What is purpose of a cd transport?

Its job is to properly read the disc and transmit the digital data. Some transports use better parts and are better built and are therefore better at doing that than others.

Pbb's post wasn't here when I posted. In theory, a well-designed one box player should beat separates, because that's one less interface and a shorter path for the electronic signal. In fact, the original CD players, with the main exception of Sony's state of the art two-box system they came out with (the 703?), were one-box units. Separate DACs came into play from the high-end designers to take advantage of the additional space in the separate box for the better power supplies and analog sections that couldn't fit in (or for pricing reasons weren't put into) the standard CD player's chassis and because the original one-box players didn't sound that great; then companies like TEAC began manufacturing no holds barred transports that were built like tanks and were far better than the cheap mass-market CD player transports. At that point, with no real high end manufacturers involved in the single box CD player business, a separate DAC made more sense to a person who wanted a better-sounding CD playback system. Now, the power supplies, analog sections and transports in the better one-box players are on a par with the better separates and may have tipped the scales the other way, though a well-designed inexpensive DAC (and there are plenty of them) is still a viable alternative for better sound from an inexpensive CD player. I personally have heard differences in some, not all, transports, but I don't really feel they come close to the magnitude of the differences I've heard in DACs.

Audio Logic has done the same thing with their DAC in the past (the new MDX version is a one-box DAC)--in fact, their prototype actually had three boxes, one for the DAC circuitry, one for the analog power supply and one for the digital power supply. With my Forsell's air pump, I have four boxes instead of one. My non-audiophile friends roll their eyes a lot. Does it all make a difference? I think so, but in objective terms the difference all of my esoterica makes is probably very little in the grand scheme of playing music, otherwise I could not be moved listening to music on my car radio.

That's how the production Audio Logic with the single power supply did it as well. A good design. I sort of miss the separate boxes from the prototype, though, because you could turn off the analog stage (and therefore save tube life) while leaving the digital stage on all the time, which seemed to help it sound better.

Hell, Sugarbrie, my PREAMP is 4 boxes. Where my friends can turn on their receiver with one button and a CD player with another and have music, I have a tuner, a CD transport, a DAC, two preamp power supplies, an active crossover and four amplifiers to turn on to get music (and then I have to wait 6 minutes until the Jadis' auto-muting goes off)! Is it any wonder people view us as "different"?

Heck, Sugarbrie, for vinyl I only have the Lamm phono to turn on for the phono stage (maybe I should consider the Blue Circle, though the Lamm is really an extra box itself because my Jadis has its own world-class phono stage), but then I also have to turn on the Walker Motor Drive, the Basis' motor and the vacuum for the vacuum holddown! And you guessed the monbloc amps, with two biwires for the highs and a lonely single wire for the bass. I won't go into the four dedicated lines....

Honestly, Jeremy's right, simpler is better if it's well-designed and executed.