Automatic Loudness Control

ALC6 Original

Years of research and standardization work on loudness and true-peak level has enabled TC to design high-resolution, low-latency loudness measurement and control equipment such as this Automatic Loudness Correction processor, ALC6.

In broadcast, digitization is driving the number of AV channels and platforms up, while the total number of viewers remains roughly the same.

Using only a dialog-based level control principle has led to ambiguous level management, more level jumps between programs, and extra time spent on audio production and management in general.

Non-dialog based level jumps are currently creating havoc in digital TV; and ALC6 helps correct that situation. For instance by utilizing the adaptive Stridency Reduction parameter that brings overly loud content under tight control, while leaving normal content untouched.


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Downmix Compliance


Measuring the loudness of a 5.1 program can give significantly different results from measuring a stereo or a mono downmix of the same program. Furthermore, these differences are systematic because measuring loudness according to BS.1770 is insensitive to inter-channel phase differences, while a downmix is not (see note 2). Tight tolerances on program loudness therefore presents a dilemma when it comes to mono or stereo listeners, generally the vast majority of a station's audience. 

ALC6 introduces a new way of dealing with this issue: A 5.1 program may be adjusted based on what it measures in 5.1, or on what it measures using a specific set of downmix coefficients. 

ALC6 also helps getting a higher audio quality and more headroom out of AC3 data reduction. The downmix part of the decoder is prone to overload, especially when AC3 DRC is set to Off. On the other hand, enabling DRC inserts a crude processor which nobody can get rid of again. Use the new "AC3 Codec" setting with ALC6's true-peak limiter to prevent downmix clipping while preserving headroom, thereby making it safe to switch off DRC and make AC3 more transparent.


Target 24

Broadcasters around the world are going through a transition towards loudness based production and program normalization. In order to comply with legislation such as the CALM Act, as a quick fix, it's tempting to insert "sausage processing" at the point of transmission. However, sausage processing is completely against the spirit and the purpose of new American, Brazilian, Chinese, European and Japanese audio standards - and also against the philosophy of TC.

With ALC6, it's no longer a choice between plague and cholera. Processing at a higher resolution than any other audio device or codec in the broadcast signalpath, DB4 or DB8 will never be your quality bottleneck.

ALC6's adaptive Stridency Reduction parameter brings overly loud content under tight control while leaving normal content untouched. Another setting allows the operator to indicate whether a program is already normalized, not normalized or sometimes normalized. If content is 5.1, ALC6 may automatically dissolve into bypass, or you may set a different Target level depending on the format. ALC6 is transparent in several senses of the word.

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Loudness Website

Keeping track of the multitude of issues around loudness can be a challenge. That's why we've created a one-stop dedicated loudness website where you'll find critical information on the most important issues.

The site is an answer to the highly relevant question:
"What Is Loudness, and Why Is It Important?"

Visit the Loudness Website

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In the Press

There's a reason that world-leading broadcast magazine regularly invite us to share our knowledge: we've been pioneers in the digital audio world since 1985.

We've taken part in research studies and the definition of international broadcast standards right from the start, donating technology which has become a crucial part in new, open international standards.


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