Clear-Com TEMPEST 2400 Dokumentacja Pobierz pdf (Strona 52)

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Tempest 2400 Wireless Intercom System

Multi-System Tempest Technology

Tempest is a digital, frequency hopping spread spectrum (FHSS) radio device. Using 80 MHz of spectrum

in the 2.4GHz ISM frequency band, Tempest changes frequency (hops) 200 times per second, following

one of 64 hopping patterns (Tempest Network Numbers 0 – 63). The hopping pattern determines the

order in which frequencies will be utilized. Each hopping pattern has the same frequencies as all other

hopping patterns; the only difference is the order of use.

Also, Tempest utilizes proprietary 2xTX technology to send each digital packet twice, once each on

consecutive hops, and so on different frequencies.

When using more than one Tempest system within a given operational area, occasionally the systems will

try to utilize the same frequency. When two systems that are located physically close to each other try to

use the same frequency at the same time, harmful interference is likely to occur. One or both systems

may loose a digital data packet due to inter-system RF interference.

Because of the ordering of frequencies in each hopping pattern and the fact that each packet is sent twice

on consecutive hops, after any packet conflict, both systems will hop to a different frequency and send a

duplicate packet on a clear frequency where the audio data may be delivered. As more Tempest systems

are operated within a given area, more inter-system RF interference will take place. If a system

encounters RF data packet loss on two consecutive hops, one packet of audio data will be lost.

Tempest utilizes an audio compression algorithm that employs advanced Lost Packet Concealment

(LPC) technology. One lost packet of audio data will be covered over by the LPC technology and the end

user will not notice any interruption in audio. Two consecutive lost packets of audio data (four to five

consecutive lost RF data packets) may be noticed by a user, but will not be objectionable. More than two

consecutive lost packets of audio will likely cause interruption of audio that will be noticeable and will

likely be objectionable to users. Even with 2xTX and LPC, when enough Tempest systems are collocated,

there will be a sufficient inter-system RF interference causing lost packets to cause adverse audio

performance.

Accu-Sync is a proprietary technology that synchronizes transmit/receive timing to avoid harmful

desensing of receivers. Desensing occurs when one transmitter is broadcasting while a nearby receiver is

trying to “listen” to a different source and the broadcaster overwhelms the signal the “listener” is listening

for. Desensing happens even when the transmitter and receiver in question are not on, or even near, the

same frequency. This effect is extraordinarily harmful to wireless systems and must be avoided at all

costs.

Accu-Sync sends a very precise timing signal to all base stations that are connected to the Base-Sync

circuit, to control when those base stations (and consequently their associated beltstations) transmit. This

forces all of the base stations to transmit simultaneously ensuring that there are no closely located

transmitters that are active during the receive cycle of any other base station, effectively eliminating

desensing. Eliminating desensing greatly improves system performance when there are collocated

systems. Whenever multiple systems are being used in a collocated situation it is strongly suggested that

Accu-Sync be utilized.

Accu-Sync does not do anything to avoid packet collisions stemming from more than one Tempest

system attempting to use the same frequency at the same time. This would require frequency hopping

pattern synchronization, and the FCC does not allow frequency hopping pattern synchronization as it

would allow a single system or group of systems to monopolize the 2.4GHz ISM band.

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