DATA TRANSMISSION AND RETRIEVAL This Chapter includes Demodulation Data Transmission and Retrieval FAX Modem Created by C. Mani, Principal, K V No.1, AFS, Jalahalli West, Bangalore

Demodulator D C C’ RL Output AF signal Amp. Modulated Signal Demodulator L1 L2 D C C’ RL Output AF signal C’ offers low impedance to RF signal and high impedance to AF signal since Xc = 1/2C’

Data Transmission AF Processor and Filter AF Power Amplifier Transmitting Antenna AF Processor and Filter AF Power Amplifier AF Amplifier AF Data Line Fi l ter Modulator Power Amplifier HF Amplifier HF oscillator

Data Retrieval HF Amplifier-I HF Amplifier-II Demodulator Amp. Modulated Signal Data Retrieval Receiving Antenna Line Fi l ter HF Amplifier-I HF Amplifier-II Demodulator AF Data Processor AF Power Amplifier AF Pre-Amplifier AF Data

FAX (Facsimile Telegraphy) * The electronic reproduction of a document at a distant place via Telephone line Original Document Optical Scanner Analog Signal Telephone Line Sensor Modem Light Signal Digital Signal Printer FAX Copy Functions Performed by FAX: i) Optical Scanning ii) Conversion of data for transmission and reception iii) Printing a copy at the receiver’s end

FAX Modem i) Optical Scanning ii) Conversion of data for transmission Written document Rollers Metallic Plate Mirror Lens Light Source Charge coupled device (Semiconductor Chips) Digital Signal Analog Signal Telephone Line Modem

MODEM (MOdulator and DEModulator) DATA SET * Converts digital signal to analog signal at the transmitting end and vice versa at the receiving end. AM Signal Digital Signal Modulator Portion Demodulator Portion Digital Signal Communication Channel Demodulator Portion Modulator Portion Digital Data Machine eg. Computer Digital Data Machine eg. Computer AM Signal Modem I Modem II Terminal-1 Terminal-2 * Modem is also called as data set because it inter connects data source with a transmitter or receiver.

Modems differ in: a) Speed of data transmission: i) low speed : upto 600 bits per sec (bps) ii) medium speed: 600 – 2400 bps iii) high speed : above 2400 bps b) Modulation methods: i) frequency shift keying (FSK) which shifts a carrier frequency to indicate a space or mark ii) phase shift keying (PSK) which shifts the phase of a carrier wave c) Bandwidth: i) low frequency range (below 4 kHz) ii) wide band (in multiples of 4 kHz)

Modes of Modems: 1) Simplex mode: It operates as either send-only or receive-only device; i.e. data signals can be transmitted between Transmitter and Receiver only in one direction. 2) Half-duplex mode: In this, data signals can be transmitted between Transmitter and Receiver in both the directions but only in one direction at a time. 3) Full-duplex mode: In this, data signals can be transmitted between Transmitter and Receiver in both the directions simultaneously. END