WHILE YOU may be tapping your foot tomusic on the All India Radio's I'm(Frequency Modulation) channel, thesame broadcast band (87.5 - 108 MHz)may also be serving to contact adoctor required to attend to anemergency call. The I'm Radio DataSystem (RDS) - a paging service - is onthe air.
Paging is a one-way transfer of precoded messages to persons concernedonly, explains P S Sundaram, director(engineering), R&D, AIR and Doordarshan. The RDS paging system was introduced early this year.
To page a subscriber, a phone call tothe paging service control room doesthe trick. The operator types the message into a computer, which is transmitted to the encoder - a device whichreceives the message in the binary codeand transmits the same to the Very HighFrequency FM transmitters. This binarycode is radiated from the transmitter asa sub-carrier in the I'm broadcast band.But its frequency (57 Kilohertz) doesnot fall in the audible range for thehuman ear, and so does not interferewith the FM music channel broadcasts,explains S K Rajagopalan, assistantresearch engineer with AIR.
Each pager has an individual number or address called Cap code preprogrammed into it. The Cap code helpsthe pager to scan through the differentradio frequencies and lock to the rightsignal.
The RDs does not require exclusivefrequencies - it uses the idle capacity ofthe FM channel, which can transmit1187.5 bits per second - and this, inturn, reduces the infrastructural costs,thus giving a low cost nationwide paging service. Registration for radio paging costs Rs 1,000 and monthly servicecharges vary with the type of pager used.
The @adio paging allows coverage ofa 60 kin radius. Intercity paging is donevia the control room operators who calltheir counterparts in the city where theservice is available and the message isdelivered to the particular pager number.
With 85 FM stations already in existence and another 140 on the anvil, theAIR appears determined to succeed in itsplans to link up the country with a fastand efficient communication modebefore the turn of the century.