC detector read the full audio input stream and output detection result for every frame, whereas the static GMM detector and decoder-based detector runs after those, only processing the audio samples that have been passed by the former two detectors. The block diagram of detection modules are as follows:

Block diagram of VAD modules

All VAD detector is disabled by default for buffered processing. For stream processing, the level and zero cross threshold detector is enabled by default. Other detectors should be set up and enabled by options.

Level and zero cross threshold detector

A basic VAD module based on signal level and zero-cross threshold. Audio signal in which both the signal level and number of zero-cross is higher than the given threshold will be treated as speech segment.

It's trivial, simple, light-weight method, but still works fine on high SNR condition.

Parameters can be specified by -lv, -zc, -headmargin and -tailmargin options. The -headmargin and -tailmargin options specifies the margin before / after the detected segment, as illustrated below: VAD by level threshold

This detector is enabled by default for stream processing (live audio and network input), and disabled for buffered processing (file input). Set -cutsilence to force enable this detector, and -nocutsilence to disable.

WebRTC detector

WebRTC VAD is a GMM based voice activity detection developed by WebRTC group for real-time speech processing. The WebRTC implementation is widely accepted as one of the modern fast VAD, and is an open-source gold standard. The original source code is open at WebRTC site as part of wider range of codes, and you can find the exact code here. Julius integrates its forked version "libfvad: voice activity detection (VAD) library".

This detector is disabled by default. Set mode from 0 to 3 with -fvad option to enable. You can also tease parameters with -fvad_param.

Note that, for internal reason, this WebRTC detector will not work when the level and zero cross threshold detector is totally disabled (for file input, or forced to be disabled by -nocutsilence), If you want to switch off the level and zero cross threshold detector and run WebRTC detector only, leave it enabled (set -cutsilence for file input) and set a minimum value to the threshold, i.e. "-lv 1" to enforce the threshold detector to always pass through the input.

Static GMM based detector

Gaussian mixture model (GMM) based speech detector. Requires the voice / noise GMM model trained beforehand. Unlike WebRTC detector, no adaptation will be performed while running.

This detector is disabled by default. To enable, build Julius with configure option --enable-gmm-vad. Note that for internal reason, this static detector and "GMM input rejection" feature is exclusive: when this detector is enabled, GMM input rejection will not be available.

The definition of GMM should be in HTK format HMM acoustic model file, contains voice models and noise models. Each model should have 3 states (1 emitting state), which has pdf as diagonal gaussian mixtures. Several models can be defined.

At runtime, specify the GMM definition file with option -gmm, and list of noise HMM names to be rejected by -gmmreject. You can tease detection margin and up/down trigger sensitivity by -gmmmargin, -gmmup and -gmmdown options.

If acoustic feature of the GMM is different from the main acoustic model for recognition, you should specify the GMM's feature extraction parameters using -AM_GMM section.

Decoder-based detector

Decoder-based VAD is an experimental function, utilizing the statistics of main speech decoding process for speech detection. The first pass of recognition process will be performed for the entire input on real-time for given input stream, and the hypothesis statistics of "silence words" at the ongoing recognition process are inspected to judge whether speech segment starts or ends at the certain frame. The silence words are the words that is assigned a sequence of silence models in word dictionary. On many speech recognition the silence words should be a beginning of sentence marker, end of sentence marker, or punctuation words.

This detector is disabled by default. To test it, set configure option --enable-decoder-vad at build time and set option -spsegment at run time. To determine the silence words, the name of silence models should be passed to -pausemodels options.