2 - The analog—digital interface

Abstract

This chapter discusses encoding and modulation schemes, pulse code modulation, pulse amplitude modulation, pulse position modulation, pulse number modulation, pulse width modulation, and pulse density modulation. There are many ways to encode the digital information into the shape of electrical signals. This process is called modulation and most common method is probably pulse code modulation (PCM). There are two common ways of transmitting PCM and they are parallel and serial mode. There are many possible modulation schemes, such as pulse amplitude modulation (PAM), pulse position modulation (PPM), pulse number modulation (PNM), pulse width modulation (PWM), and pulse density modulation (PDM). All these modulation types are used in serial transfer mode. Further, when the analog signal is quantized, it is commonly represented by binary numbers in the following processing steps. There are many possible representations of quantized amplitude values. One way is to use fixed-point formats like 2's complement, offset binary or sign and magnitude. Another way is to use some kind of floating-point format. There are a number of floating-point formats around. They all rely on the principle of representing a number in three parts: a sign bit, an exponent and a mantissa. One such common format is the Institute of Electrical and Electronics Engineers (IEEE) Standard 754.1985 single precision 32-bit format, where the floating-point number is represented by one sign bit, an 8-bit exponent and a 23-bit mantissa.