Overview Page 39 of 906Circuit Card Description and InstallationCoder/Decoder circuitThe Coder/Decoder (CODEC) performs Analog to Digital (A/D) and Digitalto Analog (D/A) conversion of the line analog voiceband signal to and froma digital PCM signal. This signal can be coded and decoded using either theA-Law or the μ-Law companding algorithm.On some analog line cards, the decoding algorithm depends of the type ofCODEC installed when the board is built. On others, it is an option selectedusing a software overlay.Variable gain filtersAudio signals received from the analog phone line are passed through alow-pass A/D monolithic filter that limits the frequency spread of the inputsignal to a nominal 200 to 3400 Hz bandwidth. The audio signal is thenapplied to the input of the CODEC. Audio signals coming from the CODECare passed through a low-pass A/D monolithic filter that integrates theamplitude modulated pulses coming from the CODEC, and then filters andamplifies the result. On some of the line cards, the gain of these filters can beprogrammed by the system controller. This allows the system to make up forline losses according to the loss plan.Balancing networkDepending on the card type, the balancing network provides a 600 ¾, 900 ¾,3COM or 3CM2 impedance matching network. It also converts the 2-wiretransmission path (tip and ring) to a 4-wire transmission path (Rx/ground andTx/ground). The balancing network is usually a transformer/analog (hybrid)circuit combination, but can also be a monolithic Subscriber Line InterfaceCircuit (SLIC) on the newer line cards.Line interface and foreign voltage protectionThe line interface unit connects the balancing network to the telephone tipand ring pairs. The off-premise line card (NT1R20) has circuitry that protectsthe line card from foreign voltage surges caused by accidental power lineconnections and lightning surges. This protection is necessary if thetelephone line leaves the building where the switch is installed.The line interface unit has a relay that applies the ringing voltage onto thephone line. See Figure 4 on page 38. The RSYNC signal from the 20 Hz
