Nortel NT1R20, Description

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Page 232 of 290 QPC192 Off-Premise Extension (OPX) Analog Line Card
553-3001-105 Standard 5.00 June 1999
Relays are provided in each unit to apply ringing onto the line, and the timing
of the application of ringing current is controlled to prevent switching during
current peaks. Signal detection circuits monitor on hook/off hook signaling.
Codecs
Each line interface unit 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.
Each channel contains a codec that performs A/D and D/A conversion of the
line analog voiceband signal to a digital PCM signal. The codec also contains
a switchable pad for control of transmission loss. This pad can be controlled
in software, and the possible gain settings are shown in Table 61.
Multiplex control
The multiplex control logic is common to all four 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.
Table 61
OPX analog line card—switchable pad gain settings
Connection Gain
OPX to OPX Connection ±1 dB
Line to OPX Connection ±1 dB
OPX to Line Connection ±1 dB