Published Date 3/19/15 3:20 PM
It looks like Opus audio format becomes popular. During last month SE received several requests to add it to the rating system. Low bitrate settings (--bitrate 59-90-96) were in the system since the first public release of the codec. Now high bitrates (--bitrate 128-192-256) are added as well.
In contrast to other VBR encoders Opus uses “non-standard” bit allocation strategy for different audio material. As a consequence encoding of less saturated music (with fewer instruments) can result in higher output bitrates than encoding of pop music for example. The table below shows that. Two different music fragments – pop-rock and classic – were encoded to Opus and mp3 formats with approximately equal VBR settings. While mp3 encoder shows usual behavior allocating more bits for more complex pop-rock material, Opus behaves exactly the opposite.
| || mp3 (kbit/s) || Opus (kbit/s) |
| -V5 || -V2 || -V0 || --bitrate 128 || --bitrate 192 || --bitrate 256 |
| Pop-Rock || 126.6 || 187.3 || 260.6 || 126.5 || 190.4 || 255.1 |
| Classic || 106.9 || 176.3 || 231.2 || 151.7 || 216.8 || 278.1 |
Table 1. Resulting bitrates of encoding pop-rock music (Midnight Oil - (1996) Breathe, tracks 7-13, 26:53) and classic music (Piazzolla and Kronos Quartett - (1991) Five Tango Sensations, 26:50) with mp3 (Lame 3.99.5) and Opus (libopus 1.1) encoders at 128-192-256 kbit/s VBR settings. Opus encoder ends up with higher bitrates for less complex music.
Nine SE testing samples are usually encoded with lower bitrates by all modern VBR encoders because most samples are pure voices or instruments. Opus is the only encoder which produced actual bitrates higher than the set ones: 152.3kbit/s for 128kbit/s, 222.5kbit/s for 192kbit/s and 290.1kbit/s for 256kbit/s. So the ratings of Opus were placed on 160kbit/s, 224kbit/s and 320+kbit/s pages respectively. This is less convenient but more correct as all SE ratings are grouped according to actual bitrates.
The encoder operates at 48kHz only, so for other sample rates (as in our case) resampling is required. Some good quality resampler is already integrated into the codec but we used the external one from Audition CS6. The complete test files production chain is as follows:
reference test files 44.1/16 -->> 48/24(Audition CS6) -->> opusenc -->> decode by foobar2000(48/32) -->> 44.1/32(Audition CS6) -->> test files production
Opus encoder/decoder can be downloaded from http://opus-codec.org/downloads/