How to process audio for DAB+

DAB+ continues to expand its footprint. With more than 25 countries benefiting from regular service and over 25 countries currently running trials on DAB+ (source: WorldDAB), it’s becoming increasingly important to adapt radio operations to the current broadcasting landscape.

Many FM programs are now available via DAB+ services, while new DAB+ programs are also being launched. So why is adequate audio processing important for DAB+ transmissions?

The goal is not to be the loudest, but to ensure your sound is clear and not distorted.

First, there are some general reasons why audio processing is necessary, also in DAB+:

• Compression of the dynamic range to make it easier for the audience to listen to the program in noisy environments, for example in cars. In other situations, e.g., at home, an expansive acoustic dynamic range might not be desirable to avoid disturbing family and neighbors with excessive sound levels.

• Consistent presentation: The program material from different producers can vary quite heavily. Most radio broadcasters agree that an overall consistency of sound texture and spectral balance contributes to a successful sonic image. Multiband compression can achieve this goal. By setting a target spectral balance and automatically re-equalizing program material that does not have this balance, the multiband compression helps the radio station achieve the desired sound.

• Increasing the relative loudness of the signal by comparison to an unprocessed signal normalized to the same peak level. This can be achieved by reducing the peak-to-average ratio of the signal. If several radio programs share the same multiplex ensemble and have the same reach, one of the few things that the broadcaster can do to stand out from neighbors/competitors is to broadcast a louder and punchier audio signal. Experience has shown that a combination of multiband compression and sophisticated peak limiting is the most effective way to do this. Of course, the goal is not to be the loudest, but to ensure the sound is clear and not distorted.

• Improving the speech intelligibility of program material, e.g., incoming telephone calls. Properly designed multiband compression like that used in Orban’s Optimods can make improvements without a need for preprocessing in a production studio.

Main Difference

In addition to the above general reasons, there are some specific aspects to consider as well. The main difference between processing for analog and digital radio is peak limiter technology. It’s important to minimize audible peak-limiter-induced distortion when using a low bitrate codec.

The solution for this is to implement look-ahead limiters instead of hard clippers. A look-ahead limiter’s gain control signal has a much lower bandwidth than that of a clipper and produces modulation sidebands that are less likely to be audible. Furthermore, pre-emphasis/de-emphasis limiters and/or clipping peak limiters which are used in conventional AM, FM, or TV audio processors do not work well with the HE-AACv2 audio codec that is used in DAB+. The pre-emphasis/de-emphasis limiting unnecessarily limits the high frequency headroom.

Orban has integrated look-ahead limiting technology in the Optimod 6300 audio processor for the special purpose of processing DAB+ programs. The company also implements it in the digital processing chain of its 8700i, 8700i LT, 8600Si and 5700i audio processors, allowing these products to be used for FM and DAB+ simultaneously. Of course, pre-emphasis/de-emphasis limiting is only used in the FM transmission chain. Also, it’s possible to adjust output levels separately for the FM and the DAB+/streaming path.

Orban has also integrated its PreCode technology into the Optimod 6300. This impacts several aspects of the audio to minimize artifacts caused by low bitrate codecs, ensuring consistent loudness and texture from one source to the next. PreCode includes special audio band detection algorithms that are energy- and spectrum-aware.

This can improve codec performance on some codecs by reducing audio processing induced codec artifacts, even with program material preprocessed by other processors.

The author is Senior Vice President Global Sales of Orban Labs Inc. and Orban Europe GmbH.