BANDWIDTH EXTENSION OF TELEPHONY SPEECH - Adaptive Signal Processing: Next Generation Solutions - page 350

Figure 7.1 (a) Analog telephone bandpass according to ITU recommendation G.151
[16] (used in analog phone connections) and (b) highpass in GSM enhanced full rate
codec according to [10] (used in the GSM cell phone network).
caused by the introduction of band limiting filters within amplifiers used to keep a cer-
tain signal level in long local loops [22]. These filters have a passband from approxi-
mately 300 Hz to 3400 Hz [see Fig. 7.1 a ] and were applied to reduce crosstalk
between different channels.
As we can see in the time-frequency analysis of a speech signal transmitted over a
telephone network [depicted in Fig. 7.3 b ] the application of such a bandpass attenu-
ates large speech portions. Digital networks, such as Integrated Service Digital
Network (ISDN) and Global System for Mobile Communication (GSM), are able
to transmit speech in higher quality since signal components below 300 Hz as well
as components between 3.4 kHz and 4 kHz can be transmitted [see Fig. 7.1 b ].
However, this is only true if the entire call (in terms of its routing) remains in those
Figure 7.2 Overall system for bandwidth extension.