t. Albans, UK. No. RD 1983/9. White, J.C. (1988) Colour LCD TV, Phys. Tech, 19. This page intentionally left blank Telephones and headsets David M Davidson Northern Telecom (Sections (58.1-58.10) Malcolm A Nugent GN Netcom (UK) Ltd (Sections 58.11-58.16) Contents 58.1 Telephones 58/3 58.10 Telephone standards 58/11 58.2 Telephone speech functions 58/3 58.11 Headsets 58/11 58.3 Telephone transmitters 58/3 58.12 Headset aesthetics 58/11 58.4 Telephone receivers 58/4 58.13 Headset technical considerations 58/11 58.5 Telephone handset design 58/5 58.14 The growing need for headsets 58/12 58.6 Telephone transmission performance 58/6 58.15 Headset approval process 58/13 58.7 Signalling 58/8 58.16 Headset design criteria 58/13 58.8 Loudspeaking telephones 58/10 58.9 Digital telephones 58/11 This page intentionally left blank Telephones 58/3 Telephones telephone and correctly received at the telephone at the distant end. We do not have to transmit every single part of the sound generated, When we speak our voice sets up sound vibrations which disturb the since the listener acts as an error correction mechanism and can fill surrounding air, and travel through the air to be detected by the in any missing elements and still understand completely what has listener's ear drums. Sound will travel through most media, air, been transmitted. The prime intelligence in the human voice is water, wood, plastic, etc. and at different speeds through the differ- contained in quite a small segment of the bandwidth of the hear- ent media. For example in air sound travels at approximately 1100 ing/voice spectrum. The human ear can detect sounds from 16Hz to feet per second (335m/s) and approximately 4300 feet per second in 20000Hz and the human voice can generate sounds from 100Hz to water (1311m/s). Electrical signals travel at the speed of light 3 X 10000Hz. 108m/s. Most of the energy in an average male voice is contained within The telephone, patented by Alexander Graham Bell in the USA in the band from 125Hz to 2000Hz and the average f