CTCSS stands for Continuous Tone Coded Squelch System. That's the generic term for what Motorola calls "PL" or "Private Line". It is what people are referring to here when we are referring to "tones". You can think of it as a selective calling system that allows radios of different agencies or groups of users to use the same radio frequency but not be bothered by each others' traffic. It works basically by sending a continuous tone of very low frequency along with the users' transmissions. A group, agency, or set of users is assigned a specific tone and their radio transmitters and receivers are set to use that tone - the transmitters will send it out during each transmission and the corresponding receivers will only unmute when receiving a transmission with that particular tone. This allows multiple groups of users to share a radio frequency without bothering each other with the wrong traffic. Random interference such as atmospheric noise and noise from other sources is also not passed through to the speaker obviously because random noise like that (usually) doesn't contain the correct subaudible tone so this is an added benefit. The tones range from about 67Hz to about 250Hz or so (I think the wiki on the subject here contains the exact frequencies). Generally, most LMR (land mobile radio) gear is designed to only pass audio frequencies above 300Hz or so and below 3KHz, which are where most human voice communication information outside of singing is contained, to the audio amplifier for transmission and reception; within limits, however, frequencies outside this range can be added for special uses such as selective calling especially at the low end. Filtering to allow only this range of audio frequencies enhances radio voice communication and reduces high and low frequency noise For non-broadcast use (i.e. non-music, voice only) this is desired. The CTCSS tones are placed deliberately below this range so that the user doesn't hear the tones; Humans don't hear these tones that well especially the very lowest of them and the radios' audio amplifier circuits add additional filtering to filter them out before the voice frequencies are amplified. Before the filtering, however, tone decode circuits will still receive and decode these frequencies.
Businesses use this alot because business radio frequencies are very busy with multiple users on the same frequency (called co-channel users). Government agencies like the CHP use it so that they can re-use their frequencies throughout the state and not hear the undesired traffic - this is coordinated so as to minimize interference by keeping a minimum distance between different districts. Despite the distance, however, especially at VHF-Low frequencies, sometimes distant traffic is still strong enough to bother local receivers so having the CTCSS tone system keeps that traffic muted.
Radio repeaters also use this type of system to selectively block non-affiliated co-channel users from accessing the repeater - very important for high level mountaintop repeaters like what the CHP uses.
In your case, you would set your Pro-96 to use "CT" mode and set the tone to match what is written in the database for your desired agency. Check your manual to see exactly how to do this. This will have the added benefit of eliminating that static you keep mentioning but still allow, ideally, you to hear the desired transmissions assuming you are in range with the proper antenna, etc.
Another variation of this system is called "DCS", Digital Coded Squelch. On your 96 I think it would be the DC mode (not sure - been awhile since I used a 96). For all intents and purposes it functions much like regular CTCSS as far as a scanner user is concerned but yields many more distinct codes - business users use this alot as well as CTCSS but, currently, the CHP only uses CTCSS. It uses low frequency subaudible digitally coded pulses sent out continually rather than one steady tone as CTCSS does.
Keep in mind that CTCSS and DCS don't really eliminate interference they just keep the users from hearing it from their radio receivers' speakers. The interference is still there and can, when strong enough, still block the desired traffic or destructively interfere with it but users with these tone squelch systems will not typically hear the resulting noise - they will just hear nothing if this is the case.
For CHP monitoring you really need to monitor the low band frequencies - the extender frequencies such as the old but still used in many areas 154.905MHz as well as the new 700MHz VRS frequencies are really only good within a few miles of the car unit when an officer is using them and may not always be turned on. They use these "extenders" to provide mobile officers with portable radio coverage when they are away from their car radio. The VHF-Low band uses a long enough wavelength that making an efficient portable radio antenna for that band is nearly impossible without making it very long and unwieldy. So the CHP uses "extender" systems that are basically low powered vehicular repeaters placed inside their cars to repeat the traffic to portable radios operating on much higher radio frequencies meaning much smaller and more efficient antennas (relative to the wavelength). The new VRS 700MHz extenders are going to replace the older VHF-High 154.905MHz equipment and are starting to be employed. The older system used (and still uses) regular analog FM and used a single frequency "sampling" repeater which simply chopped the transmission from the car at periodic intervals to listen for portable transmissions - this is why, if you do hear one of these it will have the characteristic "chopped" audio sound to it. The new VRS system will be P25 digital modulation audio and will have true dual-frequency repeaters within the cars with many channels so that multiple on-scene units won't interfere with each other as much and/or allow different districts and/or offices to use different channels. Keep in mind that these systems are intended only for short range use within a mile or less from the mobile units so, although interesting to monitor for special reasons like "just to see if you can" and to give one some idea that a CHP car is likely fairly close, they are not suitable for monitoring all of the main traffic on a continuous basis - you still need the main low band frequencies for that. Still, they may also be used for close range car-to-car traffic as well as the usual low band channels (the CHP typically uses the base frequency for car-to-car direct traffic when necessary) - definitely the VHF 154.905MHz extender channel is sometimes used this way.
Hope this helps!
-Mike
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