 Doordarshan had a modest beginning with the experimental telecast starting in Delhi on 15 September 1959 with a small transmitter and a makeshift studio.Delhi  Now more than 90 percent of the Indian population can receive Doordarshan (DD National) programmes through nearly 1,400 terrestrial transmitters.  There are about 46 Doordarshan studios producing TV programmes today  Doordarshan operates 21 channels

There are three types of Basic transmission Systems.  Cable transmission  Direct to home  Transmitter service

 Each cable channel is 6MHz wide, AM picture signal and the FM for the sound signal.  These mid band cable channels range from 88 to 176 MHz also all the low band VHF channels (7 to 13) are used for cable TV.  Power Supply:  Maximum demand/capacity: 30KW  Monthly average consumption : 6000 units  Monthly average expenditure’s 40000/month

Representation of Direct to home

 Digital picture: The picture quality in DTH is much better. The quality of the picture is uniform across all channels.  Digital audio: We get the stereo phonic sound. So if we have got a home theatre, connect it to your set-top box we will get better sound effects.  Electronic program Guide (EGP):  It’s an on-screen guide that shows the program schedule or listing of all channels.

1.High power transmitter (HPT):  Transmitter power 10KW  Distance covered by above transmitter is 60km-100km Eg: located in Rajahmundry 2.Low power transmitter (LPT):  Transmitted power 100w-500w  Local area transmitter covers distance around 21kms Eg: located in Kakinada 3.Very low power transmitter (VLPT):  Transmitted power – 10w  Distance covered is around Eg: located in Yanam

Generator power supply Receiver dish antenna P.D.A Mast antenna Transmitting rack Monitoring rack Input rack

 The parabolic dish antenna is a metal structure with the shape of a half circle, apart from that at a distance a feed arm is held with support in the air. Fig: 3.2 parabolic antenna Fig: Parabolic antenna Fig Parabolic antenna Parabolic antenna

Transmitter Rack Audio- video switcher CombinerDriver Driver amplifier Exciter

VHF transmitter Block Diagram of Exciter unit LPF Video processor Control oscillator Vestigial side band filter Power controll er IF oscillator Vision modulator Aural modulator Audio signal Video driver signal

LINEARITY CORRECTOR BASE BAND CORRECTOR EXCITERUPCONVERTER SPLITTER & PA VIDEO SIGNAL

LOW NOISE POWER LOW NOISE BLOCK CONVERTER DIGITAL VIDEO BROADCAST RECEIVER PARABOLIC DISH ANTENNA

V1 AUDIO V2 &VIDEO V3 SWITCH EXCITER DRIVER AMPLIFIER POWER AMPLIFIER ANTENNA

Antenna : Radiation pattern: Field Strength: FS= 2.85 √P Ht. Hr / ʎ D 2 milli volt/meter

Sky wave propagation Ground wave propagation Space wave propagation Tropospheric propagation

Self supporting mast Guided wire mast

The transmitter service involves great equipment that deals with monitoring section exciting system and we learn about the above equipment of the Doordarshan relay centre and it’s working. We also learned about the procedure of transmission, reception, strengthening of the signal and retransmitting the signal into space for the broadcast around the range of propagation.