Re: Seattle Radio Happenings

Andy Brown

The big issue to get there is the interim period with hybrid operations, which require more bandwidth. Clearing out a lot of stations cluttering the dial has to be done in order to roll out hybrid before fully digital AM either with IBOC or DRM. Once you go fully digital across the entire band, more stations can be added back in since the bandwidth requirement is lessened by not having the old analog modulated carrier.

The other technical note worth mentioning is that antenna parameters are highly critical to performance. No high VSWR situations can be tolerated and common point impedances must be symmetrical on both sides of center (X Ohms at +j6 @ 5 kHz below carrier must equal X Ohms at –j6 @ 5 kHz above carrier).

Both systems (IBOC and DRM) are technologies that use orthogonal frequency-division multiplexing (OFDM) technology which are employing many small digital carriers in the frequency spectrum assigned.

As a result, the “sound” is almost as good as FM analog stereo in the hybrid modes.

“I wish we had a local station that was doing the testing. It would be interesting to hear the sound quality of a pure digital signal.”

Depends on what audio codecs they go with.

Both systems have bit rate limitations, of course, so bit reduced audio codecs are required. iBiquity had its codec developed by Coding Technologies (now Dolby) and calls it HDC. It is proprietary and firm details on it are not published. Some have called it similar to AAC+ with SBR. It has a dual mode capability when using the two AM modes (core and enhanced). DRM utilizes the AAC standard, so we believe the codecs are quite similar in efficiency and audio quality at a given bit rate although iBiquity claims they might have a slight edge at lower bit rates.

Also, we don’t really know how good it will be when other stations in analog and hybrid mode are still present on co channel and adjacent channels to a pure digital station. It will take years to get to a purely digital dial, but it will take until then to really know how much improved it is.

The bean counters will love it, though, because you won’t need anywhere near as much power to get the same coverage.