Nortel NT1R20 Description page 54

Page 30 of 290 Description
553-3001-105 Standard 5.00 June 1999
Coder/Decoder circuit
digital (A/D) and digital to analog (D/A) conversion of the line analog
voiceband signal to and from a digital PCM signal. This signal can be coded
and decoded using either the A-Law or the µ-Law companding algorithm. On
some analog line cards the decoding algorithm depends of the type of codec
installed when the board is built. On others, it is an option selected using a
software overlay.
Variable gain filters
passed through a low-pass Analog to Digital (A/D) monolithic filter that
limits the frequency spread of the input signal to a nominal 200 to 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 A/D monolithic
filter that integrates the amplitude modulated pulses coming from the codec,
and then filters and amplifies the result. On some of the line cards, the gain of
these filters can be programmed by the system controller. This allows the
system to make up for line losses according to the loss plan.
Balancing network
provides a 600 ¾, 900 ¾, 3COM or 3CM2 impedance matching network.
It also converts the 2-wire transmission path (tip and ring) to a 4-wire
transmission path (Rx/ground and Tx/ground). The balancing network is
usually a transformer/analog (hybrid) circuit combination, but can also be a
monolithic subscriber line interface circuit (SLIC) on the newer line cards.
Line interface and foreign voltage protection
connects the balancing network to the telephone tip and ring pairs. The
off-premise line card (NT1R20) has circuitry that protects the line card from
foreign voltage surges caused by inadvertent power line connections and
lightning surges. This protection is necessary whenever the telephone line
leaves the building where the switch is installed.
The Coder/Decoder (codec) performs analog to
Audio signals received from the analog phone line are
Depending on the card type, the balancing network
The line interface unit