| 摘要 |
<p>A system and mechansim substantially to maintain a perceived level of speech quality upon transition from a wideband to a narrowband service environment is illustrated in (fig 2.). A subscriber unit moving from a cell where a wideband speech channel (e.g. 8kHz) is utilised to a cell where a narrowband speech channel (e.g. 4kHz) is utilised usually experiences an immediately perceivable deterioration in audio signal quality output. This is overcome by generating a pseudo wideband signal from the received narrowband signal by analysing (68) formant components of the received signal, or by duplicating the narrowband spectral profile and reflecting (74) the profile about the narrowband Nyquist point. In the former respect, low frequency components are matched (70) against low frequency signal profiles stored in a database (28), and associated high frequency components are added to complete the speech signal in a manner equivalent to an 8kHz channel. During a transition period, the pseudo wideband signal is progressively band-limited (80) to the narrowband signal of the currently occupied cell.</p> |
