Comtech EF Data SDM-100A - Manual (Page 196)

Options SDM-100A Satellite Modem
B–14 Rev. 0
B.2.1.1 Multiplexer
The ADPCM interface provides a 6-wire, RJ11 phone connector on the back panel for the
analog input. The input is a full-duplex four-wire input, with two additional pins for
E&M signaling (Refer to Section B.2.3 for the RJ11 pinout specifications). The audio
input is transformer-coupled with a 600
Ω input load. The signal then passes to a
programmable input level control circuit. The audio input level is user selectable in 1 dB
steps, from -20 to +10 dBm.
The signal then passes through a filter with a 300 to 3400 Hz bandwidth. The filtered
signal passes to a Pulse Code Modulation (PCM) encoder, which encodes the data at
64 kbit/s. The user can select either A-Law or
µ -Law operation of the encoder from the
front panel. A TX Clip indicator in the FAULT TX menu is provided to monitor
excessive input levels.
The 64 kbit/s PCM data from the encoder is compressed to 32 kbit/s, 16 kbit/s, or passed
directly to a MUX. The compression rate is selectable from the front panel.
A rate exchange now occurs as the data is input to a FIFO in the MUX, where 1/15
proportional IBS-type framing overhead is added. The multiplexed data is output to the
modulator. In pass-through mode (64 kbit/s), the data coming out of the multiplexer is in
the form of an IESS-309 IBS frame. Depending on how the modem is configured (trunk
or subscriber), either the E or M signal is multiplexed into the overhead framing structure
for reception on the other end. Spare bit 4 of byte 32 in the IBS frame is used for the E or
M signaling. For 32 kbit/s mode, a pseudo-IBS frame structure is used.
In ADPCM mode, the modulation data rate is fixed at 64, 32, or with BPSK 1/2 or QPSK
1/2, 3/4, or 7/8 coding.
B.2.1.2 Demultiplexer
The receiving section works basically reverse of the transmitter side.
Data is received and demodulated, then routed to a demultiplexer. The DEMUX
synchronizes to the IBS framing structure, and removes the IBS overhead. The DEMUX
also sends the received E or M signal to the E&M signaling circuitry for level
conversion, and on to the RJ11 connector.
The PCM data is input to a FIFO as part of a rate exchange. The FIFO output data is
expanded from 32 to 64 kbit/s.
The data passes to the PCM decoder, and is converted to analog.