The peak level of the master does not affect the cutting level.
For each cut, the signal supplied by the player is evaluated in terms of analog characteristics, and is adapted accordingly to meet the requirements.
The signal-to-noise ratio (for analog media) and resolution (for digital media) should be fully utilized; therefore a high overall level is advisable for the master.
However: In the case of digital recording media, various investigations concerning headroom requirements have yielded different results; to be on the safe side we recommend restricting the level to -3 dBfs rather than -0.01 dBfs.
For audio tapes: Peaks may reach 510 pWb/mm, or 850 pWb/mm in the case of studio tapes capable of reproducing a large dynamic range with high peak levels.
The level used when transferring to disk, referred to as the “cutting level”, determines the extent to which the groove cut into the lacquer is modulated by the audio signal, and hence the level generated by the playback system. A higher cutting level improves the signal-to-noise ratio with respect to rumbling, hissing and crackling noises, but also increases the risk of playback distortion. In addition, more extensively modulated grooves require more space, which decreases the amount of playing time available. The use of a lower level thus allows longer playing times and reduces the risk of playback distortion, however it also adversely affects the signal-to-noise ratio.
The cutting level is always related to the peaks in the audio material.
The impression of loudness is of course dependent upon the signal level; however, it is also strongly influenced by the dynamic range of the production. If there is a great difference between the peak and average level, the impression of loudness can be expected to be less than when the difference is small. The cutting level is always set in accordance with the peak levels of the production. A restriction of the dynamic range (through compression or limiting) can increase the impression of loudness. For the disk there is the advantage that very soft passages are boosted, so that they are not at risk of “disappearing” into the background noise during playback. However, highly compressed productions tend toward greater intensity in the mid- and high-frequency range, which may cause technical problems during cutting and give rise to playback distortion.
Among other things, the frequency response describes the frequency-dependent change in level associated with the transmission of a signal by an audio system.
In the case of a phonograph record, a significant alteration in the frequency level curve is undertaken in advance, before the audio signal is supplied to the cutting equipment. The frequency response is thus adjusted as follows:
Roughly speaking, prior to cutting, the low frequencies are sharply reduced and the highs are considerably boosted. The result must be “equalized” again when the record is played back, so that the playback results will approximate the sound of the master. The characteristics of these measures are defined in DIN 45543 and in the international standard IEC 98, as well as being described in the Recording Industry Association of America (RIAA) bulletin E1. (“RIAA curve” and “RIAA equalization” are widely used terms in this context.)
These measures were developed on the one hand so as to decrease the space requirements of the groove to be cut by reducing levels in the low-frequency range, thus achieving a greater maximum playing time for the disk. (Another factor is that otherwise, with the use of customary levels when cutting disks, the low-frequency modulations would result in impermissibly large deflections of the groove.) In addition, reducing the high frequencies during playback also decreases noise such as hissing and crackling noises.
Apart from the necessity of strict adherence to these characteristics, whether desired or not, two main aspects are inevitably associated with discrepancies between the sound of the master and the playback results for the finished disk:
Disk cutting and
In the process of producing a phonograph record, cutting is the most important factor in transferring the desired sound from the master to the disk.
With modern cutting equipment, the bandwidth of the signal that can be used for cutting is more than 20 kHz; however, important restrictions should be mentioned: The level for high-frequency signals that can be used for cutting is significantly lower than that for mid- and low-frequency signals. Cutting with high-frequency audio signals places increased demands on the cutting equipment, and overloading can result in damage to the equipment.
Audio information that contains many highs must therefore be transferred to disk using lower levels than are required for audio documents with a moderate proportion of highs.
In the low-frequency range, unrestricted suitability for disk cutting can be achieved only when the differences between stereo channels remain small. Poor mono compatibility in the low-frequency range can occasionally lead to excessive cutting depth, which can also overload the cutting equipment, as well as causing problems in subsequent production steps (electroplating and pressing).
Modulation associated with very low-frequency audio signals can cause oscillations (resonance) of the tonearm during record playback, making it difficult for the playback stylus to trace the groove, and causing playback quality to suffer. Signals lower than 15 Hz must not be present! They can damage the cutting equipment, and during playback such signals are very likely to result in uncontrolled resonance oscillations of the tonearm. In any case, frequencies lower than 20 Hz are acoustically meaningless.
In disk cutting, a stylus, usually made of ruby or sapphire, is placed against a blank rotating disk so that the stylus cuts a spiral groove into the surface of the disk. The disk is made of aluminum, coated with nitro-cellulose lacquer. For this reason, such disks are referred to in the industry as "lacquers" or "lacquer disks". (In the German-speaking world, this lacquer is referred to as “Folie”.) Due to the shape of the stylus, the groove has an approximately V-shaped cross-section, where the walls (flanks of the groove) are positioned at an angle of 90° to one another. Each groove wall slopes so that it forms an angle of 45° with the surface of the lacquer. The slow movement of the stylus from the outer region of the rotating lacquer-coated disk toward the center (pitch) results in the formation of a spiral groove cut into the lacquer. The stylus is mounted on a cutting head; actuation of the cutting head by an audio signal provided for this purpose produces movements of the stylus that are proportional to the course of the signal. During cutting, these movements create small deviations from the basic direction of the groove and/or the cutting depth set for the groove: This modulates the groove. The cutting head can accelerate the stylus along two mutually independent axes of movement. These axes are oriented at precisely 90° from one another; each forms an angle of 45° with the surface of the recording medium. The modulation axes are thus perpendicular to the groove walls. This means that each of the two directions of modulation has a groove wall available as a plane of modulation. Each of the two directions of motion is assigned to one channel of a stereo signal (the left or right channel), so that the two stereo channels can be modulated separately from one another in a single groove.
At the time when the process described above (referred to as “2 x 45° recording” or “a 45/45 system”) was first introduced for recording a stereo signal onto the groove of a phonograph disk, it was necessary to maximize compatibility with the mono pickups that were still in widespread use; these were capable of processing only lateral modulation (parallel to the surface of the disk). For this reason, the two individual stereo signals were assigned to the modulation directions in such a way that signals which were the same in both channels generated purely lateral modulation, and additional vertical components (perpendicular to the disk surface) were generated only by differences between the channels.
The following diagrams show the basic possible modulation directions (viewed in the direction of the groove). In each case, the adjacent illustration shows the associated groove modulation (viewed from above the recording medium):
The tape machine used for cutting to disc has two playback heads.
The tape is fed first to one head, then after a loop with several guide rollers, to the second head. We thus obtain two identical but time-shifted playback signals. The first signal is supplied to the cutting machine to control groove spacing, allowing the required spacing of the grooves to be calculated in advance and adjusted. Loud passages require more space, since they generate greater deflection of the groove than is the case with soft passages. The second, subsequent signal is used for the actual cutting of the grooves.
In order to create a master tape, before the music recording itself, it is usual and necessary to record several sine test tones that are at different frequencies (for example 60 Hz, 1 kHz and 10 kHz), and that have defined level differences with respect to one another.
There are two important reasons for this:
Firstly, studio tape recorders have the capability of shifting the recording and playback frequency response in the high and low frequency range.
This is due to the fact that different makes and types of tape exhibit different relative sensitivities in the high and low ranges. It is therefore necessary to be able to adjust the tape machine for a particular type of tape, so that the frequency response or sound of the playback corresponds to that of the recording. When a tape is played on an external machine, e.g. at a broadcasting studio or record cutting studio, the test tones are used to restore the recorded test tone level ratios, in order to match the playback frequency response to that of the recording.
Secondly, although ideally the recording head should be precisely perpendicular to the tape, deviations are always possible. The two stereo channels (left and right) are recorded side by side on the tape. If the angle of the playback head relative to the tape is somewhat different from that of the recording head, a slight time shift occurs between the two channels, because the signal from one of the channels reaches the head somewhat before or after the signal from the other channel. This causes the sound and stereo image of the playback to seem spatially diffuse with coloration particularly in the high frequency region. With the aid of a test tone preceding the music, the angle of the playback head can be adjusted so that the phase of the playback signal is identical in both channels, ensuring that the playback of the two channels is precisely synchronized.
When a phonograph recording is played back, the playback stylus is positioned between the walls (groove flanks) of the V-shaped groove, and is set in motion by the groove modulation that was generated by the audio signal during cutting of the disk.
Ideally, this motion should follow the deflections of the groove modulation perfectly: The playback stylus then precisely traces the motions made by the cutting stylus when cutting the groove. Depending upon the type of audio signal, the modulation can give rise to lateral or vertical movements. In order to ensure sufficient resistance to fracture of the playback stylus, and to avoid excessive wear of the stylus and the phonograph disk, the playback stylus must have a large area of contact with the groove walls. However, the radius of curvature of the stylus where it touches the groove walls must be small in comparison with the radius of curvature of the groove modulation. Otherwise extremely short-wave, very steep, or complex forms of modulation will not be traced sufficiently accurately by the playback stylus: In this case the playback stylus is no longer guided continuously by the groove walls, but only by the amplitudes oriented toward the stylus; this is interpreted by the stylus as a reduction of groove depth, in other words as additional vertical modulation. A signal foreign to the original is thus added to a damped version of the original signal. This results in losses, particularly in the high-frequency range, and causes the playback to sound distorted (see also Playback distortion).
The degree of fidelity to the original that can be obtained varies over the course of playing back one side of a phonograph disk, since the modulation has a longer wavelength nearer the outer edge of the disk than it does closer to the center of the disk: Because the speed of rotation of the disk during cutting remains constant, the available groove length per rotation becomes shorter as the stylus approaches the center of the disk.
The following illustrations show a modulated groove with a symbol depicting the playback stylus. The two diagrams show how the same modulation appears in two different regions of the recording area available on one side of a phonograph disk:
Thus a modulation that is complex, contains many highs, or simply has a high level, can normally be played back with greater fidelity to the original if it is positioned near the beginning of a record side than if it is located near the end. The speed of rotation used for cutting the disk significantly influences this effect; cutting at 45 revolutions per minute (rpm) results in a modulation with a longer wavelength than is the case at 33 rpm. If possible, the order of several selections for one side of a disk should be determined in part by the extent to which possible losses of sound quality, especially losses of high frequencies, are acceptable during playback, and by how intensively the high-frequency range is used in each selection.
A modulation can also subject the playback stylus to such high accelerations that due to the inertia of its mass, or due to a stylus tracking force setting that is too low, the stylus can sometimes lose contact with one or both groove walls, or can even leave the groove entirely. Depending upon the mass and shape of the playback stylus used, high acceleration caused by a modulation can lead to elastic deformation or even damage of the groove walls by the stylus in the area of contact; the playback result then becomes corrupted relative to the modulation originally produced during cutting. Modulation associated with very low-frequency audio signals can result in resonance oscillations of the tonearm which holds the playback stylus, making it difficult for the stylus to trace the groove, and causing playback quality to suffer. Due to the equalization of the frequency level curve that is required during playback (see Frequency response), noise in the low-frequency range such as rumbling or humming is particularly accentuated; therefore, to the greatest extent possible, the record player used and the place where it is set up should not introduce any vibrations or noise, and should be well isolated from environmental influences such as wind and floor vibrations.
When a phonograph recording is played back, the playback stylus is positioned between the walls (groove flanks) of the approximately V-shaped groove, and is set in motion by the deviations (modulation) generated by the audio signal during cutting of the disk. Ideally, this motion should follow the groove modulation exactly. Errors can have the following causes:
Important: Every type of playback error not only impairs the playback result, but also increases wear of the phonograph record.
Stereo or two-channel audio can be recorded on a disk. However, there are limitations:
Large differences between the channels give rise to particularly complex groove modulation, which increases the risk of playback distortion, and means that more space is required for the groove. For recordings where the use of a high cutting level is expected (such as maxi singles with short playing times for use by DJs), only moderate stereo effects should be employed; in the low-frequency range it is best to avoid these altogether! In the low-frequency range, large differences between the channels with regard to the level and particularly the phase are to be completely avoided, since the resulting groove modulation presents technical cutting problems and can cause the playback stylus to leave the groove entirely.
The degree of correlation of a stereo audio signal refers to the phase correspondence between the two stereo channels. The degree of correlation can range in value from
+1 (mono, with both channels exactly the same) to
-1 (where both channels have the same signal, with opposite phases).
The intermediate value of 0 can represent e.g. two completely different signals, or stereo channels with a signal on only one of the channels. Roughly speaking, stereo signals with a degree of correlation between +0.5 and +1 can be assumed to be sufficiently mono compatible: Mono compatibility exists if combining the two stereo channels to create a mono signal does not cause any significant impairment of the sound or level. This is important, for example, in productions that may be used for radio or television, since here it can be expected that the use of stereo will be restricted to varying degrees, up to and including mono reproduction.
The following maximum playing times are recommended for various disk formats and applications:
30 cm/12 inches:
Up to 14 minutes at 45 rpm and up to 24 minutes at 33 rpm;
for DJ applications up to 9 minutes at 45 rpm and up to 15 minutes at 33 rpm.
25 cm/10 inches:
Up to 8 minutes at 45 rpm and up to 14 minutes at 33 rpm;
for DJ applications up to 6 minutes at 45 rpm and up to 9 minutes at 33 rpm.
17 cm/7 inches:
Up to 4½ minutes at 45 rpm and up to 6 minutes at 33 rpm.
These specifications are only general guidelines! Longer playing times require a reduction of the cutting level, and may necessitate alteration of the characteristics of the audio material. Depending on the type of audio material to be transferred to disk, other combinations of format and speed of rotation may be more appropriate. We will be happy to assist you!
To create a marker space (or marker groove, identification groove), for a short time during cutting larger groove spacing is employed, permitting visual identification of the beginning of selections on the disk, for example.
A marker space has no effect on the playback result and does not create an additional pause. Thus, the beginnings of selections can be made visible even when the selections follow one another with no break. In the absence of other instructions, a marker space will be provided for the beginning of every selection that is identified on the master; the first selection on each side of a disk is of course preceded by a lead-in groove.
Here cutting of the groove is begun near the center of the disk, or near the label; the groove is then cut so that it spirals outward toward the outer edge of the disk.
This does not mean that the disk rotates in reverse; rather, at the beginning of playback the stylus must be positioned near the center of the disk, where the end of the recording on that side of the disk would normally be found. The requirements for the master to be supplied are the same for an inside-out cut as for an ordinary cut. However, it should be kept in mind that for the region near the center of the disk (the beginning of the recording in this case), the playback result may have a lower sound quality; see Record playback.
What is the purpose of an inside-out cut? The attraction of novelty may be the primary motivation for ordering such a cut. However: If a production designed for one side of a phonograph disk begins moderately or quite softly and develops so that it has the greatest intensity at the end, the playback quality could be higher if an inside-out cut is used. However, there can be problems when playing such disks with automatic record players: The attempt to place the needle at the part of the recording closest to the center of the disk may cause the tonearm to lift up and the device to shut off.
With an inside-out cut, the challenge in cutting is to utilize the available space on the disk so that the end of the material transferred to disk is located precisely where the lead-in groove would normally be found. An inside-out cut is possible for all disk diameters and speeds of rotation.
Here the groove forms a closed ring.
The audio information cut into the groove is the same length as one disk rotation, and is continuously repeated during playback. If during transfer to disk the time for one disk rotation corresponds to the measure or interval length for the audio material to be transferred, a cyclical rhythm results from playback of the loop. Since the same locked groove is played repeatedly, depending upon the playback conditions, this groove will become worn relatively rapidly; this effect is further accelerated if the material contains many highs or has poor mono compatibility (see also Playback distortion).
Loops are usually cut and played back at 33⅓ rpm. In this case, one disk rotation lasts 1.8 seconds, and the audio material to be transferred must be exactly this length. On the master this material must be provided at least five times in succession with no pause, just as it is to be heard during playback.
For transfer to disk at 33⅓ rpm, the following applies: One disk rotation has a duration of precisely 1.8 seconds, and a 4/4 measure will fit exactly into one rotation if the tempo of the master is 133⅓ beats per minute.
For 45 rpm the duration of one disk rotation is 1⅓ (1.33333...) seconds, and a 4/4 measure will fit into one rotation if the tempo is 180 beats per minute.
So as not to increase wear of the continuously replayed locked groove, to the greatest extent possible the master should use mono or only limited stereo, and should not make intensive use of highs.