p of Bell Labs physicists were experimenting with a germanium crystal they had placed between two wires. Unexpectedly, a signal that was traveling across the wires began to amplify to 40 times its original strength. This discovery of the transistor-from which all integrated circuit technology evolved-paved the way for the digital world in which we now live. PCM is the most common method used to convert an analog audio signal to a digital one. An audio channel is sampled at uniform intervals 8,000 times per second. As a sample is taken, its voltage is compared to a set of numeric values. Since the sample is going to be encoded using eight bits, one of 256 integers can be used to represent it. The integer most closely resembling the sample is used to represent it. This step is quantizing. The step that follows quantizing is encoding. In this step, the selected integer is expressed as an eight-bit word, a BYTE Streams of bytes are transmitted across networks at a rate of one every 125 microseconds. As each byte reaches its destination, a codec reconverts it to its analog form. PCM-encoded voice requires a transmission line with a speed of 64 kilo-bits-per- Video Communications: The Whole Picture second (Kbps). The signal is sampled 8,000 times per second and each sample is encoded using eight bits (8,000 X 8 = 64,000). This 64 Kbps channel is the fundamental building block of digital networks, although many telephone carriers rob 8 Kbps from a circuit and use that 8 Kbps to carry timing information. Sampled and quantized signal Envelope of signal Figure 6-1. Pulse Code Modulation (Michel Bayard). The 64 Kbps data rate forms the basis of the Integrated Services Digital Network (ISDN) bearer (B) channel. It is multiplexed with 23 other PCM-encoded channels into a wide pipe known as a DS-1 (T-1). DS-1 is a digital transmission link with a capacity of 1.544 Mbps (in which M represents million). The bandwidth of a DS-1 is divided into channels using a technique called time division multiplexing (TDM). The North American 