Barrett HF 2000 Series, Руководство пользователя

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DIGITAL VOICE AND SECURE DIGITAL VOICE - OVERVIEW
Operating with Digital Voice
To achieve the best results from the digital voice capabilities of Barrett transceiv-
ers fitted with DV/SDV modules, it is important to understand how the system
works. This section describes the system and also details operational scenarios
which will assist operators in getting the most from their equipment.
As mentioned earlier, the DV/SDV modules achieve the best results in terms of
digital voice clarity by automatically changing baud rates. This is achieved by
the DV or SDV module assessing the signal-to-noise ratio (SNR) in the current
conditions and then changing the baud rate appropriately to allow best syn-
chronization between all transceivers. This is known as auto baud i.e. the DV
modules have “auto baud capabilities”.
The DV and SDV-56 modules have baud rates of 700/1200/2400 bps. The SDV-
256 has baud rates of 600/1200/2400 bps. As the baud rates are different for
the SDV-56 and SDV-256, the modules are not compatible in terms of being
able to be utilised for DV operation. However, analogue voice would still be
available for voice communications.
The module will adjust up or down between the above baud levels for effective
use of digital voice. All the modules require a minimum SNR of -3 dB to operate
at the low level. It is quite common that the baud rates being used between
stations can vary as the SNR can be different at each station. This can be due
to the antenna types being used, fixed or mobile stations, and/or local area
interference.
The method the DV modules follow for achieving the optimal baud rate for
conditions is as follows:
• The DV modules will, by default, start at the low baud rate for the first
transmission.
• During each transmission, as well as the digital audio being transmitted,
the SNR received on the previous transmission is also sent back to the
transmitting station.
• After four successful transmissions at the low rate, the DV module will
assess the received SNR values and if favourable, the baud will increase
to the medium rate. This assessment process will repeat and if SNR has
improved it will go to the high baud rate. It is possible that if the SNR is
optimal after the first assessment it may go directly to the high baud rate
bypassing the medium baud rate.