I), a high voltage level a logic 0. A single switching point, or decision level, or a low level represents the logic level 1. A zero level represents is used to set the threshold. By comparing the coded signal to the the logic level 0. To transmit the sequence 10101 the following threshold level, the binary information can be decoded. If the signal pattern would be output: high/zero/low/zero/high This type of level is above the threshold, then a logic 1 is detected; conversely, coding is known as pseudoternary coding. It is called "pseudo" if it is lower, a logic 0 is detected. Because the signal has two levels, because the zero voltage level is not really classified as a discrete it is known as a unipolar code. Unipolar coding is a very good level. The receiver needs two decision levels to decode the incom- system for TTL systems but is not suitable for transmission systems ing data and also has to keep track of the level of the last logic 1 using copper cables over long distances. transmitted SO that the correct level of the next logic 1 is sent as the 00000 Figure 21.8 AMI and HDB3 pseudo-ternary codes Coding schemes 21/13 00000000 0000000 Figure 21.9 Typical B7 zero code suppression example code alternates. In the case in which the wrong level is detected, i.e. in such way as to maintain or reset the d.c component to a zero a high followed by a high, a code violation is recorded. The advant- value. age of this type of coding is that the d.c. level, or balance, is The American T1 system generally uses a technique called B7 maintained by the transmission of alternate high and low pulses. zero code suppression (B7ZS). To keep telephone company T1 line The disadvantage is the extra circuitry required. repeaters and channel service units (CSU) in synchronisation, the Many receivers extract clocking information from incoming data digital bit stream permits a maximum of 15 consecutive zeros. As by detecting the occurrences of the incoming signal data crossing a already mentioned, this is known as the Telco's ones de