iZotope have recently released Neutrino a free plugin that uses spectral shaping technology to process the audio. But what is spectral shaping?  In this article with the help of the iZotope team we are going to try and lift the lid on their spectral shaping technology.

What Is Spectral Shaping?

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 equalisers have not been able to achieve. 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 analysing the signal across 32 frequency bands, each band can be processed individually without needing crossovers. You could imagine spectral shaping as a 32-band dynamic equaliser, 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 visualise an approximation of this, take a look at the dynamic equaliser in Ozone 7. If you right click on the spectrum, and choose “Spectrum Options.” and then choose ⅓ Octave, as the spectrum type, from the preferences menu. This will divide the spectrum analyser 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 the iZotope design team had 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 ready for 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 iZotope R&D team went back to work, optimising the spectral shaping algorithm for use on multiple tracks simultaneously, and updating its behaviour to perform well on individual instruments and voices, not just in mastering applications.

Along with this research, the iZotope 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 very different. With an algorithm like spectral shaping, it’s possible to customise the behaviour and performance of the processing to each type of audio source and that is why iZotope choose four modes for Neutrino.

1. 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.
2. Instrument Mode smoothes resonant frequencies while preserving the authentic character of the instrument.
3. Bass Mode is designed to gently attenuate notes that stick out while adding punch and weight to electric, acoustic, and synth basses.
4. Drum Mode emphasises transient detail while minimising 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 probably 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. Don't worry, these are just starting points, 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.

Adjust And Pause

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.

Its Not Summing As We Know It Jim

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. With the Amount knob set to zero, Neutrino will not affect the signal in any way. It is strictly passing audio through as if it was bypassed.

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.