A little background;
The following is from wiki radio reference;
“For CTCSS and PL there is no difference. PL is copywrite Motorola. CTCSS is the generic name.
Other manufacturers, finding that the CTCSS (Continuous Tone-Coded Squelch System) system was absolutely necessary to stay competitive, copied the idea and technology but couldn't use a trademarked term, so they came up with "Channel Guard" or "CG" from GE, "Quiet Call" from Ritron, "Quiet Channel" or "QC" from RCA, "Call Guard" from E. F. Johnson, "Quiet Tone" from Kenwood, and "Electronic Tone Squelch" or "ETS" from Canadian Marconi Company. Other manufacturers had / have other terms. Over the years, the Motorola trademark "Private Line" (or "PL") has become a generic term despite the best efforts of their marketing and legal staffs.
The Network Access Code or NAC is a feature of
Project 25 digital radios similar to
CTCSS or
DCS(Digital Code System) for analog radios. That is, radios can be programmed to only break squelch when receiving the correct NAC. NACs are programmed as a 3 digit hexadecimal code that is broadcast along with the digital signal being transmitted.
Since the NAC is 3 digit hexadecimal number (12 bits), it gives 4096 possible NAC's for programming, which far exceeds its analog counterparts combined. It should be noted that 3 of the possible NACs have special meaning:
- 0x293 ($293) - the default NAC
- 0xf7e ($F7E) - a receiver set for this NAC will unsquelch on any NAC received
- 0xf7f ($F7F) - a repeater receiver set for this NAC will allow all incoming signals and the repeater transmitter will retransmit the received NAC.
Note that the above is true for commercial grade radios only; setting a scanner to NAC F7E is not the same as setting it to 'NAC Search'.
A high-pass audio filter was added to the receiver audio path so that only the audio above the tone range was passed on to the audio amplifier and the speaker. This got rid of the PL tone hum under the incoming voice.”
All have the same intended use.
Because there’s only a finite amount of frequencies available on any band, the FCC tries to allocate frequencies that aren’t already assigned to another agency. (It used to be as a rule of thumb 70 miles spacing as the crow flys.) PL or DPL , NAC, are sub-audible or digital tones/codes that when assigned to a particular agency, will limit what’s heard. The frequency may be busy but the PL assigned to any particular radio will limit what you hear. An example of this is when an agency has multiple divisions such as a police department, all operating on the same frequency. While the mic is in the mic holder the radio will remain quiet until another transmitter with the same PL transmits. When you want to make a radio call you remove the mic from its holder. At this point the PL is disengaged, allowing you to hear if the frequency is in use. If so, you wait until the frequency is clear and then transmit. That’s an example of just one side of the transmissions… your receive side. But it’s a two way street, when you transmit that same PL allows access to the repeater. If that PL is not present the repeater won’t accept your transmission. You are still transmitting but only those who might be listening without their PL turned on, and within range, will hear you. (Some repeaters will recognize your transmit PL and regenerate it, some will simply pass your transmit PL.) There are variations to this such as the mic holder can be set up to either hear everything when the mic is in its holder (hanger) or only that which use the same PL. I’ve also seen repeaters use PL’s that are non-standard… meaning the tone frequency is not one of the accepted, published tones. Instead, a frequency halfway between two of the established PL’s was used. You may be able to fool the repeater into passing your transmission if you were close enough but weaker signals wouldn’t get recognized.
Hope this helps…
