I started to rencode my CD Collection to high Quality : VBR 256 Kbs 44100 Khz

I would like to know what is better between :

192 Kbs 48000 Khz

or

256 Kbs 44100 Khz

Since your CDs are recorded at 44.1kHz, using 48kHz will not only not gain you anything, but the resampling needed will cause you to actually lose quality. So definitely stick with 44.1kHz.

(I think that empeg has to resample non-44.1kHz-sampled tracks back to 44.1kHz anyway, so it might always be better to go with 44.1kHz, even on sources that are not 44.1kHz, since you can conceivably control the resampling on your computer; I don't know how good the empeg's resampling technology is. I could also be wrong about this whole paragraph about the empeg needing to resample.)

At what point does an mp3 become cd quality? 192?

"CD Quality" depends on the listener. Some people are fine with 128kbit. Plus, if you have a terrible CD player, your discs might sound like someone else's bad 64kbit MP3s.

I think the most recent general consensus has been 192kbit. On some passages on some discs discs you may be able to hear very noticeable differences.

Make sure you increase the high-pass filter setting if your encoder defaults to something low like 15kHz. 19.5 is enough - CD's are 20 but most people can't hear that high anyway.

Of great improtance when using VBR is also the VBR code being used. If you were to use a really badly programmed algorithm, it might not kick the bitrate up when it should and you'll be left with an inferior file. This is where reading up on the latest advances helps. Find recent threads talking about LAME and then find the sites linked that offer help with its best presets.

Bruno

On a more technical and less psychoacoustic angle, an mp3 can never be 100% CD quality. It will never have exactly the same data as the CD, because mp3 is a lossy compression format. I suppose if you had a 705.6 kbps mp3, (which is not supported by the specification), it could be lossless, but you might as well just use a WAV at that point, as the size would be the same.

But, as Bruno said, depending on your ear, mp3s can sound as good as the original CD without getting even very close to that.

At what point does an mp3 become cd quality?

MP3 never becomes CD quality. I hate it when they say that. MP3, at any bitrate, is lossy-compressed and is therefore less than CD quality.

The question becomes "at which bitrate can I no longer hear compression artifacts?", which depends on the song and depends on you. This is all in the FAQ, so read that.