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VT-4R150 VHF Project 25 Digital Transmitter Instruction Manual
4-1
4
Repeater System Configuration
4.1
Interim Repeater
The MT-4 series Project 25 Interim Digital Receivers will typically be used in a low current
repeater configuration with MT-4 series Project 25 Interim Digital Transmitters. In its Interim
mode of operation, the receiver can demodulate both FM analog and Project 25 digital signals to
baseband audio. This audio can be passed to the Transmitter, where it can be remodulated as
either a FM analog or Project 25 digital signal. The voice signal will be repeated in either mode,
but other Project 25 digital information such as Network Addressing Code, Talk Group ID, and
low speed data will be lost. The simplest way of setting up a repeater system is to connect the
receiver to the transmitter through a Radio Interconnect Cable plugged into the CNTL BUS
connectors on the front panels of each radio module. This cable routes received audio and the
ANALOG_COR* and DIGITAL_COR* signals to the transmitter, which is able to key in either
Analog or Project 25 Digital mode.
Most users will normally want to have the digital repeater set up to re-transmit an incoming signal
in the same mode as it is received (i.e. normally, you would want a received analog signal to be
repeated as analog and a received digital signal to be repeated as digital). In the interim digital
repeater, this is accomplished in two steps.
1) The receiver determines whether an incoming signal is analog or digital and asserts either
the ANALOG_COR* or DIGITAL_COR* signal line to the transmitter.
2) When the transmitter senses an ANALOG_COR*, it keys up using one of the 16 channels
from Bank A, and when it senses a DIGITAL_COR*, it keys up using one of the 16
channels from Bank B.
The transmitter has two banks of 16 channels each, called Banks A and B. In the interim
transmitter, it is suggested that Bank A is used for analog channels and Bank B is used for digital
channels. This makes it easy to set up a repeater which re-transmits signals in the same mode in
which they were received. It is possible to program any channel in either bank to either analog or
digital mode should this be required for special applications. The transmitter has two LEDs labeled
A and D, indicating that the transmitter is keyed in either analog or digital mode. In the interim
transmitter, these LEDs actually indicate that the transmitter is keyed in Bank A or B. Thus, if
Bank A channels are used for digital channels, the Analog LED will light when the transmitter is
keyed on a Bank A channel, even though the modulation is digital. This problem will be corrected
in the firmware upgrade to fully compliant Project 25 operation.
When setting up the repeater you would typically program the transmitter’s Bank A channels with
analog settings. Then you would program the corresponding channels in Bank B with the same
frequencies but set them up for digital mode. The RSS makes this easier through the "Paste Special
to Bank A and Paste Special to Bank B" menu items which allow copying of the frequency,
channel name, and timeout value from all the channels in one bank to the corresponding channels
in the other bank.
It is important to note that the receiver and transmitter have no connection between them apart from
the audio and analog and digital COR* signals. This can lead to confusion since the transmitter
doesn’t get any information from the receiver about which frequency or channel number to key up
on. The transmitter channel is determined by the state of the CSEL0-3 channel select lines, and the
receiver can only tell it to key on bank A (analog) or bank B (digital).