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

[Jan Stary]

On Aug 08 10:29:31, petrparizek2000 at yahoo.com wrote:
> > As the thing is to encode for human ears (AFAIK), I'd say that 4kHz is
> already "quite high",
> > and I wonder who can actually hear pure 20kHz sine.
> 
> If you read the beginning of RFC 6716, you learn that Opus never encodes any
> frequencies that are higher than 20 kHz. So at some medium or high bitrates,
> anything above 20 kHz is filtered out, not because of the bitrate but just
> because the Opus format itself doesn't have "room" for describing such
> high-frequency content; it's a bit similar to the fact that fixed-point PCM
> audio is not able to describe any values greater than +1 or smaller than -1.

To be clear: the two limitations have nothing to do with each other.

> > Also if you look at the samples for (e.g.) a 20kHz sine samples at 44kHz,
> > the samples hardly resemble a sine wave very mch, and seen reversely:
> > It's not obvious that it once was a pure sine wave.

On the contrary, a sine wave of 20 kHz
can be perfectly reconstructed from samples made at 44 kHz.

> > IMHO the high frequencies just "add some shine" to the sound;

Are you sure it is "shine"? Wasn't that "spark" or "bouquet"?
Please stop this nonsense.

> I'm talking about something else. Opus combines two compression schemes; one
> is CELT, the other is a modified version of what was originally SILK. In
> certain specific situations (which are described in RFC 6716), all
> frequencies lower than 8 kHz are encoded with the modified SILK while the
> frequencies from 8 kHz upwards are encoded with CELT. If the bitrate is very
> low, it seems that Opusenc internally resamples the signal to 16 kHz and
> sends this resampled version to the modified SILK algorithm.

This is how your original post should have started :-)
Now it makes much more sense.

Jan

> But if the
> resampling procedure has a weak anti-aliasing filter, then the frequencies
> of 20-24 kHz turn into frequencies of 4-8 kHz, which may actually sound
> annoying.
> 
> Petr
> 
> 
> 
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