Clear-Com Tempest900 - Manual (Page 14)

8 Tempest®900 2-Channel Wireless Intercom System
Theory of Operation
Tempest is a DSP (Digital Signal Processor) based, full duplex, wireless intercom system. It is a digital, point-to-point
communications system, operating in 26 MHz of spectrum in the 900MHz frequency band. This band allows users to
operate with no license requirements in most locations.
The 900MHz band offers many benefits. There are, however, a wide variety of technologies and users competing for clear
spectrum in this limited band. Tempest utilizes patented and proprietary technologies to ensure a robust and reliable RF
link under a wide variety of physical and RF environments. This level of reliability is the most important attribute of any
wireless system.
Tempest utilizes proprietary, Frequency Hopping Spread Spectrum (FHSS) technology that has seven U.S. patents. Multiple
BeltStation access is achieved by implementing Time Domain Multiple Access (TDMA) with Frequency Shift Keying (FSK)
modulation. Tempest uses an Algebraic Code - Excited Linear Prediction (A-CELP) voice compression algorithm to reduce the
bandwidth necessary for transmission and yet maintain the highest possible voice intelligibility. In addition, this algorithm
utilizes advanced lost packet masking technology, which greatly reduces the affect of packet loss, possible in all Digital RF
transmission schemes. The loss of audio packets is managed by the algorithm to reduce the possibility of detecting a loss
of audio.
The incorporation of various diversity techniques provide an RF system that is robust and reliable under greatly varying
operational conditions. Because of this inherent design, Tempest delivers superior operational range and greater levels of
interference and multipath fade rejection. Covering extremely large areas or multiple coverage locations can be problematic
with traditional wireless intercom systems. Tempest utilizes iSelect™ on-command roaming to allow users to move from
one area of coverage to another. iSelect allows Tempest users to easily choose the desired coverage area by selecting
the BaseStation they want to communicate through. The BeltStation (remote) remembers the last 64 BaseStations with
which it has been Paired. When a different BaseStation is selected, the hopping pattern and key code of the BeltStation is
synchronized to the selected BaseStation and the BeltStation logs into the BaseStation.
Audio latency is a critical factor in all digital systems. The hop duration in Tempest is intentionally kept very short - under
5ms. This short hop duration limits the amount of data that may be lost in any one hop. More importantly, it reduces system
latency, which can cause undesirable echo. Total one-way (BeltStation to BaseStation) system latency is less than 50ms.
Even with this short system latency, unwanted and distracting echo will occur when interfacing with hardwired party-line
intercom systems unless echo-reduction technology is implemented. Echo is primarily the result of inefficiencies in the
conversion of the two-wire signal. This condition exists in analog systems as well, but it does not cause unwanted echo