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Oversampling Strategies

edited November 2023 in App Tips and Tricks

This month's Sound on Sound magazine has a great piece on aliasing and oversampling.

https://www.soundonsound.com/techniques/when-should-you-use-oversampling

If you're not a subscriber, you'll only be able to read the first third of the piece, but I'll summarise and add my own thoughts.

I've subscribed to SoS since the mid-80's and highly recommend that you treat yourself to an early Xmas present and nab yourself a 12-month digital subscription (currently £33 in the UK). Of the UK music-tech publications, it's easily the best. I put it up there with Tape Op (in its prime) in terms of being more than a bunch of puff pieces for advertisers.

The crux of the piece is that oversampling isn't the panacea it's often sold as.

Breaking things down, it discusses things to be cognisant of when devising strategies for dealing with aliasing with oversampling.

  • Oversampling is processor intensive, so it's often best to only use it when printing your stems.
  • Oversampling can add significant latencies, so lookout for plugin options that specifically mention that they're low latency, offer minimum phase options or that they employ IRR filtering as an alternative to oversampling.
  • Many plugins don't report latency correctly to the host, so be on the lookout for those that play badly with host plugin delay compensation. In these instances, it's best to use a host that makes it easy to apply negative track delays to compensate manually.
  • Oversampling can cause phase issues in send|return setups so it's best to avoid, but it's always helpful to employ a decent stereo phase correlation metering solution so you can track down any issues as part of your general workflow.
  • Oversampling can increase peak signal levels, so be cognisant of this as you gain stage your mix.
  • Oversampling may attenuate or phase-smear the high end of your mix.
  • Having multiple plugins in a chain that each has their own oversampling options increases the likelihood that you'll encounter the sorts of the issues mentioned above. If you're using many plugins in your production that require oversampling to sound optimal, consider running you project at a 96K+ sampling rate to minimise individual plugin oversampling instances.
  • A great alternative to oversampling as a solution to aliasing artefacts, is to employ a decent low pass filter plugin after each plugin that leads to aliasing issues. A clean latency free filter plugin like those in the FabFilter plugin family is all you need (as long as you're not using steep cut-off slopes, which risk introducing more aliasing rather than filtering existing aliasing artefacts). I use this approach a lot in my own productions, as I prefer a warmer sound with gentle filtering past 12 kHz. Any high-end sparkle I employ tends to be more organic in nature and less reliant on harmonic distortion.

Most of all, don't become over-paranoid of aliasing as aliasing artefacts are often below the noise floor, so the artefacts are out of the audible range (particularly within the context of a multichannel mix). Plus, many modern synthesis flavours employ aliasing creatively (classic FM synthesis in general terms, and many aspects of West Coast synthesis in particular employ digital distortion rich in aliasing artefacts as a creative choice). At the end of the day, rely on your ears over sine wave sweeps and suchlike in utilities like Plugin Doctor, not all aliasing is bad per se. If you like the noises you're making, go forth an alias to your heart's content.

Somewhat tangentially connected to issues regarding aliasing and oversampling. I read a great paper this week regarding the fundamentals of audio measurement. It's definitely one for the audio geeks among you, but it's really well written and much of the discussion runs parallel to sound synthesis but flipped on its head to the way we noise-makers frame the same discussion. This one I can freely share, so download if it piques your interest.

Fundamentals of Audio Measurement (PDF).

Comments

  • Nice articles, thanks for the heads up.

  • It nice to be reminded that there’s an engineering discipline to music production.

    @Blue_Mangoo used to make us videos using the VST Doctor from DDMF so we could see aliasing. How bad does it have to be to hear it?

  • @McD said:
    It nice to be reminded that there’s an engineering discipline to music production.

    @Blue_Mangoo used to make us videos using the VST Doctor from DDMF so we could see aliasing. How bad does it have to be to hear it?

    There's no hard and fast rules. But one thing to look out for is the stacking effect of multiple tracks, each with non-audible aliasing, leading to audible artefacts. But even in those instances, judge with your ears.

    Much as many of the points raised in the SoS article discuss the negatives of oversampling, it's not suggesting that oversampling is bad. It's more a case of being on the lookout for possible negative artefacts. Well-engineered plugins will oversample more cleanly than other options.

    Once you've finished tracking and have moved into the mixing stage, issues regarding latencies aren't as much of a concern. In fact, when the task is mixing or mastering, you'll often find engineers working in the digital realm lengthening their audio interface buffer to 1024 or even 2048 samples, which makes the audio interface less likely to crap out on short-term CPU processing spikes. Counter intuitively, these extended buffer sample rates make it easier to identify plugins that aren't reporting their latencies correctly. Many of the "classic analog" plugins on the market can introduce latencies of 200-500 ms when oversampling, so it's pretty obvious why your finely crafted production has a chronic case of bad DJ mix syndrome due to errant plugin delay compensation issues! :)

    One of the most popular harmonic distortion plugins, SoundToys Decapitator, has audible aliasing when a project is run at e.g. 48k of below, but in many instances the aliasing is part of the core character of the timbre/tone. Many artists think something is broken when they raise the sample rate as part of their final mix render workflows. A common complaint is that the "edginess" of the mix has turned to "mushiness" (a classic case of oversampling smearing transients).

    As I mentioned above, you've got to let your ears be the judge when it comes to aliasing. A swept sine wave may look horrendous in Plugindoctor, but that doesn't always correlate to negative audible results with normal program material.

    Another part of the jigsaw relates to the way one gain-stages their mix. A good guideline is to try and ensure you're staying as close to -18 dBFS throughout your plugin chain. It's not good enough to lower the channel fader, that has no bearing on how hard you're hitting each plugin through the chain. Most DAW's have a utility plugin which you generally use to set your level going into your effects chain. And if you ensure that the exit from the plugin chain is still around -18 dBFS you ensure you're not favouring fx processing choices that simply sound louder. I'm not digressing from the core thread discussion of aliasing here. If you're not properly gain staging your mix and each channel is nearing 0 dBFS, you're far more likely to encounter negative aliasing issues, as you'll invariably end up misusing compression and limiting either on the channel or mix busses, and this with amplify non-audible aliasing artefacts up to the audible range.

    As you say - there's an engineering discipline to music production. And it doesn't matter if you're mixing and mastering with an iOS/iPadOS setup (DAW or in the likes of AUM) or on a holy grail, Rupert Neve designed AMEK 9098i Recording Console, improper gain staging will always rear it's ugly head before too long. And aliasing will most certainly sound far worse with bad gain staging.

    Seeing as I mention proper gain staging in passing, I'll go further with the digression.

    If your final signal on each track/buss is maintaining an average close to -18 dBFS, that will help achieve a final mix pre-master of approximately -6 to -3 dBFS. Most decent mastering engineers will specify this as the ideal goal, and it needs to be achieved without slamming into a limiter on the master buss. Some gentle SSL/API style master buss compression to help glue the mix is fine - I'd actually go so far as to say it's the optimal approach to master buss compression. But leave the mastering engineer (or yourself after swapping hats) plenty of headroom to go for club mix/metal loudness gold or stream friendly LUFS targets when creating the final masters.

  • wimwim
    edited November 2023

    Thanks @jonmoore - your technical contributions over the years have always been excellent! I especially appreciate it when you help bring ignorance that affects quality, and tech-head overkill, into balance.

    I also learned a ton from this video:

  • @wim I think that sample rate/oversampling video of Dan's for FabFilter was a breakthrough moment for Dan. After that video went viral in audio production circles (close to half a million views for a nerdy audio engineering video is pretty good going), his own YT channel gained a shedload of new subscribers. And thankfully, he's continued to post great content.

  • That video really opened my eyes and ears to the concept and how I can test for aliasing on my own. Must watch for anyone who works with audio.

  • Thanks @jonmoore I also appreciate you posting this!

  • Don’t have time at the moment but will be coming back to this because it seems like there’s lots of good info here. SOS is fantastic. Even beyond the UK. Most musicians I know stateside read it or at least know of it as well.

  • @wim I just remembered a follow-up video from Dan Worral to the one you posted above that fits in perfectly with much of what we've been discussing in this thread. Titled with typical tongue-in-cheek linkbait style "Oversample Everything! Reaper FX and FX Chain Oversampling". Most definitely worth a watch.

    @HotStrange AFAIK SoS has more overseas subscribers than UK ones, with the USA being a very healthy portion of those overseas subscribers. Last man standing is the phrase that most comes to mind for me ref SoS. The SoS team got ahead of the curve with their digital strategy back in the 00's by opening up the archive to all visitors to the SoS website free of charge for any content more than 6 months old, and for me, that's where much of the best content lives. I subscribe out of loyalty as much as anything else, and of course, not having to wait 6 months to access timely content such as in-depth product reviews is pretty useful too.

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