Nortel NT1R20, Description

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QPC789 16-port Message Waiting Analog Line Card Page 269 of 290
Line cards Description
Codecs
Each SLIC also contains a filter and a coder/decoder (codec). Audio signals
received from the telephone line are passed through a low-pass monolithic
A/D filter that limits the frequency spread of the input signal to a nominal
200–3400 Hz bandwidth. The audio signal is then applied to the input of the
codec. Audio signals coming from the codec are passed through a low-pass
D/A monolithic filter that integrates the amplitude modulated pulses coming
from the codec and then filters and amplifies the result.
Message waiting lamp logic
Each of the 16 hybrid subscriber line interface circuits (SLIC) also contains a
–150 V dc driver for the message waiting lamp and a lamp fail detector. The
lamp has three states: ON, OFF, and FLASH. The message waiting lamp
driver circuit is disabled whenever the phone is in the off-hook state. The
lamp fail detector switches high whenever the lamp is bad (open).
The lamps flash at a rate of 1 Hz. When flashing is enabled on all 16 of the
lamps, the lamp driver circuit is enabled on only 8 of the lamps at any one
time. The other eight lamps are enabled when the first eight are off (alternate
flashing).
Multiplex control
The multiplex control logic is common to all 16 channels. This logic
interfaces the individual line circuits to the peripheral bus signaling channel.
Circuits are provided in the multiplex control logic to retime the digital
signals received from the peripheral bus. Circuits are also provided to decode
the address information contained on the peripheral bus to enable the
individual line circuits during their selected time slots. Further, logic is
provided to enable or disable the front panel LED to indicate the service state
of the card.