54 OverviewFigure 14Typical trunk interface unit block diagramCoder/Decoder circuit The coder/decoder (codec) performs Analogto Digital (A/D) and Digital to Analog (D/A) conversion of the line analogvoiceband signal to and from a digital PCM signal. This signal can be codedand decoded using either the A-Law or the μ-Law companding algorithm.On some trunk cards the decoding algorithm depends of the type of codecinstalled when the board is built. On others, it is an option selected using asoftware overlay.Variable gain filters Audio signals received from the analog phone trunkare passed through a low-pass A/D monolithic filter that limits the frequencyspread of the input signal to a nominal 200–3400 Hz bandwidth. The audiosignal is then applied to the input of the codec. Audio signals comingfrom the CODEC are passed through a low-pass A/D monolithic filter thatintegrates the amplitude modulated pulses coming from the CODEC, andthen filters and amplifies the result.On some of the trunk cards, the gain of these filters can be programmedby the system controller. This allows the system to make up for line lossesaccording to the loss plan.Nortel Communication Server 1000Circuit Card ReferenceNN43001-311 01.04 StandardRelease 5.0 23 May 2008Copyright © 2003-2008, Nortel Networks.
