The physical aspects of the various CD formats are essentially the same. The disc is 12cm in diameter and is supported by a rotating disc table that is attached to a spindle motor and clamped from above. Data is read from the disc by a laser, which traverses from inner edge to outer edge.
Data is recorded in one long spiral track from inner edge to outer edge. To maintain the same data rate off disc at all times, it is necessary for the rotation speed to reduce as the laser moves towards the outer edge. This approach, more commonly known as CLV (Constant Linear Velocity) means that more data can be packed on to a disc. If the rotation speed were constant (CAV--Constant Angular Velocity), then the data would have to be stretched out towards the outer edge of the disc, resulting in less storage potential. The CLV ranges from 1.2 to 1.4 m/s.
The 44.1kHz, 16-bit digital audio data stream is not suitable for storing directly to disc because it would not be possible to retrieve the data reliably. This data is first ACIRC (Advanced Cross-Interleaved Reed Solomon Code) processed for error correction purposes, then sub-code data (track, time and other information) along with its own error correction data is added. Each byte of the subsequent data stream is then converted to a 14-bit word, a process known as EFM (Eight to Fourteen Modulation).
EFM must meet the following criteria: no two 1s will be consecutive; the minimum spacing between two 1s will be two 0s (3T pit/land where T= 1 clock period); and the maximum spacing between two 1s will be ten 0s (11T pit/land)
These criteria ensure that the frequency bandwidth of the signal on disc is reduced; the DC content of the signal is reduced thereby improving stability of servo circuits; the pit length is wider than the track width; and with regular appearance of 1s (no more than 10 consecutive 0s) the Phase Locked Loop of the clock recovery circuit will remain locked.
The EFM process uses a look up table for converting each 8-bit byte to 14-bit word. As there are 256 different combinations for an 8-bit word and 16384 combinations for a 14-bit word, the 14-bit words which best meet the criteria above have been selected. These are listed in the Red Book. Each 14-bit word or symbol is joined to the next symbol by three merging bits. This is necessary because it cannot be guaranteed that when one 14-bit symbol ends in a 1, that the next 14-bit symbol would not commence in a 1, therefore disrupting the EFM criteria.
The EFM data +3 merging bits are recorded along the spiral track in the form of 'pits' and 'lands':--a transition from pit to land or land to pit being the moment a digital '1' occurs.
The decode or playback process for CD-R is essentially the same as that for standard CD.
A laser is focused on the surface of the spiral data track on the disc and the change in intensity of reflected laser light caused by the different reflectivities of a pit and a land is detected by a 4-way photo-diode array (ABCD). A pit has a lower reflectivity than a land and results in a lower voltage level at the RF output.
The three-beam system is adopted by most CD systems as this provides better tracking stability. The first CD players used a 1-beam system where the ABCD diodes were used to provide tracking information.
The three beams are derived from a single beam by passing it through a diffraction grating and an optical assembly guides the resulting beams to the disc. In addition, this optical assembly guides the reflected beams that are modulated by the surface of the disc, to the photo-diode array.
Focus of the laser beam is achieved by ensuring that the focus error = 0. This occurs when the intensity detected by photo-diodes (A+C) equals that of (B+D), that is when the detected beam is circular. If the detected beam is elliptical, this results in a non-zero focus error. In the figure bottom left, the intensity detected by (B+D) is greater than that detected by (A+C). This results in a negative focus error voltage that is used to control the focusing system.
It is essential that the laser beam does not stray from the track being read--this is achieved by ensuring that the tracking error (E-F) is zero, that is when the intensity on E = intensity on F. If E > F or F > E then the laser is not tracking the centre of the track.
Data information is retrieved from disc by summing the outputs from the four photo-diodes A, B, C, D. The resulting RF signal is amplified and then compared to a voltage threshold level known as the slice level. Any level above the slice level results in a logical 'high' and any level below the slice level results in a logical 'low'. The resulting data stream is fed into a Phase Locked Loop circuit to lock the decoder to the data rate coming from disc. This ensures that data is decoded at the correct rate otherwise huge data retrieval errors would occur. The data is clocked into the decoder circuits at the correct rate and is then subject to the opposite of the encode path that is, EFM demodulation, sub-code removal then error correction. If the system is working correctly, we should have a 44.1kHz, 16-bit audio signal that is identical to the signal that was recorded.
However, if there is no EFM data on the disc to provide lock for the spindle servo, that is, recording on a blank disc, how is the rotation speed controlled? Every blank disc is prestamped with a 'wobble' signal. The spiral data track wobbles from side to side with a displacement amplitude of 0.03um and at a 22.05kHz rate. The photodiode array is used to detect this wobble signal, which is subsequently processed to produce a 22.05kHz square wave. If the wobble decode circuits detect a frequency other than 22.05kHz, then the spindle motor rotation speed is adjusted until the frequency detected is the correct value.
Since the rotation speed of the disc changes from inner to outer edge, the actual wobble frequency prestamped on the disc also changes from inner to outside edge.
Absolute time in Pregroove (ATIP)
The ATIP information is encoded on the blank disc by frequency modulating the prestamped, wobbled spiral track. The 22.05kHz carrier frequency is modulated by a 1kHz signal such that a 23.05kHz (22.05+1) frequency conveys a '1' and a 21.05kHz (22.05-1) frequency conveys a '0'. The resulting decoded stream of ones and zeros provides the ATIP in the form of minutes, seconds and frames of which there are 75ps. The ATIP increases monotonically from inner to outer edge of the disc.
The start time of the Lead-in Area is encoded in the ATIP and this varies slightly from one manufacturer's disc to another's. The end of the lead-in area is always defined as 99:59:74 (mins: secs: frs) and the start of user recording area or program area begins at 00:00:00. The PCA and PMA regions start at pre-defined times relative to the beginning of the lead-in area.
Program Calibration Area (PCA)
OPC is performed when a blank or partially recorded disc is loaded into the CD burner. Some burners will in addition, wait for the first record command. The OPC begins by retrieving the recommended record laser power value from the ATIP in the lead-in area of the disc. This value becomes the starting point for the calibration process, which involves recording data in the PCA at various laser powers above and below the starting value. This data is then read back to determine which laser power achieves best symmetry around the 'slice level' (see above). Perfect symmetry results in equivalent pit and land lengths. In practice, a parameter known as beta is measured to achieve optimum symmetry. The optimum laser power is set and is used for recording as long as the disc is not ejected.
The PCA is large enough to allow up to 100 OPC procedures and this is adequate since a CD can only have up to 99 tracks.
Some older CD-R burners did not perform OPC and instead only used the recommended laser power value written in the ATIP. Hence, there were more compatibility problems in the early days of CD-R.
Some new CD-R burners also incorporate 'Running OPC', a system which continually monitors the recorded signal throughout a record process in order to adjust the laser power to achieve optimum signal at all times. For example, a fingerprint on a disc will effectively reduce the amount of laser power reaching the dye layer. Running OPC will therefore increase laser power to compensate.
The Program Memory Area (PMA) or temporary table of contents starts about 13 seconds before the lead-in area and is used to store information about the partially recorded disc. More specifically, the start and stop time of every track on the disc is stored here. When a disc is finalised, the PMA data is copied to the real table of contents (TOC) in the lead-in area in order to make the disc Red Book format and thus playable on a standard CD player.
THE DESIGNERS OF CD were surprised by the reliability of the CD system because of the complexities of the various optical and electronic sub-systems--it worked better than anticipated in practice. However, slight misalignments and dirty or worn parts can result in corrupt data especially in a CD-R system.
In order to write a CD-R it is necessary to melt an organic dye, which requires about 10x higher laser power than that required for playback. As a laser ages, it deteriorates and requires a higher drive current to maintain the same power levels. It is therefore recommended that laser power alignment is checked every 500 hours or so. If laser power is too low, the organic dye may not melt properly causing an inaccurate transition from pit to land. If the laser power is too high, the organic dye may be over heated causing poorly defined pit and land lengths. Laser power is therefore crucial in maintaining a low jitter signal.
As far as the user is concerned, cleaning the laser lens is the only maintenance that they can do, although only those with a delicate touch should tackle this. In many cases, accessibility to the laser lens is difficult, in which case cleaning should be left to qualified service personnel. There are commercial lens cleaners on the market, but HHb believes cleaning is safer and more effective when using a cotton bud with cleaning fluid. If the lens is glass, use an alcohol based cleaner, if plastic, use distilled water. Beware of leaving cotton bud hairs on the lens surface!
Most modern CD players do not require any adjustment as the system software performs alignment. If a calibration problem exists, the cause is usually the laser. CD-R machines, however, do still require alignment. The following adjustments are necessary to ensure reliable operation.
Playback laser power--if out of alignment can cause playback skipping, digital noise on audio (glitching) and an inability to recognise discs, especially CD-R discs, as these have lower reflectivity compared to commercial CDs.
Record laser power--If too low, the dye layer will not reach melting point and therefore no recording is possible. If the laser power is slightly out of alignment, inaccurate burning of pits may result in a high-jitter RF signal. This CD playback RF signal is often referred to as an 'eye pattern' which is shown. A well-recorded signal is shown together with a badly recorded signal. Notice that the diamond shapes (eyes) in the well-recorded signal trace are clearly identifiable, whereas they are extremely blurred in the badly recorded signal trace. The latter is caused by inaccurate and widely varying pit and land lengths (high jitter) and would probably result in a high block error rate (BLER).
Why does the RF signal resemble an 'eye pattern' during playback of a CD? What you are actually seeing is the intensity of laser light reflected from the 3T to 11T pits and lands overlaying one another. This is shown more clearly in the other figure.
Focus servo offset voltage, gain--If out of alignment, may prevent the machine from detecting a disc or cause a noisy eye pattern, which may result in audible errors.
Tracking servo offset voltage gain--If out of alignment, will reduce the machine's ability to lock to the spiral data track on the disc, thereby causing skipping, and inability to locate different parts of the disc.
Wobble adjustments--If these are out of alignment, the machine will not be able to read the preformatted blank disc properly, thereby preventing disc identification and reliable recording.
In some cases, playback problems occur because of additive errors. As an example, consider the situation where a user has a CD-R burner, A, which is only just out of alignment; perhaps the record laser power is slightly too low. In addition, the user plays back his recorded discs on a standard CD player, B, which has not been serviced for years and its playback laser power is slightly too low. The user experiences problems playing back discs on B, which were recorded on A. He does not have problems playing back commercial CDs or CD-Rs, which were recorded using another burner on B. In addition, the discs burnt using A, play back reliably on another CD player. Here the problems only arise with a particular combination of recorder and player, A+B. The errors created by each alone are fairly innocuous, but when the errors are combined, the error correction systems are unable to cope.
CD-R users should also be aware that there are compatibility issues with CD-R media. In the last 18 months or so, we have seen the emergence of the 80 minute or 700Mb CD-R. [Note: Users may have noted that some 74-minute discs can store 650Mb of data and others can store 680Mb. The difference is due to the way in which the manufacturer defines a megabyte. Some use 1MByte = 1024kbyte and 1kbyte = 1024 bytes, others use 1MByte = 1000kbyte and 1kbyte = 1000 bytes]. The extra storage on an 80-minute disc is achieved by narrowing the track width from 1.6µm to 1.5µm. Some older recorders and players are unable to reliably lock to the narrower tracks and others may not be able to read the data on the disc because of its lower reflectivity. Most recent machines are able to handle the tighter specification of 80-minute discs.
In the audio CD-R field, there is the added complication of whether to use Professional or Consumer CD-R. The difference has nothing to do with the physical quality of the disc. Consumer CD recorders can only use consumer discs which are more expensive than professional discs. A consumer disc is identified by information in the ATIP. Professional recorders can use both types of disc.
In addition, caution should be exercised when selecting discs with regard to allowable 'write' speeds. Disc packaging usually informs the user of the minimum and maximum write speed specification. Some discs state a minimum write speed of x2. These are obviously not intended for real-time audio burners, but are suitable for computer burners, which are able to write at various speeds. Disc write speed is dependent on type of organic dye, dye layer thickness, reflectivity layer and groove structure, however most discs are manufactured to provide reliable results at a range of write speeds.
Often, CD readers may have problems reading CD-R media but are fine reading commercially pressed discs. Commercial CDs have physical pits and lands rather than an organic dye storage layer. These result in higher reflectivity than a CD-R and thus are easier to read.
The question of media quality and compatibility is a big one beyond the scope of this article, but it should be stated that not all discs meet the same standard of quality. Even the top brand manufacturers are susceptible to occasional batch problems although they are very rare. Considering the fact that hundreds of millions of discs are produced every year, there are bound to be the occasional failures. In general, assume a machine error or alignment problem before blaming the media.
With the proliferation of cheap computer CD burners, this technology is becoming a disposable one. Is it worth having your £100 CD burner serviced to ensure that it is within specification? Probably not. However, is it then reasonable to expect it to give you years of untroubled use? Again, probably not. Five years ago, a CD recorder would set you back thousands of pounds--certainly worth having serviced. Now you can buy 20 or 30 burners for the same price. Even if each one were to last only one year (the warranty period), you have the ability to burn discs reliably for 20 to 30 years.