Tone remote

: General Electric Mobile Radio, unknown year). This GE manual covers tone- and DC-remotes. ] [To confirm the use of the phrase "tone remote" as accurate in describing this type of device, please see US Patent and Trademark Office patent ID 6950653, "Scanning tone remote adapter for land-mobile radio dispatch for use with dispersed dispatch stations." The article does not describe this tone remote but confirms the use of the phrase to describe this system of signaling. ]

A tone remote may be a stand-alone desktop device in a telephone housing with a speaker where the dial would have been located. It may look like a desk top base station. Or, it may be an integral part of a computer-based console system with touch-screens in a dispatch center.

History

The first two-way radio remote controls utilized a harness of wires extending speaker, microphone, and controls for options such as channel selection or CTCSS switches. This limited a base station to being within tens to hundreds of feet from the user's workstation. Early systems often had volume unit meters, clocks, and switchboard keys. [A picture of one example is shown in Crane, Bob, "Highway Patrol Radio Communications Development," "California Association of Highway Patrolmen Golden Chronicle 1920-1970", (Sacramento, California: California Association of Highway Patrolmen, 1970,) pp. 71.]

1960s two-way radio remote control consoles used direct current loops. Users would run ordinary telephone wiring from the remote consoles to the radio base station chassis. [One example describing DC-remotes is found in, "Section 8.4, Console Electronics" in "Trunked Radio System Request For Proposals", (Oklahoma City, Oklahoma: Public Safety Capital Projects Office, Oklahoma City Municipal Facilities Authority, 2000) pp. 157. Unlike tone remotes, there are no default currents that map to specific functions. Service manuals for a specific type of equipment will reveal how many functions are available and what currents actuate a specific function. These may be modified from one system to another.] In some cases, the base station was located at the same address as the control console. In other cases, the base station was located at a distant site. For distant sites, a dry pair of telephone wires called a "private line-" or "RTO circuit" was leased from the telephone company.

As pair gain electronics and microwave radio came into widespread use throughout the public switched telephone network, telephone companies filed tariffs to eliminate their past responsibility of providing leased circuits with direct current continuity. If the base station were located across town in an area served by a different telephone exchange, the only available circuits reaching the distant exchange might be a single voice-grade channel in a D-4 channel bank on a DS-1 or a single microwave radio baseband channel. Tone remotes became necessary with the wide use of telephone carrier or multiplexing equipment. They require only a voice-grade audio path with roughly flat equalization from 300-3,000 Hz. A circuit that could pass audio in both directions could be used for remote control.

Tone format employed

Listen
filename=Tone_remote_function_tone_examples.ogg‎
title=Function tones
description=Three function tone sequences: 2175+1950 Hz, 2175+1850 Hz, and 2175+1750 Hz.
format=OggTone remotes send commands to a base station using "function tones", a series of two tones in sequence. The first tone is 2,175 Hz and is 100-300 milliseconds in length. [2,175 Hz tone lengths vary depending on the decoder's time constants and any delays present in the audio path. About 150 milliseconds is a good median value. Too long a tone causes an unnecessary delay from push-to-talk until the base station begins transmitting.] The most common second tone is 1,950 Hz. The most commonly-used tone sequence in tone remote controls is the channel 1 transmit command. The default for this command consists of a high-level 2,175 Hz followed by a lower-level 1,950 Hz. A continuous, low-level 2,175 Hz tone follows. Voice is multiplexed over the tone. So long as the 2,175 Hz tone is present, the transmitter remains on. A notch filter is supposed to eliminate the 2,175 Hz low-level tone from the transmit audio. General Electric Mobile Radio called the low-level tone, "Secure-it tone". In the industry, the low-level continuous tone is often called, "low-level guard tone". The low-level tone is present at the same time as transmitted voice.

Listen
filename=Tone_remote_sound_across_phone_line.ogg
title=Example of voice traffic between a base station and tone remote.
description=The sound of a tone remote if you bridged a speaker across the phone line.
format=Ogg

Receive

Some function tones are sent without the continuous low-level tone. These are commands that change a state of the base station. An example is turning off the receiver CTCSS decoder (the "monitor" function).

Receive commands use a function tone only. They may include things like:
* Turn off CTCSS decode or "monitor"
* Switch base station to channel 2
* Set squelch threshold tight/loose
* Repeater set-up/knock-down
* Turn on second receiver in dual-receiver base station.

Transmit

Transmit commands use a function tone followed by low-level guard tone, which holds the transmitter on. They may include things like:
* Transmit, channel 1
* Transmit, channel 2
* Transmit, channel 3
* Transmit, channel 4
* Transmit with CTCSS encode disabled (used to send two-tone sequential paging tones)
* Transmit site 1 [Site selection can be referred to as transmitter steering. Using a channel select button, in some systems the dispatcher can select a transmitter that is likely to reach a field unit with the best signal. This is used most often in regional systems where one transmitter will not create an adequate signal over the whole coverage area. The dispatcher must know geography and radio signal behavior well enough to pick an appropriate site. In some systems, the calling mobile radio user will queue the dispatcher to respond using a specific transmitter site. For example, "Downtown 6 to base on South Mountain." ]
* Transmit site 2
* Transmit site 3
* Transmit site 4

: for example, the second tone of a sequence might be 10% of the level of the first tone. The highest level tones are set to the maximum allowable for the DS-1 channel or telephone line. The figure at right shows the envelope of a function tone's two-tone sequence.

Maintenance

Levels and equalization

Level discipline, setting the tone output levels at the remote control, affects the reliability of a tone remote. Audio levels set too low may cause the transmitter to drop out when speech peaks occur. The combination of voice and signaling tones is supposed to lie in a balance. If everything is set ideally, a notch filter at the base station blocks the steady 2,175 Hz tone from going out on the air.

Level discipline varies from one system to another. In order to get the transmitter to key reliably, sometimes the continuous tone that holds the transmitter on may have to be set so it is heard in the transmit audio. Sometimes equalization problems with a telephone circuit make it necessary to run higher levels than a circuit with a flat response from 300 to 3000 Hz would require.

Audio level compression

Remote control consoles use audio level compression in transmit and receive audio paths.

In documentation for most any console, receive audio is described as having a 30 decibel (db) knee of compression: for a 30 db variation in input audio level, the speaker volume changes by 3 db. [There are many examples all citing this figure. One of many would be "Maintenance Manual: Deskon II Remote Control Unit for Standard and GE Mark V Trunked Mobile Radio Desk Top and Wall Mount", part number LBI30968B, (Lynchburg, Virginia: General Electric Mobile Radio, unknown year.) ] This is desirable because persons calling on the radio who are whispering or yelling would ideally be intelligible and be presented to the console user at roughly the same volume. It helps reduce the need to constantly adjust the volume control to accommodate different voices. It also helps the console user talk on the telephone without loud sounds coming from the remote control speaker. A professional dispatcher will often set the console volume at a setting similar to the telephone's perceived volume. This allows the dispatcher to shift her attention between the radio and the phone without continually adjusting volume controls during a conversation.

Transmit audio is compressed so that field units can hear the base station user over background environmental noise present in cars and in the street scape around walkie-talkie users.

Tone conflicts

In designing a system, it is important to pick selective calling tones that do not conflict with remote control tones in use.

Analog voting systems usually utilize either 2,175 Hz or 1,950 Hz as an indication a receiver is squelched or idle. In the case of 2,175 Hz, the steady tone leaking from a receiver phone line, or from the output of the voting comparator, may cause the transmitter to lock on.

Two wire versus four wire

The simplest tone remote system uses a two-wire audio circuit to operate a simplex base station. If simultaneous transmit and receive is required, a four-wire or full duplex audio path to the base station is required. Systems using diversity combining (voting) require four-wire circuits.

Some systems where transmitter steering is employed may also require a separate audio path for transmit and receive, (four-wire circuits).

Notes