Since the introduction of telephony, not much has changed where the transmitted frequency range of audio signals is concerned. For more than 100 years, the range has been between 300 and 3400 hertz. A bandwidth of 3100 hertz allows for an easily comprehensible transfer of spoken words, but imposes severe restrictions on sound quality. Even digital processes such as ISDN didn’t change the bandwidth for voice transmission. At a data rate of 64 kilobits per second, even the G.711 codec used by ISDN devices only makes use of this restricted bandwidth.
HD telephony expands the frequency spectrum to almost double the audio bandwidth of almost seven kilohertz, and supports a frequency range of between 50 to 7000 hertz. This incorporates roughly the entire natural sound spectrum of spoken communication, from between 20 and 8000 hertz, and allows for extremely high-quality speech transfer. HD telephony makes use of audio codec G.722.
VoIP (Voice over IP) software is in principle capable of supporting HD telephony. If phone data is transmitted between the participants directly via the IP network, both terminal devices must support codec G.722. If media gateways are used for transmitting the data, these must also be able to accept codec G.722. In order to make phone calls in HD quality with a cordless phone, DECT telephones are available which support G.722 for wireless transmission. This software is often called CAT-iq (short for Cordless Advanced Technology - internet and quality). Many mobile phone network providers market HD telephony under the title of HD Voice.