Codan Radio P25 Training Guide - Manual (Page 14)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 1: Introduction To P25 Page 6
The P25 Radio System Architecture can be broken down into three main areas.
A to D / D to A and Speech Coding / Decoding
An Analog to Digital conversion is performed on the audio before speech coding and a Digital to Analog
conversion is performed to create audio after the speech decoding. P25 uses a specifi c method
of digitized voice (speech coding) called Improved Multi-Band Excitation (IMBE™). The IMBE™
voice encoder-decoder (vocoder) listens to a sample of the audio input and only transmits certain
characteristics that represent the sound. The receiver uses these basic characteristics to produce a
synthetic equivalent of the input sound. IMBE™ is heavily optimized for human speech. Older IMBE™
vocoders didn’t always do well in reproducing other types of sounds, including dual-tone multifrequency
(DTMF) tones and continuous test tones. Since 2009, the enhanced IMBE™ vocoder works signifi cantly
better with DTMF and continuous tones.
The IMBE™ vocoder samples the microphone input producing 88 bits of encoded speech every 20
milliseconds. Therefore, the vocoder produces speech characteristics at a rate of 4400 bits per second.
Channel Coding / Decoding
Channel Coding is the method in which digital RF systems utilize forward error protection / error
correction techniques to ensure that the data (voice or control) arrives and is recovered correctly.
The forward error protection / error correction are designed to improve the system performance by
overcoming channel impairments such as noise, fading and interference. Channel Coding can also
include the addition of all overhead data that is included with the voice information, including NAC,
TGID, SID, MFID, KID, ALGID and many others.
P25 error protection / correction channel codes include; interleaving and linear block codes such as
Hamming codes, Golay codes, Reed-Solomon codes, Primitive BCH, and shortened cyclic codes.
Modulating / Demodulating and Filtering
In Phase 1, a 12.5 KHz channel is used to transmit C4FM modulated digital information. C4FM
modulation is a type of differential Quadrature Phase Shift Keying (QPSK) where each symbol is shifted
in phase by 45 degrees from the previous symbol. Although the phase (frequency) is modulated for
C4FM, the amplitude of the carrier is constant, generating a constant envelope frequency modulated
waveform.