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The APCO Project 25 Common Air Interface or "P25 CAI" is used to describe the APCO (Association of Public Safety Communications Officials International) specified standard for digital voice modulation (digital audio). This standard specifies the type of digital signals transmitted by compliant radios.

One radio using P25 CAI should be able to communicate with any other P25 CAI radio, regardless of what manufacturer produced the radio. P25 CAI uses a specific method of digitized voice called Improved Multi-Band Excitation (IMBE). The IMBE voice encoder-decoder (vocoder) samples the audio input at the microphone and produces a digital stream that represents the sound, this digital stream is then transmitted. The receiver sends this digital stream to the vocoder in its radio and it is used to produce a synthetic equivalent (of analog audio) of the input sound. The IMBE vocoder is optimized for human speech and does not reproduce other types of sounds well, so analog (DTMF or multi-tone) alerts, such as fire tone outs, cannot be used with P25 CAI.

Common Misunderstandings

P25 CAI means a digital trunking system

P25 CAI is a digital voice (modulation) type and therefore can be used on conventional simplex or repeater radio systems or in a trunking radio system. System type is not dependent on the use of P25 CAI except in the case of a Project 25 Trunking Radio System; in that specific case only P25 CAI modulation is used (no analog allowed.)

P25 CAI means a 9600 bps control channel

P25 CAI is not related to the bps rate of any trunking system at all. P25 CAI is used on several Motorola ASTRO systems (3600 bps) both shared with analog talk groups or as an exclusive modulation type. Examples of the latter are the South Dakota State TRS and the Ohio State MARCS systems. Neither of these systems are Project 25 systems, but both are P25 CAI exclusive.

Motorola ASTRO and P25 CAI are not the same

There is much confusion regarding about Project 25 and Motorola's ASTRO product line. Some of this confusion comes from the scanner manufacturers themselves and some from often repeated, but incorrect, information.

Motorola ASTRO actually comes in two versions:

  • Motorola ASTRO -- Motorola, early in the digital modulation age came up with a proprietary digital form of modulation called Vector-Sum-Excited Linear Prediction or VSELP. This digital modulation type is fast becoming obsolete in Motorola's product line; however, there are still some radio systems using it and it is not compatible with P25 CAI. There are no scanners capable of monitoring VSELP.
  • Motorola ASTRO-25 -- Motorola contributes to the confusion by offering P25 CAI in its product line but referring to it generically as ASTRO-25. These radios can be used conventionally or on a Motorola TRS (3600 bps) system. These radios do use P25 CAI digital modulation and any scanner capable of P25 CAI decoding can be used with these systems (using these radios). Motorola also uses this same equipment when installing a Project 25 Phase I & II trunking system (9600 bps).

The key to remember is that Motorola ASTRO and Project 25 are not synonymous.