WEEK 5 DAY 1 : PUBLIC ADDRESSING EQUIPMENTS

5.1 PUBLIC ADDRESSING EQUIPMENTS

        These equipments are used to address public and providing them information about trains etc. Our major focus would be on the announcement system used in stations. We will also briefly talk about wireless walkie talkie.

5.1 Public Announcement System used in stations

        

Public Announcement System

It consists of three parts :-
1) Mic and Mic cord
2) Amplifier System
3) Speaker System

5.1.1 Mic and Mic cord

        As you know Mic is an input device. The announcer announces on the mic to make announcements. There are two types of mic :- 

        1) Mic with cord 

It is wired mic as shown in figure. We use a mic cord to connect the mic to amplifier system

        2) Assist Auxillary ( Cordless mic)

        It is a wireless mic. Simply a receiver is put in the amplifier system. There is a transmitter in mic which is used to transmit the data to the amplifier system. . Some of the salient features are:-

• Requires 9 V cell as supply.
• Range 15m to 100m.(If it works on VHF then range is 500m
• For the receiver it can be powered by either 12 V battery or direct supply. 

5.1.2 Amplifier System

        Amplifier system simply boosts the voice signal received by it through the mic system and transfers it to the speakers.Salient features are:-

• A typical Amplifier used in station can support upto 8 mics (5+3) (5 wired and 3 wireless).
• For controlling more than one mic at once master is used. It basically switches between the ports of the mic to make available only the announcement through a desired mic.
• We can also connect multiple amplifiers through one another. In that case a master control called saloon is used to control them.
• Apart from AC supply it can also work on 24V DC supply.

Here is the basic parts of Amplifier that was demonstrated for us

Basic Ports of Amplifier

1) Battery :- Terminals at which 24 V supply battery is connected

2) GND :- Ground is connected here

3) 100 W :- We can connect up-to 100 W in that terminal. If the power of speakers is less than 100 W then we can connect them in parallel to the terminal. For example we have 25W speakers then we can connect four 25W speakers in parallel to the terminal

4) 70 W :-Same as above only the limit is 70 W.

5) 4Ω :- We can connect speakers that sum up to 4Ω in total. Again connect speakers in parallel. For example if we have 16Ω speakers we will require 4 such speakers to connect to the terminal IN PARALLEL
6) 8Ω :- Same as above only the limit is 8Ω.
7) Recording:- It is a terminal by which we can connect it to a recording device
8) Prout :- used to connect auxiliary cable of mic. We can also connect multiple amplifiers by connecting auxiliary to auxiliary.

5.1.3 Speaker System

        Two types of speakers are used. 

1) Horn Speakers :- They are normal horn shaped speakers that are usually seen at stations for announcements

2) D.J Speakers :- They are large speakers that are used on special occasions. They cannot be directly connected to amplifier as they power rating is high. So, a booster is used before connecting it to amplifier. Apart from that they also contain "Line matching Transfer (LMTC)" which transfers line (i.e it taps ohms and ground).

        That's all about the PAE. There are some wireless instruments that we will talk about briefly.

5.2 Wireless technology (Walkie Talkie)

        Railways employ some wireless techniques also. Some of the examples are:- 

5.2.1 25 W VHF set

• It is generally used for communication between Station master and guard in case of emergencies.
• uses SMPS (SWITCH MODE POWER SUPPLY - has a very high power frequency)
• 3 km range

5.2.1 5 W VHF set

• It is generally used for communication between guard and driver
• Press to talk (PTT)
• 12V supply (DC or rectified AC)
• can also run on 12V batteries.
• 20m antenna

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