low compared to the top of the human hearing range, is sufficient to convey speech. Different aircraft, control towers and other users transmit with different bandwidths and audio characteristics. Digital radio A switch to digital radios has been contemplated, as this would greatly increase capacity by reducing the bandwidth required to transmit speech. Other benefits from digital coding of voice transmissions include decreased susceptibility to electrical interference and jamming. The change-over to digital radio has yet to happen, partly because the mobility of aircraft necessitates complete international cooperation to move to a new system and also the time implementation for subsequent changeover. Unauthorised use It is illegal in most countries to transmit on the airband frequencies without a suitable license, although an individual license may not be required, for instance in the US where aircraft stations are "licensed by rule". Many countries' regulations also restrict communications in the airband. For instance, in Canada, airband communications are limited to those required for "the safety and navigation of an aircraft; the general operation of the aircraft; and the exchange of messages on behalf of the public. In addition, a person may operate radio apparatus only to transmit a non-superfluous signal or a signal containing non-profane or non-obscene radiocommunications." Listening to airband frequencies without a license is also an offense in some countries. However, in certain countries, such as the UK, it is permissible to listen to as it is covered under navigational and weather related transmissions. Such activity has been the subject of international situations between governments when tourists bring airband equipment into countries which ban the possession and use of such equipment. See also ACARS Air traffic flow management Avionics Control tower Future Air Navigation System Line-of-sight propagation References Airbands