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

[Jan Stary]

On Aug 07 00:41:52, petrparizek2000 at yahoo.com wrote:
>     #1. To test encoding at low bitrates, I encoded a sine sweep at 12 kbps
> with Opusenc and then decoded the resulting file with Opusdec.

What sine sweep exactly? How did you obtain it,
and how exactly did you encode and decode it?

	Jan

> The strange
> thing was that even though the output wave file was at 48 kHz, it showed
> clear marks of being resampled from 16 kHz and where my original frequency
> reached 16 kHz, the frequency in the output file practically fell down to
> zero.
>     Unfortunately, it seems that the lowpass filtering is somehow a part of
> the actual resampling algorithm. Why do I think so? Opus normally doesn't
> encode any frequencies above 20 kHz, which would suggest that 20 kHz should
> be aliased down to 4 kHz followed by silence. However, right at the spot
> where my original sine sweep went all the way up to almost 24 kHz, the
> aliased frequency wen to almost 8 instead of being absent altogether.
>     Now I can hear someone say that natural sounds are not composed solely
> of some very high frequencies. But I still think that aliasing is an
> undesirable effect. After all, when the final sample rate is supposed to be
> 48 kHz anyway, why should low bitrates make it necessary to first downsample
> the audio and then upsample again?