(click thumbnail)The processing paths of an AM station are shown on this screen shot of an Omnia5.EX.Cris Alexander is director of engineering for Crawford Broadcasting. This is one in a series of tips about HD Radio implementation. The series is archived atradioworld.com.
HD Radio audio processing has a lot in common with conventional broadcast air processing, but the HD path processing does depart considerably.
In an ideal world, the HD Radio audio path would not be processed at all. We would feed unprocessed audio and allow the listener to enjoy the full dynamic range and spectral balance of the source audio that the producer intended. But that’s not the world we live in.
Were we to do that, we would find that when our HD Radio receivers made the transition from analog to digital, the perceived audio level would drop and the digital audio would lack “punch.” Listeners would have to turn up the volume to hear in mobile or noisy listening environments. Competitors who do process their digital path audio would have an immediate leg up.
The world in which we live and operate requires us to process our HD Radio audio to some extent, and while we don’t need all the clipping that we use in our analog audio paths, we do need something. Processor manufacturers including Omnia and Orban have provided for this. For station engineers trying to make things sound right, we need to know a few things.
Adjust output level
FM stations generally have it easier. With FM HD Radio stations, it is possible to use most of the same audio processing chain used for the analog signal path for the HD Radio signal path. The station can use all the wideband AGC and multiband limiter/compressor stages in both paths.
Where we can depart is in the output section. In the analog world, we use clipping, composite or otherwise, to generate loudness and maintain absolute control over peak modulation. A byproduct of clipping is distortion. The final clipper setting is always a compromise between loudness and distortion.
In the FM HD Radio world, we do not have the requirement for absolute peak modulation control, and because we do not operate in real time, there is no reason why we can’t take advantage of that and “look ahead” a few cycles. We can make instantaneous advance adjustments in the output level to maintain loudness and level control.
By using common processing for both the FM and HD audio paths right up to the clipper and look-ahead limiter respectively, the station can maintain its signature sound on both paths. The only real processing challenge for the engineer, then, is to match the FM and HD-R levels so that the transition from analog to digital in receivers is seamless.
On the AM side, things get a bit more complicated. The AM and HD Radio audio paths are considerably different. We deliberately band limit the AM path to 5 kHz, but there is no such requirement for the HD-R path. Generally speaking, the AM band limiting in audio processors is smoother than the “brick wall” 5 kHz limiting in the Ibiquity Digital up-converter.
The limitations and aggressiveness of the AM HDC coder do require us to pay attention to what we feed it. Source material, particularly speech, with excessive highs will produce a lot of coding artifacts.
The best way to handle this is to eliminate unneeded high-frequency content prior to the HDC coder. Band-limiting talk format source material to 12 kHz in the processor is an excellent way to keep things clean while maintaining very pleasant HD Radio demodulated audio.
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