ve problems with definitions. The digital subscriber line (DSL) continues increased presence. One can argue whether it is digital or analog. ISDN, of course, is another notable exception. 5.2 DEFINITION OF THE VOICE CHANNEL The IEEE (Ref. 1) defines a voice-band channel as "a channel that is suitable for trans- mission of speech or analog data and has the maximum usable frequency range of 300 to 3400 Hz." CCITT/ITU-T also defines it in the range of 300-3400 Hz. Bell Tele- phone Laboratories (Ref. 2) defines it in the range of 200-3300 Hz. We remain with the CCITT/IEEE definition. 5.2.1 The Human Voice Human voice communication depends on the voice-generating mechanism of mouth and throat being the initial transmitter, with the acuity of the ear being the receiver. Frequency components of the human voice extend down to some 20 Hz and as high as 32,000 Hz. The lower frequency components carry the voice energy and the higher frequency compo- nents carry emotion. Figure 5.1 shows a distribution of energy and emotion of the typical human voice. The human ear and many devices and components of the telecommunication net- work tend to constrain this frequency range. Young people can hear sound out to about 18,000 Hz, and as we get older this range diminishes. People in their sixties may not be able to hear sounds above 7000 Hz. It is not the intent of the PSTN operator to provide high fidelity communications between telephone users, only intelligible connections. Not only is there the frequency con- straint of voice communications brought about by the human ear, there are also constraints brought about by the subset transmitter (mouthpiece) and receiver (earpiece) and the sub- scriber loop (depending on its length, condition, and make up), but then purposely the electrical voice signal will enter a low-pass filter limiting its high-frequency excursion to 3400 Hz. The filter is in the input circuit of the multiplex equipment. Thus we say that the voice channel or VF (voice-frequency) channel occupies the band from 300 to 