radio and television

The broadcast media industry is at a critical turning point in its development, with many
countries starting to move from analogue to digital broadcasts. The chief advantage of
digital broadcasts is that they prevent a number of complaints with traditional analogue
broadcasts. For television, this includes the elimination of problems such as snowy
pictures, ghosting and other distortion. These occur because of the nature of analogue
transmission, which means that perturbations due to noise will be evident in the final
output. Digital transmission overcomes this problem because digital signals are reduced
to binary data upon reception and hence small perturbations do not affect the final output.
In a simplified example, if a binary message 1011 was transmitted with signal amplitudes
[1.0 0.0 1.0 1.0] and received with signal amplitudes [0.9 0.2 1.1 0.9] it would still
decode to the binary message 1011 — a perfect reproduction of what was sent. From this
example, a problem with digital transmissions can also be seen in that if the noise is great
enough it can significantly alter the decoded message.

Using forward error correction areceiver can correct a handful of bit errors in the resulting message but too much noisewill lead to incomprehensible output and hence a breakdown of the transmission.In digital television broadcasting, there are three competing standards that are likely to beadopted worldwide. These are the ATSC, DVB and ISDB standards and the adoption of
these standards thus far is presented in the captioned map. All three standards use MPEG-
2 for video compression. ATSC uses Dolby Digital AC-3 for audio compression, ISDB
uses Advanced Audio Coding (MPEG-2 Part 7) and DVB has no standard for audio
compression but typically uses MPEG-1 Part 3 Layer 2. The choice of modulation
also varies between the schemes.

Both DVB and ISDB use orthogonal frequency-division
multiplexing (OFDM) for terrestrial broadcasts (as opposed to satellite or cable
broadcasts) where as ATSC uses vestigial sideband modulation (VSB). OFDM should
offer better resistance to multipath interference and the Doppler effect. However controversial tests conducted bythe United States’ National Association of Broadcasters have shown that there is little difference between the two for stationary receivers.In digital audio broadcasting, standards are much more unified with practically allcountries (including Canada) choosing to adopt the Digital Audio Broadcasting standard also known as the Eureka 147 standard).

The exception being the United States which has chosen to adopt HD Radio. HD Radio, unlike Eureka 147, is based upon a transmission method known as in-band on-channel transmission — this allows digital information to “piggyback” on normal AM or FM analogue transmissions.
Henceavoiding the bandwidth allocation issues of Eureka 147 and therefore being strongly
advocated National Association of Broadcasters who felt there was a lack of new
spectrum to allocate for the Eureka 147 standard.[43] In the United States the Federal
Communications Commission has chosen to leave licensing of the standard in the hands
of a commercial corporation called iBiquity.[44] An open in-band on-channel standard
exists in the form of Digital Radio Mondiale (DRM) however adoption of this standard is
mostly limited to a handful of shortwave broadcasts.