[opus] High Frequency Hiss with Opus at 48 kbit/s

[Andreas Stöckel]

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Hi to everybody.

First of all I hope this is the right place to discuss such an
(nitpicky) issue.

I've just been testing the current Opus release and for mere curiosity
compared its performance to WMAPro with CD quality music at low
bitrates (48 kbit/s).

While Opus generally does a very good job, I found one particular
example (a high pitched chorale) where Opus produces some strange
high-frequency hiss/resonance, whereas WMAPro does not seem to have
this issue (although it audibly seems to change the bitrate after a
few seconds). The hiss is (at some extend) already present in the
original file, yet Opus seems to amplify it.

You can find the example uploaded here (please ignore the certificate
warning):

https://somweyr.de/public/opus_test.tar

The files were produced using the following command lines:

> opusenc --bitrate 48 test.wav test.opus

> wine xWMAEncode.exe -b 48000 test.wav test.wma

opusenc version is opusenc opus-tools 0.1.9 (using libopus 1.1)

I used ffplay (version 2.1.5) for playback.

Is there any way I can tweak opusenc to not show this behavior (except
for lowpass filtering/downsampling), or is this a bug/a general issue
with the encoder at this bitrate and high frequency noise being
present in the input?

Greetings and thank you for your excellent work,
Andreas
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