What is purpose of a cd transport?

There is another big reason that hasn't been mentioned yet. With digital processors, such as digital EQ and upsampling, or processors built into to surround processors (such as room correction and surround processing), you're better off keeping the signal in the digital domain without introducing multiple DA & AD conversions. You can use the digital out of a CD Player, but why use that if the CD player's DACS will never be used? My preference is to use a high quality transport routed to a Digital EQ processor, then a DAC, then on to the preamp.

The other posters have addressed the "techno" part of your question, I would just like to share I have tried different reference transports and they do offer different sonic signatures just like high end turntables.

I have had in my system the following transports: 1) ML 31.5 2) CEC TL-1 3) Ensemble reference. Each had its virtues and did offer overall sonics that affected the overall signature of my system.

For more details regarding what these differences were you could go to a thread entitled: Reference DACS: An overall perspective.

CD data is encoded using Reed-Solomon error correction. The job of the transport is to read the bits (1's and 0's pits/no pits) off the disk including the additional data for error correction and place it all into a buffer for software error correction and for eventual feed to a DAC.

After error correction the result is two channels of 65536 bit word data streams at 44.1 KHz or a data stream rate of 1411200 bits/sec.

There is not much rocket science to this and, as it is all digital, the data will either be correct or incorrect. Incorrect words will cause skipping of the sound as their remains a gap or blank in the data stream.

The relative difference in transports therefore will be related to how easily it skips and how robustly it reads all the words from say a scratched or damaged disk.

The output bitstream from one transport to another should therefore be the same provided there are no skips.

It is downstream of the transport itself where differences can occur. The DAC conversion requires an accurate clock speed to clock out the buffer and very slight (often inaudible) differences can occur due to different DAC designs/specifications/quality, such as immunity to clock jitter. Things like filters, over sampling, one bit sampling, and multi-bit sampling are all methods to achieve the most accurate conversion and all have advantages/disadvantages and can cause slight differences in the ANALOG output. These differences occur in the DAC after the buffer data from the transport.

The quality of the transport itself should not normally make a difference in the quality of the output provided it is capable of properly reading the bits from the CD. It would require a very bad transport design or a very badly damaged disk in order for the system to fail to get all the correct words off a CD disk under normal conditions.

Yes all CD player/transports can read the data, but the problem is timing and distortion. Imagine what happens to the sound if the speed of the data flow is off and/or it fluxuates because of vibration, irregular speed of the platter, poor cabling, and/or fluxuations from an inadequate power supply (and a little of "all of the above" at the same time).

The data correction in the DAC can only do so much, and of course the quality of the DAC affects this also.

Speed of data flow is related to the accuracy of the clock on the data buffer being clocked out to the DAC....so it is not related to the transport itself as in a conventional turntable...as the buffer makes a disconnect in the flow of bits. Think of the buffer as a hot water storage tank in your home - the boiler/heaters job is to keep it with enough hot water for your needs and then the speed of flow is controlled by your faucets. The transport job is to read the bits accurately from the CD and at speed that ensures the buffer does not empty.