What is spectral shaping?

Tips & Tutorials  |  September 14, 2016


Spectral shaping is a new way of applying dynamics processing across the frequency spectrum that can help bring balance to the sound of instruments and voices in a way that traditional compressors and equalizers have not been able to in the past. In the same way that transient shaping applies focused dynamics processing to just the transient portion of a waveform in the time-domain, spectral shaping applies focused dynamics processing to just certain areas within the frequency spectrum. It is a form of subtle, low-ratio compression that is employed individually across dozens of frequency bands as necessary, with unique time constants and automatic adjustment of thresholds based on the incoming audio signal.

When compared to other frequency-dependent dynamics tools, like multiband compression, spectral shaping can offer far more resolution across the spectrum. By analyzing the signal across 32 mel-spaced frequency bands, each band can be processed uniquely and without applying crossovers. You could imagine spectral shaping as a 32-band dynamic equalizer, with individual band shelf filters for every band, each automatically setting thresholds, time constants, and reduction amounts based on tuned models for each sound source. The result is a more transparent form of dynamic control that is constantly aware of the frequency content present in the incoming signal, and adjusting its processing accordingly.

To visualize an approximation of this, open the dynamic equalizer in Ozone 7, right click on the spectrum, and choose “Spectrum Options.” From this preferences menu, choose ⅓ Octave as the spectrum type. This will divide the spectrum analyzer into 30 bands, spaced logarithmically, whereas spectral shaping deploys processing across 32 bands spaced based on the Mel scale, which is based on human perception of pitch differentials. Now imagine an individual dynamic EQ node for each of these bands. That’s a powerful dynamic EQ!


How spectral shaping was developed

The original research that contributed to spectral shaping centered around iZotope’s latest Maximizer algorithm, IRC IV, which was first released in Ozone 7. The goal was to develop a more transparent form of limiting that was frequency dependent, without radically affecting the timbral character of the music when used judiciously.

When using a broadband limiter, the limiting is applied across the entire spectrum, even if the peaks in the signal were focused in just a particular frequency range. With spectral shaping, the limiting could detect frequency bands which are producing the most peaks, and then apply limiting only in those areas and only when peaks were present. The result was refined and tuned extensively before implementation in Ozone 7.

However, while considering this technology for limiting applications, the question arose about how this could be implemented with lower ratios and gentler attack/release characteristics for use in mixing contexts. The R&D team went back to work, optimizing the spectral shaping algorithm for use on many tracks at once, and updating its behavior to perform well on individual instruments and voices, not just in mastering applications.

Along with this research, the product and sound design teams collaborated with mix engineers and producers to figure out the best way to interact with this technology. This involved extensive wireframing, prototyping, usability testing, and iteration on both the interface design and its sonic characteristics. After final tuning and extensive testing in a variety of musical settings, the most practical implementation seemed to be the simplest: four modes and two knobs.

Neutrino’s four modes

As engineers, we’ve grown accustomed to using a single processor on a variety of sources, like an 1176 for compressing vocals, drums, bass, and guitars. However, if you think about how each of these instruments sound, they are really quite unique. With an algorithm like spectral shaping, it’s possible to customize the behavior and performance of the processing to each type of audio source. Therefore, Neutrino offers four modes.

Voice Mode focuses processing on mid and high frequencies for adding clarity and detail that helps vocals sit on top of the mix without becoming harsh or strident. Instrument Mode smoothes resonant frequencies while preserving the authentic character of the instrument. Bass Mode is designed to gently attenuate notes that stick out while adding punch and weight to electric, acoustic, and synth basses. Drum Mode emphasizes transient detail while minimizing frequency buildups that can make percussion tracks sound “muddy” or “flabby.”

Because the modes are subtly focused on different areas within the frequency spectrum, you’ll likely notice that Neutrino subtly changes the timbral balance of the audio depending on which mode you have selected. You may find that for a particular guitar track, the Bass Mode works well. No problem! Use your ears and enjoy what works best for your music.

Neutrino’s controls

The Amount knob adjusts how much of Neutrino’s dynamic processing is applied. The Detail knob adjusts the granularity of processing across the frequency spectrum. Because Neutrino is constantly listening and adjusting to the incoming signal, we suggest that you make a small adjustment, pause, then listen to the effect it’s had on your signal.

It’s important to note that Neutrino is not designed to impart its own particular “sonic character” to your audio in the way analog summing, transformers, or tape might. These analog devices often contribute some unique saturation characteristics. Neutrino is designed specifically to not add any distortion, preserving the original signal as transparently as possible. 

Gain-staging with Neutrino

It’s important to verify that the output signal of the last processor ahead of Neutrino is not clipping, so it does not overload Neutrino’s input. To ensure Neutrino does not clip the signal at the output, a DC offset filter and brickwall limiter are the final stages of Neutrino’s processing. If you hear an extreme amount of dynamics processing coming from Neutrino, it is likely that the input signal is to high, and driving up against the limiter on the output stage. Try reducing the output of the plug-in just prior to Neutrino.

Ready to try Neutrino for yourself?

Download Neutrino for free and see how it can help your mixes stand out from the crowd.

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