Asymmetric Waveforms: Should You Be Concerned?

In order to understand the attributes of asymmetric waveforms, it’s important to clarify the differences between DC Offset and Asymmetry …

Waveform Basics

A waveform consists of both a Positive and Negative side, separated by a center (X) axis or “Baseline.” This Baseline represents Zero (∞) amplitude as displayed on the (Y) axis. The center portion of the waveform that is anchored to the Baseline may be referred to as the mean amplitude.

wf-480

DC Offset

DC Offset occurs when the mean amplitude of a waveform is off the center axis due to differing amounts of the signal shifting to the positive or negative side of the waveform.

One common cause of this shift is when faulty electronics insert a DC current into the signal. This abnormality can be corrected in most file based editing applications and DAW’s. Left uncorrected, audio with DC Offset will exhibit compromised dynamic range and a loss of headroom.

Notice the displacement of the mean amplitude:

dc-offset-ex-480-png

The same clip after applying DC Offset correction. Also, notice the preexisting placement of (+/-) energy:

dc-offset-removed-480

Asymmetry

Unlike waveforms that indicate DC Offset, Asymmetric waveform’s mean amplitude will reside on the center axis. However the representations of positive and negative amplitude (energy) will be disproportionate. This can inhibit the amount of gain that can be safely applied to the audio.

In fact, the elevated side of a waveform will tap the target ceiling before it’s counterpart resulting in possible distortion and the loss of headroom.

High-pass filters, and aggressive low-end processing are common causes of asymmetric waveforms. Adding gain to asymmetric waveforms will further intensify the disproportionate placement of energy.

In this example I applied a high-pass filter resulting in asymmetry:

asymm-matural-480

Broadcast Chains

Broadcast engineers closely monitor positive to negative energy distribution as their audio passes through various stages of transmission. Proper symmetry aides in the ability to process a signal more effectively downstream. In essence uniform gain improves clarity and maximizes loudness.

Podcasts

In spoken word – symmetry allows the voice to ride higher in the mix with a lower risk of distortion. Since many Podcast Producers will be adding gain to their mastered audio when loudness normalizing to targets, the benefits of symmetric waveforms are obvious.

In the event an asymmetric waveform represents audio with audible distortion and/or a loss of headroom, a Phase Rotator can be used to reestablish proper symmetry.

Below is a segment lifted from a distributed Podcast (full zoom out). Notice the lack of symmetry, with the positive side of the waveform limited much more aggressively than the negative:

podcast-asymm-480

The same clip after Phase Rotation:

asymm-podcas-fixed-480

(I processed the clip above using the Adaptive Phase Rotation option located in iZotope’s RX 4 Advanced Channel Ops module.)

In Conclusion

Please note that asymmetric waveforms are not necessarily bad. In fact the human voice (most notably male) is often asymmetric by nature. If your audio is well recorded, properly processed, and pleasing to the ear … there’s really no need to attempt to correct any indication of asymmetry.

However if you are noticing abnormal displacement of energy, it may be worth looking into. My suggestion would be to evaluate your workflow and determine possible causes. Listen carefully for any indication of distortion. Often a slight EQ tweak or a console setting modification is all that may be necessary to make noticeable (audible) improvements to your audio.

-paul.

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12 thoughts on “Asymmetric Waveforms: Should You Be Concerned?

  1. Very interesting indeed, I’ve seen this phenomenon myself sometimes but never knew the reason why. I’ll have to have a fiddle with the high-pass EQ and perhaps try speaking in a falsetto voice 🙂

    I’ll definitely have a look into the phase rotate thing though. You’ve really peaked my interest; thanks.

  2. Richard,

    Cool. What’s interesting is if the audio in question is pushed hard and suffers from compromised headroom – you can confirm the benefits of Phase Rotation very easily.

    Analyze the source file and note the amount of available headroom. Apply Phase Rotation, and reanalyze. You should see a difference.

    Always good to discuss this stuff with you. 😉

    -paul.

  3. Paul, Thanks for this! Over the last little bit I found that whenever I used EQ, I ended up with some pretty extreme asymmetry. Turns out I was applying a pretty aggressive high pass filter as a matter of course like so many recommend to eliminate unnecessary low-end energy. Looks like I may just skip the high-pass filter for now!

  4. Hi Curtis,

    You’re welcome.

    Try rolling off at 50 or 60Hz. In most cases this will serve the purpose in terms of eliminating problematic low-end activity.

    Note that the use of high-pass/low-pass filters serve a secondary purpose. Perceptual Encoding (like MP3) filters out frequency activity that is outside of the normal frequency range that we as humans are capable of hearing.

    And so by presenting the encoder with a sort of limited frequency range source, the actual process of encoding may benefit by not having to deal with bits that the codec/encoder was designed to remove in the first place.

    -paul.

  5. Dear Admin I wolud love to thank you realy for such a wonderfull topic, but I have a question here lets say I am EQing a male VO by cutting below 50~60Hz to avoid missing with the waveform symmetry, so what would be a proper Q curve would you recommend, like a brickwall qurve would be called agressive?
    Thank you in advance.

  6. You’re welcome.

    I would suggest looking into a Linear Phase EQ. This type of EQ will allow you to tweak the curve without introducing phase shifting. Note it will not correct previous indications of asymmetry.

    Good luck.

    -paul.

  7. Thank you Paul for your quick reply realy apreciated, so appling adaptive phase rotation before and after EQing, is that the right thing to do?

  8. Happy to help. You want to apply Adaptive Phase Rotation after any processing stages that may be causing the asymmetrical shifting. If you use a linear phase EQ, the processing order shouldn’t matter because of how this type of EQ minimizes phase shifting.

    I find this concept to be especially important when preparing audio for Internet/Mobile distribution, where in most cases a fairly significant amount of gain will be applied in order to meet a suitable Integrated Loudness target. The Phase Rotation optimizes headroom and makes it easier for the producer to avoid excessive limiting.

    I tweeted this a few days ago. The displayed documentation contains additional information.

    https://twitter.com/produceNewMedia/status/807587184492560384

    -paul.

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