Telephony is voice communication via public networks. The voice is converted into electrical signals, transmitted in analog or digital form, and converted back into audio signals at the receiving end.
POTS (Plain Old Telephone Service) refers to classic analog telephony, which has a bandwidth of 3.1 kHz and lies in the frequency range from 300 Hz to 3.4 kHz. This frequency range is selected so that the speech is understandable and the speaker can be recognized with his voice characteristics, but nothing more. In addition to syllable intelligibility, which is over 90%, sentence intelligibility, at about 99%, is crucial.
Since the frequency ranges below 300 Hz and above 3.4 kHz are not transmitted in telephony, speech quality and intelligibility suffer. In the case of missing low frequencies, this is expressed by the fact that speech volume and naturalness suffer. In the case of the missing high frequencies, a reduction in syllable intelligibility can be observed. To overcome these disadvantages, methods have been developed to artificially extend the voice bandwidth to a frequency range between 50 Hz and 7 kHz. In addition, various mobile networks support voice services with HD voice and full HD voice as HD telephony.