W 5 T X R - Company Message
Digital Audio Emissions
Amplitude modulation (AM) was devised in the 1920s, frequency modulation (FM) in the 1930s, and single sideband (SSB), actually pioneered by Amateur Radio experimenters, also in the 1930s. AM, FM and SSB modulation as used today are all virtually unchanged since their inception.
We own an AOR ARD9800 and ARD9000 Fast Radio Modem
The Emission is OFDM and it uses, you guessed it, the DVSI AMBE 2020
the same Vocoder used in P25, D-Star, Alinco Digital
This is the Alinco DR-135MKIII with the optional
EJ-47U digital voice board
(also using the exact same DVSI IC)
the Alinco DR-135MKIII
Icom's D-Star
Spectral efficiency:
The DV format of D-STAR has a bandwidth of 6 kHz, compared to 16 kHz for analog FM with 5-kHz deviation.
This is Via Carson's Rule, BW = 2 × (Peak Deviation + Highest Modulating Frequency) = 2 (5 kHz +3 kHz) = 16 kHz.
Modulation methods: GMSK, QPSK, 4FSK
Data rate: Maximum of 4.8 Kbps
Voice encoding method: AMBE (2020) converting at 2.4 Kbps, FEC at 3.6 Kbps
Occupied bandwidth: Maximum of 6 KHz
Vocoder: DVSI AMBE 2020
Codec: proprietary!
D-star is a 4800 baud total data stream equivalent signal. Where: 2400 bps is reserved for actual digital voice, 1200 bps is reserved for FEC (forward error correction) on the digital voice. This is for callsign and short message data. And 1200 baud is reserved for serial data. This is for APRS, and text messages/text query's.
Alinco Digital 
9600bps, synchronous/asynchronous, TCM. 4800bps, synchronous/asynchronous, QAM
The processed signal modulates the VCO in GMSK direct frequency modulation using a GMSK-Modem. It is then transmitted as a 20F3E conventional FM signal.
Vocoder: DVSI AMBE 2020
Codec: Uses the ITU-TV.32 protocol
In the receiving mode, the GMSK modulated RX-AF signal comes out of a demodulation circuit and is processed at the GMSK-Modem. This signal is again processed in the CPU, then decoded in Continuous Code Delta Codec (CVSD) to obtain the original analog voice signal (Digital-Analog conversion). As in a normal FM receiver, the signal is amplified for output to a speaker.
Spectral efficiency:
Phase 1 radios use the IMBE vocoder and Continuous 4 level FM (C4FM) modulation for digital transmissions at 4800 baud and 2 bits per symbol, yielding 9600 bits per second total channel throughput. Where: 4400 baud are associated with the digital voice, 2800 baud are used for error correction on the voice signal, 2400 baud are devoted to signaling overhead.
Receivers designed for the C4FM standard can also demodulate the "Compatible quadrature phase shift keying" (CQPSK) standard, as the parameters of the CQPSK signal were chosen to yield the same signal deviation at symbol time as C4FM while using only 6.25 kHz of bandwidth.
Phase 2 is currently under development with concurrent work being done on 2-slot TDMA and FDMA (CQPSK) modulation schemes. Phase II will use the AMBE vocoder to reduce the needed bitrate so that one channel will only require 4800 bits per second.
APCO P25 Phase I is the present version that is in used across the country for Digital Public Safety, the P25 “open” standard has been reworked by some manufacturers limiting some of the standardization that the P25 was hoped to present..
P25 Phase I repeaters have the ability to function as a analog system or digital system.
P25 Presently operates via FDMA (Frequency Division Multiple Access) with the plan for P25 Phase II to use TDMA (Time Division Multiple Access), P25 Phase II will also have the capability to “roll-back” to FDMA for “conventional emergency operations.”
Vocoder: DVSI AMBE 2020
Codec: P25 (Proprietary)
Significant attention is also paid to interoperability with legacy equipment, interfacing between repeaters and other subsystems, roaming capacity and spectral efficiency/channel reuse. In addition, Phase 2 work involves console interfacing between repeaters and other subsystems, and man-machine interfaces for console operators that would facilitate centralized training, equipment transitions and personnel movement.
Amateur radio usage: Check out the website www.p25ham.com
Motorola "Astro" is NOT P25! Motorola Astro is a VSELP (Vector Sum Excited Linear Prediction)
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