ANSWERS TO FREQUENTLY ASKED QUESTIONS

Why do I get different results on different test systems?

Why does this disc work fine on some players, and not others?

What do you use to calibrate the systems?

How do you calibrate error rates?

What kind of pickup do you use?

What are the differences between single-spot and three-spot pickups?

What are the advantages of each?

Can you test just portions of the disc?

Q. Why do I get different results on different test systems?

A. When you measure error rates by playing a disc, you are not only testing the disc, but the player as well. The resulting error rates are a measure of how well the total system (player + disc) is working. Errors are not necessarily "things" on the disc, but are generated in the playback process. This is why there are no calibration discs for error rates; the error rate is dependent on the player.

Errors can be caused not only by local defects on the disc, but by the inability of the player to focus, follow the track, or recover the data clock. Small variations in the player servo systems can cause large variations in error rates. In general, discs that meet Red Book specifications will produce repeatable results. Large variations in results are invariably caused by discs that are outside the Red Book specifications. In addition, if one or more parameters are close to the Red Book limits, results can be unpredictable. There is however, a playability test disc (Philips SBC444A) which has a variety of built-in errors. This is a good way to test whether your playback system is working correctly.

Q. Why does this disc work fine on some players, and not others?

A. Typically, this is caused by one or more marginal parameters. Since there are no standards for players, and each player is slightly different, the results will be different on each player. Again, for discs well within specs, this is not a problem. The problem typically arises when one or more parameters are marginal.

Q. What do you use to calibrate the systems?

A. We use Philips test disc 5B for calibration. The QA-301 comes with a test disc that is calibrated to disc 5B. We can also supply Philips test disc 5B to our customers. All of the analog measurements (I11, I3, asym, radial noise, push-pull) can be adjusted to match Philips 5B calibration disc. There is however, no such thing as calibration for error rates. Errors are not necessarily physical things on the disc. Errors are generated in the playback process. There is no way to measure error rates without playing the disc. Therefore, error rates may vary depending on how good your playback process is. Playing Red Book discs on Red Book players will give pretty consistent results. This is why we use Philips single-spot pickups which are the generally accepted method for CD testing.

Q. How do you calibrate error rates?

A. Many customers have asked if there is a calibration procedure for error rates. Strictly speaking, there is no such thing as calibration of error rates. This is because errors are not “things” that live on the disc, but are generated in the playback process. As a result, error rates are highly player dependent. There is however, a way to confirm that your QA CD Analyzer is working properly.

This can be done using two discs; a “good” disc, and a disc with known defects. To confirm that your system is operating properly, test both discs at 1X and compare the results with the test results we have provided. If you don’t get the expected result, you know that something is wrong. Obviously, your results will be affected by dirt, dust, and scratches, so you should be careful to avoid damaging these discs. Testing these discs on a regular basis will provide confidence that your equipment is working properly.

We can provide such discs with measured results from your player before it leaves the factory. Disc 6 is an ordinary disc that is known to have very low error rates. Playing this disc will confirm that the unit is capable of generating low error rates. Many subtle problems with the player can be detected using this disc. Disc 6 sells for $160, or comes included with the QA-301.

The other disc we use is Philips test disc SBC444A. This disc has built-in defects that can be used for checking error rates. In addition to providing known errors, it tests the player under maximum stress. We can provide this disc with test results for $185, or it can be purchased direct from Philips.

Disc SBC444A provides two kinds of defects: Missing information, and black spots. The tracks with missing information provide repeatable results since these errors are encoded into the data. The sections with Black Spots have the information intact, but obscured by the black spots. In this case, not only is there information lost, but the servo mechanisms are stressed. For example, when the readout beam encounters the black spot, focus, track following, and clock recovery servo signals disappear. After the beam has passed the black spot and the signal is restored, the pickup is out of focus, off track, and the bit clock is at the wrong frequency. This causes many additional errors to be generated in an unpredictable way.

Therefore, the number of E32 and Burst errors generated by the Black Spots in particular may vary. This is because these errors are “soft errors.” That is, they are caused by disturbance to the player’s servo systems, rather than loss of data. Each time the disc is played, the disturbance is slightly different, and the results cannot be predicted.

It is not always necessary to test the entire discs. Regular checks of the system can confirm that the system is capable of producing low error rates on a good disc, and can also play a disc with large defects.

Q. What kind of pickup do you use?

A. The QA-101 now uses a Philips CDM12 three-spot holographic pickup. QA-301 uses the Philips CDM4 single-spot pickup, although we can supply it with CDM12 also. The pickup, plus spindle, tracking, and focus servos are supplied by Philips.

Q. What are the differences between single-spot and three-spot pickups?

A. single-spot and three-spot pickups generate the track following signal differently. The single-spot design uses the difference in intensity between the left and right halves of the returning beam. The three-spot design uses the difference in intensity of the two separate "outboard" spots generated for this purpose. The single-spot pickup tracks the center of the track, whereas the three-spot pickup tracks the edges of the track. There may also be other differences, depending on the implementation, which can vary considerably. For instance, single-spot pickups typically used a half-silvered mirror to separate the incident and reflected beams, whereas the three-spot pickup typically uses a polarizing beamsplitter. Also, single-spot pickups use a galvanometer-type positioning mechanism, but the three-spot must use a linear sled which is typically driven by gears.

Q. What are the advantages of each?

A. The single-spot pickup has the advantages that it has fewer parts, better signal to noise ratio (because all the laser power is concentrated into one beam), faster access time (no gears), and lack of beam distortion due to moving the axis of the objective lens. Most importantly, it also conforms to the measurement standards in the Red Book. This is why most analyzer manufacturers, including The QA-101 all use a CDM4 single-spot pickup in their systems, and advertise it as "the standard".

The three-spot pickup on the other hand, has the advantage of maintaining a constant orientation between the beam and the pits, due to the linear tracking mechanism. It also is less sensitive to the strength of the track following signal on the disc. As a result, three-spot pickups tend to have less trouble playing certain CD-Recordable discs, which often do not meet Orange Book or Red Book specs for tracking signal. The CDM12 three-spot pickup we use is ideal for measuring jitter, since it is the pickup used by Philips to measure jitter, and meets the Orange Book specs for measuring jitter. Because the polarization of the beam is perpendicular to the tracks, it provides a sharp cutoff in the tangential direction, making for low jitter measurements. Jitter measurements made with this pickup are significantly lower than those made with single-spot pickups. There is also an argument that the three-spot pickup agrees better with the results one is likely to find in the field, since all new CD-ROM drives use three-spot pickups.

One drawback to the three-spot pickup is that it does not meet the Red Book specifications for measuring Push-Pull Tracking signal and Radial Noise. There is some debate about whether it is possible to make these measurements using a three-spot pickup, but Philips states that it is not possible.

Q. Can you test just portions of the disc?

A. You can start (and stop) the test at any location. Just select the location to start using the “<<, >>, <, >“ buttons and press play. You can start and stop the player at will, and the data will continue to be accumulated. You could for instance, play the tracks in any order. One limitation is that QA-Net does not sense where the player is playing, so the graph will not reflect the correct ATIME unless you start at the beginning. The QA-301 does sense the player location, so the graphs will follow the true location.