2. Signals

We all send and receive signals. A letter or a phone call, a raised hand, a hunger cry—signals are our information carriers. Pervasive computing systems also send signals (to wheels, loudspeakers, or displays), and sense signals (like speech, images, heart activity, or temperature). These signals are by nature continuous, meaning that their level is known always and everywhere, and they are analog, meaning that this level is an infinite-precision real number. The problem is that these properties are in conflict with the discrete and digital character of computers. The price the real-world signals have to pay to be admitted and processed by a computer, is to undergo a process called digitization. Digitization happens in two stages. First, sampling periodically measures the signal’s level, and produces a finite-length sequence of samples of infinite precision. Next, quantization converts these samples into a sequence of finite-precision numbers. Digitization is the task of an electronic device, called analog-to-digital converter (ADC). The reverse process, needed by a computer system to decipher the carried message and send signals back to the real world, is called reconstruction. Reconstruction happens in another electronic device, called digital-to-analog converter (DAC). High-quality digitization and reconstruction are needed, in order to preserve the information the signal is carrying. This chapter is an introduction to signals and their representation in pervasive computing systems, with a focus on audio and video signals.