The Rise of the Nulling Upmixer

How INU is Changing Immersive Audio Production

Note: In an earlier version of this post, the plugin was called DANU. The developer has since renamed the plugin to INU. The text of this post has been changed accordingly.

In the evolving landscape of immersive audio, maintaining the integrity of stereo mixes while creating compelling multichannel content has been a persistent challenge. The introduction of INU (Immersive Nulling Upmixer), the world’s first and currently only nulling upmixer, marks a significant shift in how we approach this challenge.

The concept of nulling in audio production isn’t new, but its application in immersive audio creation through INU offers a novel solution to a common problem. As Sam Hocking, the developer of INU, explains, “The whole idea of INU is to distribute the stereo signal that you’ve already got and try to massage it around the room without unbalancing what the engineer originally intended in the stereo.” This approach ensures that when the immersive mix is folded down to stereo, it perfectly matches the original stereo mix – a process known as nulling.

Understanding the technical foundation helps appreciate why this matters. Traditional upmixing solutions, such as Penteo and similar tools, often alter the original stereo mix in subtle but meaningful ways. When these mixes are folded back down to stereo, they don’t perfectly match the original. This discrepancy can be problematic when working with carefully crafted stereo mixes that need to maintain their original character while gaining the benefits of spatial distribution.

INU’s Approach to Signal Processing

INU approaches this challenge through a sophisticated signal processing chain that sets it apart from traditional upmixers. The system initially separates the direct and diffuse components of the incoming stereo signal. In this context, the direct signal, similar to but not identical to the mid signal in mid-side processing, represents what’s common between the left and right channels. The diffuse signal conversely, captures the unique aspects of each channel. This separation allows for precise control over how distinct elements of the mix are distributed in three-dimensional space.

The implementation of INU’s signal chain is particularly elegant. As Hocking describes it, imagine two balloons in the center of your listening space: one for the direct signal and one for the diffuse signal. These balloons can be independently “stretched” toward different speakers, allowing for nuanced control over the spatial distribution of your audio. This metaphor helps visualize how INU maintains perfect downmix compatibility while creating engaging immersive experiences.

The Signal Flow and Controls

The signal processing in INU follows a specific order that maximizes control and flexibility. First, the low-frequency content can be selectively routed to the LFE channel, with precise control over the cutoff frequency and Q value. This allows for natural bass management while maintaining the integrity of the full-range stereo signal. Next, the center channel processing includes a sophisticated shaping system that can focus on specific frequency ranges – particularly useful for isolating and controlling vocal content in the center speaker.

After the LFE and center processing, INU provides independent control over how the remaining direct and diffuse signals are distributed throughout the immersive space. The direct signal push determines how much of the mono-compatible content moves into the surrounds and heights, while separate controls for the diffuse signal allow for precise front-back and up-down positioning. This granular control enables the creation of natural-sounding immersive mixes that respect the original stereo production.

Democratizing Immersive Audio Production

What makes INU particularly revolutionary is its development approach. Built using Pure Data through the PlugData platform, INU represents a new model of audio software development – one that prioritizes accessibility and transparency. The plugin’s source code is available to users, encouraging a community-driven approach to feature development and improvement. This open nature, combined with its affordable price point, democratizes access to high-quality immersive audio tools.

The implications for the industry are significant. While traditional upmixing solutions often come with price tags in the hundreds or thousands of dollars, INU offers superior technical capabilities for stereo compatibility at a fraction of the cost. This democratization of immersive audio tools opens new possibilities for independent creators and smaller studios looking to enter the Dolby Atmos space.

Practical Implementation and Future Potential

The integration of INU into the workflow has been meticulously designed. While currently optimized for use with Reaper and the Fiedler Audio Dolby Atmos Composer, work is ongoing to ensure compatibility with other digital audio workstations. This combination of Reaper, INU, and Fiedler Audio Dolby Atmos Composer creates an incredibly cost-effective entry point into professional Dolby Atmos production.

However, it’s crucial to understand the monitoring requirements when working with INU. Because the stereo downmix remains constant regardless of spatial adjustments, proper immersive monitoring becomes essential for making informed decisions about spatial placement. This might mean working in a full Dolby Atmos studio or, at minimum, using head-tracked binaural monitoring to evaluate the three-dimensional image effectively.

INU’s applications extend beyond simple stereo-to-Atmos conversion. As Hocking explains, the plugin was designed with stem processing in mind. When working with separated stems (whether through traditional multitracks or AI-powered source separation), multiple instances of INU can be used to create sophisticated immersive mixes where different elements occupy distinct spaces in the three-dimensional sound field, all while maintaining perfect stereo compatibility.

Final Thoughts

The emergence of INU represents more than just a new tool in the audio engineer’s arsenal – it signals a fundamental shift in how we approach the creation of immersive content. By preserving the integrity of stereo mixes while enabling creative spatial distribution, INU helps bridge the gap between traditional stereo production and the expanding world of immersive audio. Its innovative approach to signal processing, combined with its accessibility and transparent development model, sets a new standard for what we should expect from immersive audio tools.