Dear colleagues,

You'll find my translation of the French article about Clareety beneath (and completely for free!).
I hope there are not too many errors as French nor English is my mother tongue. And in fact I'm neither a specialist of these disks.
I'd like to draw everybody's attention to the fact that this is an article of October 2007.
I don't know what the status of the commercialization of this device is, but I hope that the fact that it was again presented in Paris at AES a month ago means it will soon be available.


Brecht Declercq
Media Manager DiVA-project
VRT - Room 2F3
1043 Brussels

The digital image serving the digitization of old disks
Jean-José Wanegh (1st October 2007)
Translation by Brecht Declercq

As a photography and phonography passionate, Charles Cros wanted to go beyond time. It's curious to see that today, sound meeting image gives us the possibility to make his dream come true. In this way, millions of phonographic records, threatened by years of time and victims of the fact that each play-out wipes out a little more the memory of these supports, are going to find a new youth, thanks to optical procedures without contact. One of these, Clareety, developed by a research team of INA, has numerous characteristics responding to the needs of archivists and restoration-specialists.

Although the audio quality of CD's is considered very good by the majority of listeners, there is a population of audiophiles who consider the encoding at 44.1 kHz of CD's not sufficient for a trustful reproduction of the musical treasures recorded with analog means. As an answer to this expectation, there is a tremendous offer of turntables for microgroove records. But even with the high level performance of these devices and the caring of these audiophiles for their vinyl records, every play-out still means a kind of wear-out. And although the pressure force of the read head is adapted to a minimum, there still is a mechanic contact of the needle with the sides of the groove. With every passing by, this slow scratching leads to an irreparable deterioration of the disk.

That's why as early as 1982, a student of Stanford University, Robert Stoddard, proved the possibility to use a laser, connected to a special arm, to realize an optical read-out without contact with the vinyl. In 1983, he created a firm called Finial Technology and with the help of Robert Stark he developed the first vinyl turntable reading out with laser. In 1989 Finial sold its patents to ELP from Japan. Only in 1991 the first ELP turntable was commercialized, while already for two years the CD sales had doubled LP's.

An audio heritage in danger

But the universe of the disks should not only be considered from an audiophiles' view, neither by the view of a nostalgic in search of lost time, when it was Teppaz to entertain our juvenile parties. As the years passed by, millions of recordings have been made around the world. For many years, phonographic recording was the only way to consign a musical, artistic, historic of political event to a physical support. Thus, an considerable amount of recordings is part of our heritage. Let's not forget that although it was invented in 1935, the magnetophone tape only appeared at radio stations after the Second World War. 

From the thirties until half the fifties, radio stations used directly carved disks, called disques souples, commonly known in France as Pyral. We should preserve all these recordings, but we should also make it possible to listen to them in the best possible conditions, without compromising the integrity of the supports. The problem is that we're talking about original recordings with often only one known copy. The weight of time made them fragile to the point that even one reading-out, even with a performant phono-pick-up, would destroy it forever. This motivated strongly throughout the whole world the development of no-contact read-out systems, with a diversity of optical solutions, from simply capturing the image of the disk with a scanner, to 3D-capturing of the topography of the groove using the principles of confocal laser microscopy. But sound archives' leaders don't choose an appropriate solution following the same rules as those followed by an audiophile. The reason is that at INA, the collection to be safeguarded counts 276.000 grooved disks (fig. 3). Even if optical read-out is the only way to extract the precious content of these disks without compromising their eternal life, still this chosen technique should make it possible to work without delay, with an optimal quality, using tools that can be industrialized and thus at an affordable cost of purchase and exploitation. If one knows that some technologies ask up to ten times the real time playing duration of the support for capturing them in 3D, no doubt this may be called a handicap that a lot of institutions would not be able to overcome. All these arguments were thus considered, when a team of INA researchers completed an original system for the optical analysis of the groove of a disk by colorimetric encoding of the variations of the gradient, in relation to the axis of the track.

ELP, the first laser turntable serving wealthy audiophiles

In this article, unfortunately it is not possible to make an overview of the range of all the solutions making use of optical read-out, going from those that are planned for the future, from whom some already have been commercialized, or at the point of industrialization, to those still in the prototype stadium. All these systems can be classified in multiple ways.

Firstly, there are systems working in real time, with the optical information directly being converted in an analog audio signal. In this system there is no signal processing. The ELP turntable is of course a typical example here.  The light produced by a laser is divided in five rays. Two of them are meant to follow the track of the disk by detecting the 'shoulders' of the groove (the tops of the sides). After the reflection, these rays are digitized and then treated to serve tracking. Two other rays lighten the flanks of the groove on an adjustable height, to benefit from a zone where the needle did not leave traces of wearing the groove out (fig. 6). 

The reflected and modulated light of the sides of the groove is sent back to a detector that converts the light signal into an analog audio signal. The fifth ray allows to control the altitude of the optical pick-up arm, in function of deformations of the disks and to adapt to different widths. If the barrier of the cost of these turntables is overcome, this solution may have several advantages to convince audiophiles and one would be tempted to believe that already with this device we have the ideal tool for the transfer of all phonographic collections of the whole world. But nothing is less true in fact. It is an excellent tool for reading out vinyl and 78 rpm LP's in excellent condition. The majority of tests by audiophiles speak with praise about this technique, but the systems has huge problems to cope with scratches and dust. The system is unable to read from transparent or colored vinyl, or materials transparent to the wavelength of the laser. We see the same impossibility with directly carved acetate disks. It is impossible to read the 78 rpm's with a vertical groove, as the rays have been conceived to detect the lateral modulation. It's also important that the form of the needle is V-shaped. Round-carved sides of the groove could create distortions. To assure a correct following of the track, it is necessary that the top of the needle and the surface of the disk come together in a perfectly defined angle. In case of a border rounded off, the tracking will be impossible. The last issue is that scratches or bits of dust could deviate the system from the groove, in some cases à cheval (spread) over two adjacent tracks, one ray reading the side of one groove, the other reading the opposite one of its neighbor.

In the nineties Juraj Poliak, an associated researcher of the Ecole Polytechnique Fédérale de Lausanne (CH), developed a real time read-out system using an optical fiber, with a glass ball on its edge, to follow the groove of a disk or a cylinder. This fiber allows to take a laser ray into the groove, that after the reflection towards a photo-detector takes into account the lateral and vertical shifting of the original cell. This system is very simple in its concept and it has the advantage of being able to read the lateral grooves as well as the vertical ones. Poliak is now retired, and his method seems never to have gone beyond the prototype stadium. 

Read-out without contact by digital imaging

After these real time systems came the systems that in a first stage produce an image of the disk or the groove, measuring then its width, to finally extract the digitally processed audio information. On this terrain, two projects are taking place. One of them, named IRENE, is lead by a team of two researchers, Vitaliy Fadeyev and Carl Haber of the Lawrence Berkeley National Laboratory. The other project one was born at the Lausanne School of Engineering, and lead by Ottar Johnsen, with active participation of Sylvain Stotzer, who made his PhD in information science on this subject. The team of Berkeley works in the field going from  applied optical metrology to particle physics, the latter being the source of the idea to make a trace of micro-images of the surface of the disk, that after processing will allow to reconstruct the complete image of the groove. Typically these are 700 µm to 540 µm pictures, containing multiple parts of grooves (fig. 7). In case of a 78 rpm disk of 25 cm, this means 100.000 snapshots have to be made for a data-volume of 1 Gb to 1 Tb. On this level it is possible to process the image, to eliminate certain defaults caused by particles, scratches or the disk being worn-out. After photographing, it's all about measuring in every point the position of the border of the groove, compared to the middle of it.  With this technique, only the disks with a lateral groove can be treated. Disks with a vertical groove, like cylinders, are analyzed with a confocal microscopy-device using laser, which is a very heavy technique to setup. Ottar Johnsen and Sylvain Stotzer's approach is very different. They make one high resolution picture of the disk, which is digitized afterwards via polar coordinates, with a scanner specially developed for this purpose named VisualAudio. Here too a precise identification of the borders of the groove has to be done to measure their position. 

Whether it's about this Swiss project or the one of Berkeley, in the two cases the subject of the captured information is the position of the sides of the groove. But, as many of you know, the amplitude of the audio signal is not proportional to the position of the needle, but to the speed of its lateral shifting (fig. 8). And thus, to know this speed, we have to calculate the derivation of the curve that determines the position of the groove. This makes it necessary to have an excellent resolution to measure the position of the groove at its beginning, typically 1 micron per pixel.

In France, INA prefers to see a mirror in every disk

In France a research team of INA directed by Jean-Hugues Chenot has chosen an original direction, starting form a relatively simple but long known constatation, to develop its optical digitization unit for old disks. The idea is to keep in mind that every side of a groove behaves as a mirror.

It's important always to remember that using a lateral modulation, the radial speed of the grooving needle is proportional to the amplitude of the signal. So every angle formed by the side of the groove and the tangent of the circle that describes the rotation of the disk is a result from the combination of the radial speed of the graver and the tangential speed of the disk (fig. 10 and 11). If now we lighten the flank of this groove with a bundle of light with certain characteristics, and we analyze then the reflected light, we should be able to discover the sound information that is recorded on the disk.

There is a precedent of this method, known by all sound engineers who have engraved disks as the Buchmann-Meyer method. This method was used to verify the engraving-characteristics of their device. Rather than measuring with a microscope the amplitude of the groove, one measures the width of the luminous strip, reflected by the side of the groove when the disk is lit from a distance with a parallel bundle of light. In every point the width of this strip is proportional to the speed of the engraving, and thus to the amplitude of the sound. This measuring technique was already used for the engraving of 78 rpm's. These engraving test disks were called waxographs.

But the difference with the method developed by the research team of INA is that this method is used only as a way to control things. It doesn't allow to optically read-out the content of the disk.

Reading out the sound by a colored, structured illumination

The great originality of the method proposed by Jean-Hugues Chenot is to encode the incident light in various colors. So, at the inside of the continuous colored spectrum coming from a filter at the end of an optical condenser, every colored ray is characterized by his incident angle. While this rainbow-colored bundle, of whom all rays that converge into one point, hits the side of the groove, every elementary colored ray is reflected in an angle that is the same as its incidental angle (always conparing to the norm) in this point of the groove (cf. Descartes' law of reflection). If we put a sensor in a well-determined angle on the path of the reflected bundle, only the colored ray with the right incidence (compared to the gradient of the side of the groove) will be reflected in the direction of the sensor (fig. 12 a, b, c). Doing so, we have an encoding related to the color of the angle of the groove.

If instead of a simple sensor we put a color video camera, it's not a point per point analysis anymore. Within the field covered by this camera, multiple parts of the groove can be simultaneously analyzed. Typically every image covers a zone of 2mm by 2mm and includes eight tracks (in the case of a 78 rpm), of which six are taken into account with every analysis (fig. 13). We thus obtain a useful image of 400 by 400 pixels. But one of the big advantages of this method, compared to the others that have been described earlier, is that this camera carries out a high speed measurement of the whole disk. From a measurement from point to point we thus go to acquiring several millions of points per second. With a rate of 60 images per second, a whole disk can be done in a time-span close to the real time duration of the disk (18O seconds). Thanks to this method, it is not necessary anymore to have the high resolutions needed in the methods of Carl Haber or Sylvain Stotzer. In this method, we have enough with 5µm per pixel, considering that it's not the position of the groove that is being measured. To this, another considerable advantage can even be added: the height of the side of the groove is sufficiently important to cover multiple (ca. 30) pixels. As the groove thus seems to be composed by different parallel tracks, redundancy is entering the signal.

Error treatment by the image

Errors that have been established in the grooves may be due to a bit of dust, or a contamination coming from the exudation of a plasticizer, as is often the case with acetate disks. Errors like these diffuse the light, instead of reflecting it. In some cases the sides of the grooves are marked with almost parallel scratched as a result of an abrasion caused by a wrongly adjusted needle or an excessive pressure. In the case of a directly engraved disk, this phenomenon may be caused by the harmful effects of a damaged needle. In similar circumstances a calculation of the average depth of the trace in the total length of the groove may free the result from traces of abrasion. In general these errors can easily be detected. A cartography of these can even be drawn up to cancel them or to reduce their effects during the processing of the image, even before extracting the audio signal.

This cartography of errors will eventually help to draw a file with metadata, that during the digital processing of the signal will provide the exact position of the errors. For these errors some restoration technique will have to be applied. But bearing in mind that a correction is never completely neutral, we avoid to tackle the signal where it is healthy. On the longer term, the system should also be able to carry out an automatic detection and correction of cracks as seen on the 'marble-lined' disks, as reconstructing these puzzles is no more than a software issue (fig. 14). In the case of a 78 rpm disk, this system offers 240.000 samples per second on the outside. At the centre of the disk there are still 78.000 samples per second. This means that compared to a digital signal of 48 kHz, we still have a comfortable over-sampling. The response in matters of frequency is very good and allows to detect details of a signal down to 15 kHz on a 78 rpm. Of course, in case of a 33 kHz micro-groove, this sampling is seriously being reduced because of the slower rotation speed of the disk. This effect can easily be compensated by an appropriate magnification of the optics of the camera. But, as everybody knows, before ending up with a complete reconstruction of the audio signal, it is a collection of images of whom a certain number is deliberately operated as an overlap zone. To cover a 30 cm disk, circa 24.000 images are made. During this acquisition the camera is positioned on a radius that remains the same during the complete rotation of the disk. In this way a series of images is made forming a crown. From this crown the image processing software calculates the junction points of the six observed tracks, thanks to the overlap zones of these images. After decoding, we obtain thus the equivalent of a multi-track piece of audio, covering a duration corresponding to the time needed for one complete rotation of the disk, or 769 ms. In this way the whole disk will be decomposed into crowns of multi-track audio with a length of 769 ms.

Don't hesitate to follow the gradient

Today this system named Clareety has already been tested at INA and the results are promising. It's development is part of the PrestoSpace project. In 2006 INA associated with an industrial partner, INDEEP from Annecy (F), aiming to industrialize and commercialize it. The system responds completely to the goal it was designed for: no-contact reading-out acetate disks that were recorded from the thirties to half the fifties, especially those for which the fragile state doesn't allow any read-out using a needle (fig. 15). The test particularly proved that direct digitization from the gradient of the grove offered a bigger sensibility of the system than the sensibility coming from calculating the width of the groove followed by a calculation of its derivative function. But way beyond this first goal, this system has proven its capacity to read-out 78 rpm's with lateral engraving that have been commercialized in the fifties, just as the first Berliner disks characterized by a very particular groove profile. A deviation at 90° of the optical read head allows moreover to read Pathé-Saphir disks with a vertical engraving. Also the read-out of 33 rpm's is proven. Inevitably we come to the question of stereo disks. Two solutions are possible. The first is reading for example the outside of the groove (right channel), then the inside (left channel) and then synchronizing the two tracks to get a stereo signal. This solution doubles the time to take the images. The second solution consists out of installing a second optical head to scan simultaneously the other side of the groove. In that case the system is a little more complex and costly, but also realizable. To read bent disks (victims of a 'curtain effect') without a hitch, the system will be equipped with an automatic correction of the altitude, keeping a perfect focalization during the whole rotation of the disk. Normally, at the end of this year, a first industrial system will be available.

A new vision on the analog world

Finally we can say that the solution proposed with this INA-system is as far as we know the only system for optical analysis and digitization of disks that's being industrialized. It's major asset is the availability on the market of a product that meets the technical and economic requirements of archiving institutions, as well as those of audio restoration professionals. As far as we understood, this system would cost little more than an ELP turntable. Of course we're still far from a system that's affordable for aficionados of rare recordings. But what's important, is that from now on we can be sure to find a system capable of extracting thousands of hours of recordings out of these phonographic pieces. There's good hope that finally a lot of archiving institutions will be able reveal these fragile memories. Thus, for once, the marriage of the digital and the optical doesn't consign the old analog carriers to the attic, but helps them to assure an eternal life.

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