FAQ | Clover Systems

If you are testing only CD’s, CDX is the best choice, as it is optimized for CD’s. DVX is optimized for DVD’s, with some compromise of CD performance.
DVX can also make Beta and Jitter measurements without additional equipment.

Q. What are the differences between CDX and QA-101?

A. CDX is designed for CD-R duplicators, and others that need to test a lot of discs in a hurry, and want to check the logical formatting of CD-ROM’s. QA-101 appeals to recording studios and others who don’t have room for a PC and want something simple to use.

CDX is a PC-based system, whereas QA-101 is a self-contained stand-alone system. QA-101 does not require a computer, is portable, and can mount in a rack.
CDX runs faster, so you get higher throughput.
CDX can also do bit-for-bit comparison between two discs, and CD-ROM format checking; useful for checking incoming masters.
QA-101 can test the lead-in area of a disc, plus it detects HF dropouts and tracking loss (skipping). This can be helpful in diagnosing disc defects.
QA-101 has AES/EBU digital audio outputs, providing high-quality audio listening.
QA-101 can be used to measure jitter with the optional jitter measurement package.

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

A. There are no “right” or “wrong” error rates. It’s just that one player plays the disc better than the other. 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.

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. In many cases, test systems using single-beam players will produce different results that three-beam players because of the difference in ability to track the pits in the presence of various types of defects.

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. If a disc tests good, is it sure to play on every player?

A. The goal of disc quality control is to have every disc play in every player every time. The answer depends on what you mean by “tests good”. Obtaining low error rates on one player will NOT guarantee that a disc will play in all players. There are no standards for CD players; only for discs. The way it’s supposed to work is that the players should be able to play any disc that meets Red Book specs. Therefore, if you make the discs to Red Book specs, they should play in all players. To be certain that the discs meet all Red Book specifications, you need to measure at least 50 different parameters – a costly and time-consuming endeavor. Disc manufacturers of course, must do this to ensure compliance with CD Licensing agreements.

The more things you can measure, the greater confidence you can have that a disc will play in all players. The QA-101 and CDX have HF outputs so that you can measure things like I11, I3, Asymmetry, Jitter, and Reflectivity using an Oscilloscope or other test equipment. These are some of the more important signals that show if the pits are properly formed. If these parameters are not well within Red Book specs, the disc cannot be counted on to play reliably in all players.

As all parameters approach their allowed limits, the likelihood of playing in all players diminishes. The best strategy is to make sure that all discs are as close to optimum as possible. By looking at the HF signal (“eye pattern”), you can learn to recognize discs that are likely to fail on some players.

Q. How repeatable should the test results be?

Repeatability is excellent on good disc. Repeatability depends mostly on the disc, rather than the drive. If some parameter is outside Red Book limits (or even if one or more parameters are marginal), the results may be unpredictable. Once a disc is outside Red Book limits, you cannot predict how the player will react. Also, many errors are known as “soft errors”. They are not always repeatable because they are caused by disturbances to the player’s servo systems, rather than a loss of data. The disturbance can be slightly different each time you play the disc, with different results each time. Also, different players will react differently to these disturbances. Remember, the goal is not for every disc to test “good”; the goal is to detect problems! If you cannot get repeatable results on a disc, that is a sure sign that something is wrong with the disc. Almost certainly it has to do with pit geometry, because that is what determines how well the player’s servos will work.

Q. Is BLER the best measure of disc quality?

Measuring error rates is the easiest way to measure disc quality. It works because any serious problem will affect the error rates. Just looking at BLER alone though, is not enough. You also need to look at the severity of the errors. BLER only tells you how many errors were generated, not how serious they were. It is possible to have a disc with low BLER, but have many uncorrectable errors. This disc is not good, even though it has a low BLER. Low error rates indicate that the systems is working well, but only on one player. If you need to have high confidence that the disc will play on all players, you need to look at the pit geometry related signals like I11, I3, Asymmetry, and Push-Pull. If these signals are near optimum, there is a high probability that it will play on all players.

Q. Can I use your equipment to test blank CD-R media?

A. Although there are a few things that can be measured on a blank CD-R, none of those things will predict whether the disc will work or not. Since there is no data or pits on a blank disc, you cannot measure error rates or pit geometry. Also, the recording process takes place in the dye layer, and there is no way to test the dye without recording something. Everything can test perfect on a blank disc, but for all you know, the dye layer could just be green paint, and therefore won’t record anything!

As discussed above, the key to disc quality is good pit geometry, which is dependent on the interaction between the writer and the media.

In order to QC incoming media (which is an excellent idea), you will have to record something. It is justifiable to waste one disc from each batch to see how it works. The media should be pretty consistent within a given batch, so once everything is working properly, you don’t have to test every disc. When evaluating media, it is best to record the entire disc, since problems are more prevalent at the outside of the disc (due to manufacturing considerations).

Q. What is the best speed to test at?

A. In general, it is advantageous to test at the fastest speed available to maximize throughput. Also, some applications require that the disc work at higher speeds. Really good discs will usually produce pretty much the same results at all speeds. However, there are many situations where you may need to test at a slower speed. For one thing, many discs will generate lots of serious errors at high speed, or not play at all. Certain types of defects will be greatly magnified at higher speeds. In this case, you may wish to try re-testing at a slower speed. This doesn’t necessarily mean that the disc is no good. CD-ROM drives will fall back to a lower speed if the disc is not readable at the current speed. They also use up to 10 re-tries to recover the data. In most applications, the user is completely unaware of this.

Generally if the disc performs well at high speed, it is a pretty good bet that it will work at lower speeds, although this is not always the case. Since CD-ROM drives are optimized for higher speeds, drive performance may be compromised at lower speed. Also, results can be affected by vibration, which varies with speed.

Q. Is an oscilloscope necessary for thorough testing?

A. Using the oscilloscope is not absolutely necessary, but it depends on how thorough you want to be. The degree of confidence you can have in a CD is dependent on how many things you can measure. To have complete confidence, you need to measure at least 50 different things, requiring a million dollars worth of test equipment and a Ph.D. to run it. Depending on your needs, you may be able to get by with less.

The easiest way to measure disc quality is to measure error rates. Most serious problems will affect the error rates. But this only tells you how well this disc plays in ONE player. To have confidence that the disc will work in ALL players, ALL the time, you need to look at the pit geometry. In order to follow the track of pits and stay in focus, the player depends on the way the laser beam is bounced back from the disc. This is totally dependent on the size and shape of the pits. In order to have confidence that all players will play the disc reliably, you need to confirm that the pit geometry is well within Red Book specs. This is where the oscilloscope comes in.

Measuring pit geometry is not something you need to do all the time. Most people get away with ignoring it. Where it is really useful is in evaluating CD-R media, writers, and discovering the cause of poor performance. It’s a very useful tool to have, but you don’t have to use it.

Q. What do you use to calibrate the systems?

A. We use Philips test disc 5B for calibration of HF parameters, and Philips SBC444A to verify error rates.

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 Clover Systems CD Analyzer is working properly.

This can be done using the two Qualification Discs that come with the QA-101 or CDX. Disc 6E is a “good” disc with low error rates, and SBC444A is a disc with known defects. To confirm that your system is operating properly, test both discs 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. When comparing with the factory test results, be sure that you use the same speed used in the original test.

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 drives do you use?

A. The CDX is made from a modified Plextor CD-ROM drive. We add circuitry to the drive to capture error rates and the HF signal. The QA-101 is made from Philips components. It uses a Philips CDM12 three-spot holographic pickup. The pickup, plus spindle, tracking, and focus servos are supplied by Philips.

Q. I get a high figure for the E12 peak, even for very good CDs. Is that normal?

A. Yes, this is normal, especially on CD-Recordables. The CIRC error correction takes place in two stages (C1 and C2). Errors which are uncorrectable at the first stage (E31) are passed to the second stage. However, the data is “de-interleaved” between the two stages. Each block of data at the C1 stage is distributed over 28 blocks at the C2 stage. This causes one large error burst to be broken up into many small errors, which are easily corrected.

Therefore, if every byte of the block is bad at C1, it would cause 28 single-byte errors (E12) at the C2 stage. So the E12 error rate can be as much as 28 times as high as E31. In severe cases, some of those errors may then become E22 and E32 as well. Even on the best discs, one usually gets some E31’s that then become E12’s. The numbers seem large because of the de-interleaving. Unless you know exactly which bits of the block are bad, you cannot predict how much larger E12 will be compared to E31.

Peak E12 of up to 500 is common, but as long as they are not E22 it is OK. Typically, this is caused by small local defects on the disc. My experience is that E12 rates tend to be higher on CD-R discs compared to molded discs.

Q. I get a high figure for the PO8, even for good DVDs. Is that normal?

A. Yes, this is normal. The DVD error correction takes place in two stages (PI and PO). Each block of data (32kB) is arranged in 208 rows and 182 columns of bytes. Each row and column has added parity bytes which allow detection and correction of errors. First the rows are corrected. This is called “inner parity” or PI. Then the columns are corrected. Any errors that were not corrected in the rows may now be corrected in the columns. As you can see, if there is one uncorrectable row (PIF error), that can result in as many as 208 bad columns. Therefore PO8 errors can be quite high.

Q. What kind of printer do I need for my QA-101?

A. QA-101 will work with any printer that accepts ASCII text and has a parallel (Centronics) interface. However, many inexpensive printers will only work with MS Windows and have only USB interface. There are literally hundreds of models of printers available, and they change often, so it is impossible to evaluate them all. When choosing a printer for QA-101, be sure that it states that it will accept plain ASCII text. Unfortunately, this is not always clear when you buy a printer.

It is reputed that many laser printers will work, and most all dot-matrix printers. Older HP DeskJet printers will work, but not the newer ones. Here is a list of printers that are known to work with the QA-101:

Epson LX800

Epson Stylus Color 800

Epson LX300

Epson 740i

Epson 880i

Epson 780

Epson LQ-570+

HP DeskJet 500

HP DeskJet 520

HP 700 Series

HP 5550

HP II

HP III

Panasonic KX-P1124

Brother Laser Printer

We appreciate any feedback you can provide regarding other printers that will work with QA-101.

Q. Where can I find specifications for CD’s and DVD’s including physical and logical data formatting?

A. The physical data format for all CD’s is described in international standard IEC 908. The sector structure for CD-ROM’s is described in ISO/IEC 10149. The Logical format for CD-ROMs is described in ISO 9660. The physical format of DVD’s is specified in ECMA-267. The structure of DVD discs is described in ISO/IEC 16448. These international standards can be acquired from various international standards organizations. Try:

Some of these sites may also refer you to others which offer these documents. The documents are not free, but are usually available at reasonable cost.

Q. How does the Balance Analyzer work?

A.The Balance Analyzer works by measuring the force produced by rotating the disc. If the disc is not perfectly balanced, centrifugal force is created. This force is constant, but varies in direction as the disc turns. The spindle motor is mounted on a bearing that moves in one axis. The centrifugal force causes the motor & spindle to vibrate back & forth. We measure this force with an accelerometer attached to the motor & spindle. This force is exactly proportional to the unbalance. We use two Philips calibration discs to calibrate it.

Q. How do you calculate Moment of Inertia?

A. The Moment of Inertia for a flat disc with a hole in the center is:

I = M/2 · (R1² + R2²)

Where

I = Moment of Inertia

M = Mass (weight) of the disc

R1 = Radius of inner hole

R2 = Radius of the disc

/ = divide

· = multiply

So, a typical disc with R1 = 7.5 mm (radius of inner hole) and R2 = 60 mm (radius of entire disc), and a weight of 15 grams will have a moment of inertia of 27422 g-mm² .

The DVD spec uses units of gram – meters² , so divide by 1,000,000 to get g – m² (there are 1,000,000 square millimeters in a square meter). So this disc would have a moment of inertia of 0.027 g-m².

If the disc is not perfectly uniform (balanced), then the moment of inertia is increased by M·h². M is the weight of the disc, and h is the distance between the geometric center of the disc and the center of mass (which is exactly what the Balance Analzyer measures). Ud as measured by the DBA is Mh, where M is in grams and h is in millimeters. You can find h by dividing Ud by M.

Ud = Mh so h = Ud/M

So if the unbalance is 10 g-mm, then h = 10 g.mm divided by 15 g = 0.67 mm. So now Mh² = 6.7 g.mm² = 0.0000067 g.m². As you can see, an acceptable amount of imbalance adds very little to the Moment of Inertia. Using the maximum allowed weight (20g) and maximum allowed unbalance (10 g-mm), gives a moment of inertia of 0.037 g-m², which is less than the allowed 0.040 gm² for moment of inertia.

Answers to Frequently Asked Questions About Disc Testing

Q. What are the differences between CDX and DVX?