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Promotionsvortrag: HD-Telephony by Artificial Bandwidth Extension
The audio bandwidth of digital landline and mobile telephone networks is still mostly restricted to the frequency range of 200 Hz to 3.4 kHz. This is due to compatibility requirements in the transition phase from analogue to digital transmission technology. The resulting characteristic "telephone speech" is widely accepted, but the intelligibility of syllables is only 91%.
Meanwhile, improved coding standards for so-called "HD voice" or "Wideband Speech" are available which are gradually being introduced into the networks. They support an audio frequency bandwidth of 50 Hz to 7.0 kHz with significantly increased audio quality and speech intelligibility. For a very long time however, new HD-telephones and old narrowband telephones have to co-exist. If an HD-terminal is connected over a narrowband link to an old telephone, the improved coding scheme cannot be used.
In this thesis, signal-processing concepts are developed for improving audio quality and intelligibility of narrowband speech by artificial bandwidth extension (ABWE). From a system theoretical point of view, this seems to be questionable. However, in the case of speech signals, redundancy of the source-filter model of speech production as well as the mechanisms of human hearing can be exploited to recover or mimic missing parts to a certain extent. These algorithms can be applied in HD terminals or in the network, to transform narrowband speech to HD voice.
In comparison to the state-of-the-art of ABWE approaches, the main contributions are:
- a new concept of estimating the wideband spectral envelope in terms of the model filter by interpolation in the acoustic tube domain
- novel algorithms for spectral extension of the excitation signal
- new insights concerning the relative importance of the excitation, the temporal envelope and the spectral envelope
- remarkable improvements of the speech quality and computational complexity
The improvements are verified by objective evaluation and subjective listening tests.