[opus] Opus Tools -- low bitrates, new features in 1.5, "expect-loss"

[Jan Stary]

On Aug 07 22:04:21, petrparizek2000 at yahoo.com wrote:
> > The encoded opus file is 48kHz,
> > so how would the output wav be resampled from 16kHz?

To be clear: did you mean the opus output of opusenc
or the wav output of opusdec?

> > What are those "clear signs" exactly?
> 
> The things that I can hear while listening at 1/2 or even 1/4 of the
> original playback rate. Where the frequency reaches 8 kHz, suddenly it
> starts going down, not up.

Ah, so you _hear_ when the frequency reaches 8k exactly?

> Also, when the original frequency is getting
> closer and closer to 8 kHz from the bottom, another frequency appears
> softly, is getting louder and is getting closer and closer to 8 kHz from the
> top(!) until the two finally meet at 8 kHz. The original frequency then
> quickly disappears while the unwanted new frequency is never completely
> filtered out, keeps falling and falling and at the exact time spot where the
> original frequency would reach 16 kHz, the undesired frequency goes down to
> zero.

Yes, I think I see that in spectrogram-opus.png too.
And then it goes back up to 8k.

And the original frequency keeps rising to around 12k AFAICS.

> > As far as I can tell (see the attached spectrograms),
> > the sound dies out around the point the freq reaches 12kHz.
> 
> You're probably talking about the other frequency that came in, the one I
> called the undesired or unwanted frequency.

I am talking about the original sweep.

> The spectrogram is one thing,
> but switching to 1/2 or 1/4 or the playback rate and pressing Play is
> another.

Exactly. The spectrogram shows you the frequencies present in the signal.
Pressing Play relies on Golden Ears (TM).

> Okay, I've read that. Seems the encoder and the decoder on your side behave
> the same way they do on my side.

For completreness, mine is
opus-tools 0.2, libopus 1.3.1, libopusenc 0.2.1

> > What makes you think a sampling frequency of 16 kHz
> > is involved anywhere in the signal path?
> 
> First, the fact that the original frequency and the undesired frequency
> actually meet at 8 kHz.
> Next, the fact that the undesired frequency is
> falling, not rising. Finally, the fact that first the undesired frequency
> comes in pretty quickly and then the original frequency disappears very
> quickly; so at one point, they're both there (and this point happens to be
> at 8 kHz).
> 
> > What makes you think there is any downsampling involved?
> 
> The fact that frequencies higher than 20 kHz are not filtered out but are
> rather replaced with frequencies higher than 4 kHz.

Well let's hear it from the horse's mouth,
if there are opus developers reading this list:
given a 48 kHz wav file, does opusenc downsample to 16 kHz?

	Jan