and long call (> 2,000 ms) In well-designed systems, repeaters or radios usually included an audio notch filter that reduced the volume of the tone at the speaker. A variation to the single tone scheme was seen in one-way paging receivers. In some two-tone sequential systems, sending 4–8 seconds of the second tone pages all receivers which have a code including the second tone. This is sometimes referred to as long tone B. Receivers made by Plectron and often used to page volunteer firefighters use a long single tone. The decoder in the typical Plectron receiver would not decode the tone as a valid call unless it was present for at least two to four seconds, (a very long variation of the burst tone). Conventional analog individual calling In individual calling, a specific radio is called. Most individual calling schemes involve a sequence of tones. Most schemes have a dozen to thousands of possible individual codes. As a practical matter, more than about two hundred radios on a single channel make an unusable level of traffic. So 1,000 individual calls will usually be more than needed. Individual calls are usually event-based. For example, a tow truck may be called to give the driver an assignment or an ambulance may be called with an emergency call. Some Motorola pagers could decode four different individual 5-tone signals (see SelCall above). Some fire departments used this feature to implement an individual signal (using the first of the four signals), a station based signal (i.e. paging everyone from one fire station, using the second signal), a region-based signal (i.e., everyone in the northwest region, using the third signal), and an all-call (every fireman, using the fourth signal). DTMF In dual-tone multi-frequency (DTMF) selective calling, the radio is alerted by a string of digits. Systems typically use 2- to 7-digits. These can be dialed from a traditional telephone dial connected to a radio or may be generated as a string of DTMF digits by an automatic encoder. In some systems, a dispatching computer is connected to a DTMF encoder via a serial (RS-232) cable: the computer sends commands to the encoder that generates a pre-defined digit string that is then sent to the transmitter. On FM two-way radios, digits are usually sent at a level that equals two-thirds, (66%,) of system deviation. For example, in a ±5 kHz deviation system, the DTMF encoder is set