unter. In this situation the digital count is proportional to the analogue input value. Notice that the frequency of the clock corresponds to the quantization step size; that is, a higher clock frequency results in a smoother quantization. The digital counter is reset to its initial condition after the end of each sample. After an analogue signal is converted into a digital format, it can be easily combined with other digital signals by interleaving in the time domain. This is referred to as multiplex- ing and is covered in the next section. Multiplexing Multiplexing is a term used to refer to a process where multiple analogue message signals or digital datastreams are combined into one signal over a shared medium. It is often practi- Chapter 1 Voice Transmission Sawtooth Reference Signal Message Signal Pulse Position Modulation Pulse Width Modulation Reference Clock Pulses to Counter Fig. 1.16 Implementation of a coder using a sawtooth signal. cal to combine a set of low-bitrate streams, each with a fixed and predefined bitrate, into a single high-speed bitstream that can be transmitted over a single channel. This technique is called Time Division Multiplexing (TDM). Time Division Multiplexing Principles Time division multiplexing is the earliest and simplest form of digital multiplexing. It was initially developed to provide multiaccess to the computing resources for groups of co- located users during the 1960s. Using a serializer the parallel input signals of different sources are combined into a single serial output. In fact, each channel (input) is allocated a timeslot, which defines the channel information within the serial output. Based on the channel time- slot duration we can have two types of multiplexing: If each timeslot is long enough to hold the full length of the input codeword, the multiplexed signal is referred to as word interleaved. Alternatively, the multiplexer may split each input code into binary bits and take one bit from each input, channel at a time; in this case, the multiplexed signal 