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

[Petr Pařízek]

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

I meant during the decoding. There's one significant difference between 
how Opusdec deals with resampling and how, let's say, MP3 decoders 
usually deal with resampling.
If I make an MP3 at a very low bitrate and if the encoder decides 
(because it's too low) to internally resample my audio before starting 
encoding, the decoder usually converts the MP3 to PCM at the effective 
lower sample rate chosen by the encoder, not at the original sample rate.
Opus files can store the information about the original sample rate. 
Therefore, if I choose a very low bitrate and if Opusenc decides 
(because it's too low) to resample my audio before starting encoding, 
Opusdec looks into the Opus file and reads: "Aha, the original sample 
rate was 48 kHz, not 16." So even though the effective sample rate of 
the encoded data might be something like 16 kHz, Opusdec actually 
resamples it to 48 kHz.

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

Yes. When I turn the playback rate down to 12 kHz, the original 8 kHz is 
changed to 2 kHz, which makes the pitch clearly recognizable.
And now that I've discovered what happens if I set either something like 
"--expect-loss 60" or "--bitrate 10" (see my last message), then it's 
much easier.

 > I am talking about the original sweep.

The original sweep stops pretty close to 24 kHz.

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

Since I aimed for a degree in music composition, it was in my best 
interest to learn to hear even the tiniest details. The only drawback is 
that nowadays my hearing stops somewhere around 16 kHz and I can still 
clearly remember the days when I was able to hear things up to 20 kHz.

Petr