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

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Joined: 13-October 12
From: Germany
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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 13 2012, 16:58
Post #2

Group: Members
Posts: 835
Joined: 17-September 06
Member No.: 35307

Welcome to the forums.

Interesting. I also note that in the music sample it's a notch cut, and in the sweep it's raised (or was the decoder was different each time as you said you used opusdec and foobar2000). If this is a bug, it's unlikely to have been picked up in listening tests, as many or most listeners struggle to identify at 16kHz low-pass filter in real music so would probably fail to notice a 3 dB variation at 16 kHz too. Nonetheless, it's potentially a sign of a bug.

Can I ask what target bitrate you specified in opusenc? It could help to get to the bottom of it.

This post has been edited by Dynamic: Oct 13 2012, 16:59
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