Tone Private Line (TPL)

Tone Private Line (TPL) transmits sub-audible (not able to be heard) tones along with the carrier when the receiving radio is to only receive calls from radios with specific TPL codes. This allows for more use and privacy on a frequency by creating communications groups while operating in Conventional communications mode. TPL and Digital Private Line (DPL) are often referred to generically as PL. Also known as Channel Guard, Quiet Call, and CTCSS.