Oversampling Table of Contents
Summary
The controversy about whether people can tell the difference between audio recorded at 96 kHz, but played back at 44.1 kHz or 48 kHz instead of 96 kHz, hasn’t been conclusively resolved. However, increasing a project’s sample rate can improve the audio quality of some sounds generated “in the box.” This is because the higher sample rate eliminates the potential for foldover distortion caused by harmonically rich synthesized waveforms, saturation that creates strong harmonics, and even dynamics processors and transients with ultra-fast attack times.
Is Oversampling fact, or fiction?
The controversy about whether people can tell the difference between audio recorded at 96 kHz, but played back at 44.1 kHz or 48 kHz instead of 96 kHz, hasn’t been conclusively resolved. However, increasing a project’s sample rate can improve the audio quality of some sounds generated “in the box.” This is because the higher sample rate eliminates the potential for foldover distortion caused by harmonically rich synthesized waveforms, saturation that creates strong harmonics, and even dynamics processors and transients with ultra-fast attack times.
However, projects with high sample rates can stress out your computer more, require more memory, and may not allow you to run as many plug-ins. Some older plug-ins may not even work properly at higher sample rates.
Fortunately, there’s a novel solution—and at the end of this post, you’ll find out how to obtain the sound quality benefits of working at 96 or 192 kHz, even in 44.1 and 48 kHz projects (oversampling). Really!
Oversampling: The Origin Story of Foldover Distortion
A digital system can accurately represent audio at frequencies lower than half the sampling rate (e.g., 24 kHz in a 48 kHz project). This frequency is the Nyquist limit. Harmonic content above this limit—e.g., at 42 kHz—can’t be reproduced properly, resulting in foldover distortion (a type of aliasing) when this tone “folds down” below the Nyquist limit and into the audio range.
An analogy is how in movies, spinning wheels with spokes can look like they’re rotating backward. This is because the frame rate is too slow to capture when the wheel completes a full rotation, so the spoke looks like it’s at an earlier position compared to the spoke in a previous frame.
High sample rates reduce the possibility of foldover distortion. In digital audio’s early days, foldover distortion was quite common. Today’s virtual instruments or amp sims often don’t have as serious a problem for any of four reasons:
- The audio isn’t harmonically rich enough to cause foldover distortion.
- The plug-in itself has internal This runs the plug-in at a sample rate higher than the project’s sample rate. As far as the plug-in is concerned, the project is running at a higher sample rate. Any foldover distortion occurs outside the audio range.
- The project sample rate is high enough to provide the same benefits as oversampling.
- The plug-in designers have built appropriate anti-alias filtering into the plug-ins themselves.
Many modern virtual instruments and amp sims have an oversampling option, and DAWs can handle high sample rates. However, oversampling or raising a project’s sample rate both have limitations. Increasing sample rates requires more CPU power. So, you may not be able to run as many instances of instruments that oversample internally, or latency might become excessive. Also, a plug-in’s sample-rate conversion algorithm that operates in real time might be more constrained than what’s available with a DAW’s offline processing.
The Oversampling Solution
Suppose you’re recording a project at 44.1 or 48 kHz, and a virtual instrument or plug-in (e.g., an amp sim or saturation) produces foldover distortion. If your DAW allows changing sample rates in the midst of a project, then you can:
- Change the project to a higher sample rate.
- Render (bounce) the virtual instrument or track with the plug-in.
- Return to the original sample rate.
It seems counter-intuitive that the aliasing would be gone after returning to the lower sample rate. But remember, it was rendered as audio. When audio plays back at the lower sample rate, it no longer contains the frequencies generated in the box that caused aliasing. So, you have the benefits of recording projects at a lower sample rate, with the fidelity of a higher sample rate.
Check out the oversampling audio examples. Normally, the difference oversampling makes isn’t as dramatic, but a subtle example wouldn’t get the point across as well. The first audio example uses Cherry Audio’s Voltage Modular synth preset to vary a high-frequency sine wave in a 44.1 kHz Studio Pro project. After rendering, it sounds like multiple tones are being generated, although only the very highest-frequency tone is the real sine wave.
For the next audio example, I raised Studio Pro’s sample rate to 96 kHz, rendered the synth part, then reset the sample rate to 44.1 kHz. Here’s what the same preset sounds like after rendering at the higher sample rate, then returning the project to 44.1 kHz and playing back at the lower sample rate.
The aliasing is totally gone, which is pretty cool. Studio Pro is particularly good at doing this easily. You’re on your own for translating how this technique works for other DAWs, but you get the idea: render at a higher sample rate, and then you can play back at the lower sample rate (assuming that the DAW has decent sample rate conversion).
Final Thoughts about Oversampling
The difference from oversampling is often subtle, if it’s even audible at all. Yet the slight degradation that occurs with foldover distortion is cumulative. An analogy is like a painting with a thin layer of dust. The painting looks fine as is…but when you remove the dust, it looks just that much better.
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Craig Anderton is a music industry legend—from his teenage touring years, to production and mastering projects for artists from classical to hardcore, to his current work in disrupting the publishing industry. Recent e-books include Innovative Techniques for Pro Tools, The Huge Book of Studio One Tips & Tricks, The Big Book of Cubase Tips & Tricks, The Ultimate Guide to Vocal Production, and How to Record and Mix Great Guitar Tracks. Visit his free educational website at craiganderton.org, and hear Craig’s latest music releases on his YouTube channel.