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Everyone knows how important two-way radios are to emergency services - the 'blue light' operations where fast response and accuracy can mean the difference between life and death.
In the late 1 980s, business invested
heavily in communication toots for off-site mobility (cellular phones) but a bottleneck
has now been revealed in the workplace itself.
PBR Business and industry has been using the services of private business radio since the end of the second World War when frequencies and products became more widely available. It was initially used for wide area coverage using mobiles fixed in vehicles, eg haulage, taxi, service companies, etc. Over the last decade, more and more on-site hand portables are being used on PBR frequencies because they are exceptionally rugged, high powered, high in features with improved battery duty cycles and they provide the operator with true mobility. PBR is ideal for companies looking to add value to their communications, either in the form of more coverage or extra calling features to improve overall business efficiency.
SRBR/PMR446 Short range business radio is a newly launched service which is designed to provide close voice communications, eg small estates, office blocks, hotels, etc. The hand portables are generally light-weight, low in features, low power and low cost. The radios are available off-the-shelf for immediate use. This service is ideal for new users looking for simplicity and can be used on a pan European basis without licensing. Selective Calling (Selcall) In general, the more coverage and the greater the number of radios in a fleet, the more the need for effective calling management. This is because multiple users sharing a radio channel cannot all speak at the same time and do not need to hear constant radio traffic. To help overcome this, a method of sequential tone signalling is built into the radios to enable individual radio calling. Plus, the radios tend to incorporate displays and keypads that aid the user on the status of both incoming and outgoing calls in an orderly and easily understood manner; just like today's cellular phones. More complex systems may include telephone interconnect, alarm functions, remote stun/revive/kill and incoming identity display with auto acknowledge. Naturally, the added value these features offer increases the price of both the portable and mobile units as well as the base and control infrastructure. DCS and CTCSS DCS (Digital Coded Squelch) is digital data or code word that is transmitted with the voice audio. This data is sub-audible with most of it's energy below 300Hz. However is does have a wide bandwidth from 2 to 300 Hz. Unlike CTCSS (Continuous Tone Coded Squelch System) which uses continuous tones below 300 Hz., DCS uses digital data or code words. Each code word is unique and all code words may be used on the same channel without interference. At the end of the radio transmission and about 1/2 second before the transmitter un-keys, the radio will encode a 134 Hz tone that serves as a turn off code. The FM deviation level of DCS data should be in the range of 500 to 800 Hz. Unlike CTCSS, DCS signal spectrum occupies considerable more bandwidth. A poor low frequency response in the transmitter or receiver may not seriously distort a single frequency tone signal but may seriously degrade a wide band signal containing multiple frequency components. The distortion risk is especially high if the frequency response delays the wide band frequency components. DCS is operated at a low baud rate (137 bits per second) and because DCS may have extended periods of all ones and zeros almost all components in the transmitter and receiver chain must be coupled down to at lease 2 Hz or lower. This requirement means that certain transmitters and receivers must be modified before they are capable of DCS operation. Phase modulators, in particular, need special consideration because they theoretically are incapable of being directly modulated by dc, unlike direct FM modulation methods. Low frequency response is the primary requirement for DCS systems. CTCSS (Continuous Tone Controlled Squelch System) a sub audible tone in the range of 67 to 254 Hz. The FM deviation level of CTCSS tones should be in the range of 500 to 800 Hz. These tones is encoded with the voice audio an all times during the transmission. |