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The great CD green pen controversy Compact discs are supposedly unimprovable, aren't they? Well, to the average user that is virtually true. Which is why claims for improvement after the application of a green paint or dye to the outside of a compact disc are intriguing to say the least. Well, we've done the tests and we reckon it's a load of garbage. By LEO SIMPSON Want to improve the sound of your compact discs? Of course you do. After all, no matter how good your hifi system, you will always be keen to improve the sound quality, particularly if it can be done for only a small outlay. 14 SILICON CHIP And so what would your reaction be if I told you that you could improve the sound of your compact discs for an outlay of only $10 or perhaps a little more, say $39.95? If it gave an audible improvement to a system costing a thousand or more dollars, it would be well worthwhile wouldn't it? The modification is that you apply a green dye from a special felt tip marker pen, to the inside and outside edge of your compact discs. This is claimed to give real sonic improvements. In fact, I'll quote directly from the literature for the "CD Stop Light" pen: "... will improve sound in all areas - depth, imaging, better bass, smoother mids and highs, less graininess and more musicality. When it's applied to a well recorded Compact Disc the difference can be detected immediately. The first dynamic note will be tighter and crisper than before". There is a lot more along these lines from the distributor of this product. -.·· ·1 This idealised diagram shows the optical system of a compact disc and the laser pickup. Invisible infrared light from the semiconductor laser at bottom is focussed by a lens system onto the disc's recorded surface. Light reflected from the disc passes back through the lens and is diverted by the prism and into the photodiode. Some of the laser light will be scattered and will reflect from the disc edges back to the photodiode. But will that cause any additional errors? So why is it supposed to work? The literature for these pens is not absolutely clear on this point, but let me quote directly from the CD Stop Light literature again: "A laser reads light, if it reads any light other than a direct reflection of its own transmitted light it will read it as a distortion. Also if its own transmitted light is reflected around the disc and it is then read by the laser it will also be interpreted as a distortion." Are you any the wiser? What I interpret this to mean is that some of the light from the laser in your CD player is scattered once it passes through the clear plastic on the underside of the disc. That scattered light will travel out to the shiny edge of the disc and be reflected back, whereupon it will add in a spurious way to the light reflected from the aluminium recorded layer of the disc. In so doing, it will degrade the signal from the optical detector (which is not a laser but an infrared detector diode) and therefore make it more prone to give errors in the data signal. If you accept that concept, then you should have no problems accepting the next part. The light from a CD laser is infrared and therefore invisible. It will be absorbed by a green paint or dye. So if any stray light from the laser is absorbed by the green dye, that should make the data signal from the CD that much cleaner. In other words, there should be less errors in the data and that should, ultimately, mean cleaner sound. OK, so if this interpretation is correct (and what other interpretation can there be?), then it should be possible to test for the reduction of errors off a CD, once it is coated on the edges with one of these green pens. How do you test for data errors from a compact disc? We're not talking here about steady state distortion of an analog signal by the way. We are talking about errors in the digital data stream. Data errors, if they occur, and are not corrected, will not show up as steady harmonic distortion. Instead they will be momentary imperfections in the signal which will be virtually impossible for any analog instrument to measure. So, we need a compact disc analyser to do the tests. Compact disc analyser The only organisation in Australia that we know of which has a compact disc analyser is Disctronics Ltd, in Melbourne. They are the only manufacturer of compact discs in this country. So we approached them with a view to using their compact disc analyser to do a whole series of tests to prove conclusively whether or not the CD Stop Light and other green pens actually work. Disctronics Ltd has four compact disc analysers in their Quality Assurance laboratory but they are not portable and so we could not borrow one. We would have to go to them. And so, on 16th October this year, I visited Disctronics for the whole day. I was armed with a selection of compact discs both good and faulty, a number of green dye pens and various cameras. I also had the full-time assistance of Disctronics' Quality Assurance staff member, Bill Dines, without whom these tests would not have been possible. Projected tests I had planned a number of tests. The first of these was fairly simple and involved looking at the signal waveform from the optical detector (the infrared detector diode) before it is processed by the decoder circuitry. Contrary to what you might think, this is not a simple "squared up" digital signal. Rather, it is rounded, because the laser light is being reflected Since the laser pickup is a mechanical system, it will be subject to variations in the way it tracks the disc surface. This makes it extremely difficult, if not impossible, to detect any variation caused by the marker pens. DECEMBER1990 15 the effect of signal hash due to scattering of the laser light by scratches on the underside of the disc. The photo of Fig.1 is from an absolutely pristine disc which is also from a very good master. It has an almost ideal "eye pattern". Fig.2 is from an average quality CD with not very many scratches or surface defects. Fig.3 is from a CD which is very badly scratched and virtually unplayable in parts. Hash on the digital signal Our objective tests of the CD stop light pens was done in the quality assurance laboratory of Disctronics Ltd, in Melbourne. The author is on the right, decked out in clean room gear, while QA technician Bill Dines is on the left. Between them is one of the Sony CD analysers. from pits in the CD metallisation which are not perfect (ie, they don't have vertical walls) and because the laser spot diameter is not extremely small with respect to the pit diameter. The signal I looked at is known as the "eye pattern" and reveals, to the practised observer, the depth of the pits and the general quality of the pressing from the master. Further- more, because it is an unsynchronised signal (ie, not stationary on the oscilloscope screen) and one which is hard to photograph, I looked at the signal from the lead-in track of a number of CDs. Three photographs of eye patterns from three separate CDs are featured in this article - see Figs.1, 2 and 3. Not only do these photos show the pressing quality but they also show Badly scratched discs such as this abused example do cause the laser to produce a lot of errors, as was clearly shown in our tests. However, even badly scratched discs like this can be played on most CD machines. A worse problem is scratches on the label which make the disc unplayable. 16 SILICON CHIP Notice how there is a lot of "hash" on the signal of Fig.3 compared with the very clean pattern in Fig.1. If you're not sure what I'm talking about, the hash is the blurring at the top and bottom of the signal pattern. This is a clear demonstration that laser light is scattered by scratches and surface defects on a CD and that the resultant waveform from the optical detector is noisy. Noisy signals like these are bound to have more errors than clean signals. Just for the exercise I compared the eye patterns of the same discs when they were treated with the CD pens but if there was any effect there was no way you could see a difference in the patterns. Clean signals stayed clean; dirty signals stayed dirty. Measuring error signals So far, I had confirmed that laser light can be scattered by surface imperfections on CDs and that this scat- Applying the CD Stop Light pen is a little easier because it has a groove in the tip. It's still a messy job, though. FIG.1 FIG.2 These three "eye" patterns show the signal directly from the laser photodiode of the CD player. Fig.1 is from a first quality disc with an almost ideal "eye" pattern. Fig.2 is from an average good quality disc, while Fig,3 is from the very badly scratched disc shown in a photo on the facing page. These photos clearly demonstrate that scratches (and presumably internal reflections) can degrade the signal from the photodetector. But the CD pen did nothing to improve matters in this regard. tering caused distortion (ie, noise and hash) of the recovered digital signal. So the first part of the claim for the CD Stop Light pen is undoubtedly true. Now I had to test whether the green dye treatment had any effect. Measuring errors in the data stream from compact discs can be done in a number of ways. Disctronics has two types of CD analyser in their QA lab, three made by Sony and one made by Shape, a US company. The Sony machines, type CDP-5000, can give a printout of every block error and interpolation on a disc. And this is where I came to the first hurdle. The printout for a typical good disc with an average block error rate might easily run to 100 to 200 pages, with 66 lines per page. For a bad disc, a complete printout would run to thousands of pages! Comparing two such printouts, before and after treatment with the green pens, was clearly going to be impossible. I had quite a few discs to do, so I had to compromise. I decided that for each disc I would only do an analysis of the innermost track (track one) and the outermost track. Because these were closest to the inner and outermost edges of the CD, they would be the most likely to benefit from the green pen treatment, if it worked. I wanted to do at least four discs to begin with. Two of these would be pristine, first quality discs. Two would be really rubbishy, with lots of scratches. After all, if scratches cause light scattering, then the green pens should have an even more beneficial effect. I also wanted to test whether the pens worked better on moulded or pressed compact discs. Those pressed by Disctronics have a clear centre section you can see through and a sharp outer edge. By contrast, those made by PDO (Philips Dupont Optics) and used for many of the Philips and European labels have the metallisation right across to the centre of the disc and a somewhat more rounded outer edge. So I had to test at least four discs. I started with a very good quality PDQ disc. As expected, it had a very low block error rate right through the disc but right at the end it suddenly had a bunch of interpolation errors. Interpolation errors are those that the compact disc player cannot correct by using the standard CIRC (Cross Interleave Reed Solomon Code) method developed by Sony. Instead, it virtually "guesses" what the data should be, from the data immediately before and after the interruption in the signal. Interpolation errors are serious because they represent a definite departure from the original signal and therefore they are distortion. In serious cases, as on badly scratched discs, interpolation errors become audible the disc sounds distorted. Block errors are corrected And that brings me to a most important point. All compact discs have block errors, lots of them. There are 7350 blocks (ie, parcels of serial data) per second and therefore around 30 million blocks on a disc. The average block error rate on a really good disc FIG.3 is likely to be about 20 in every 10second period. Therefore, such a disc is going to have around 9000 block errors. And that's a good disc! For a badly scratched disc, the average block error rate can be 400 to 500 in every 10-second period. This disc is going to have between one and two million block errors over the hour or so of its playing period! But the good news is that block errors are not only detected but that they are all corrected. The resultant data is exactly the same as ifno block errors had occured in the system at all. Many people have trouble with this concept of data correction. It means exactly what it says - data is corrected to precisely the same value as it would have been if the error had never occurred. So good is the CIRC error correction that it can completely correct for data interruptions of up to 4000 bits that's equivalent to a spot on the disc of 2.5mm in diameter. Most CD players cannot track over such a large interruption by the way, but the error correction can cope with it. However, the story becomes even more complicated. Never twice the same The trouble with measuring and comparing block error rates read off discs is that the errors will be different for each disc each time it is played! Why? Because the laser tracking mechffnism just cannot track the disc the same way each time it is played. It is a servo-mechanical system. It can no more track a disc identically each time it is played than you can drive down a road in an identical way each time. Your car will always take a slightly different track than before - and so will the laser. And therefore the block errors are bound to be different, even though the block DECEMBER 1990 17 01:00:13:00 NOISE B.E.R 01:00:13:14 NOISE B.E.R 01:00:13:30 NOISE B . E.R 01:00:13 : 44 NOISE B.E.R 01:00:13:59 NOISE B.E.R 01:00:13:74 NOISE B. E . R 01:00:13:74 NOISE INTER 01:00:14:14 NOISE B . E.R 01:00:1,4:29 NOISE B.E.R 01:00:14:44 NOISE B.E . R 01:00:14:59 NOISE B.E . R 01:00:14:74 NOISE B.E.R 01:00:14:74 NOISE INTER 01:00:15:14 NOISE B.E.R BLOCK ERROR RATE THRESHOLD INTER POLATION THRESHOLD BLOCK ERROR RATE (MEAN) BLOCK ERROR RATE (MAX.) 01:00:13:06 01:00:13:21 01:00:13:21 01:00:13:36 01:00:13:51 01:00:13:66 01:00:14:06 01:00:14:21 01 : 00:14:22 01:00:14:36 01:00:14:51 01:00:14:67 01:00:15:06 01:00:15:22 BLOCK INTER BLOCK BLOCK ERROR RATE POLATION ERROR RATE ERROR RATE NOISE NOISE NOISE NOISE NOISE NOISE NOISE NOISE NOISE NOISE NOISE NOISE NOISE NOISE POLATION POLATION B901001300033703710984 B901001314033903200848 B9010013300353039908F7 B901001344032703230869 B901001359033403290891 B9010013740306017408C7 C003060174020040 B901001414031202230925 B9010014290292024508DA B9010014440289021508BC B9010014590281020908BD B9010014740294020908EB C002940209020063 B901001514027701010866 0070 0071 0072 0073 0074 0075 0076 0077 0078 0079 0080 0081 0082 0083 B901001306031102150806 B901001321033002150840 C00330021502000C B90100133603290324085E B901001351033403220882 B90100136603060217085D B90100140603250217081D C002930217020070 B9010014220293021708A6 B901001436028401080 89 B B901001451025700030883 B9010014670276000308B8 B901001506025500030837 B90100152202800003097F :0200 :0100 :327 . 5 :0479 B.E.R B.E . R INTER POLATION B.E.R B.E.R B.E.R B.E.R INTER POLATION B . E.R B.E . R B.E.R B. E.R B.E.R B.E . R THRESHOLD THRESHOLD (MEAN) (MAX.) 0069 0070 0071 0072 0073 0074 0075 0076 0077 0078 0079 0080 0081 0082 : 0200 :0100 :325 . 2 :0479 Shown here are two CD analyser printouts of a scratched compact disc. Both printouts are for the same compact disc, played twice in succession on the same machine, without any treatment or even being removed from the machine. Note how none of the tracking errors are the same! Clearly, our tests showed that not only does the green dye treatment not work but there is no way that anyone could claim that it does! error rates will be much the same. I had not thought about this before but it was brought home to me strongly when I tried to compare two printouts for a CD, before and after treatment. I expected to find that many, if not all, block errors would be much the same. They're not. For example, I compared a scratched disc at time 01:00:02, before and after treatment with the green pen. In this very short time interval 18 SILICON CHIP (one hundredth of a second), there were two block errors, followed by an interpolation, followed by another three block errors. But while the block error rate was identical for both readouts, none of the block errors or the interpolation were the same. Now I can't say this for sure, but having carefully examined and compared dozens of pages of block error printouts, I don't think there is much likelihood of any given block error occuring twice in succession, for two successive playings of a disc. Think about it: each time your car goes over a pot-hole, it will respond in a slightly different way each time. And so, when the laser encounters an interruption in the data of a CD, the error is detected (and corrected) in a slightly different way each time. Comparing the printouts After doing a series of printout comparisons, I concluded that not only did the green pens not appear to have any measurable effect on the block error rate but they had no effect on the number of interpolations either. Still, I had to pursue the corn- parisons for at least the four discs I mentioned above. So for the remainder of the tests done on the Sony CD analyser, I just took the printout summary. This gives the maximum and average block error rates for the disc. As I have just stated, the differences between the before and after treatment results were negligible. There was one anomaly though. You might remember that I mentioned measuring a very good PDO disc which had a bunch of interpolations at the end, printed out on the summary. Since this was early in the process and I had not thought enough about the difficulties of the test, I decided to see if the green pen had any effect on this bunch of interpolations. The trouble was, I did not think to carefully examine the disc for dust or fingerprints at that stage. Messy business I then applied the green pen to the outer and inner edges of the disc according to the instructions. Now this is a tricky process. If you are not absolutely careful and do not have a steady hand, it is quite easy for the pen to wander over onto the label or the playing surface. On this occasion I made rather a mess of things, so I cleaned it off thoroughly with isopropyl alcohol and then reapplied the green pen. On the subsequent test, the disc went through with no interpolation errors at all. Voila! The pen really worked! Or did it? I then thought that perhaps there was a fingerprint on the disc for the first test and when I had subsequently cleaned the disc, after smudging it with the pen, I had removed it. So I thoroughly removed all the green dye from the disc, made sure it was spotlessly clean and then put it through the same test again. And what do you think? Not only were the interpolations still absent but the maximum block error rate for the disc was now slightly lower. The pen had had no effect at all! But cleaning the disc thoroughly had made a big difference to the interpolations. Block error histograms Just in case the Sony CD Analyser was not giving the full picture, I decided to do some discs on the Shape CD Analyser. This unit uses a Revox According to the instructions which come with the pens, the green dye must also be applied to the inner edge of the compact disc and to any grooves that may also be present in the central area. CD player linked to a special card in an IBM PSZ computer. As well as giving block error summaries, this gives a histogram of the block errors on a disk. Two of these histograms are shown with this article. They show that even though the maximum and average block errors may be similar, the actual histograms, showing a 10-second moving average of block errors, are slightly different. But that is what you would expect, knowing that the laser cannot track the disc identically in each case. We also did some bi-refringence tests on the discs before and after treatment. These tests, performed with red light from a helium-neon laser, show the stresses and strains in a compact disc. Since this test is certainly affected by laser light reflections inside the plastic of the disc, we thought that it might just show the effect of the green dye. But you guessed it, there was no measurable effect. Conclusions So what are the conclusions? You might say that the green pen treat- ment is a load of old rubbish and you'd be correct but let's take it a bit further than that. I started by accepting the theory that internal reflections of laser light might distort the signal coming off the disc. Certainly, a badly scratched disc gave a lot more hash on the signal than that from a clean disc. But digital systems are designed to be largely immune to superimposed noise and so should not be affected in the subsequent decoding. Even so, I still went along with the theory, considering the possibility that, at times, noise on the digital signal from the disc might just cause more errors. I did all the comparison tests and could find no significant difference in the block errors and no difference at all in the number of interpolations. As stated above, block errors are all corrected so even if there had been a difference in their number, there still would have been no difference at all in the final sound quality. There was no difference in the number of interpolations, so the sound quality could not be improved DECEMBER1990 19 due to this source either. It really is a load of garbage. What about listening tests? Some hifi writers have stated that they can clearly hear the difference of 11..0CK ERROR HISTOGRAM 70- rn10- the green pen. Now we regard this as straight out self-delusion. How do you do a fair test? You can't just listen to a disc, treat it with a pen and then listen to it again. It takes a couple of minutes to apply the green pen and 16/10/90 ID: BEACH BOYS LINE: "BAD" STAMP:BEFORE RUN: IRK ONE NI T_HOLD: 1 BURST T_HOLD: 7 NI ERROR:0 BURST ERROR:0 AUG BLER: 48 MAX BLER: 54 30- *iHHHtPASS***** i0- 10- Minutes: BLOCK ERROR RATE (1 0 sec. ave~ages> BLOCK ERROR HISTOGRA M 16 / 10/90 I D: BEACH BOYS LINE: "BAD " STAMP:AFTER RUN: IRK. 1 70- '9-- 50-- 10- rn-- NI T_HOLD: 1 BURST T_HOLD: 7 NI ERROR:0 BURST ERROR:0 AUG BLER: 48 MAX BLER: 56 *****PASS***** 20-- 10- Minutes: I 160 I 7 ~1 then another five minutes to let it dry. And you have to be absolutely sure that you have not put any fingerprints or dust on the disc in the process because that will definitely change the number of errors. The time difference between your two listening tests is likely to be around 10 minutes. With that sort of time gap, how can you be sure of anything? Nor can you directly compare two identical discs, one treated and one untreated, in two identical players. For a start, no two players are absolutely identical and neither are two CDs, even if they are from the same batch. And as we have already said, the same disc is never played through in exactly the same way by the laser, even though the resultant sound quality is the same. Are you getting the picture? Not only does this process not work but the people who are promoting it don't have the evidence to support their claims. If the process really did work, it would be supported by experimental evidence from some official testing organisation. These pens have no such supporting data, just a lot of uncorroborated statements. Our advice to people wanting the best sound quality from their CDs is this: (1) Buy a good quality CD player with a well known brand. Avoid the cheapies - they're an unknown quantity. (2) Take great care with your CDs. Always keep them in their cases when not in use. When handling them, take care not to touch the playing surface the less fingerprints and dust you get on them, the less you have to clean them. (3) Keep the discs as clean as possible. Fingerprints cause interpolation errors - they are worse then scratches in this respect. (4) Take even greater care not to damage the label in any way. Never write on it with any sort of pen or pencil. The screen printed label and the surface immediately under it is the only protection for the metallisation layer. BLOCK ERROR Rfi YE ( 10 sec . a ve~ages ) These two block error histograms show the first track of a <:ompact disc before (top) and after treatment with a CD green dye pen. In this case, the histogram after treatment is slightly worse but this can easily be caused by a fingerprint or a slightly different tracking error in the mechanism. 20 SILICON CHIP Acknowledgement Our thanks to the staff of Disctronics Ltd for their assistance in the preparation of this article and specifically to Alan Bremner and Bill Dines.