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Topic: How does a CD-player work? (Read 10434 times) previous topic - next topic
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How does a CD-player work?

Can anyone guide me to good info on the signal path of a "typical" CD-player? I am involved with lots of discussions about this topic, but can contribute only theoretically about how things should work.

The articles at wikipedia and howstoffworks are very thin and describe obvious stuff.

I am interested in the buffering of data from the laser, feedback-mechanisms for controlling read speed, (re)clocking of data etc. In short, what is needed to conclude that a "bit is a bit" or that it is not.

In my mind, a CD-player must have a crystal that clocks samples with sufficient precision that it is percieved jitter-free. Any variation in disk physical condition should be eliminated by buffers.

Therefore, as long as error-free reading is possible, a CD-player should be uniquely described in terms of audio quality by the precision of its clock, the ability to output error-free samples and possibly the frequency response of its spdif transmitter.? Moreover, there should be no need to analyse the physical condition of the disc as long as error free samples are recovered?

-k

How does a CD-player work?

Reply #3 –
I don't know much about the internals, but I do know that a CD has a huge ammount of redundant information to correct errors. You might find more info about this if you search for Reed-Solomon's error correction.

How does a CD-player work?

Reply #4 –
Quote from: legg on
I don't know much about the internals, but I do know that a CD has a huge ammount of redundant information to correct errors. You might find more info about this if you search for Reed-Solomon's error correction.

I know, the CIRC used is described quite thoroughly at wikipedia. I also have som practical experiments such as these:
http://www.exactaudiocopy.de/eac13.html

A CD with intentional physical errors

This is the same drive but extracted with a speed setting of 4x. Now you can see why Plextor drives are often recommended. The Plextor was able to remove nearly all errors on extraction and the few of them are below -60 dB(A)!

Besides, I have tested and documented that my Sony external DVD player is capable of playing back a ripped and burned CD with exactly zero sample errors compared to the original. Trivial, perhaps, but when dealing with religious people, nothing is to far out it seems.

But what I am asking for is some more detailed description of clocks, timing, feedback control loops etc.

How does a CD-player work?

Reply #5 –
Maybe you should check your library.  Books with "Compact Disc(s)" in the title will probably deal with all the details, down to the type of CRC formulas used.

How does a CD-player work?

Reply #6 –
Pohlmann's Principles of Digital Audio goes into some detail.

How does a CD-player work?

Reply #8 –
The CD -Player [ or DVD player, ... even blu ray is still an optical disc system  ] is major devided into 2 main parts. 1) The Front End Servo (Electro-mechanical ) mechanism and the 2) Back-end Decoder part.

The front end usually involves application of the feedback control theory, from practically simple Lead-Lag compensators either in S-domain (analogue servo) or Z-Domain (Digital servo) to PID or even Adaptive Filtering (mostly for issue papers or researchers) can be used for a single purpose; Optimally control the laser spot over surface of the information layer of the Optical disc.

This is done by controlling a 2 axis actuator , namely Focus and Tracking actuator which control the movement of the optical lens which brings the laser into focus. The precision for the focus, say in CD -Player is within 1um  ( 10e-6 m) for normal operations, while the track pitch is 1.6um.  That is infact quite a challenge to electrical as well as mechnically inorder to keep tracking a precision spot of 1um in an environment of say a few milimeters.  That is why we need some real-time feedback control system for the above Front End mechanism.


How does a CD-player work?

Reply #9 –
I found this document:
CD audio decoder, digital servo and filterless DAC with integrated pre-amp and laser control (PhonIC)
http://www.ortodoxism.ro/datasheets/.../SAA7824HL.pdf

This far:
See attachement for simplified diagram of dataflow. Notice the audioflow from "dataslicer" (?) to the left, PLL/demod, FIFO, EC, interpolator and oversampling digital filter.

Notice the simple error concealement algo: single sample error -> linear interpolation. >1 sample error -> hold last known value, then interpolate last erroneous sample

"7.15 Spindle motor control
7.15.1 MOTOR OUTPUT MODES
The spindle motor speed is controlled by a fully integrated
digital servo. Address information from the internal ±8
frame FIFO and disc speed information are used to
calculate the motor control output signals. Several output
modes, selected by decoder register 6, are supported:"

I believe that this means we have a feedback from the FIFO buffer to motor speed, presumably trying to avoid buffer under/overflow. This should be a good mechanism to decouple physical variable timings from the crystal clocked audio path?



How does a CD-player work?

Reply #10 –
Quote from: knutinh on
I found this document:
.....

I believe that this means we have a feedback from the FIFO buffer to motor speed, presumably trying to avoid buffer under/overflow. This should be a good mechanism to decouple physical variable timings from the crystal clocked audio path?


Yes, u are correct. The control of the disc rotational speed (usually called Spindle Servo) are using the address pointer generated from the FIFO 16K/32K SRAM buffers. However, the accuracy of the CLV control (Constant linear velocity) still making use of the PLL loop filters, to compares the incoming data stream 11/22T signals, with the PHASE AND FREQUENCY of the reference crystal frequencies.