What is purpose of a cd transport?

Like anything else that spins, such as a turntable, how well it works depends on how well it spins. The better transports spin with less vibration and noise, the circuitry and laser are better, and they more accurately read the disc and send the digital data to be turned into analog sound.

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.

Go listen and judge for yourself.......

To supplant Rcprince with an example -- some cdr/dvd-r are better than others. Some power supplies are more elaborate than others. Why separate? Good question -- sometimes it;s just a matter of space, sometimes marketing.

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.