Ian Andrew Ahner writes:

Two of the most annoying things that radios can do is make noise
when you don't want them to, and not make noise when you do want them to. This,
too, is something that the cool engineers have found some solutions to.

Historically radios used a "level detecting squelch." Basically what this means
is that when another radio talks, the receiving radio will choose to listen
based on how loud he is talking. (loud being used in RF terms, not actual audio
level.) If the sending radio is low signal, the receiving radio will simply
ignore it. We can adjust the threshold level that this occurs at using the
"squelch" setting on the radio. This is usually a value from 1-10 or something.

The problem with this method is two-fold. If you set the squelch high enough
that random noise doesn't make your radio go off (the annoying thing where the
radio randomly emits static) then occasionally you miss important transmissions
from people like Britton who are probably very far away telling you where
thermals are. If you set it low enough that you always hear those transmissions,
you can get annoyed by other electronic interference causing random static all
the time.

There is a solution. The solution is using "PL tones", also known as "CTCS"
(Continuous Tone Coded Squelch) or "Digital PL tones (DPL)", also known as "DCS"
(Digital-Coded Squelch). When this is used, the transmitting radio sends an
additional signal alongside the actual voice transmission. The receiving radio
listens for this specific extra signal. If the signal is present, the radio
breaks squelch, but if it is not, the radio remains silent. This solves all the
above problems.

The only challenge is that everyone has to agree on some additional radio
settings for it to work. Not only do you have to agree on frequency, as always,
you also have to agree on what additional signal the radios use. My
recommendation is to use DCS. To make things easy, I'll pick a value for you:
73.

This means two additional settings need to be touched in your radio. "Transmit
DCS = 73" meaning that your radio sends the additional signal, and "Receive DCS
= 73" meaning that your radio listens for the signal.

One thing to understand about these settings is that they can make for some
weird stuff if people don't have both settings done. For instance: if one person
has the settings but the other person doesn't, the person who doesn't will hear
everything, but not be heard by people who do.

Having "transmit DCS" set on your radio can never hurts. If you have it set,
then everyone can hear you, even if they don't have DCS set up, and you can hear
everyone. It is only once you set up "Receive DCS" that your radio now only
listens to radios that have "Transmit" set up.