In the previous Friday Tip of the Week, we covered how recording soft synths and amp sims at higher sample rates (like 96 kHz) can give higher sound quality in some situations. However, we also discussed some issues involved with recording at higher sample rates that aren’t so wonderful.
So this week, it’s time for a solution. Offline upsampling to higher sample rates can let you retain the CPU efficiencies of running at a lower sample rate, while reaping the sonic benefits of recording at higher sample rates… and you can do this in Studio One by upsampling in a separate project, rendering the file, and then importing the rendered file back into your original project.
But wait—wouldn’t you lose the benefits of upsampling when you later convert the sample rate back down to 44.1 kHz? The answer is no: Rendering at the higher sample rate eliminates any foldover distortion in the audio range, sample-rate converters include an anti-alias filter to avoid this problem, and 44.1 kHz has no problem playing back sounds in the audio range.
However, note that upsampling can’t fix audio that already has aliasing distortion; upsampling audio to 96 kHz that already contains foldover distortion will simply reproduce the existing distortion. This technique applies only to audio created in the computer. Similarly, it’s unlikely that upsampling something recorded via a computer’s audio interface will yield any benefits, because the audio interface itself will have already band-limited the signal’s frequency range so there will be no harmonics that interfere with the clock frequency.
UPSAMPLING IN STUDIO ONE
We’ll assume a 44.1 kHz project sampling rate, and that the virtual instrument’s MIDI track has been finalized but you haven’t transformed it to audio yet. Here’s how to upsample virtual instruments.
That’s all there is to it. If you want to upsample an amp sim, the process is similar: export the (presumably guitar) track, save the amp sim preset, render at 96 kHz, then import the rendered file into the 44.1 kHz project.
Listen to the audio example “Upsampling with Amp Sim,” which plays the sound of an amp sim at 44.1 kHz and then after upsampling to 96 kHz. The difference isn’t as dramatic as last week’s synth example, but you’ll still hear that the upsampled version is clearer, with more presence.
Do bear in mind you may not want the difference caused by upsampling. When I did an upsampling demo at a seminar with a particular synthesizer, most people preferred the sound with the aliasing because the upsampled sound was brighter than what they expected. However when I did upsampling with an amp sim, and with a different synth, the consensus was that the upsampled version sounded much better. Regardless, the point is now you have a choice—hear the instrument the way it’s supposed to be heard to decide if you like that better, or leave it as is. After all, distortion isn’t necessarily that horrible—think of how many guitar players wouldn’t have a career without it!
Although upsampling isn’t a panacea, don’t dismiss it either. Even with synths that don’t oversample, upsampling may make no audible difference. However, sometimes synths that do oversample still benefit from upsampling; with some sounds, it can take 4x or even 8x oversampling to reproduce the sound accurately. As always, use your ears to decide which sound works best in your music.
The controversy about whether people can tell the difference on playback between audio recorded at 96 kHz that’s played back at 44.1 kHz or a higher sample rate (such as 96 kHz) has never really been resolved. However, under some circumstances, recording at a higher sample rate can give an obvious, audible improvement in sound quality. In this week’s tip we’ll investigate why this happens, and in next week’s tip, tell how to obtain the benefits of recording at a higher sample rate in Studio One with 44.1 and 48 kHz projects.
REALLY? CONVINCE ME!
A Song’s sample rate can make a difference with sounds generated “in the box,” for instance using a virtual instrument plug-in that synthesizes a sound, or distortion created by an amp simulator. Any improvement heard with high sample rates comes from eliminating foldover distortion, also known as aliasing.
Theory time: A digital system can accurately represent audio at frequencies lower than half the sampling rate (e.g., 22.05 kHz in a 44.1 kHz project). If an algorithm within a plug-in generates harmonic content above this Nyquist limit—say, at 40 kHz—then you won’t hear a 40 kHz tone, but you will hear the aliasing created when this tone “folds down” below the Nyquist limit (to 4.1 kHz, in this case). Aliasing thus appears within the audible range, but is harmonically unrelated to the original signal, and generally sounds pretty ugly.
Foldover distortion can happen with synthesized waveforms that are rich in harmonics, like pulse waves with sharp rise and fall times. (Amp sims can also be problematic; although their harmonics may be weak, if you’re applying 60 dB of gain to create overdrive or distortion, the harmonics can be strong enough to cause audible aliasing).
SO IS IT A PROBLEM, OR NOT?
Not all plug-ins will exhibit these problems, for one of four reasons:
Many modern virtual instruments and amp sims oversample, and DAWs can handle higher sample rates, so you’d think that might be the end of it. Unfortunately, there can be limitations with oversampling and higher project sample rates.
Furthermore, with plug-ins that oversample, the sound quality will be influenced by the quality of the sample-rate conversion algorithms. It’s not necessarily easy to perform high-quality sample-rate conversion: check out comparisons for various DAWs at http://src.infinitewave.ca (where, incidentally, Studio One rates as one of the best), and remember that the conversion algorithms for a plug-in might be more “relaxed” than what’s used in a DAW.
So what’s a musician to do? In next week’s Friday Tip of the Week, we’ll cover how to do upsampling in Studio One to reap the benefits of high-sample-rate recording at lower sample rates. Meanwhile, if you still need to be convinced recording at different sample rates makes a difference, check out this audio example of a synthesizer recorded in Studio One first at 44.1 kHz, and then at 96 kHz: