CD Testing: The Digital and Analog Sides
Originally published in CD-ROM Professional, February
as part of an article entitled is there a CD-R media problem?
by Katherine Cochrane
There are several ways to measure what a CD-ROM reader's laser pickup "sees" as it reads data. The first are the digital tests, which detect errors in the data stream being read; analog tests are designed to detect problems with the physical structures of the pits of the media themselves. Within these tests and their various components lie the heart of CD-ROM and CD-R media evaluation.
CD-ROMS and CD-R discs are encoded with Cross-Interleaved Reed-Solomon Code (CIRC). This code uses interleaving to distribute errors and parity to correct them. With a bit rate of over 4.3 million bits per second, the need for robust error correction is obvious. The error rates in the low-level decoding strategy are monitored over two levels -- referred to by most hardware manufacturers as C1 and C2. A third level of "Extended" Error Correction (ECD/ECC) is used in many (but not all) CD-ROM formats.
A disc's "Block Error Rate" (BLER) is the sum of corrections and passes made in the C1 decoder. The C1 decoder is designed to locate and correct up to two bytes of information on a CD block. If more than two bytes are detected, the entire block is passed to the de-interleaving stage and the C2 decoder. BLER is rarely an issue on CD-R media. It represents correctable random error and is generally not used as an indicator of failure or lost information. In most cases, only a small amount of BLER represents uncorrectable blocks. The Red Book allows for a raw error rate in the C1 decoder of up to 3 percent of the possible blocks in errors per second over a ten-second range.
BRST (Burst Error) is a localized group of missing data, perhaps caused by a speck of dust or a scratch -- a burst of errors in one spot. It is the same data as that tested for BLER, but unscrambled (de-interleaved) before it is checked. Interleaving is aimed at correcting BRST. It is easier to correct one bit out of 10 bytes than 10 bits out of one 16-bit word, which is why the data is encoded or interleaved across an entire block.
Often referred to as E32 or E42 errors, a disc's uncorrectable error count represents the number of blocks that could not be corrected by the de-interleaving and C2 decoding stage. The block errors corrected or passed through the C2 decoder by and large tend to represent non-random or physical flaws, which cause the most concern in CD-R testing. While CD-R discs frequently have lower BLER rates than pressed discs, they far exceed their replicated brethren in E32s and other uncorrectables, since by definition the Red Book specification does not allow any errors to pass the C2 decoder.
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The analog tests are I3 and I11 to ITOP, Symmetry, Radial Noise, Push-Pull Magnitude, and Cross Talk. Analog tests read the laser beam reflection as sine waves as the signal changes its binary state by passing from pit to land, or land to pit. "T length" is a function of the player's timer, where "T" is a variable defined by the ration of pit length to the constant linear velocity at which the laser beam travels over the disc's reflective surface. This variable is used as a factor in analyzing analog test results. 3T is the minimum length of pit allowed, and 11T is the maximum length. The size of "T" varies according to the physical length of the pit, and is dependent upon the definition of the pit in the control bytes in each sector. The length of the pits that represent a given number of binary bits are the same regardless of placement on the disc, but the velocity of the disc's rotation varies according to how long it takes to read these defined pit lengths. In other words, the rotational speed is controlled by the definition of the pit lengths.
ITOP, the distance from the signal baseline to the top of the amplitude of the I11 signal, is used to calculate the ratios of I3 and I11 to ITOP. The I11 signal is that sent by the laser reading a pit 11T long, and the I3 signal is the result of reading a pit 3T in length. The magnitude of the signal is the important part because the timer is sending zeros to the data stream until a change at least 3 zeros long is encountered. The Red Book says I3?ITOP must be between 0.3 and 0.7, while I11/ITOP must be above 0.6. Within the I11/ITOP test, the general rule of thumb is the higher the reading, the better, since this generates a well-defined high-frequency signal.
The pulse width in the RF signal or "eye pattern" can be affected by a DC offset superimposed by the pit/land conditions. All CD readers contain a correction circuit to eliminate this DC offset and maintain the proper pulse width entering the decoder. It is desirable to minimize this offset or asymmetrical condition by maintaining the proper pit-to-land definition in manufacturing. It is also desirable to attain 0 symmetry on a molded or replicated disc to minimize compensation by the player. Often, as you move through the manufacturing process, the pit-to-land ratio will change. Most manufacturing facilities will adjust the manufacturing process to attain a value close to zero on the final molded disc.
A Radial Noise test is a measure of how much the tracking system must move back and forth to stay on track of the pits. The straighter the pit track, the lower the number, and the better the reading. The maximum value for this measure is 30, according to the Red Book specification.
Push-Pull is a measurement of tracking signal magnitude. There are three analog signals measured from a player. The HF/Eye Pattern signal, the Focus signal, and the tracking signal. In mastering, we get max HF signal with deep pits and max push-pull signal with shallow pits. We have to find the comfortable situation in the middle to get a little of each. If the pits are too shallow, we lose HF. If the push-pull is too high, a one-spot laser pick-up will oscillate or over-drive. For this reason, an upper limit is specified. The minimum value allowed is 0.04, and the maximum is 0.09.
Cross Talk is a measure of the amount of interference coming from neighboring pit tracks. As track pitch is tightened (when tracks are packed closer together to put more data on a disc), Cross Talk increases. A maximum value of 50% is allowed by Red Book Specs.