HD radio gears up for expansion

 

Besides improved audio quality, the high-bandwidth available with HD radio provides ample room for broadcasters to transmit additional information, such as a song’s artist and title, which is displayed directly on the digital radio’s display. Broadcasters can even choose to expand this service to include weather updates, traffic alerts, or other local information that adds value and helps to cement listener loyalty. 

 

The high capacity of digital signals also allows stations to support multicasting by dividing their digital signal to facilitate the broadcast of two or more programmes simultaneously. For example, by using the same radio station and frequency, a digital broadcaster can now air talk radio on one channel while music airs on another channel. This translates into greater opportunities for increased advertising revenue for the broadcaster, and more options for the listener. 

 

When digital radio was first launched in the United States, radio stations were required to broadcast via an IBOC (in-band on-channel) transmission in which digital and analogue signals shared the same channel and -20 dB power rating. The initial main method used for transmission, accomplished with a coupler, is called “high-level combining.” 

 

While high-level combining proved efficient in requiring less equipment to broadcast both analogue and digital content, it is not without problems. Due to the low power level, these -20 dB transmissions experience poor signal penetration in metropolitan areas where building interference is a concern.  

 

An additional limitation of high-level combining is its inherent power inefficiency. Because digital radio broadcasting requires significantly less power than analogue transmission, broadcasters must “dump” the excess digital power into a dummy load (90% of the digital signal and 10% of the analogue signal is wasted). The excess power generates significant amounts of heat, which must be compensated for with additional cooling (HVAC) systems at the transmission site. 

 

At the -20 dB power level, this amount of wasted power and added HVAC costs have been tolerable to broadcasters. However, after experiencing poor signal penetration in key metropolitan markets, digital radio broadcasters successfully petitioned the US government to expand the allowed power rating of digital signals to -14 to -10 db. Once signal levels were considered at -10 dB, an increase in signal by 10x, the high-level combining technique was no longer feasible due to the high HVAC operating costs and high energy waste. 

 

In addition, because the analogue and digital signals are broadcast at the exact same frequency, signal interference between the two has proven to be common. If the high-level combiner that is selected for this tandem transmission lacks sufficient isolation, the signal quality of both the digital and analogue stations significantly suffer. 

 

Addressing signal interference between the digital and analog signals is not a simple, nor inexpensive, problem for the broadcaster to overcome. When the signals interfere with each other to the point that neither station can be clearly received by its listeners, broadcasters are forced to purchase new antenna systems, broadcast from a new location lower on the tower, or even rent new space on an alternate tower. Until the problem is corrected, the broadcaster is likely to lose listeners, their ranking in the market, and — ultimately — advertising revenue. 

 

High-level combining is not the only available method for transmitting both digital and audio radio signals. Alternatively, many broadcasters have sought out “common amplification” transmission. In this method, combining of the signals is accomplished in the transmitter instead of the coupler. This has the benefit of having all of the transmission work being completed on the ground, with easy access for engineers. Common amplification, however, still generates substantial heat and power consumption (transmitters are inefficient in this mode, in some cases with a power loss of 50% or more). Additionally, it’s power limited, and fails to reach the high power levels often required for broadcasting.

 

A growing trend in HD radio broadcasting is to favour a third option, “space combining.” By focusing on maximising antenna isolation to as much as 40 dB, today’s most advanced antenna manufacturers have discovered new ways to combine both analogue and digital signals directly in the antenna. This reduces the risk of interference between analogue and digital signals, and supports power levels that do not require such extensive power dumping and cooling. While these interleaved and dual-input panel antennas often place more weight and wind load on a tower, thus potentially requiring an additional investment in tower reinforcement, these up-front costs are more than balanced out by reduced operating costs. Space combining has emerged as the most efficient way to get on the air with an IBOC system. 

 

While advancements are well under way in the infrastructure required to deliver digital radio, broadcasters still face challenges in gaining widespread adoption of this technology at the consumer level. Currently it’s estimated that in the United States, less than 20% of cars are equipped with HD radios. Because this is the environment in which digital radio is expected to be the most popular, broadcasters and HD radio manufacturers are lobbying the government to encourage more car manufacturers to incorporate digital radios as a standard part of their new vehicle designs. Other manufacturers are developing set-top boxes that receive digital radio signals as well. 

 

Perhaps a greater challenge for broadcasters is in determining the best way to solicit customers in new markets. Without a digital radio signal already broadcasting in an area, radio listeners find it difficult to evaluate and embrace this new technology. Broadcasters are forced to make a speculative investment in a new market, installing equipment to support digital broadcasting under the presumption that once prospective customers are able to hear a digital signal in technology stores, they’ll purchase HD radios and become loyal listeners. 

 

As it did in the US, HD radio worldwide must begin with governmental regulations on how to handle analogue and digital transmission frequencies. The challenges will be reduced if governments provide separate frequencies. But even in an IBOC broadcasting scenario as the US has experienced, technologies now exist that will make HD broadcasting a success. Testing and trials of these systems are already under way in many countries. 

 

While it will still be many years before the adoption of digital radio worldwide — and the widespread use of digital radio receivers by consumers is  realised —  by selecting space-combining interleaved or dual-input panel  antennas, broadcasters can take steps now to build an equipment infrastructure that will support even the “worst-case scenario” of shared-frequency IBOC broadcasting.