Telecommunications Analog-to-Digital Conversion James K. Marsh March 11, 2010
Analog Signals An analog signal is a smoothly and continuously varying voltage or current. Sine waves are single frequency analog signals. Voice and video voltages are analog signals that vary in accordance with the sound or light variations that are analogous to the information being transmitted.
Analog Signals An analog signal contains two parts, an envelope and two frequencies. One of these frequencies are low and referred to as a message frequency. These two frequencies combined create the signal that an antennae must decode before sending the signal to the device you are using, such as a television or radio.
Broadcast Frequencies Television broadcast can be ultra high frequency (UHF) or very high frequency (VHF)analog signals. Ultra high frequency is for television broadcast of the UHF television channels fourteen through sixty seven. Very high frequency is in the thirty to three hundred megahertz range. It is used to broadcast television channels two through thirteen, as well as frequency modulated (FM) radio broad cast.
Advantages and disadvantages of Analog Signals The disadvantages outweigh the advantages of the analog signal. Analog signals are very easily distorted causing attenuation or signal loss. The one major advantage of analog transmission is it uses less bandwidth than a digital signal.
Digital Signals Digital signals, in contrast to analog signals, do not vary continuously, but change in steps or discrete increments. The digital code uses ones(on) or zeroes(off) to create a code and transmit data, much like a computer. The telegraph used Morse code, with its short and long signals (dots and dashes) to designate letters and numbers.
Binary Code Data used in computer is also digital. The most commonly used digital code in communications is the American Standard Code for Information Interchange.
Nyquist Theory Unlike analog signals digital signals can be interpreted by only having a small part of the signal. The Nyquist Theory states that a digital signal can be reproduced by taking samples at twice the analog sample frequency.
Analog-to-Digital Conversion Most analog signals must be converted to digital signals before they can be interpreted by today’s technology. The device used to perform this translation is known as an analog-to-digital converter.
Analog to digital Conversion (cont.) Analog-to-digital conversion is the process of sampling the analog signal at regular time intervals. At each time interval a binary number is generated to represent the sample.
Television Conversion A very good example of analog-to-digital conversion is the recent event that took place in television broadcast. All US full-power analog TV broadcasts came to an end on June 12, 2009.
Television Conversion Advantages DTV has several advantages over analog TV, the most significant being that digital channels take up less bandwidth. This means that digital broadcasters can provide more digital channels in the same space. Disadvantages Even though digital has many great advantages over analog there are still draw backs to this conversion. Based on this survey and considering that 13–15% of TV viewers depend on over-the-air TV, Three percent of the overall TV viewer ship might be lost due to the DTV conversion. Potential negative impacts on TV stations include; reduced television advertising and pledge drive revenue.