How I reverse engineered a commercial spatial audio effect
A Linux enthusiast embarks on a quest to replicate a cherished commercial spatial audio effect, meticulously reverse engineering its behavior through clever signal processing techniques. This story captivates HN by showcasing impressive technical ingenuity, the power of systematic experimentation, and the satisfaction of open-sourcing a solution to a previously proprietary audio experience.
The Lowdown
Driven by a desire for a Linux equivalent of a proprietary spatial audio effect, the author, Austin Shijo, embarked on an ambitious reverse engineering journey. After initial frustrations with standard approaches, he shifted his focus to systematically analyzing the commercial filter's behavior, leading to a profound understanding of its underlying DSP.
- Early Stumbles: Initial attempts involved experimenting with Head-Related Transfer Functions (HRTFs) from databases like MIT KEMAR and SADIE II, which proved inadequate due to their anechoic nature or inability to replicate the desired psychoacoustic effects. Leveraging LLMs like Gemini for guidance provided theoretical concepts but failed to yield practical solutions.
- Strategic Pivot: Realizing the difficulty in reverse-engineering the exact algorithms, the author cleverly decided to treat the commercial effect as a black box. The new approach focused on observing the filter's output in response to specifically designed audio inputs.
- Orchestrated Tests: A battery of audio probes was devised to characterize the filter, including:
- Impulse Responses: Single nonzero samples at varying amplitudes to check for linearity, time-invariance, and decay characteristics.
- Sine Sweeps: Long logarithmic sweeps to capture frequency response more robustly against noise.
- Steady and Stepped Tones: To analyze gain changes, frequency-dependent behavior, and time constants.
- Multitones and Repeats: To detect non-linearities, intermodulation distortion, and time-variance.
- Filter Recovery: The author developed methods to precisely align recorded outputs using synchronization clicks and then extracted the four impulse responses (from left/right input to left/right output) for the commercial effect's main surround setting. The filter was found to be largely linear time-invariant for this mode.
- Open-Source Triumph: The extracted filter characteristics were successfully implemented in
Saq, an open-source audio enhancer for Linux, achieving the desired 'out-of-head' spatialization effect and fulfilling the author's long-held goal.
Through meticulous experimentation and a deep dive into DSP principles, Shijo not only demystified a commercial audio effect but also provided a publicly available alternative, demonstrating the power of grassroots technical exploration.