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. The most frequent cause of this is crosstalk in the telephone cables, frequently caused by a too-high audio level in the line (called "overdriving" the line). 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 Remote control