Commentary: Full Digital Is the Way to Go
Full Digital Modes of HD Radio, DRM Are Good Neighbors for First-Adjacents
by Edward "Ted" Schober
There has been much teeth gnashing over IBOC and adjacent-channel interference. I believe everyone has completely missed the point. The hybrid IBOC system is a nasty compromise designed to bridge the gap from analog to digital and was never intended as a permanent solution.
Full digital medium-wave broadcasting, without hybrid analog compatibility, whether HD Radio or Digital Radio Mondiale, fits easily in the +/- 5 kHz channel we have in the United States, and also can fit in the +/- 4.5 kHz European channel. Only the hybrid system is troublesome; the full digital modes of HD Radio and all modes of Digital Radio Mondiale are excellent neighbors to first-adjacent-channel stations.
Adjacent-channel interference between non-hybrid digital stations is negligible, and between a non-hybrid digital station and an analog AM station is low - lower than analog-to-analog. Interference from IBOC hybrid digital signals results from the fact that you cannot squeeze the digital carriers under the analog AM signal; you have to put them on your neighbor's turf.
The bottom line is the transition to non-hybrid digital must be accomplished in the shortest time possible. Hybrid digital operation is only a stopgap measure, with no improvement in coverage and troublesome adjacent-channel interference.
I propose we overcome the poor interference performance of the hybrid system by leapfrogging to full digital medium-wave broadcasting.
There are two strategies that can make this come to pass.
Road to conversion
The first strategy requires that radio broadcasters look further than the next rating period. Economically, it is reasonable that large group owners with multiple AM stations in a market would duplicate their top-performing AM station on their weakest station in non-hybrid digital. The digital-only coverage of a station is easily twice the service area of an analog AM station at the same average power. This means the group operator will have good digital and a good analog signal in the market while eliminating much of the operating expense of the "dog."
As digital receivers enter the market, it becomes economically viable to convert the next-poorest performer to digital and put on a new format, or duplicate the next-best analog AM station. Eventually, and the sooner the better, there will be no reason to broadcast in analog AM, and the broadcaster will have its channels back in glorious digital with giant service areas and crisp noise-free audio.
Governmental policy decisions could dramatically hasten the conversion to all-digital medium-wave broadcasting. If the FCC ruled that no new analog stations would be authorized in the medium-wave band, all-new authorizations would be for digital-only stations. Opening an auction window for digital-only stations, especially if it included the expanded band, would provide the opportunity for many new digital stations to drive the manufacture and sale of digital medium-wave radios.
The FCC also could require that long-form applications for new stations from the recent auction window specify digital-only operation, or as an alternative give a large bidding credit for those applicants who specify they will operate digital-only. The implementation of 500 to 1,000 new digital-only medium-wave stations would generate demand for digital receivers. The FCC also should waive regulatory fees for five years for stations that converted to digital-only operation.
A question we all have to face: How long can AM analog radio survive the competition of satellite radio and improved FM service? Conversion to all-digital will eliminate the "poor relative" status of analog AM and foster the return of medium-wave broadcasting to its halcyon days. It will provide a noise-free service of slightly better audio quality than regular FM.
Full digital medium-wave promises a larger service area and far better consistency in mountainous terrain and urban canyons than either FM or IBOC VHF broadcasting. Why do we want to hang on to a sound that was barely okay in the 1920s, when medium-wave digital service promises great sound and vastly improved coverage?
How fast can we make the transition? AM broadcasters are now in the position of the FM broadcaster in 1964. The FM broadcasters were bold and installed stereo generators and formed the NAFMB to promote their case. Look what happened. Each group AM broadcaster should be bold and convert one station in each market now, and promote the new service widely.
If we implement only the half-measure of hybrid IBOC, we will eventually lose much of the present audience to the newly improved FM and satellite services. Why will consumers buy medium-wave digital radios? They will if there is something new to hear, or if the signal doesn't drop out in the urban canyons and in the mountains.
IBOC further despoils the quality of AM analog radio with interference while it fails to encourage the manufacture and sales of digital medium-wave radios. The switch to full digital is the way to go.
Ted Schober, PE is a consulting engineer for Radiotechniques Engineering LLC in Haddon Heights, N.J.
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