signal value within the segment. Despite all similarities there are some differences between these two schemes. A-law provides a slightly larger dynamic range compared to u-law, while u-law is more efficient in terms of quantization noise reduction for small signal levels. A-law is also used for inter- national connections. If any conversion between these two companding schemes is required, it should be accomplished by the u-law country. Also notice that this conversion is a lossy process because it involves requantization of the signal. Encoding After sampling and quantization, the PCM signal is encoded into the bits of 0 and 1. As mentioned before, the coding will be more efficient if the number of quantization levels is a power of 2. Most PCM systems use 8 bits to represent each sample (256 quantization levels). There are different coding schemes that can be used to represent the data. The binary number code is an example of a more general category known as weighting codes. In binary number coding, each binary digit (either 0 or 1) carries a certain weight according to its position in the binary number. The binary code can be found in different forms. For example, in 8421 Binary Coded Decimal (BCD) representation, each decimal digit is converted to its 4-bit pure binary equivalent. The 4221 BCD code is another example where each bit is weighted by 4, 2, 2, and 1, respectively. Mention should also be made of another important class of codes called unit-distance codes in which the distance between two code characters is the number of digits by which they differ. In other words, a code is considered to be unit-distance if it changes at only one digit position when going from one number to the next in a consecutive sequence of numbers. Multiplexing Clock Clock Clock Clock Input Clock Digit 1 Digit 2 Digit 3 Digit N Fig. 1.15 N-digit binary counter using master-slave flip-flops. Gray code (also referred to as reflected binary code) is an example of a unit-distance code that facilitates the error correction process 