[opus] Antw: Re: 133 kbps stereo killer sample

[Ulrich Windl]

>>> Agustín Dall'Alba <agustin at dallalba.com.ar> schrieb am 14.04.2017 um 22:53
in
Nachricht
<CAHBqS-w3v44WM5x+_4XdFMkD42A2iYTbEWKEBmvJc2P3Y-LJGA at mail.gmail.com>:
> I halved the volume of the sample before encoding with
> `sox -v 0.5 floex.wav quiet.wav` and now I can't ABX it succesfully
anymore.
> So the artifact I heard was just clipping when encoding or decoding.

Hi!

Somewhere I read the recommendation that the peak level of input material
shopuld not be 0dB, but something like -3dB. The reasoning was that encoding
reconstructs the audio signal from the sampling points, and the reconsruction
could actually exceed the sampling point, which my result in an overdrive,
which will result in clipping.

I just wonder whether the encoder (or the decoder at least) can or should warn
in such situations.

Regards,
Ulrich


> 
> Opus remains unbeaten for me at 133 kbps. That's totally awesome.
> 
> Cheers
> 
> On 10 April 2017 at 17:14, Agustín Dall'Alba <agustin at dallalba.com.ar>
wrote:
>> Hello! I found a sample I can ABX successfully when encoded at
>> 133.333 kbps. I was targetting 1 MB/min.
>>
>> https://drive.google.com/drive/folders/0B8KWShoIrA1kQzR1Z0FFRUlfcEU 
>>
>> floex.wav is 4:54–5:04 of a lossless copy of 'Forget-me-not' by
>> Floex, downloaded from http://store.floex.cz/album/zorya 
>>
>> floex-133.opus was created with `opusenc --bitrate 133.333333 floex.wav
>> floex-133.opus`, opus-tools 0.1.9 (using libopus 1.1.4).
>>
>> I used a Sony MDR-V6 pair of headphones and my laptop's oonboard audio. I
>> detected the artifact when I was listening casually from my phone with the
>> same headphones.
>>
>> In the second to last note of the crescendo (at ~5 s in the sample)
there's
>> a slight flutter in the original that turns more prominent and tape-like
>> after encoding.
>>
>> I also tried encoding at 128 kbps and 150 kbps.
>>
>> ABX results:
>> - floex-133.opus.wav 10/10, p-value<0.001
>> - floex-150.opus.wav 4/10, p-value<0.83
>> - floex-128.opus.wav 17/20, p-value<0.002
>>
>> Only at 150 kbps is Opus transparent to me for this sample.
>>
>> Cheers
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