COFDM is a modulation scheme that divides a single digital signal across 1,000 or more signal carriers simultaneously. The signals are sent at right angles to each other (hence, orthogonal) so they do not interfere with each other. COFDM is used predominately in Europe and is supported by the Digital Video Broadcasting (DVB) set of standards. In the U.S., the Advanced Television Standards Committee (ATSC) has chosen 8-VSB (8-level Vestigial Sideband) as its equivalent modulation standard.
The main reason for Europe's decision to use COFDM is its ability to completely overcome multipath effects. When a signal is transmitted, it is met with obstructions such as canyons, buildings, and even people, which scatter the signal causing it to take two or more paths to reach its final destination, the television. The late arrival of the scattered portions of the signal cause ghost images. Multipath effects can occur simply by an individual walking into the room. For this very reason, some consumers in metropolitan areas or areas with rugged terrain opt for cable or satellite television instead of fighting their antennas for better reception. COFDM is resistant to multipath effects because it uses multiple carriers to transmit the same signal. Instead of the signal scattering when met with an obstacle, it flows around the obstacle like a river flows around a rock making it perfect for free DTV programming and for mobile television viewing. Problems with multipath effects were often cited in early evaluations of 8-VSB, although it is expected that devices such as internal antennas will overcome them.
In Europe, stations transmit the same signal 100 percent of the time across many borders using single frequency networks. A single frequency network is a network of several stations that broadcast the same signal simultaneously using multiple transmitters. This allows television viewers to watch the same broadcast anywhere in Europe without interference. COFDM is ideal for single frequency networks.
Continue Reading About COFDM
- The BBC's Research & Development Department's paper, "Explaining some of the magic of COFDM," by J.H. Scott explains how COFDM works.
- Digital Television: The Site has information on the controversy surrounding VSB and COFDM and on DTV in general.