Why do we analyse CD masters and copies?
As we are a CD manufacturer and duplicate or replicate discs for our clients we have found that the investment in a proper CD analyser has been vital. It prevents lost time on projects by detecting problems early and gives us the time to have them rectified and still get the jobs out on time. With our inhouse CD work it has meant that we can monitor blank disc quality regularly and spot problems with batches of discs before they get to the customer.
The easiest way to get a picture of disc quality is to measure error rates. Most serious defects will cause an increase in error rates. By looking at both the quantity and severity of errors, you can get a pretty good idea of overall disc quality. In general Playback errors can be caused by two things: Localized defects, and poor pit geometry.
Some people assume that CD burning programmes (with verification software) are enough to detect errors. Unfortunately this is not always the case. - In most CD drives including burners there is sophisticated error correction capability - this actually works against us when we want to know the quality of the burnt master as it will cover up potential problems. Therefore the only way to detect disc problems is to use a proper CD analyser.
CD Analyser Report
So to summarise:
- Data comparison (software verification) does not measure disc quality.
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Just because you can play it on your CD player it doesn’t mean it’s good.
- Poor disc quality can cause unreliable operation and short lifetime.
Error Correction issues:
All CD's incorporate very powerful error detection and correction. Data discs have an additional layer of error correction as well. This error correction is extremely effective, and CD's could not work without it, as there would be far too many errors.
However, the error correction also hides a multitude of sins. In principle, it is possible to drill a 2mm hole through the disc, and still recover every single bit! But even though this disc is readable, you could not consider it a "good" disc. It will be slow and unreliable (due to the ongoing error correction). The data could become unrecoverable at any time.
The only way to judge the quality of the disc is to make sure it conforms to Red Book specifications. Measuring error rates also shows the location and severity of defects. From this information we can then determine whether or not we need a new master supplied or if it is good enough to produce bulk copies from.
Key to reliability of your CD master and copies:
In order for a disc to play, the reflected laser beam must generate track-following and focus servo signals so that the player can follow the track and stay focused. Generation of the proper servo signals is completely dependent on the size and shape of the pits on the disc.
Without the proper "pit geometry", the disc will not play reliably. So usually the key to disc reliability is "pit geometry". The player's ability to read the disc is wholly dependent on the size and shape of the pits and the creation of good pit geometry is dependent on the interaction between the media and the writer. With our analyser we can determine if this is ok.
Other factors that will affect the quality of duplicate copies include: Quality of the blank media, burning software, computer and the burner (especially if the laser is getting a bit old & tired!).
What are we testing for?:
With our analyser we can check the lead-in area of a disc, plus it detects HF dropouts and tracking loss (skipping). As well as BLER, E11, E21, E12, E22 and E32 errrors All of these tests can be helpful in diagnosing disc defects.
Error Reports
So what do the results mean?:
Q. Why do some discs 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.Will a disc that tests good, 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. If necessary our analyser 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.
So what do with the results?
For a manufacturer such as ourselves if we begin to detect repetitive problems then it's time to investigate why they are happening. it could be a poor batch of stock or time to change some CD burners. With replicated discs it may be that a faulty stamper has been created. If it's a master supplied by the client then we try to help them solve the issues at hand.
So by having the analyser operational we maintain our equipment on a regular basis, keep on top of stock issues and we constantly get feedback on the quality of our copies. We also help our customers improve the quality of work that they can do, Ultimately it means we can supply our customers with the best copies possible for both short run burnt copeis and bulk pressed CDs.
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