Research Area: | Speech Recognition | Year: | 2010 | ||||
Type of Publication: | Mastersthesis | Keywords: | Lombard effect, fundamental frequency, strength of excitation, zero-frequency resonator, loudness, intelligibility, speaker verification, imposter, playback speech | ||||
Authors: | G. Bapineedu | ||||||
Abstract: | |||||||
Speaking in the presence of noise changes the characteristics of the speech produced
which is known as the Lombard effect. This effect is perceptually felt with an increase in
intensity of speaking. These changes in the characteristics of speech production is to en-
sure an intelligible communication in noisy environment. These changes also result in the
performance degradation of speech systems like speaker recognition, speech recognition,
etc. Human speech production mechanism is affected due to the Lombard effect, and is
reflected mainly in the excitation source. Previous studies have focussed mainly on the
changes in system level features. In our work, we examine the excitation source to study
its changes due to Lombard effect.
The excitation source features studied in this work are the instantaneous fundamental
frequency F0 (i.e., pitch), the strength of excitation at the instants of significant excita-
tion and a loudness measure reflecting the sharpness of the impulse-like excitation around
epochs. Another feature, called the normalized energy is used to study the articulation
variability of speech due to Lombard effect using the high variations of energy within an
utterance. Analysis is performed using the distributions of the features which are seen
to distinguish between normal speech and Lombard effect speech, and also to explain
the speaker-specific nature of Lombard effect. The extent of Lombard effect on speech
depends on the nature and intensity of the noise that causes the Lombard effect. Charac-
teristics of speech produced without a feedback and difference between loud speech and
Lombard effect speech are also studied using the excitation source features. Duration is
used to study the phonetic changes due to Lombard effect. Intelligibility due to Lombard
effect at a sentence level is studied and the mechanism of the Lombard effect is described
based on the analysis performed. This analysis is extended into an application where the imposters during speaker verification can be detected with the theory that speech pro-
duced under noise is different from that spoken under normal conditions. Further, the
performance of text-dependent speaker verification system due to Lombard effect is also
described. |
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