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### Enhanced Speech Modelsfor Robust Speech Recognition

Results

Juan Arturo Nolazco-Flores

Dpto. de Ciencias Computacinales

ITESM, campus Monterrey

Talk Overview

- Introduction
- Enhanced-Speech Models
- Coments and Conclusions

Introduction

- Problem:
- Automatic Speech Recognition performance is highly degraded when speech is corrupted for noise (additive noise, convolutional noise, etc.).
- Fact:
- In order to have real speech recognisers, ASR should tackle this problem.
- Knowledge.
- ASR can be improved either:
- Enhancing speech before recognition
- Training models in the same environment the ASR is going to be used.
- Challenge:
- Find a simple and efficient technique to solve this problem.

Input Data

It needs a model

for unit of recognition.

M1

M2

Probability of each model.

MQ

Higher Probability

Recognised word

Recognition using CD-HMMRecogniser

Enhancing Speech

- Features:
- Models are trained with clean speech.
- Corrupted speech is enhanced.
- There are a number of well studied techniques:
- Subtract an estimated noise found during nonspeech activity.
- Adaptive noise cancelling (ANC).
- Successful for low to medium SNR (>5dB).

Problems:

- Enhancers are not perfects, therefore
- the speech is distorted and
- there are residual noise.

Training models in the same environment

- ASR systems which uses this technique can deal with low to high SNR (>0 dB).
- In example, for an isolated digit recognition task where digits are corrupted for helicopter(Lynx) noise, you can get the following performance:
- For TIMIT
- Problem:
- There are many possible environments (no practical).

However, using continuous HMM is possible to combine the clean speech model and noise model and obtain a noisy speech model.

- Techniques:
- Model Decomposition
- Parallel Model Combination-PMC (Mark Gales, 1996).
- Cepstrum-Domain Model Combination-CDMC (Kim & Rose, 2002).

Changing to linear domain using PMC

- Introduction
- Scheme
- Diagram

Introduction

- It is an artificial way to simulate that the system has been trained in the adverse environment the system is going to work.
- The clean speech CHMM and the noise CHMM (estimated with the noise before the word is uttered) are combined in the linear domain to obtain models adapted to the adverse environment.
- The combination is based in the assumption that that pdf of the state distribution models are completely defined by the mean and variance.

Scheme

- For simplicity, it is convenient to combine these models in a linear domain.
- Problem:
- High performance speech recognition is obtained in a non-linear domain (i.e. mel-cepstral domain, auditory-based coefficients).
- Solution:
- Transform coefficients to a linear domain.

Diagram

Clean

speech

HMM

Linear

domain

C-1()

exp()

PMC

HMM

C()

+

log()

Noise

HMM

C-1()

exp()

Simulates training in noise.

Enhanced Speech Models

- Introduction
- Hypothesis prove
- Enhanced-Speech Models Combination
- Changing to linear domain using PMC
- Diagram
- Results

Introduction

- When we train in the same environment, we obtained the following upper boundry values:
- Since PMC or CDMC (Cepstrum-Domain Model Combination) tries to simulated recognition in the same environment, hence this are the best expected results for these kind of techniques.

Introduction

- How can we improve recognition performance in adverse environments?

Fact:

- The enhancer returns a “cleaner” speech, but distorted.
- Therefore the question is:
- Is it possible to improve recognition performance if the models where trained with this enhaned speech?

Hypothesis

- Enhanced-Speech models improve ASR performance in noisy environments.

In order to prove this hypothesis:

- A signal enhancement scheme has to be selected.
- Models has to be trained with the enhanced speech.
- Observation vectors input to the recogniser has to be processed for the selected enhancement scheme.

Hypothesis Prove

- Introduction
- Spectral Subtraction definition
- Experiments and results
- Conclusions

Introduction

- Since it is a simple (and successful) scheme, Spectral Subtraction (SS) was selected.

Spectral Subtraction Definition

- Before filterbank
- After filterbank.

Experiments and Results.

- CHMMs were trained with speech enhanced by SS.
- Recognition performance was developed over speech enhance by SS in the same conditions.

Example 1

- Task: isolated digit Recognition
- Vocabulary Size: 10
- Training: Using enhanced speech
- Noise: Helicopter (Lynx)
- Database: Noisex92
- Real noise is artificially added to clean speech, such that no Lombard effect can bias recognition performance.

Example 2

- Task: continuous digit Recognition
- Vocabulary size: 30 words
- Training: Using enhanced speech
- Noise: White
- White noise is artificially added to clean speech, such that no Lombard effect can bias recognition performance.

Example 3:

- Task: continuous speech Recognition
- Vocabulary size: 6233 words
- Training: Using enhanced speech
- Noise: white
- Database: TIMIT
- Real noise is artificially added to clean speech, such that no Lombard effect can bias recognition performance.

Conclusions

- Hypothesis was prove to be true.
- Challenge:
- Tried these experiments using other databases.
- How can we combine
- Enhanced Scheme,
- the Noise Model
- and the Clean models
- such that we do not need to train for all enhancement conditions.

Conclusions

- Are all the enhancement schemes suited for combination?

Conclusions

- Now, we know that ASR can be improved either:
- Enhancing speech before recognition
- Training CHMM in the same environment the ASR is going to be used.
- Training CHMM with the same enhancement technique that is used to get “cleaner” speech at recognition.
- Advantage:
- Moreover, training with a better enhancement technique means a potential better recognition performance.

ES-SS Model Combination

- Introduction
- ES-Spectral Subtraction Scheme

Introduction

- How can we combine CHMMs without having to train for each enhancement and noise condition?
- Observation: For CHMMs the state’s pdfs are completelydefined for their means and variances.

ES-Spectral Subtraction Scheme

Assuming Y and YD can be modelled as parametric distributions

with means E[Y] and E[YD] and variances V[Y] and V[YD].

It can be shown that these parameters are distorted as follows:

pdf of Y

ES-PMC Diagram

Adaptation

calculations

Clean

speech

HMM

ES-PMC

HMM

C->log

exp()

C()

log()

+

+

PMC

Noise

HMM

C->log

exp()

Speech is pre-processed using SS.

Results

No compensation scheme

Spectral

Subtraction

PMC

Spectral

Subtraction and parallel model

combination

Results

No compensation scheme

Spectral

Subtraction

PMC

Spectral

Subtraction and parallel model

combination

Results

No compensation scheme

Spectral

Subtraction

PMC

Spectral

Subtraction and parallel model

combination

No compensation scheme

Spectral

Subtraction

PMC

Spectral

Subtraction and parallel model

combination

Coments and Conclusions

- Since training and recognition with the same speech enhancement scheme have not been tried before, hence a new area of research has been open.
- How can we combine CHMM, such that we do not need to train for all enhancement conditions.
- Are all the enhancement technique suited for CHMM combination?
- We show how to combine enhanced-speech, noise and clean CHMM for SS scheme.
- It was shown that equations for ES-PMC-SS were straightforward.

We expect that training with a better enhancement technique we can also obtain better recognition performance.

- Future work:
- Develop equations and experiments for other enhancement techniques.
- Obtain the optimal alpha for SS scheme.
- Compensate in the Cepstrum Domain.

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