Codan Radio P25 Training Guide - Manual (Page 106)

TRAINING GUIDE | P25 RADIO SYSTEMS
Chapter 8: IMBE™ and AMBE+2™ Vocoders Page 98
The IMBE™ encoder uses sophisticated algorithms to estimate a set of model parameters for each
segment of the incoming speech signal. These parameters consist of: (1) a fundamental frequency,
to represent the pitch of the speaker; (2) a set of Voiced/Unvoiced (V/UV) decisions, to represent the
mixture of voiced and unvoiced energy; and (3) and a set of spectral magnitudes, to represent the
frequency response of the vocal tract. The encoder computes a Discrete Fourier Transform (DFT) for
each segment of speech and then analyzes the frequency content to extract the model parameters for
that segment. These model parameters are then quantized into 88 bits, and the resulting voice bits are
then output as part of the 4.4 kbps of voice information produced by the IMBE™ encoder. An additional
2.8 kbps of error correction information is then added to the voice information to produce the 7.2 kbps
bit stream that is transmitted over the CAI.
The IMBE™ decoder reproduces analog speech from the 7.2 kbps digital bit stream that is received
over the CAI. The decoder fi rst uses the error correction information included in the received bit stream
to attempt to correct any bit errors that may have been introduced by the radio channel. The decoder
then reconstructs the model parameters for each segment and uses these parameters to synthesize
both a voiced signal and an unvoiced signal. The voiced signal represents the period portions of the
speech and is synthesized using a bank of harmonic oscillators. The unvoiced signal represents the
noise-like portions of the speech and is produced by fi ltering white noise. The decoder then combines
these two signals and passes the result through a digital-to-analog converter to produce the analog
speech output.
DVSI has also introduced new Enhanced Vocoders for P25 based on DVSI’s latest AMBE+2™ Vocoder
technology. The Enhanced AMBE+2™ Full Rate Vocoder is fully interoperable with the current P25
7.2 kbps (IMBE™) standard. The AMBE+2™ can also operate at half rates of 3.6 kbps for Phase 2.
The AMBE+2™ provides improved voice quality, better noise immunity, tone capability, and other new
features. The Enhanced Vocoders signifi cantly improve the voice performance of the P25 system, while
facilitating the migration and interoperability between new and existing P25 equipment. DVSI’s vocoder
technology is used extensively in digital radio systems and in mobile satellite telephony worldwide.