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The Truth about True Peak Limiting for Audio with Dr. Paul Tapper of @NugenAudio


The True Peak measurement is now part of the delivery specification of legislation such as the CALM Act, which has brought audio front and center in the spotlight of the video world, but what does this actually mean?

We’re all familiar with the idea of digital samples, digital audio, and digital peak level. But what is True-Peak?

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The smooth blue line in the graph represents the original analog audio signal – the compression in the airwaves. The dots represent digital audio samples. The green lines show the reading when we measure only how far away the samples are from 0dB – this is the digital peak level. If this was our only reference, we would think everything was fine – they’re below 0dBFS, perfectly legal. The problem is that the underlying analog signal that was represented by those samples is not OK. It goes over 0dBFS, and this is indicated by the red lines. You can see that the underlying signal has gone beyond the clip level. Why should you be concerned about that if the digital samples are OK?

If your audio is ever going to have a codec conversion, it is important that the True-Peak level is legal. If your audio is going to be broadcast, or get converted to AAC or MP3 or if its ever going to get streamed out through a streaming service, then there is going to be a codec conversion. Codec conversion makes use of the analog waveform between those digital samples. If you have a level that is beyond 0dBFS in the underlying analog wave, you will probably get clipping or fizzing or a ‘busyness’ in the sound, which would cause rejections from the broadcaster, or even if the content isn’t being checked for loudness compliance, it becomes fatiguing for the listener. To try and get around this problem, the True Peak level is now a fundamental part of international loudness delivery standards, such as the CALM Act.

The True Peak level is a measure of how far the analog signal gets, not just relying on the maximum level of the digital samples – but taking into account the maximum level of the underlying analog signal. It is ‘true’ in the sense that it is a measure of the underlying signal, not just the digital samples. There are a few things that you need to be aware of with True Peak. Certainly if you’re dealing with broadcast, the true peak will be measured, not just the peak level, and meeting exact targets is required. If you get a true peak clip, then that audio will be rejected, because it will cause problems further down stream.

If you’re using a limiter, the chances are that the more you push your audio into the limiter, more true peak overs will result. Normal limiters tend to push the audio out of the legal True-Peak limits. It is important that you meter the true peak level, not just the peak level. In the situation where you’re just relying on your ears in a monitoring environment, you are not necessarily going to hear whether there is a true peak clip, but if the audio then gets converted later on, to AAC or mp3 or some other compression format, then it is going to become a problem which you won’t be aware of just using your ears because you are not auditioning the compressed file. You need to use metering tools that provide you with information that your ears wouldn’t give you on their own. Or you could use a limiter that is designed to limit the true peak level, not just the digital peak level, such as NUGEN Audio’s ISL 2 True Peak Limiter.

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NUGEN Audio’s ISL 2 Inter Sample True Peak Limiter

Metering is a key aspect of how your eyes can help your ears – a numerical reading from the computer can be a valuable tool, so that there is not just a reliance on the subjective listening process, but there’s a bit of science that can assist you to avoid costly rejections, and ensure high quality audio all the way down the play-out chain. True Peak metering can provide important information about how your work is going to get played out, which you won’t be aware of just by listening to it.

New loudness regulations can actually be liberating for content creators, as they adapt to this new, changing competitive environment for the production industry as audio increasingly gets loudness normalized and regulated across different distribution platforms. There’s a change in what is going to work or what is going to sound good, and what we should be aiming for.

Personally, I believe that the loudness regulations imposed on broadcast content will eventually reach all parts of audio because the end result is that it improves things for the consumer. Despite additional complexities that are introduced for the production process, the competitive pressure to over compress is removed, and the end result is more freedom for the audio engineer. Audio that is played out in a loudness-normalized environment ends up just being better.

To learn more about ISL, or NUGEN Audio, please visit www.nugenaudio.com

This article is based on excerpts from a lecture by Dr. Paul Tapper, from the Los Angeles Audio Engineering Society Convention.

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