True Peak Limiting: Accuracy and Impact on Digital Audio

True peak limiters offer greater accuracy by minimizing underreads from 3 to 0.668 decibels, improving gain reduction and reducing aliasing distortion.

### True Peak Limiting: The Key to Accurate Signal Measurement in Digital Audio
* 🔊 True Peak limiters interpret the signal by measuring the level of a signal from the PCM encoded sample Peaks, but at conventional sampling rates, there can be up to **three decibels of difference** between the discrete PCM sample values and the resulting waveform, leading to **underreads**.
* 📏 To solve this issue, true Peak limiters emulate the D to A converter and measure the output digitally by **interpolating new sample values in between the present ones**, aiming to achieve the same physical behavior as analog true Peak limiters.
* 🎯 The ITU-R BS.1770.4 standard recommends **four times oversampling** to minimize the underread down to **0.668 decibels**, while FabFilter Pro-L 2's true Peak limiter uses **eight times oversampling** to achieve **0.169 decibels** of underread.
* A null test conducted by using FabFilter Pro-L 2's true Peak limiter and Pro-L 2 without oversampling showed that the **true Peak limiter recognized sample points accurately**, while the **normal limiter did not until oversampling was turned on**.
* 🎚 In practice, true Peak limiting doesn't have a sound, but due to its **greater accuracy in measuring the signal**, it can lead to a **considerable amount more gain reduction** compared to native PCM encoder limiting.
* 🔮 When using true Peak limiting but aiming for the same amount of gain reduction as native limiting, it's recommended to **dial back the gain or threshold of the limiter by around two to two and a half decibels**.
* 🔄 The best practice is to start by **oversampling your limiter at four or eight times**, as it not only minimizes underreads and overshoots but also helps the limiter function with **greater accuracy and reduced aliasing distortion**.

I have to admit, I've been wrong about something. Many believe that true peak limiting sounds worse. The matter of the fact is we've been using it wrong. And on my last video, I had multiple people ask if I could debunk or look into true peak limiters. I had a theory that I'd present to mastering engineers. I knew a bunch of blind A/B tests. I'd plot the results and then I'd call it a day. But that didn't happen because I reached out to Fabian from TDR Labs. And he's an incredible developer and has a great knowledge of DSP. He explained to me everything he knew about true peak limiting. And that put me down a rabbit hole. A rabbit hole of research, learning sync wave functions, Whitaker Shannon interpolation formula, having a friend compile some code for some tests I needed to do, get a confirmation from iZotope's engineering team on their true peak limiter, and putting this video together. So stick with me. This is a meaty video filled with my research, interpretation, tests, and recommendations.

I invite anyone that has information on the topic to share so in the comments. Challenge my ideas, and we can learn and be better Engineers together. These are videos which I get excited to make, and if you get excited to watch them, don't forget to smash the like button and subscribe for more. So all limiters measure their Max Peak and reduce gain relative to it. The difference is in how various limiters interpret that signal. Interpreting the signal now, this is what differentiates one limiter from the next. An analog limiter measures the level of a signal, which is a continuous waveform, so it's always got a true reading on it, whereas digital limiters measure the level of a signal from the PCM encoded data. Pulse code modulation is the method used to digitally represent sampled analog signals. Now you might be familiar with looking something like this. However, each of these samples don't represent an instant moment in time. In fact, each of these samples are a scaled sync wave which extends in theory infinitely wide both past and present. The resulting waveform we see in our Daw.

from a stream of PCM samples is a sum of these scaled sink waves, and the sum of the sync wave values represented as a waveform. Which we digitally see in our Daw can produce over and undershoot below and above the given peak of the given sample value. Now this presents a significant issue with the accuracy of digital limiters. At most conventional sampling rates 44.1 or 48k, there can be up to three decibels of difference between the discrete PCM sample values and the resulting waveform. Which means there's often audio passing through the threshold of a digital limiter without a corresponding PCM sample point for it to react to. Now this is an issue, and in fact a major flaw in the function of normal digital limiters. So how does a true Peak limiter seek to solve this? To achieve the same physical Behavior as analog true Peak digital limiters, emulate the D to a converter and measure its output digitally. Speaking one way to emulate the d2a conversion process is to interpolate more discrete values in between the

present ones so the limiter can measure the level of a signal from a more accurate sample. Point itu rbs1770.4 lays out a consideration for accurate Peak metering of digital signals. Now, with the standard of four times oversampling, this minimizes a sample piece underread from three decibels down to 0.668 decibels. And at eight times oversampling, this minimizes a sample peak's underread down to 0.169, which is the same standard used for fabfilter pro l2's true Peak limiter. I found this article published in 2015 by Isotope's principal software engineer, where he lays out some code in Python to compile this sync wave offset a fraction of a sample. The sample Point has been offset by 3 3 8 of a sample or 0.375. Now, I actually failed to compile this code, so I reached out to a few friends within the programming space, and one of them was able to compile the code and produce me with the necessary PCM encoded sync.

wave for my tests which is this one here. So a big shout out to Kevin, thanks for that. The test is simple. Given that we've now established that limitus function by how they interpret the signal, a digital limiter interprets that signal by the PCM encoded sample Peaks. PCM encoded data can under read up to three decibels at 48 kilohertz. By interpolating new sample values, we minimize this under read within 0.668 decibels and 0.169 decibels using four and eight times oversampling respectively, as calculated and presented in the itu r documentation. So can I get fabfilter Pro L2 true peak limiter to null with fabfilter Pro 2 using eight times oversampling when setting the threshold to the PCM encoded sample peak and in a second test matching the threshold to the reconstructed or interpolated true peak. Now with this shifted sync wave, we have a sample point at negative 8.13. So here I have a true peak limiter set to 8.13 and here I have

a normal limiter set to 8.13 fan filter Pro 2. Interpolates the data at eight times oversampling. This one does not. So what we're going to do is we're going to play this back in a loop. And then what I'm going to do is in the middle of that Loop, I'm going to switch eight times oversampling on here and null it. And you'll see that it almost nulls absolutely perfectly well below the noise floor. So let's do that now. Okay, so one thing we notice is the true peak limiter is definitely doing some limiting. The normal one isn't recognizing that sample point. But what happens when we turn oversampling on? Ah, we've reconstructed sample points or interpolated sample points in between the existing ones. And it's now recognizing that it has to be limiting. It's actually missing that information. So take a look at this null test. When I, so it's about negative 140 decibels is the difference indistinguishable to the naked ear if I turn this off.

look how much higher it is up eight negative 80 negative 75. It's absolutely doing it's actually doing nothing there so again. I'll put the oversampling on completely nulls. So that's me doing a test there between the over sampling here and eight times. I mean the true Peak limiting here and over sampling here. So what happens if we set this to 6.37. Okay and then we're going to interpolate this data by up sampling it because what's actually going to happen is the other limiter won't pick up anything at all. So if we resample this we're going to have new data points up here and one of those data points just happens to be at 6.1 which is perfect because it's just right over that little threshold here. The way it's interpolated that now look at this so this one here is set to 8.1 8.6.37 sorry set to 6.37 a true Peak there and this one here we'll bypass that we'll turn the

oversampling off, so we can hear what a normal one and a true peak one is doing. Not here, but analyze what it's doing, and then we can put the oversampling on, and we'll get a new data point to work from. So here again, because there's much less gain reduction, they almost null anyway, but the whole idea of this test and exercise is to show that the oversampling then allows this limiter to start working and recognizing those sample points above it. The null becomes much more like it's almost nothing there. So yeah, so what does this mean in practice? Well, true peak limiting doesn't have a sound. The gain reduction algorithms look ahead, attack, release, and ceiling all theoretically should function in the same manner. Simply put, it measures a signal with greater accuracy, and when that accuracy reduces underreads from three decibels to 0.668 decibels at four times oversampling, that can create a considerable amount more of gain reduction that wasn't present in the signal before. Now, if you choose to turn true peak limiting on, but

want the same amount of game reduction dial back the gain or threshold of your limit up around two to two and a half decibels. Then you'll have a similar ballpark amount of game reduction as the native PCM encoder limiting. But the best bet is to throw the voodoo of true Peak limiting out the window and just start by oversampling your limiter at four or eight times. Not only because it minimizes underreads and overshoots, but it helps the limiter function with greater accuracy and reduced aliasing distortion. Consider especially the aliasing distortion. Now everything I've spoken about is pretty fresh information to me, and I'm going to do my best to answer all the comments as best as I can in the comment section below. So please feel free to share with me your experiences with true Peak limiters and any knowledge you have on the technology below.

Is True Peak Limiting a Lie? What You Need to Know!

Hello. I'm Nicholas Di Lorenzo. I'm a mixing and mastering engineer and owner of Panorama Mastering Studio in Melbourne, Australia. I’m an Italian-Australian. born and raised in Melbourne.

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The True Peak Limiting Debate: A Journey of Discovery

Many people have misunderstood true peak limiting, but a developer's explanation led to in-depth research and testing.

True peak limiters interpret the signal differently, with analog limiters measuring the level of a continuous waveform, and digital limiters measuring the level from PCM encoded data.

The video is a comprehensive exploration of the topic, inviting discussion and challenging ideas to improve engineering practices.

The Challenge of Digital Limiters in PCM Sample Waveforms

PCM sample values can lead to over and undershoot in waveforms, causing accuracy issues for digital limiters.

True Peak limiters aim to solve this by emulating the D to A conversion process and using oversampling to minimize underreading of sample peaks.

A standard of four times oversampling can reduce underreading to 0.

668 decibels, while eight times oversampling can minimize it to 0.

169 decibels, as seen in the fabfilter pro l2 limiter.

Isotope's principal software engineer published an article in 2015, providing Python code to offset the sample point and compile the sync wave, ultimately addressing the challenge in digital limiters' function.

The Impact of Oversampling on True Peak Limiting

Oversampling helps minimize under reading of PCM encoded data, reducing it to 0.

Fabfilter Pro L2 true peak limiter with eight times oversampling effectively nulls the signal, while the normal limiter struggles to recognize and limit the sample points, showing the significant impact of oversampling on true peak limiting.

Oversampling can significantly impact true peak limiting, allowing the limiter to recognize sample points above it more accurately.

True peak limiting doesn't necessarily affect the sound, but it does increase gain reduction with greater accuracy in signal measurement.

The use of oversampling can create a considerable amount more gain reduction in the signal.

Adjusting Game Reduction and Oversampling for Better Limiting

Increase the limit up by two to two and a half decibels to reduce game, similar to the native PCM encoder.

Oversample the limiter at four or eight times to minimize underreads, overshoots, and aliasing distortion, improving accuracy.

Share your experiences and knowledge of true Peak limiters in the comments below.