How Opus Audio Handles Frequencies Above 20 kHz

The Opus audio codec is highly regarded for its versatility and efficiency, but its handling of ultra-high frequencies is governed by strict psychoacoustic design. This article explains how the Opus format manages audio frequencies above 20 kHz, detailing its sampling rate limits, internal bandwidth configurations, and the technical reasons why these high frequencies are filtered out during the encoding process.

The 20 kHz Bandwidth Limit (Fullband)

While the Opus codec supports an input and output sampling rate of up to 48 kHz, it does not encode frequencies up to the theoretical Nyquist limit of 24 kHz. Instead, Opus caps its maximum audio bandwidth at 20 kHz, which is designated as “Fullband” mode.

To achieve this, the encoder applies a low-pass filter that effectively cuts off all audio information above 20 kHz. Any frequency components present in the original source material above this threshold are discarded during the encoding process and are not reconstructed upon decoding.

Why Opus Discards Frequencies Above 20 kHz

The decision to filter out frequencies above 20 kHz is rooted in psychoacoustics and data efficiency:

• Human Hearing Limits: The nominal range of human hearing is 20 Hz to 20 kHz. Most adults cannot hear frequencies well below the 20 kHz threshold, making the retention of ultra-high frequency data practically useless for standard listening.
• Bitrate Optimization: Encoding inaudible frequencies requires data. By discarding everything above 20 kHz, Opus frees up valuable bitrate. This allows the encoder to allocate more bits to the audible spectrum (such as mid-range frequencies and transients), resulting in a much higher perceived audio quality at lower file sizes.
• Coding Modes: Opus dynamically switches between two internal compression technologies: SILK (optimized for voice) and CELT (optimized for music). Under the CELT engine, which handles Fullband audio, the codec focuses strictly on the 0 Hz to 20 kHz range to maintain low latency and high fidelity.

In summary, the Opus audio format handles frequencies above 20 kHz by filtering them out entirely. Even when operating at a 48 kHz sample rate, Opus prioritizes human auditory limits, using a low-pass filter at 20 kHz to deliver maximum coding efficiency and superior sound quality within the audible spectrum.