 radio An additional feature is that the control station can be switched to regulations are required, telephone coupling equipment will also talkthrough, so that one receiver can talk direct to another. This is a have to be approved by telephone line and safety authorities. In the useful facility for first aid teams or security guards. UK BABT (British Approvals Board for Telecommunications) is The European Selective Paging Association (ESPA) are currently responsible for type approval of telephone line connection equip- considering putting forward proposals to ETSI to replace the cross ment and for safety compliance to BS6301. In Germany the line band talkback frequencies by 10 additional frequencies at u.h.f. approval is undertaken by ZZF. In the USA FCC has responsibility as does the Department of Communications in Canada. 45.6 Large on site systems 45.5.6 Talk back paging Figure 45.2 is an example of a large on site paging scheme and is Portable units comprising a paging receiver and a transmitter ca- fully featured. It has a remotely linked section of the system as pable of speech acknowledgement are used on about 10% of all on would be found on a two sited hospital complex or a large brewery. Large on site systems 45/7 Table 45.3 Typical talkback pager specification Transmitter Transmitter Encoder Receiver Receiver frequency range 25MHz to 406MHz to 54MHz 490MHz Remote Contact Directory Absence Channel spacing 10, 12.5, 20, 20kHz to 25kHz Interface Control Unit 25kHz Computer Interface Sensitivity (typical call) 20uV/m 15uV/m Local Speech (12dB Sinad) 40uV/m 30uV/m Control Telephone Interface Call Logger Remote Adjacent channel select Modem Controller (typical) Spurious response (typical) P.A.B.X. Spurious emissions Tie Line (max.) Transmitter frequency 146MHz to 174MHz range Remote Channel spacing 10, 12.5, 20, or 25kHz Modem Transmitter Controller Directory Output power (typical) 20mW ERP Control Talkback time limits Unrestricted/restricted according Encoder to regulations Audio distortion Absence Spuriou