Guardian G25AMK005, Technical Manual

Page: 43 / 85

G U A R D I A N V H F 1 1 0 W M O B I L E 3-13
3.5.3.3 CAI Receive Voice Mode
The DSP takes the 9.6 kbps CAI-compatible data stream and framing, and splits out the voice data for passing to the
VOCODER. The DSP decodes the link control words so that the host H8 can read the link control fields, and the
encryption synchronization information is available to the encryption process. CAI receive processing is initiated by
the frame synchronization correlator trigger. When this is asserted the next 64 bits of network identifier (NID) data
are decoded and checked. If the NAC code matches the one selected for the channel, voice or data processing
proceeds, otherwise the physical layer is forced into search mode.
3.5.3.4 CAI Receive Voice Test Mode
The DSP software is capable of testing the 9 voice code words received in a CAI frame against a known 9 x 144
PRBS segment. The total number of errors in that frame is then output to the H8 controller. The H8 controller
controls this test mode.
3.5.3.5 CAI Transmit Data Mode
The DSP selects the user data (encrypted or clear) and a number of link control fields set by the host H8 or from the
DES system, and formats logic link data units compatible with the CAI at 9600 bps.
3.5.3.6 CAI Receive Data Mode
The DSP selects the 9.6 kbps CAI-compatible data stream and framing, and splits out and decodes the data for
passing to the user data port. The DSP decodes the link control words so that the host H8 can read the link control
fields, and the encryption synchronization information is available to the encryption process.
3.5.4 Transmit Physical Link Layer
This software uses common modulator interface software and a number of mode-dependent, physical link layer
software modules.
3.5.4.1 Transmit Modulation Interface
The software provides a common interface to the dual modulation, DAC in the transceiver through its SSI serial port
1 for all transmit modes. In transmit modes, the SSI uses an externally sourced clock at 3072 kHz. This interface
takes frequency deviation samples at 48 kHz, and writes each value scaled by a fixed number set by the host to both
the reference oscillator DAC and the VCO DAC. Additionally a host-controlled dc offset is added to the reference
oscillator DAC value.
3.5.4.2 Transmit CAI Physical Link Layer
This software takes the 9.6 kbps CAI-compatible data stream and converts it to 48k samples of frequency deviation
data. To achieve this the software implements the dibit for symbol mapping, Nyquist, and shaping filters as
described in the CAI.
3.5.4.3 Transmit Analog FM Physical Link Layer
This software takes the 8 ksps filtered audio and converts it to 48 ksps of frequency deviation data compatible with
TIA/EIA-603 in 12.5 kHz and 25 kHz modes. The signal processing uses additional high-pass filtering to reduce the
energy in the DCS tone band, audio band pre-emphasis if required, DCS tone addition, and deviation limiting and
smoothing. The software is capable of operation in 12.5 kHz, and 25 kHz channel spacing with appropriate
deviation scaling. It is capable of appending a phase reversed tone burst of 180 ms as defined in EIA-603,
generating DCS and audio turn-off codes, and companding the voice signal in 12.5 kHz mode.
3.5.4.4 Transmit CVSD Physical Link Layer
The transmit CVSD physical link layer converts the 12/16 kbps CVSD data stream and converts it to 48 ksps of
frequency deviation data. The software implements a pre-modulation filter with raised cosine time response and
100% eye height.