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I just recently got my scanner and have been listening to SPD. On occassion the control operator will 'close' the channel and you will hear intermittent beeps. The control operator will eventually get a call to open the channel and it's back to normal chatter.

Can someone explain what's going on? Are they blocking transmissions from being heard by the public during this time? Technically speaking is the signal being scrambled or are they just not talking?

In my neck of the woods we call that a "code-10" which means the frequency is closed to anyone who is not involved in the incident. Usually that's used in pursuits, building searches, and calls with weapons involved just to name a few. The beeps just tell others on the ch. to stay off unless it's an emergency. My guess is that is the same procedure used with SPD.

Ah that makes a whole lot of sense. Thanks.