What is P.L.
The term 'pl' comes from "private line" a form of squelch control, I believe
was developed by GE. Most speakers used to reproduce audio on two-way
radios are unable to reproduce bass frequencies, ie 300hz and below.
Taking advantage of this, clever engineers decided to transmit low frequency tones
along with the speech audio in the radio, as a way to control interference.
For example, if you have a radio in conventional mode, runningon 'carrier squelch'
any old electronic noise might set it off. For example, I find gas stations
very 'noisy' in the VHF hi spectrum. But if the repeater I am monitoring is set
to unmute the squelch only if it receives the right PL tone, then I won't hear the noise.
Another application is to allow multiple user groups to share the same frequency,
and not hear each other if they don't want to, by using different tones.
PS on some radios, if you hook them up to a suitably large speaker, you will hear the PL tone as a humming in the background.
Jean B.