[opus] Antw: Re: Antw: Re: Possible bug in Opus 1.3

[Jan Stary]

On Nov 05 11:16:54, Ulrich.Windl at rz.uni-regensburg.de wrote:
> >> >> Why using 96kHz in the original: AFAIK Vorbis and Opus both use
> >> >> frequency components to encode the file.
> >> > 
> >> > What frequency components are those,
> >> > in a sweep from 0 to 20kHz?
> >> 
> >> The frequency component at any time should be more or less exactly one
> >> (THE frequency).
> > 
> > Exactly. So what frequencies above 48kHz are there
> > to be sampled at 96kHz?
> 
> None. Do we want to discuss the Nyquist theorem?

I meant what frequencise _in_your_signal_.
Your signal does not contain anything above
20kHz, so why use a 96kHz sample rate?

> (AFAIR it's for pure sinus

No.

> waves and is says "at least").
> 
> > 
> >> >> With higher sampling frequencies in the original,
> >> >> I was expecting to reduce the aliasing effects for higher frequencies.
> >> > 
> >> > What higher frequencies?
> >> 
> >> If you plot a 44.1kHz sine wave
> > 
> > But you stop at 20kHz.
> 
> OK, Nyquist again. Maybe really look how the waveform looks like. Or are you
> saying Opus really ignores any extra samples at that frequencies?

At what frequencies?

> > 
> >> at 16kHz or higher,
> > 
> > The sample bit width has nothing to do with it.
> 
> ? 16kHz is a frequency, not a bit resolution.

Ah, sorry, I mis-read.

Anyway: a sample rate of 44.1kHz is enough to completely capture a signal
which does not contain frequencies above 22 kHz. Like your signal doesn't.

> > 
> >> it does not look much like a sine wave any more.
> >> So my expectation was that applying DCT on that could add
> >> some "ghost frequencies" that did not exist in the original.
> > 
> > It would be quite sad if the processing used during compression
> > added frequencies ABOVE the orginal, forcing the user to upsample.
> 
> Yes, but I'm afraid that's how it works.

I very much doubt that, but someone more knowledgeable about
the actual Opus compression would have to step in here.

	Jan