Ask the Radio Nerd Guy

A Primer on PL tones and DPL Codes

What are PL Tones and DPL Codes?
PL tones (generically referred to as Continuous Tone Controlled Squelch System – CTCSS) and Digital PL codes (generically referred to as Digital Tone Control System – DTCS) are encoded with the voice audio at all times during transmission, and are used to control repeaters, exclude unwanted or uninvited traffic throughout the Public Safety, business, amateur and GMRS two-way world.

What is the difference between PL tones and Digital PL codes?
A PL tone is a sub audible tone in the range of 67 to 254 Hz. The FM deviation level of CTCSS tones should be in the range of 500 to 800 Hz. Any one or more of the 50 tones can be utilized by the communications system.

A Digital PL code is digital data or code word, expressed as a three digit octal number, that is transmitted with the voice audio. Unlike CTCSS (Continuous Tone Coded Squelch System) which uses continuous tones below 300 Hz, DCS uses digital data or code words. Each code word is unique and all code words may be used on the same channel without interference.

Is it important to have the exact PL tone or DPL code on my radio, or can it be "close enough"?
It is extremely important to use the exact PL tone or DPL code, otherwise other users on your radio network will not hear your transmissions.

Why do some of my department channels have different PL tones for transmit and receive, and in some cases, there’s only a PL tone on transmit, but no PL tone on receive?
Communications planners decide ahead of time how they can minimize interference between multiple users on a single channel, or multiple radio systems assigned to the same frequency. Using PL tones and DPL codes is an effective way to minimize unwanted interference.

Won't people hear the "hum" from the PL tone on my channel when I transmit?
No, because the tone is added after the audio circuit in the transmit portion of the channel, and removed ahead of the audio circuit in the receive portion of the channel, so speech and analog data are not affected.

A Primer on Radio Repeaters

What is a radio repeater?
A radio repeater, or commonly just "repeater", is a combination of a radio receiver and a radio transmitter that receives a weak or low-level signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. A unique feature that defines repeater operation is the fact that it transmits and receives at the same time, (simultaneously). This simultaneous transmit and receive function is called "duplex" operation.

A duplex repeater uses two radio frequencies: an "Input" frequency, which it monitors for signals, and an "Output" frequency, on which it retransmits the received signals at a higher power or higher amplitude. The input is the frequency on which mobile radios, or handheld radios, transmit to reach the repeater. The output is the frequency that mobile and handheld radios are tuned to in order to receive the signal from the repeater. Point-to-point radio systems that don’t use repeaters are called "simplex" systems, whereby all users transmit and receive on the same frequency.

What frequencies should I program my radio to?
A common source of confusion stems from the inverse nature of the repeater and the radios that use the repeater: the repeater "transmit" frequency is the radio "receive" frequency, and the repeater "receive" frequency is the radio "transmit" frequency. Thus, when having your radio programmed, be sure to specify transmit and receive frequencies for your radio, not the repeater!

Why is a repeater needed in the first place?
Repeaters are used for a variety of reasons. If all the radio users for an agency always stayed within a 5 to 10 mile range, then a repeater probably wouldn’t be necessary. But, if an agency is spread over a large area, like a county, or in hilly terrain, or in a crowded metropolitan area, then the point-to-point radio signals between users would probably be blocked.

A repeater is typically installed in a high location, like on top of a hill, mountain, water tower or tall building. The idea is that the repeater antenna has far greater coverage over a wider area when installed in a high location. The repeater can "hear" the weak signals from users much more effectively, since the high location increased the likelihood of a line-of-site path between the user and the repeater. The repeater then rebroadcasts the signal from the user on the output channel, at a much higher output power level. All the users in the system can "hear" the repeater much better, again, because it’s likely that a line-of-site path exists between the repeater and the user.

What's the difference between a base station and a repeater?
A base station is usually located near the user, like a dispatcher or headquarters location, although a base station can also be located remotely. A base station, however, is not necessarily a repeater, although a base station can operate with a repeater.

For example, a taxi dispatcher might use a base station transmitting on Channel 1. All the taxis in the company listen to Channel 1, and transmit back to the dispatcher on Channel 2, and the dispatcher receives, or listens to, Channel 2. What's interesting is that the taxi drivers can only hear the dispatcher, but not each other. Why? Because the taxis are transmitting back to the dispatcher on Channel 2, but listen to the dispatcher on Channel 1, they only hear traffic from the dispatcher. This is an example of a half-duplex communications system. Taxi drivers don’t need to communicate between themselves, since they only get orders to pick up fares from the dispatcher.

Why is a repeater system called duplex?
In the example above, the taxi system is half-duplex because the drivers can only hear the dispatcher, but not each other. A duplex system uses the repeater to its full advantage. All users (including the base station dispatcher) transmit on the repeater’s "input" channel, and all users (including the dispatcher) receive on the repeater’s "output" channel. That means that all users hear all the traffic all the time: users can communicate with the dispatcher, and users can also communicate with all other users.

Duplex communications is vital in public safety applications, as everyone needs to know what’s going on without having to rely on dispatcher instructions only. And ham radio and GMRS repeater systems allow all users to have equal access to conversations at any time.

Why do some repeater systems use PL tones?
PL tones, or CTCSS tones, are used to prevent unwanted or uninvited users from accessing a repeater. If a user hasn’t selected the right PL tone for a particular repeater, then that user can’t access the repeater and tie it up or transmit maliciously.

What is repeater talk-around?
Repeater talk-around means setting the mobile or portable radio to transmit and receive on the same channel as the repeater output frequency. Some agencies refer to this operation as "truck-to-truck". For example, a fire crew member might be on a portable, handheld radio with only 3 watts of output at a scene, while others on the scene might be using the 40 watt radios in the truck. Let’s assume the truck radios can reach the repeater just fine, but, the handheld doesn’t have enough power to reach the repeater. In that case, by switching to "talk-around", the handheld user can communicate directly to the trucks by transmitting on the repeater output channel. Sine the handheld user can hear the repeater, he can also hear the trucks that are using the repeater.

Can I have both repeater channels and simplex channels programmed into my radio?
Yes, absolutely. All land mobile radios can be programmed to operate with simplex and repeater (duplex) channels.