Remote controls are used any time a two-way radio base station is located away from the desk or office where communication originates. For example, a dispatch center for taxicabs may have an office downtown but have a base station on a distant mountain top. A Tone remote, also known as an EIA Tone remote, is a signaling system used to operate a two-way radio base station by some form of remote control. 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. 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. 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 DC loop, private line- or RTO circuit (Radio Telephone Operation) was leased from the telephone company. Older burglar alarms used the same type of DC wiring from subscriber location to the alarm office. As pair gain electronics and point-to-point microwave radio links 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
Tone 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. 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. An audio notch filter removes the 2,175 Hz low-level tone from the actual transmit audio. General Electric Mobile Radio called the high-level tone, '"Secur-it tone", and called the low-level tone "hold 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.
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 paging tones) Transmit site 1 Transmit site 2 Transmit site 3 Transmit site 4 Tones used for remote controls are described in ratios called decibels: 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. Proper levels for tone remote systems are 0dbm for the high level tone, -10dbm for the function tone and -30dbm for the low level guard tone (Motorola's older manuals often confused technicians with some levels listed as high as +14dbm. Analog telephone circuits used by tone remote systems normally only allow 0dbm as a maximum) All levels are based on 600 ohm impedance termination at the radio itself and needed gain after a long circuit to bring the levels back up to the levels noted above. 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. Normally a phone circuit ordered as a PLPA or conditioned type results in a 300 to 3400 Hz response which easily handles all tones used in a tone remote radio system. Normal LMR practices of running the levels hotter than normal is something to be avoided to prevent an audio problem later if equipment is changed out.
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