n commu- nications and electronics work, the term analog is used to mean a particular type of circuitry or manner of dealing with voltage and current. A good analog signal is absent distortion when compared to the original. A microphone converts sound waves (acoustical energy) into electrical energy-voltage and current. A speaker converts electrical energy into acoustical energy. Distortion of any kind along the path between source and destination is undesir- able technically and may not be commercially acceptable to the viewer or listener. Electrically or electronically, an analog wave- form is a representation of voltage at any given instant or over a period of time. It can be stable or varying in amplitude. The polarity may be constant or reverse in regular or irregular inter- vals. In practical broadcast engineering, analog is a label to character- ize NTSC, PAL, SECAM video, and non-digitized audio signals. In today's digital world it's important to recognize and remember the following two things: (1) content or the signals representing the content must be digitized or present in digital form to be carried on digital transport media; and (2) video is structured, while its coun- terpart, audio, is relatively unstructured. Program content-audio and video signals in analog form and digital media mix about as well as oil and water. Practical IP and Telecom for Broadcast Engineering and Operations Copyright © 2004 by Fred Huffman. All rights of reproduction in any form reserved. PRACTICAL IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS Analog-to-digital and digital-to-analog conversion If a signal is in analog form and needs to be carried on digital media, it must be converted to digital. If the signal is in digital form and must be displayed or heard, it must be converted to analog form. Conver- sion from one form to the other requires care; otherwise, the process can produce undesirable or unwanted side effects rendering the signal useless for commercial purposes. Carrier Carrier is a truncation from cl