ISP - Intersample Peaks

What are Intersample Peaks and how do you control them.

Gain Staging


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Intersample peaks can play havoc with your mix, tripping compressors and limiters and causing all manner of gain structuring problems.

Before we jump in to how we detect and manage them let us take a quick look at Headroom as ISPs can affect the afforded headroom dramatically.

Headroom

Headroom is the difference between an audio tracks peak level (when the meter is displaying its highest value) and 0 level (ceiling) on the output meter. Let me give you a simple analogy I always use to explain headroom to my students. You are 6 feet tall and you enter a room that is 10 feet high. You have 4 feet of headroom. I know it’s simplistic but it works for me.

Now that we know what headroom is you can see how errant spikes or peak transients can eat into the valuable headroom. At least with these types of peaks we can detect them. ISPs are far harder to detect as they can be single events that sit between samples and sub samples or errant spikes that appear in random places depending on the process used.

Peak transients

Peak Transients are simply the loudest (highest) peaks that you see when you view an audio waveform magnified.

Intersample Peaks – ISP

Inter Sample Peaks can also be grouped with this fine set of problematic transients but we have ways to catch and manage ISPs. A simple yet effective solution is to set your limiter’s output ceiling to -0.3. That usually ensures that all ISPS are caught in time.

Now that we have looked briefly at headroom and why we need it we need to explore one problem that arises in almost all projects: that of Inter-Sample Peaks (ISP). Although these errant spikes are barely visible or audible in a mix content they will have a huge impact on the ensuing dynamic processes, a good example being ISPs tripping compressors and limiters.

Basically, and this really is keeping it simple, the DA (digital to analogue) process will use a reconstruction filter to recreate the analogue equivalent of the digital waveform and this can result in some audio waveforms reaching peaks above 0dBFS in-between the sampled values. This means, for example, that although all the sample values are limited to a maximum of 0dBFS in the digital domain, the reconstructed waveform may have sections that will exceed this. The reconstructed waveform will have what we term as Inter-Sample Overs. Is this a huge problem and if so are there workarounds? If I had to be honest almost all ISP problems can be negated with sensible headroom margins, but in the instances of wanting your mix to hit ‘hot’ some of these errant spikes will cause problems. I tend to always check the ISPs using either a True Peak Meter or a plugin that will display ISPs like FabFilter’s Pro-L (which I will be using for the ISP video). Most DAW meters are PPM (Peak Programme Meters) and they are not known for capturing fast or errant transients. To punish them we call them Quasi Peak Meters. There are two specifications that govern this type of meter, Type 1 and Type 2. Because of their integration times neither Type will be adequate enough to catch and display fast, errant peaks. Use a good true peak meter to detect ISPs.

Once we have detected the ISPs we need to find a way to manage them and it is here that limiters shine through.

Limiter

A limiter keeps signal peaks from exceeding a pre determined level.

While a compressor reduces the overall dynamic range, a limiter affects only the highest peaks. Limiters have very fast attack times, very high compression ratios and a high threshold.
You can turn your compressor into a limiter by using a very high threshold and ratio.

The ‘classic’ definition is that a limiter ‘flattens’ all peaks above a certain level, but leaves lower-level sounds intact.

Limiters are excellent tools for capturing and taming errant spikes and ISPs.

In the video I explain what intersample peaks are how to detect them. I use different software specifically designed to detect and capture ISPs and I run through how to use these devices in full. Finally, I show you an industry technique of using a limiter to capture and attenuate ISPs. I explain all the processes from capture to treatment in detail.

Plugins used in this video:

SSL X-ISM

FabFilter Pro L

Stillwell Audio Bitter Meter

Topics covered in this video are:

  • What are Intersample Peaks
  • How is the audio signal processed
  • Using Limiters
  • Controlling ISP Margins
  • X-ISM and Control
  • Metering and Headroom
  • Clipping