I don't know nothing of the tech behind mp3 but that is what a codec comparison said in www.modatic.net/audio (the site is down now) of xing:
Xing: Is a blistering fast mp3 encoder that is owned by Real and can only be found in Real Jukebox and AudioCatalyst. (both of which are not free) Unlike Blade, being outdated isn't its problem, but being absolute rubbish definitely is!!!
"Haste makes waste" certainly applies here. First, frequency reproduction above 16Khz is terrible, even at 320kbps.
Second, their code is buggy. It's not uncommon to have an mp3 mangled from time to time. (clicks and other artifacts)
Third, bad joint stereo implementation. Like with FHG's encoders, joint stereo is only defaulted for 128kbps or lower.
Fourth, a known weakness of the mp3 format is that it has problems with handling sharp impulses (also known as transients) like drumbeats and cymbals. When encoding to mp3, these transients can become less sharp, this is what is referred to as pre/post echo. To help prevent pre/post echo, most encoders use short blocks (Lame, FHG and Blade included, though Blade's implementation is pretty pathetic) which gets rid of some of the pre/post echo problems. Xing does not use short blocks at all.
It does have a few positves. Out of the mp3 encoders compared here, it is by far the fastest. Also, unlike FHG's encoders, the Xing encoder writes a VBR header. If you really want speed, use FHG's FastENC, it may be a little bit slower than Xing, but it certainly keeps a lot more quality.
Recommended MP3 Encoders for quality
(In order from best to worst)
1st best Lame (Free)
2nd best Fraunhofer Mp3ENC(Commercial)
3rd best Fraunhofer AlternateHQ (Commercial)
4th best Fraunhofer FastENC (Commercial)
5th best Blade (Free)
6th best Xing (Commercial)
Obviously this guy hate xing , there is a lot of of new reviews that discredit this codec. It's not as good as lame, this is a fact, but I don't believe that xing isn't as bad as this review says,