esent each country ing ounces and of spider-like proportions, yet which are amazingly issues its own national standards for telephones, either via the PTT strong and flexible. There are even plastics which allow the headset as the controlling authority or via a national standards organisation to be formed to fit the head simply by heating with friction from the such as BSI (British Standards Institute) in the UK. hand. Headsets come in a variety of forms, over-the-head, under- In North America the position on standards for telephones is much the-chin, on-the-ear, but the market is tending to standardise on two simpler. The FCC (Federal Communications Commission) issues major types: the over-the-head and the on-the-ear versions as shown the basic standards for all of the USA and in Canada the DOC in Figures 58.14 and 58.15 respectively. The tip-in-ear variety is in (Department of Communications) issues similar standards. decline, partly due to the fact that it can irritate the inside of the ear, as well as cause aural hygiene problems. Hygiene is also an import- ant consideration in respect of ease of cleaning and maintaining. 58.11 Headsets Rapid growth in the use of computers and telemarketing means that 58.13 Headset technical considerations more and more people in many walks of life are finding themselves before a visual display unit, operating a keyboard accessing data, The ergonomics and mechanical design of modern headsets are of and using a telephone terminal with a headset. Headsets, once the paramount importance. Different styles and configurations of head- domain of the telephone switchboard operator, are playing the major sets, perhaps with a modular concept, are essential. Limited choice 58/12 Telephones and headsets Noise cancelling versions of both electret and electromagnetic microphones are also available. However the operation of noise cancelling microphones can often degrade transmission perfor- mance if users are not trained properly. There is a tendency to presume that a noise cancelling micro