Applied Technology: Continuous Loudness Control

The State-of-the-Art in loudness and dynamic control for live applications has brought ground-breaking transitions to pro-audio technology. A main driver of these changes has been the paradigm shift from voltage-based peak meters toward a performance metric: the introduction of loudness metering.

Of course, the development of this new technology won’t stop the need for audio processors. Up to now, the operation of traditional systems such as single-band or multi-band compressors or limiters has been based on the peak-limiting requirements. Today, however, the audio landscape all over the world increasingly demands devices for loudness-optimized processing in a way that considers the technical limits of the transmission system at hand—think of TruePeak, for example.

At the same time, there is a requirement for supplying audio and video through increasingly more distribution paths with sometimes highly diverging channels. As an example, let’s compare the linear ultra-dynamic audio on a Blu-ray disc to the brickwall-limited data-compressed offerings for mobile devices.

Consequently, a processing system must be capable of adjusting the program material at hand to a loudness target inaudibly (i.e. without any artifacts) while limiting the signal dynamics and preserving the electrical properties of the transmission medium. If the audio to be processed is a file, modern processing techniques allow for achieving this with varying degrees of quality.

But what about live material? Is it possible at all to adjust the loudness or dynamics of program material to a target value when the future progression of the material is entirely unknown at the time of adjustment?

This seemingly insurmountable challenge has now been addressed by the Institut für Rundfunktechnik (IRT) and RTW from Cologne. Continuous Loudness Control (CLC) is the latest addition to RTW’s Masterclass PlugIns range and makes the patented loudness-correction algorithm that was developed and introduced by the IRT in 2014 available as a plug-in and a stand-alone application. The unique low-latency look-ahead algorithm implemented in CLC allows for adjusting the program material to a target loudness value, as well as to an adjustable TruePeak value, with or without correcting the original loudness range. CLC is capable not only of applying loudness and dynamic compression using a single adjustment factor, but also dynamically processing those parameters. This allows for effective handling of loudness variations in audio without “brickwalling” the material. Obviously, the hardest part is processing the signal during the first seconds or minutes of content because there is no other reference data but the current values to be used for projecting future signal behavior. In this case, the control algorithm uses historical data for the selected genre that are based on the analyzed behavior of several hundred pieces of content. During playback, this data is continuously matched against the real statistical data to update the control process at all times.

While CLC was developed with live-signal handling in mind, its algorithms are perfectly suitable for processing pre-produced material, which often includes unwanted loudness changes resulting from putting together audio from various sources.

Operator display for the RTW Continuous Loudness Control operating in the playback mode.

Operator display for the RTW Continuous Loudness Control operating in the playback mode.

What Makes CLC Real-time Processing Stand Out?

When comparing the different ways to process audio for loudness, one can see why CLC stands out. File-based loudness normalization is a technique that hardly affects audio quality, and there are hardly any differences between results produced by existing tools. This technique is different than loudness processing in real-time plus dynamics adjustment, which is a processing technique subject to considerable differences in quality.

There is no standard approach—all vendors use proprietary techniques leading to highly diverging results. CLC is the most complex real-time process used for loudness and loudness-range correction or for adjusting the dynamic behavior of the signal, respectively. The regulation process is the distinguishing feature. For example, adjustments triggered by abrupt volume increases (e.g. when suddenly the audience starts applauding) are hardly audible.

By continuously capturing the loudness and dynamics of the audio, the CLC algorithm “learns” about the natural dynamics of the material. Traditional systems perform loudness normalization using data that was generated from short time windows (e.g. 10 seconds); however, if a significant deviation from the target value (such as the above-mentioned applause) occurs during that window, those systems will take what they consider appropriate countermeasures. Thus, signal dynamics are falsely detected as loudness issues. In contrast, the CLC algorithm knows the real signal dynamics from measuring the loudness range (LRA) and other parameters and will therefore consider a loudness increase over a specific period of time (e.g. caused by applause) a natural signal feature—and thus not correct it. This way, the natural dynamics and the intention of a program as designed by the audio engineer are optimally preserved. However, to compress the perceived dynamics of a signal on purpose, you may lower the LRA value to the desired upper target limit or set static dynamic reduction. The CLC algorithm performs the LRA correction at almost inaudible speed, thus allowing for effectively limiting abrupt loudness changes (for example, when the program switches from a commercial break to a movie).

The almost perfect absence of artifacts is achieved mainly by performing dynamics compression only when abrupt changes in dynamics occur. Low-dynamics passages are sent through unaltered. With conventional compression at a fixed ratio, the overall dynamics of the audio is unnecessarily affected. 

Control parameter panel

Control parameter panel

When developing CLC, another focus was on user-friendliness. The operator just needs to configure the target loudness and upper limits for LRA and TruePeak—and the system will perform loudness normalization of standard audio signals at an outstanding quality. The processing of highly sensitive signals with ultralow volumes or drastic peaks can still be optimized by fine-tuning other parameters. For that purpose, the software provides easy-to-use operating modes suitable for many applications.

CLC is the only real-time solution currently known that performs time-accurate loudness and loudness-range processing, meaning that the appropriate dynamic processing is performed in exactly the point in time where the relevant signal change occurs.

Continuous Loudness Control display surface

Continuous Loudness Control display surface

Use cases for the CLC options range from DAW plug-in processing to fully automated processes to standalone hardware-based solutions in broadcasting and distribution centers.

With the development of the innovative Continuous Loudness Control process RTW and IRT created a system that offers a giant leap forward to better sound quality for live transmission and all other applications in the audio industry.

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