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Saturday, November 22, 2014

Better speech compression with Opus

In a previous post, we looked at configuring Audex for ripping CDs to the superior Opus format. While that was good for general purpose audio encoding, it doesn’t let us take advantage of Opus’ immense versatility, it’s ability to get excellent results across a wide range of bit rates.

You will notice at the low end of that graph are codecs that typically specialize in speech, because speech is more compressible that complex audio sources like music. Speex, for instance, can get excellent compression of speech with acceptable quality, yielding much smaller file size than MP3 or Vorbis. One of the benefits of Opus is that it should be able to replace this usage as well.

As you might have guessed, the opusenc commands we gave Audex can also be used directly on the command line. If you want to target a lower bit rate for speech encoding, all you need to do is add the --bitrate option. For speech, try something in the range of 16-48. In Audex, you might add two different profiles for compressing speech with Opus, just as there are multiple profiles for different qualities of compression with Vorbis.

opusenc takes raw, Wave, AIFF, or FLAC input (FLAC is still the preferred format for archival storage). If you pass a FLAC file that is already tagged with metadata, opusenc will incorporate that metadata automatically, so there is no need to pass it on the command line.

opusenc --bitrate 16 input.flac output.opus

For comparison, a 40 minute audio recording ripped as an uncompressed Wave file was about 403 MB, compressed with FLAC it was about 103 MB, opusenc with default settings got it down to about 26 MB, while the above command reduced it to about 4.5 MB. So targeting the lower bit rate was a space savings of over 80% and made it small enough to share by email.

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