5.2.4 Replay Equipment
22.214.171.124 Grooved recordings were made to be replayed with a stylus and pickup. Though optical technology has some special advantages which are discussed below (see section 126.96.36.199), and though advances in optical replay are bringing closer the likelihood of a practical system which does not require physical contact, currently the best and most cost effective approach to retrieving the audio content from such a recording is with the correct stylus. For lateral recordings a set of styli with different radii in the range of 38 µm (1.5 mil2), to 102 µm (4 mil), with an additional focus on 76 µm (3 mil) and 65 µm (2.6 mil) for early and late electrics respectively, is essential. The correct stylus for the particular groove will ensure best possible replay by fitting properly into the replay area, and avoiding worn or damaged sections of the groove wall. Records in good condition will reproduce with greater accuracy and reduced surface noise with elliptical tips; records in visually poor condition may be better suited to conical tips.Wear from previous use may well be to a particular region of the groove wall leaving some undamaged areas. Choosing an appropriate tip size and shape will allow these undamaged sections to be reproduced without including distortions caused by the damaged sections. A truncated stylus of either shape will better avoid any damaged areas in the bottom of the groove. Care should be taken in the replay of Pathé lateral discs as they typically have a larger groove width, and may require larger tip radii to avoid damage to the groove bottom.
188.8.131.52 Mono pickups are available, but it is more common to use stereo pickups as these allow separate capture of each groove wall. Moving coil pickups are often highly regarded because of their enhanced impulse response which aids in improving the separation of groove noise from audio signal. However, the range of various tip sizes for moving coil pickups is not as wide as that for moving magnet, are integral to the pickup, and those that can be ordered are around four time more expensive. Moving magnet pick ups are more common, more robust, lower cost, and generally more than adequate for the task.When replaying shellac discs a tracking force in the range of 30-50mN (3-5 grams) is often appropriate. It is recommended that a lesser tracking force be applied to lacquer discs. An advantage in using a stereo pickup is that this allows the two resultant channels to be stored separately, enabling future selection or processing of the separate channels. For listening the two channels may be combined in phase for a lateral recording, and out of phase (with respect to the pickup) for a vertical recording.
184.108.40.206 Selection of a suitable stylus in vertical recordings is governed by different criteria to lateral recordings. Rather than choosing a stylus to sit in a particular space on the side of a groove wall, playback of cylinders and other vertical cut recordings requires that a stylus be chosen that is a best match for the bottom of the groove. This is critical with instantaneous cylinders, where even very light tracking forces will cause damage if the incorrect stylus is chosen. A spherical stylus is generally preferred especially if the surface is damaged, though an elliptical stylus may well avoid frequency dependent tracking error. Typical sizes are between 230 (9 mil) and 300 µm (11.8 mil) for standard cylinders (100 grooves/inch) and between 115 (4.5 mil) and 150 µm (5.9 mil) for 200 grooves/inch cylinders. Cylinders should be replayed with a stylus whose tip has a radius a little smaller than the bottom radius of the groove. A truncated stylus will damage the groove because tracking will take place at the edge rather than the tip, resulting in increased pressure to that part of the groove.
220.127.116.11 When it comes to making decisions about what equipment to acquire, knowledge of the content of a particular collection will be the primary guide to determining the type of equipment required. Different types of carriers will obviously require different types of replay equipment, but even within similar carriers some specialist needs may arise.
18.104.22.168 Generally, historical equipment should not be used, mainly because of its poor rumble performance and in the case of cylinder players, greatly increased tracking force compared with equivalent modern replay equipment. Some problematic cylinders may not be playable on this type of equipment as modern cylinder players normally track the grooves with auto- controlled feed retrieved from the motion of the needle.When using this set up it is virtually impossible to properly track locked grooves, or scratches nearly parallel to the groove. This problem can be solved by using a modern player with fixed feed, or a modified historical cylinder player.
22.214.171.124 Radio transcription discs commonly have a diameter of 16 inches. If such discs are held in a collection, it will be necessary to procure a turntable, arm and pickup for discs of this size. For standard discs up to 12 inch records generally a modern precision turntable, modified to allow varispeed in a wide range, is required.
126.96.36.199 Negative metal stampers manufactured for mass replication of discs can themselves be replayed if an appropriate bi-point or stirrup stylus is available. This type of stylus sits astride the ridge (which is a negative impression of a disc groove) and needs to be placed carefully so as to avoid falling between adjacent ridges. As the stamper holds an inverse spiral to the discs it was designed to replicate, it should revolve anticlockwise, that is, in the opposite direction to a replicated disc, in order to be played from start to finish. To do this correctly would require a fully reverse-mounted tone arm. Much simpler and just as effective would be to play the stamper from finish to start on a standard clockwise turntable, and reverse the resulting digital transfer, using any current high quality audio editing software.
188.8.131.52 Bi-point styli are now extremely difficult to obtain, and fall into two categories, namely low- and high-compliance. The former are designed to repair manufacturing defects in metal stampers and as such are not ideally suited to archival transfer work. The latter, employing a significantly lighter tracking force are designed for audible replay rather than physical modification of the stamper, and so can be considered more suitable.
184.108.40.206 Turntables and cylinder phonographs for archival transfer purposes need to be precision mechanical devices in order to produce the minimum transmission of spurious vibrations to the record surface, which acts as a receiving diaphragm for the pickup. Low frequency vibrations are called rumble, and these vibrations frequently have a considerable vertical component. To reduce rumble generated by external vibrations,the replay apparatus must be placed on a stable foundation that is not likely to transmit structural vibrations. The replay machine should have a speed accuracy of at least 0.1 per cent; wow and flutter (DIN 45 507 weighted) better than 0.01 per cent; and an unweighted rumble of better than 50 dB. The turntable will be either belt or direct drive; friction drive wheel machines are not recommended as suitable speed accuracy and low rumble is not possible with these devices.
220.127.116.11 Any power supply wiring and the electric motor must be shielded to prevent injection of electrical noises into the pickup circuit. If required, additional Mu-Metal plates may be used to shield the motor from the pickup. The connecting cable to the pre-amplifier must be within the specifications regarding the loading impedance for the pickup. The installation should follow best analogue practice and adequate grounding procedures must be adhered to in order to ensure noise is not added to the audio signal. All of the above suggestions and specifications should be quantified, by analysing the output from test discs (see 5.2.8).
18.104.22.168 Both turntables and cylinder phonographs should be capable of variable replay speed, with the possibility of half-speed replay being particularly desirable (see 22.214.171.124), and feature a speed readout to allow documentation, possibly as a signal suitable for automatic logging for metadata. The pickup arm must sit on a base that can be adjusted, not only as regards distance from the turntable centre, but also in elevation.
126.96.36.199 In order to evaluate and decide on the most appropriate equipment and settings, comparisons must be made between the different options. This is best achieved through simultaneous, or A/B comparison, and audio editing software should be chosen which allows multiple audio files to be compared simultaneously. Transferring portions of a recording with different parameters and aligning the different resulting audio files in the editor for listening purposes, allows repeated direct comparison and reduces the inherent subjectivity of the process to a minimum.
188.8.131.52 A decision will need to be made as to the application of an equalisation curve prior to digitisation (see 5.2.6 Replay Equalisation).Where this is desirable, an appropriate preamplifier will be required, adjustable to recreate all necessary settings.
184.108.40.206 As an alternative to contact pickups the entire surface of a disc or a cylinder can be scanned or photographed at high resolution then converted to sound.Various projects have been developed up to a (quasi-) commercial level (ELP LaserTurntable; IRENE by Carl Haber,Vitaliy Fadeyev et al; VisualAudio by Ottar Johnsen, Stefano S. Cavaglieri, et al, Sound Archive Project, P. J. Boltryk, J.W.McBride, M. Hill, A. J. Nasce, Z. Zhao, and C. Maul). However, all of the techniques investigated so far present some limits (optical resolution, image processing, etc.), resulting in poor sound quality, if compared to using standard mechanical devices. A typical application for optical retrieval technology is for records in very bad condition, where mechanical replay devices would fail, or where the recordings are so fragile that the replay process would cause unacceptable damage.