dyrtjunkie
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When I hear a conversation (PD and Dispatch), does the signal coming from the police car come right to me or does it go back to the main tower and then back to me?
Where's that signal coming from?
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rvictor
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One police department that I monitor used to have a practice of turning off the rebroadcast of the transmission from the mobile units when they wanted to pass somewhat private data. Lately I haven't heard them doing it. I suspect that they just use their cell phones for that purpose now.
Dick
Dick
RKG
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The typical modern police or fire department will operate on what is known as a “voted” “repeated” system. The system will consist of:
Two transmitters (a “main” and a “standby”);
Four to eight receivers;
A “comparator” (a/k/a “voting selector” and sometimes “SpectraTac”);
Two or more dispatch “operator positions;” and
A dispatch central electronics bank (“CEB” (Motorola) or “CCU” (Zetron).
The main transmitter is typically located at the best high site in the city or town; the standby transmitter is located at some other (albeit lesser) site; and only one transmitter is active at any time. Each transmitter is connected to the comparator by a “wireline” (i.e., a dry copper circuit, an audio-grade circuit leased from the phone company, an audio-grade fiber optic circuit, or an audio-grade microwave circuit).
The receivers are located: one at the main transmitter site; one at the standby transmitter site; and the rest at sites selected so that a portable radio outdoors on the street at any location in the town should be able to reach at least one receiver with a full quieting signal. Each receiver is connected to the comparator by a wireline.
The CEB contains (among other things) a channel card for each channel controlled by the dispatch console and an operator position card for each operator position available on the console. Each channel card has a Tx line and an Rx line, which are connected to the comparator (usually by dry copper, as the comparator and the CEB are typically co-located). Each operator position is connected to an OP card in the CEB (usually by dry copper). Typically, the CEB is located in an equipment room in the same building as the operator positions.
When a “subscriber unit” (a portable or mobile radio) transmits, the system works as follows:
The subscriber transmits on the “input frequency,” and it is heard by one or more of the receivers. Each receiver sends what it hears to the comparator.
The comparator selects the receiver that is producing the best sounding audio and it sends the audio to whichever transmitter is active, together with EIA function tones that cause the transmitter to key up and transmit the received audio over the “output frequency.”
At the same time, the comparator sends the received audio to the CEB channel card, which causes it to be heard on a speaker at each console operator position.
When a dispatch operator transmits, the system works as follows:
The dispatcher’s operator position sends the dispatcher’s audio to the OP card to which his position is connected. This card determines which channel the operator has selected and sends the audio to the channel card for that channel. The channel card sends the audio to the comparator, which sends it to the active transmitter for that channel, together with keying tones. The active transmitter transmits the dispatcher’s audio over the output frequency.
Each console operator position will have a button or icon called “Main/Standby.” This permits an operator to select which of the two transmitters will be active.
Each console operator position will also have a button or icon called “Repeater Disable.” When active, this function prevents the comparator from sending audio received from a subscriber to the transmitter, while allowing it to be sent to the operator position speakers.
If a subscriber switch is mobile or portable to “direct,” he will be transmitting on the output frequency (instead of the input frequency). Since the system receivers listen only to the input frequency, “direct” transmissions are not repeated by the system transmitters and are not heard by the dispatchers. They will be heard by any subscribers who are close enough to the subscriber transmitting on direct.
All of this may be more detail than you wanted, but it is the answer to the question posed.
Two transmitters (a “main” and a “standby”);
Four to eight receivers;
A “comparator” (a/k/a “voting selector” and sometimes “SpectraTac”);
Two or more dispatch “operator positions;” and
A dispatch central electronics bank (“CEB” (Motorola) or “CCU” (Zetron).
The main transmitter is typically located at the best high site in the city or town; the standby transmitter is located at some other (albeit lesser) site; and only one transmitter is active at any time. Each transmitter is connected to the comparator by a “wireline” (i.e., a dry copper circuit, an audio-grade circuit leased from the phone company, an audio-grade fiber optic circuit, or an audio-grade microwave circuit).
The receivers are located: one at the main transmitter site; one at the standby transmitter site; and the rest at sites selected so that a portable radio outdoors on the street at any location in the town should be able to reach at least one receiver with a full quieting signal. Each receiver is connected to the comparator by a wireline.
The CEB contains (among other things) a channel card for each channel controlled by the dispatch console and an operator position card for each operator position available on the console. Each channel card has a Tx line and an Rx line, which are connected to the comparator (usually by dry copper, as the comparator and the CEB are typically co-located). Each operator position is connected to an OP card in the CEB (usually by dry copper). Typically, the CEB is located in an equipment room in the same building as the operator positions.
When a “subscriber unit” (a portable or mobile radio) transmits, the system works as follows:
The subscriber transmits on the “input frequency,” and it is heard by one or more of the receivers. Each receiver sends what it hears to the comparator.
The comparator selects the receiver that is producing the best sounding audio and it sends the audio to whichever transmitter is active, together with EIA function tones that cause the transmitter to key up and transmit the received audio over the “output frequency.”
At the same time, the comparator sends the received audio to the CEB channel card, which causes it to be heard on a speaker at each console operator position.
When a dispatch operator transmits, the system works as follows:
The dispatcher’s operator position sends the dispatcher’s audio to the OP card to which his position is connected. This card determines which channel the operator has selected and sends the audio to the channel card for that channel. The channel card sends the audio to the comparator, which sends it to the active transmitter for that channel, together with keying tones. The active transmitter transmits the dispatcher’s audio over the output frequency.
Each console operator position will have a button or icon called “Main/Standby.” This permits an operator to select which of the two transmitters will be active.
Each console operator position will also have a button or icon called “Repeater Disable.” When active, this function prevents the comparator from sending audio received from a subscriber to the transmitter, while allowing it to be sent to the operator position speakers.
If a subscriber switch is mobile or portable to “direct,” he will be transmitting on the output frequency (instead of the input frequency). Since the system receivers listen only to the input frequency, “direct” transmissions are not repeated by the system transmitters and are not heard by the dispatchers. They will be heard by any subscribers who are close enough to the subscriber transmitting on direct.
All of this may be more detail than you wanted, but it is the answer to the question posed.
The easiest way to tell is with your own ear. If the signal from the field units always comes across as strong as the base, then a repeater is in use. Note that the audio quality of the transmission might not be so good if the unit is transmitting from a bad location or is on portable, but the signal strength will always be strong with a repeater. By contrast, if you don't hear certain units that the dispatcher is communicating with, then they're probably not using a repeater.
You should also look up the jurisdiction you're wondering about, as the database will likely show if a repeater is in use. An "input frequency" is a dead giveaway. Failing that, you can also look the license up in the FCC ULS database and see what station class is in use. A repeater is indicated by class FB2.
You should also look up the jurisdiction you're wondering about, as the database will likely show if a repeater is in use. An "input frequency" is a dead giveaway. Failing that, you can also look the license up in the FCC ULS database and see what station class is in use. A repeater is indicated by class FB2.
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