t interfaces. We can, for example, imagine that the signal value stays at a small negative value that produces the encoded word 00000000, and inversion of every other bit pro- duces the word 01010101. Without the inversion of every other bit, we would transmit continuous zero and might have difficulties synchronizing the receiver with the received data stream. Other coding schemes used to ensure proper synchronization are discussed in Chapter 4. Signals Carried over the Network 11111111 11110000 11100000 11010000 11000000 Positive segments of the A-curve 10110000 10100000 10010000 10000000 1/8 1/4 Sample value Position Polarity Segment in a segment Code word representing the value of a sample Figure 3.14 Binary coding. 3.6.7 PCM Encoder and Decoder The PCM coding schemes for digital voice communications were standard- ized by CCITT (now ITU-T) in the early 1970s. The standards were based on the technology of those days. The European standard was defined to be slightly different from the American standard, which is why conversion equipment is needed when communicating over the Atlantic or from Europe to Japan. Most countries in the world use the European A-law standard. As a conclusion to our discussion about PCM coding, we now look at the block diagrams of the PCM encoder and decoder that contain the processes that we have discussed in previous sections. 3.6.7.1 PCM Encoder Figure 3.15 presents a block diagram of a PCM encoder based on the Euro- pean standard. Before actual encoding, the analog signal is filtered into the frequency band from 300 to 3,400 Hz. This bandwidth was defined to be acceptable for sufficient quality human voice SO that the speaker can be rec- ognized at the other end. This filtering is mandatory to ensure that the Introduction to Telecommunications Network Engineering Other PCM-coded speech signals 256-level Parallel Encoder to serial 8 kHz quantizer multiplexer converter 8 bits 64 Kbps parallel 2,048 Kbps Timing serial circuit LPF = Lowpass filter S/H = Sample and hold circuit TDM = Time