Mp3 converter are a sensitive topic.

This is where the sound is made....

Or should I say, this is where no sound should be made. The problem with converter is, that you don't take notice or think about them, unless you suddenly hear some unwanted artifacts in your music or speech files.

The harder and more complicated part of converter lies on the encoding side while decoding (playing) is much simpler.

How to decode mp3 is exactly specified in the ISO standard. It only provides sample implementations (one simple and one complex) for encoding. That means that the encoding quality differs much more from codec to codec, than the decoding part.

You can test the decoding quality in a very scientific way by comparing bits. A decoder cannot sound "good" or "bad" but just decode correctly or make mistakes.

The limits for encoding regarding "good" or "bad" sound are set much wider and the door for opinions on this is wide open. It mainly depends how smart the developer implements the basic concept and how good they can handle the trade off between speed and quality.

A good encoder doesn't necessarily have to be slow, but it tends to be that way because the process is actually very complex.