Bandwidth Extension of Telephony Speech
The public switched telephone network (PSTN) restricts the acoustic bandwidth of telephony
speech to less than 4 kHz. For compatibility with analog telephone networks, a 0.3 − 3.4 kHz pass
band is common. This bandwidth reduction has a signiﬁcant impact on perceived quality, and
is especially noticeable and even distracting when PSTN users call into, e.g., video conferencing
systems in which the other participants may use wideband (50 − 7k Hz) speech codecs. To reduce
the gap in quality, one may attempt to resynthesize the missing spectrum. Techniques for this
are referred to as bandwidth extension (BWE).
For this thesis, two systems for BWE of speech into the high band (f ≥ 3.4 kHz) were imple-
mented in Matlab, based on systems proposed in literature. The extension was done according
to the linear source-ﬁlter model for speech, meaning estimation of the excitation and spectral
envelope from the narrowband (0.3 − 3.4 kHz) signal were done separately.
BWE System 1 made use of linear prediction (LP) analysis in combination with modulation for
extension of the excitation. Its wideband spectral envelope estimation was primarily based on
linear prediction cepstral coefficients (LPCC) and artiﬁcial neural networks (ANN).
BWE System 2 made use of bandpass-modulation of Gaussian noise (BP-MGN) for extension of
the excitation. Its wideband spectral envelope estimation was based on Mel-frequency cepstral
coefficients (MFCC) and Gaussian mixture modelling (GMM), which was the most complex
estimation method of the two systems.
Objective analysis of the two systems? spectral envelope estimation and informal listening tests
were carried out. These analyses showed that BWE System 1 performed best, though both
systems improved the perceived quality. BWE systems based on LP analysis therefore seem to
be preferrable due to the superior excitation, and efficient computation of the cepstrum.
Place, publisher, year, edition, pages
Institutt for elektronikk og telekommunikasjon , 2009. , 79 p.
IdentifiersURN: urn:nbn:no:ntnu:diva-25084Local ID: ntnudaim:4707OAI: oai:DiVA.org:ntnu-25084DiVA: diva2:730486
Svendsen, Torbjørn, ProfessorPettersen, Svein Gunnar