MadSci Network: Computer Science

Re: How does a CD 'transport' differ from a standard computer cd-rom drive?

Date: Tue Jan 2 04:42:58 2001
Posted By: Bruno Putzeys, Staff, Electroacoustics and Analog Electronics, Philips ITCL
Area of science: Computer Science
ID: 977795694.Cs


For either audiophile CD transports and CD rom drives the data error rate 
is very low, regardless of cost of the device.
The main issue for audio transports is to get the data out at precisely 
timed intervals (=keep clock jitter low). Depending on the knowledge of 
the designer, two techniques are used to insure clock stability:

1. To reduce ripple on the power supply that powers the clock and 
transport, mechanically stabilise the rotation and minimise vibration such 
as to keep the servo's from operating jerkily and drawing heavily 
modulated DC currents from the power supply. This approach is cumbersome 
and only limited in effectiveness. It is also quite expensive because of 
the mechanical work.

2. To give the crystal oscillator and the output circuit their own power 
regulator so that there can be no interaction from the servos. Power 
regulators cost 10c each and are a much more effective way to reduce 

The second option works with any cd drive, including the ones used in CD-
roms. I have modified my $50 S*N* player by adding a circuit board 
containing a clock and a correctly designed S/PDIF output circuit, each 
with their own regulator. The whole mod cost me $10 and the player is on a 
par with the most expensive cd transports.

Wiser still would be to have the clock generated on the D/A board and feed 
it back into the CD player electronics. That places the clock where it's 
supposed to be: at the DAC.



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