The Art of remastering is basically the art of increasing the amount of information perceived to be on a CD.
When artists record music they have a choice of using analogue or digital recording equipment, both of which are superior to home recording equipment, usually. This equipment captures a vast amount of imformation with regards to the music being recorded, more than almost every playback system is capable of reproducing. In order to get this music onto a medium that can be listened to at home record producers take "liberties" with the information. Usually this involves some degree of compression, whereby the information is limited in order to fit it onto the medium CD/vinyl/cassette(remember them?). In the case of CD 24bit digital information is usually compressed into 16 bit information, in the case of vinyl the amplitude of the music is compressed(literally to stop the needle being ejected from the record by dramatic passages in the music).
All of this means the sound gets degraded somewhere. A digital recording is also just an imitation of a soundwave. If a sound wave fed into a microphone is analysed it looks like the alps gone mad. If this wave form was then digitised it would look like the alps gone mad made from neatly stacked piles of bricks. The sharp peaks and troughs would be replaced by flat edges. This is due to the analogue signal being cut up into 44100 slices every second and being turned into a binary number.
So the skills of the remastering expert bring as much music back out of the recordings as possible. They also use new equipment which can eek out nuances and character that was not picked up by early analog to digital converters.
Tim de Paravicini, hi-fi designer extraordinaire - EAR Yoshino top man, has produced a custom valve EQ unit which gets stunning results when used in conjunction with dB Technologys 24 Bit anologue to digital converters. Going to the EAR Yoshino website is an education in itself.
HDCD is a method which uses software to allow an ordinary CD to hold extra information, up to 20 bits instead of the usual 16 bits. This extra information helps bring significant detail to music, the sound of brushs on snare drums, the zing of a cymbal, inflections in a singers voice and so on. The music has to be encoded in HDCD and the CD player has to have the HDCD decoding software built into it. On an ordinary CD player HDCD sounds just like an ordinary CD, but on a HDCD player they sound so much more lifelike than any ordinary CD can. Remastering allows an engineer to add this extra feature, used recently by ZZ Top on their Smoke, Chrome and BBQ box set to stunning effect. Neil Young has also used it in order to give his remasters a more natural sound.
XRCD is a system designed by JVC to give their CDs a more natural sound. Going to the JVC XRCD website will explain all! Only one factory in the world has all of the prerequisites necessary for this form of remastering, which is why they are so expensive and rare.
Great examples of what a good remastering can bring out of a recording can be heard on the following CD's -
Echoes - Pink Floyd , all of these tracks are noticeably better than even the last remastering
Dire Straits - any of the recent remasters
Bowie - all of his last set of remastered albums sound much better than the RCA issues(but are not as collectable!)
ZZ Tops - Smoke, Chrome and BBQ boxset, stunning quality and in HDCD which makes it even better!