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Flaws in recent Opus codec?
post Oct 13 2012, 11:53
Post #1

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Posts: 18
Joined: 13-October 12
From: Germany
Member No.: 103833

Hi everybody,

this is my first post here, even if i read this forum for a quite long time already.

Recently i did some testing on the Opus codec an found a strange behaviour.
I encoded a file with the official 0.1.5 binaries and had a look the frequency response.
Knowing that judging quality with this is useless, i discovered a ~1000Hz /-3dB drop
around 16 kHz which is not present e.g. in Ogg encoded files.

This lead me to encode/decode a linear sweep for comparison and this finally showed
a really messed up response for the Opus encoded sweep in the HF region.
I am not quite sure if it is encoder or decoder related.
For decoding i used the official opusdec and the foobar built-in one.

Opus music sample:

Oggvorbis music sample:

Opus linear sweep:

Oggvorbis linear sweep:

What do you think? Could someone try to replicate or has this been
noticed already?

Thank you.
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post Oct 16 2012, 14:40
Post #2

Group: Members
Posts: 717
Joined: 16-January 09
Member No.: 65630

I noticed similar, maybe same "issue" which I guess it's just opus temperament feature

With any bitrate method,
- at 128 kbps, if I downmix opus file to mono (overlap channels), around 16 kHz there is visible boundary region (maybe related to OP observation)
- at 96 kbps larger low density strip is visible in ~ 12-16 kHz range:

- at 80 kbps there are two large strips

I tested with both opusdec file (44.1k) and opus decoded from foobar (48k).
This "signature" was present on all (10) test files in larger/smaller amount then in example screenshot

scripts: http://goo.gl/M1qVLQ
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