Scalable Quality Extension for the Opus Codec

Scalable Quality Extension The Opus codec was designed to operate at sampling frequencies up to 48 kHz, with an audio bandwidth up to 20 kHz. The CELT mode that is used for high bitrate coding uses ector quantization with a mostly implicit bit allocation system that is dictated by the bitstream definition. Opus can allocate up to 8 bits per MDCT bin in some of the bands. While Opus capabilities listed above are sufficient to reach achieve perceptually transparent audio coding, there is a use for codecs that scale beyond those specs. That includes the current market for 24-bit/96 kHz codecs, but also any application where the intended receipient is not (only) a human being, e.g. ultra-sonic applications. This document proposes a scalable quality extension layer that both increases the resolution of existing Opus quantizers below 20 kHz, and defines a way of coding audio above 20 kHz, with a sampling rate of 96 kHz. The extension is designed to be forward and backward compatible with . All extra bits use the Opus extension mechanism defined in and a 96 kHz decoder is designed to be able to decode a regular 48 kHz RFC 6716 stream and vice versa. The code corresponding to this draft (work in progress) is available on the exp_qext17 branch of the Opus repository at https://gitlab.xiph.org/xiph/opus/ .

Extended resolution To reduce the coding error, we need to increase the resolution for 3 different quantizers: the fine energy quantizer (scalar), the band pyramid vector quantizer (PVQ), and the band splitting angle quantizer. More on extra resolution here

Extended frequency range To extend the audio bandwidth, we need to define more frequency bands. Because psychoacoustics is no longer involved past 20 kHz, all new bands are defined to have the same width. More on band definitions here

Time-domain processing at 96 kHz CELT includes two time-domain filter pairs that require updating for 96 kHz: the preemphasis/deempahsis filters, as well as the pitch prefilter/postfilter. The CELT deemphasis filter is currently defined as D(z)=1/(1 - a1*z^-1) for a 48 kHz signal, where a1=27853/32768. To obtain approximately the same response in the 0-20 kHz range using a sampling rate of 96 kHz, we instead use D(z)=g*(1 - b1*z^-1)/(1 - a1*z^-1), where g=5415/8192, b1=7209/32768, a1=30245/32768. For the pitch pre-filter/post-filter, we use zero-insertion upsampling of the 48 kHz filters, which results in the same frequency response below 24 kHz and a "folded" image above 24 kHz. For example, if for a pitch period T (in 48 kHz units) the postfilter was P(z)=1/(1 - a0*z^-T+1 - a1*z^-T - a2*z^-T-1), then for the same pitch, the 96 kHz filter becomes P(z)=1/(1 - a0*z^-2T+2 - a1*z^-2T - a2*z^-2T-2).