So then what is DPL?

DPL is an analogue signaling technique used on FM and was designed as the digital extension for PL (otherwise known as CTCSS). DPL has the same function as PL: radio users that would need to communicate with each other would share the same DPL. Users with radios programmed to another DPL code, would not be heard. LIke PL, DPL can be used on shared Community Repeater systems or where who neighboring systems may interfere with each other (see my post on PL).

DPL uses similar techniques to PL: the modulation is below the receiver’s audio passband thereby preventing it from being heard on the radios speaker.

When a radio, configured for DPL operation transmits, a 134.4 bps bitstream is superimposed (at 10% of maximum deviation) on the transmitted audio. The DPL code is embedded in this bitstream.
The bitstream consists of a repetitive 23-bit Golay codeword consisting of 12 data bits (of which three are fixed) followed by 11 CRC bits. In theory this means that there are 512 possible DPL codes. However, due to potential synchronization errors, only 84 of them are used.

When programming a radio, the technician (or user) would select one of these 84 codes, which are represented as 3-digit octal numbers in the CPS. In some Motorola models, it is possible to select Inverse DPL - this is merely a 1’s complement of the DPL code and allows another ~84 codes to be used on a system.

Most Motorola radios also support a DPL Turn Off Code (DPL TOC). Like PL Reverse Burst, this prevents the squelch crash sound from being heard by the radio user.

Outside Motorola, DPL is known as DCS (Digital Coded Squelch);  CDCSS (Continuous Digital-Coded Squelch System); DCG (Digital Channel Guard); DQT (Digital Quiet Tone) or DTCS (Digital Tone Code Squelch).