[opus] Antw: Re: OPUS vs MP3

[Jean-Marc Valin]

On 10/31/2017 07:08 PM, Ulrich Windl wrote:
> as MP3 and Opus have very similar objectives, I think the original poster's
> question was a valid one: Why does Opus have more artefacts in the lower
> frequency ranges than MP3 has?

I'm not sure, but my best guess would be "because MP3's window is very
leaky and MP3 has to waste a lot of bits in the LF because of that". It
could also be just the MP3 encoder being silly, or other things. Most
logical explanations would be related to MP3 being bad than anything else.

> The spontaneous suspect that lower frequency
> artefacts may be more noticeably than higher frequency artefacts seems
> plausible, also. Is it a matter of energy (which is higher for higher
> frequencies)?

Most signals have more LF energy than HF, so it's normal for the noise
to look like that as well. If the noise is flat, then you have too much
HF noise and you're wasting bits in the LF. In fact, that's exactly what
I'm noticing in the spectrograms that are posted.

> When your own ears are no longer in their best possible condition, you may try
> a spectrogram, just to make sure you don't miss anything.

Actually, that's the wrong way. Especially when the spectrogram is
computed on a signal difference. For example, some codecs can alter the
phase (or add a small delay) in a way that's imperceptible, and yet
causes a large difference signal. At the same time, artefacts such as
pre-echo will not be noticeable on the spectrogram of the difference
signal -- even when it's audible and annoying.

The least bad way of estimating how good a codec is at very high bitrate
is to just measure the point where you can't ABX and assume that all
codecs improve by about as much per kb/s once that point is reached. And
that's mostly true.

Cheers,

	Jean-Marc