A common narrow-band speech signal is expanded into a wide-band speech signal.
The expanded speech signal gives the impression of a wide-band speech signal regardless
of what type of vocoder is used. Extending the narrow-band speech signal into a
lower range involves analyzing the narrow-band speech signal to generate one or
more parameters, and synthesizing a lower frequency-band signal based on at least
one of the one or more parameters. The synthesized lower frequency-band signal
is then combined with a signal that is derived from (e.g., via up-sampling) the
narrow-band speech signal. In preferred embodiments, a pitch frequency parameter
is generated, and generation of the lower frequency-band signal includes generating
continuous sine tones that are frequency shifted with the pitch frequency parameter.
