Digital radio technologies convert sound into digital signals which are then compressed, transmitted, and decoded back into sound by digital radio receivers. There are several ways of transmitting and receiving digital radio today. These include the DAB, DAB+, DRM (digital radio mondiale) – DRM30 (AM), and DRM+ (above 30 MHZ) and HD radio standards.
HD radio technology allows broadcasters to transmit a high-quality digital signal alongside traditional analog radio signals. It operates on both AM–MW and FM VHF band II either in digital only mode or in hybrid-digital plus analog mode. The HD radio mode places low-level digital carriers in the upper and lower side bands of the analog spectrum. The most common format includes the original analog signal, a simulcast digital signal, and one or more additional digital stations. Since the analog signals are in close proximity to the digital signals, great care must be taken to prevent unwanted interference between them.
The HD radio standard is based on technology owned by iBiquity. In 2002, the FCC approved iBiquity’s HD radio technology for use in the United States. HD radio is the only FCC-approved digital radio technology.
In HD radio the original analog signal is broadcast with two digital sidebands at the bottom and top of the allotted frequency, therefore, if the power allotted to the sidebands is high enough, it can cause interference in the adjacent channels as well as its own associated analog signal. This was a problem due to the fact that low signal strength results in an HD radio broadcast that nobody can receive, while a strong one can interfere in the reception of the analog signal.
The ratio of power of the analog signal to the digital signal was initially standardized at 100:1 (–20 dBc), making the digital signal 1 percent of the analog carrier power. The National Association of Broadcasters (NAB) requested a
10 dB (10×) increase in the digital signal from the FCC. This equates to an increase to 10 percent of the analog carrier power, but no decrease in the analog signal. This was shown to reduce analog coverage because of interference, but results in improvement in digital coverage.
In 2010, the FCC approved to voluntarily increase the maximum digital effective radiated power (ERP) to 4 percent of analog ERP (-14 dBc), up from the previous maximum of 1 percent (-20 dBc). There is also a conditional approval from the FCC for a –10 dBc power level if the station can show that no interference to other stations would result.
Stations supporting HD radio simultaneously broadcast analog and digital versions of the same programming over the same frequency. With a regular radio, we can hear the usual analog version andwith a HD-compatible radio, the radio tunes in to the digital programming on HD stations and the analog signals for non-HD stations. If the HD radio signal becomes too weak, the radio will automatically switch over to the parallel analog signal.
The most visible benefit of HD radio is higher audio quality. HD radio takes FM sound quality closer to that of CDs and makes AM broadcasts resemble those of analog FM.
Another benefit of HD radio is the radio station’s ability to transmit additional information along with the audio signal. This could be song’s artistes, titles, station name etc., which are displayed on the receiver’s screen. Real time updates like traffic and instant information about changing weather conditions, latest sports scores, and emergency messages are possible to be broadcast along with audio signals.
Moreover, by broadcasting digitally over traditional radio waves, a single frequency is now capable of delivering up to four stations of content in crystal clear sound, also known as multicasting. After tuning to a particular frequency the HD radio receiver indicates that the station is multicasting thereby enabling the listener to select alternate channels being broadcast on the same frequency. These additional digital stations (HD1, HD2, HD3, HD4) can be used to provide specialty content. It makes the HD radio format more spectrum efficient like other digital radio techniques.