D4 Digital Channel Bank Family: Thin-Film Dual Active Filter for Pulse CodeModulation Systems

The first major Bell System application of resistor-capacitor (RC) active filters was in the D3 channel bank. These filters were fabricated using tantalum thin-film resistors and capacitors1 and beam-leaded silicon integrated circuit (sic) operational amplifiers. They have been manufactured since 1971. Evolution of the technology has reduced the cost, size, and power of these filters and has expanded their application to other systems. The filters are currently being used in D3, D4, DCT, 2815 SLC*-96 subscriber loop carrier, LT1, and several other PCM systems. This article briefly reviews the history find application of these filters in PCM systems and describes the latest design: a dual filter fabricated for single in-line package (SIP) mounting. Most PCM systems that encode and decode a voice-frequency (VF) signal require two filters for each channel. A simplified view of the D4 channel bank is shown in Fig. 1. A filter is required to band limit the signal before it is encoded to avoid aliasing from the 8-kHz sampling rate. This is often called the transmit filter. Another filter, called the receive filter, reconstructs or smooths the output from the decoder. Similar filters are required if per-channel coders-decoders (CODECS) are used. The typical transmit-end and receive-end four-wire response limits for the transmit and receive filters are shown in Figs. 2 and 3, respectively. Since there are other sources of channel-frequency shaping, somewhat tighter limits are applied to the filters alone.