Why is low DC-content desirable for data stored on a CD?

I think Bob's second answer is right. Normal information content is random and by keeping it balanced (low DC content) it appears more white noise like and is sure to have a frequency component at the data clock frequency or a multiple thereof say, bit rate * 14 bits, maybe. You reconstruct this clock frequency and compare it to a reference clock to get an error signal which "controls" the servo.

That assumes the author considers the spindle speed as part of the servo system.

As for radial tracking, I thought that was part of the focus system where the photo detectors keep the laser over the data track. At any rate, if it capacitively coupled, you don't want DC in the data, so

I'll take a guess at this. I assume the servo system they are talking about is the one responsible for tracking the signal on the disk. It tries to maximize signal strength by moving the sensor head radially as needed. Assuming it looks at the raw signal and is AC coupled, long stretches of 1s or 0s would give an erroneously low response that might interefere with tracking. There might also be issues if it is trying to derive a data clock based upon the frequency of edge transitions.

Again, just a guess. Maybe somebody who actually knows what they are talking about will step forward...

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Thanks for the answer.

I don't really get it - could you explain further?

This is true, and is one reason for wanting to avoid long streams of 1's and 0's. But as I understand the explanation I read, the other reason is more important.

Anyone else is also welcome with other ideas, or support to the above answer.

CDs use CIRC and EFM.... Cross Interleave Reed-Solomon Coding and Eight-to-Fourteen Modulation. CIRC is an error correction scheme that adds about 50% redundancy to the other channel one frame behind, so hopefully, an error can be corrected with this addtional info (somehow?). But the EFM is a lookup table of 256 of the avilable 16 thou 14 bit codes carefully chosen so there is now run of 0s greater than 3. This allows the pll to stay synched even on quiet passeges with lots of 0s being transmitted. The analog data is 2 8 bit bytes... they feed each byte into the efm looker-upper. This is explained pretty well in Pohlman Digital Audio.