Design of Low Cost Modem Using Tele Typewriter Signals - Ijcns.com

International Journal of Computer Communication and Information System ( IJCCIS)– Vol2. No1. ISSN: 0976–1349 July – Dec 2010Figure 1: Frequency Shift Keying ModulationSo for transmitting digital data using Tele TypewriterSignals, we use f H=1070Hz and f L=1270Hz. The majorreason for choosing Frequency Shift Keyingmodulation technique is due to the small bandwidthrequirement of the Tele Typewriter signals. TheBandwidth is 200Hz which is very low. The usageother modulation techniques like amplitude modulationor other frequency modulation techniques like ASK(Amplitude Shift Keying), PSK (Phase Shift Keying),the input binary message signal is not effectivelyreproduced. The obtained message signal is distortedand does not resemble the input message signal. Theaccuracy of FSK technique is better than othermodulation techniques. The Probability of Error ofFSK technique is also good. So because of thesereasons Frequency Shift keying has been selected as themodulation technique in our modem.II. BLOCK DIAGRAM DESCRIPTIONFigure 2: Block DiagramThe basic block diagram of the Wireless DataModem is shown in Figure 2. It consists of five stagesnamely a) Modulator Stage, b) Transmitter Stage, c)Receiver Stage, d) Filter stage and e) DemodulatorStage. The aim is to transmit the Input digital dataeffectively from one network to another network orfrom one network to other peripheral devices. The firststep is to modulate the input digital data with the carriersignal using Frequency Shift Keying ModulationTechnique. This is achieved using IC 555 Timer, whichproduces the FSK modulated signal whose f H=1070Hzand f L=1270Hz, which means that when the inputmessage signal is of logic 1 the frequency of themodulated signal is 1070Hz and when the inputmessage signal is of logic 0 the frequency of themodulated signal is 1270Hz. The modulated signal isthen transmitted using IR LED. Thus Wireless Datatransfer is achieved by the using IR LED. Thetransmitted signal is received at the Receiver side bymeans of a spectrally matched Phototransistor whichthen sends the obtained modulated signal to the FilterStage. The Filter stage consists of a Second order LowPass Filter (LPF). The final stage is the Demodulatorstage which consists of Phase Locked Loop (PLL) anda RC ladder Filter. The Phase Locked Loop isimplemented using IC 565. Thus the demodulatorStage separates the message signal from the carriersignal and hence the digital data signal is obtainedsuccessfully. The Baud rate which denotes the rate oftransfer of data for this modem is given asBaud rate (1)Here =( 1+ 2)=1.722ms, 1=0.935ms,2=0.787ms, where 1 is the time during which bit 1lasts and 2 is the time during which bit 0 lasts andhence the Baud Rate is 580 bps (Bits per Second).III. MODULATIONModulation is the process of altering thecharacteristics of the carrier signal in accordance to themessage signal. Here Frequency Shift KeyingModulated signal is obtained using IC 555 Timer. Thecircuit diagram for FSK Signal generation using IC555 Timer is shown in Figure 3. It consists of a IC 555Timer which works in Astable Mode. The resistors R a,R b and capacitor C determines the Frequency of theFSK modulated signal. The standard digital data inputfrequency is usually 150Hz. When the input is HIGHi.e. when the input binary data is of logic 1, the PNPtransistor Q is off and IC 555 Timer works in thenormal astable mode of operation. The frequency ofthe output FSK modulated signal is given by theequationf 0= 1.45/(R a+2R b)C (2)Figure 3: FSK Generation using IC 555 timer138