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,