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

[Emily Fox]

Applicable to the entire conversation, just chose this bit to quote:

On Fri, Aug 9, 2024 at 1:10 AM Jan Stary <hans at stare.cz> wrote:

> 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?
>

Opus resamples everything to 48kHz (aside from custom modes, where there be
dragons); it never "downsamples" to anything else, but it does *bandpass*
the input first. The default bandpass is based on bitrate, and is 20kHz for
most of the range, but can drop to 12, 8, or even 4. At 12kbps you have hit
the point where it'll try to switch to 12kHz bandpass. (And could switch in
and out, since it's right on the cusp.) This is plainly visible on the
output spectrogram.

Petr, as you noticed, adding FEC (expect-loss) is equivalent to adjusting
the usable bitrate down.

The other half of the band is a 3Hz highpass for DC rejection, in case you
wanted to test the very bottom of the spectrum for some reason.

Now, the reflection slightly above 8kHz is a different issue. It's
*possible* that it's an opus issue, such as the frequency masking, or even
an unavoidable artifact of the format. But the biggest wildcard imho is how
your "1/2 or 1/4 rate" resampler actually works, whether it could have a
bug in its algorithm. There's also the entire output chain (system mixer,
driver, dac, etc). There are many people who claim "golden ears" who have
extensively tested the codec, and have found issues, but afaik this is the
first time I've heard of a peculiar reflection for pure signal right at the
8kHz boundary.

Emily Fox
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