Opus audio codec achieves transparent music compression at 160 kbps
Evaluating perceptual transparency of lossy music audio compression with Opus codec
Sound
Summary
This study examines how well the Opus audio codec compresses music without listeners noticing quality loss. At 128 kbps bitrate, most listeners can hardly tell the difference from uncompressed music, but some slight degradation is still detected under tough listening tests. Increasing the bitrate to 160 kbps makes the compressed music indistinguishable from original CD-quality sound, even for critical listeners and challenging music samples. The researchers tested this with many listeners and different songs, and they share their data for others to check.
What this means in practice
- •For audio streaming services: Optimize music bitrate settings to ensure listeners cannot perceive quality loss while reducing bandwidth use.
- •For audiophile headphone manufacturers: Use validated bitrate thresholds to guarantee transparent sound quality in embedded digital audio players.
Authors
Benjamin Martin, Romain Hennequin
Abstract
Although the open-source Opus audio codec is widely used for digital audio streaming, its perceptual transparency threshold for high-bitrate music compression remains insufficiently evaluated. To establish this threshold, this study evaluates Opus across a two-phase ITU-R BS.1116-3 double-blind framework, analyzing Difference Grades using a Linear Mixed-Effects Model (LMM) and Two One-Sided Tests (TOST) for statistical equivalence. In a first phase with 21 qualified listeners and 23 music excerpts, 128 kbps achieves statistical in-distinguishability ($p = 0.456$) but narrowly fails strict equivalence to the uncompressed reference ($p = 0.064$). In Phase 2, Best Linear Unbiased Predictors (BLUPs) isolate the 10 most critical listeners and 8 most challenging samples for a stress-test. Under these worst-case conditions, 128 kbps is conclusively detected as degraded ($p = 0.030$), whereas 160 kbps successfully reaches perceptual transparency ($p = 0.747$) and strict statistical equivalence ($p = 0.024$). The study concludes that 160 kbps demonstrates perceptual transparency to uncompressed CD quality in critical listening environments. For reproducibility, we publicly release results for both phases, anonymized subject background info and sample characteristics.