HD Radio - Overview - FM - Bandwidth and Power

Bandwidth and Power

Currently, FM stations in the United States and Canada are licensed to carry 100 kilohertz of bandwidth, requiring approximately 200 kilohertz of spectrum. Only 15 kHz of the modulation bandwidth is used by analog monaural audio (baseband), with the remainder used for stereo, RBDS, paging, radio reading service, rental to other customers, or as a transmitter/studio link for in-house telemetry.

In regular hybrid mode a station has its full ±100 kHz of analog bandwidth and adds an extra ±30 kHz guard band and ±70 kHz for its digital signals, thus taking a full 400 kHz of width. FM stations also have the option to discontinue existing subcarrier services (usually at 92 kHz and 67 kHz) in order to carry extended HD Radio, though such services can be restored through use of the digital subchannels that are then made available. However, this requires the replacement of all related equipment both for the broadcasters and all of the receivers that use the discontinued services.

The ratio of power of the analog signal to the digital signal is standardized at 100:1 (−20 dBc), making the digital signal 1% of the analog carrier power. Unlike with subcarriers, where the total baseband modulation is reduced, there is no reduction to 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% 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 a dramatic improvement in digital coverage. Other levels have also been tested, including a 6dB or fourfold increase to 4% (−14dBc or 25:1). National Public Radio is opposed to any increase because it is likely to increase interference to their member stations, particularly to their broadcast translators which are secondary and therefore left unprotected from such interference. Other broadcasters are also opposed (or indifferent), as increasing power would require expensive changes in equipment for many, and the already-expensive system has so far given them no benefit.

There are still some concerns that HD Radio on FM will increase interference between different stations even though HD Radio at the 10% power level fits within the FCC spectral mask. North American FM channels are spaced 200 kHz apart. An HD Radio station will not generally cause interference to any analog station within its 1 mV/m service contour, the limit above which the FCC protects most stations. However, the IBOC signal resides within the analog signal of the first-adjacent station. With the proposed power increase of 10dB, the potential exists to cause the degradation of the second-adjacent analog signals within its 1 mV/m service contour.

On January 29, 2010, the U.S. FCC approved a report and order to voluntarily increase the maximum digital effective radiated power (ERP) to 4% of analog ERP (−14dBc), up from the previous maximum of 1% (−20dBc). Individual stations may apply for up to 10% (−10dBc) if they can prove it will not cause destructive interference to any other station. If at least six verified complaints of ongoing RF interference to another station come from locations within the other station's service contour, the interfering station will be required to reduce to the next level down of 4%, 2% (−17dB), or 1%, until the FCC makes a final determination. The station to which the interference is caused bears the burden of proof and its associated expenses, rather than the station that causes the problem. For grandfathered FM stations which are allowed to remain over the limit for their class, these numbers are relative to that lower limit rather than their actual power.

Read more about this topic:  HD Radio, Overview, FM

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