The answer lies in the difference between analog and digital recordings. A vinyl record is an analog recording, and CDs and DVDs are digital recordings. Take a look at the graph below. Original sound is analog by definition. A digital recording takes snapshots of the analog signal at a certain rate (for CDs it is 44,100 times per second) and measures each snapshot with a certain accuracy (for CDs it is 16-bit, which means the value must be one of 65,536 possible values). 

From the graph you can see that CD quality audio does not do a very good job of replicating the original signal. The main ways to improve the quality of a digital recording are to increase the sampling rate and to increase the accuracy of the sampling. This means that, by definition, a digital recording is not capturing the complete sound wave. It is approximating it with a series of steps. Some sounds that have very quick transitions, such as a drum beat or a trumpet's tone, will be distorted because they change too quickly for the sample rate. In your home stereo the CD or DVD player takes this digital recording and converts it to an analog signal, which is fed to your amplifier. The amplifier then raises the voltage of the signal to a level powerful enough to drive your speaker. 

A vinyl record has a groove carved into it that mirrors the original sound's waveform. This means that no information is lost. The output of a record player is analog. It can be fed directly to your amplifier with no conversion. This means that the waveforms from a vinyl recording can be much more accurate, and that can be heard in the richness of the sound. But there is a downside, any specks of dust or damage to the disc can be heard as noise or static. During quiet spots in songs this noise may be heard over the music. Digital recordings don't degrade over time, and if the digital recording contains silence, then there will be no noise. good job of replicating the original signal.

 

Disc Cutting  (THE  INFORMATION BELOW IS  30 YEARS OLD BUT IS PRETTY MUCH THE STANDARD OF  TODAY AND SHOULD ANSWER SOME VINYL QUESTIONS) taken  from  a book on recording  by Robert Runstein As the master tape is played on a tape machine, its signal output is fed through a disc mastering console to the disc cutting lathe.  Here the electrical signals are converted into the mechanical motion of a stylus and are cut into the surface of the lacquer –coated recording disc.     Unlike tape which maintains the chronological order and duration of recorded information by relating periods of time to physical lengths of tape, discs relate periods of time to the angle of disc rotation (Fig. 12-1). As the turntable rotates at a constant angular velocity such as 33 1/3 or 45 rpm, the stylus gradually moves closer to the disc center, cutting a continuous spiral into the disc surface.  The time relationship of the recorded material can be reconstructed by playing the disc on any playback turntable that has the same constant angular velocity as the one used to record the disc.     The system of recording used for stereo discs is the 45/45 system.  The recording stylus cuts a  90 degree angle groove into the disc surface so that each wall of the groove forms a 45 degree angle with the vertical.  Left channel signals are cut into the inner wall of the groove, and right channel signals are cut into the outer wall.  The stylus motion is phased so that a signal, which is in phase in both channels (a mono signal or a signal centered between the two channels) produces lateral motion of the groove.  (Fig. 12-3A); out of phase (channel difference information) signals produce vertical motion, i.e., changes in groove depth  (Fig. 12-3B).  Because this system is compatible with mono disc systems which use only lateral groove modulation, a mono disc can be accurately reproduced with a stereo playback cartridge.  However, unless monophono cartridges are designed for stereo compatibility, they will reproduce only lateral motion and resist movement in the vertical direction.  This resistance will damage the stereo information contained in the vertical motion of the groove if a stereo record is played with a mono cartridge.

Cutting Lathe The main components of the modern (1971) disc cutting lathe are the turntable, the lathe bed and sled, the pitch/depth control computer, and the cutting head.  The Neumann VMS-70 lathe is illustrated in Figs. 12-4 .  The tube entering the front of the cutter head provides helium cooling to permit extended operation at high cutting velocities. The turntable is very heavy in order to reduce speed variations via the flywheel effect.  It is driven by a special motor and linkage system to eliminate flutter and rumble from the recording.  Three sets of stroboscopic rings on the outer rim calibrate the four switch-selectable speeds of the turntable: 16 2/3, 22 ½, 33 1/3, and 45 rpm.  A vacuum suction system secures the recording blanks to the turntable via holes in the turntable surface.  The holes can be selectively opened or closed to provide proper suction to hold lacquer discs from 7 to 16 inches in diameter.  The suction is introduced through a flexible pipe connected to the hollow center post of the turntable.

Cutting Head The cutting head (Fig. 12-7) translates the electrical signals applied to it into the mechanical motion of the recording stylus.  The stylus gradually moves in a straight line toward the center hole of the disc as the turntable rotates, creating a spiral groove on the surface of the record.  This motion is achieved by attaching the cutting head to a sled.  A spiral gear known as the lead screw drives the sled in a straight track called the lathe bed.  The speed of the cutting head motion toward the center of the disc determines the playing time of that side of the record.        The head speed is called the pitch of the recording and is measured by the number of grooves or lines per inch (lpi) cut into the disc.  As the head speed increases, the number of lpi decreases, so the pitch and playing time also decrease.  Several methods of changing pitch are possible:  the lead screw can be changed for one with a finer or coarser spiral;  the gears that turn the lead screw can be changed to change the speed of the lead screw rotation;  or the lead screw rotational speed can be varied directly by varying the speed of the motor driving it. This latter method is used in the Neumann lathe and provides continuously variable pitch.     The space between grooves is called land (Fig. 12-8).  Unmodulated grooves are equally spaced at all points.  Adding modulation to the grooves produces lateral motion proportionate to the in-phase signals contained in the two channels being cut.  If the cutting pitch is to high (to many lines per inch, making the grooves very closely spaced) and high-level signals are cut, it is possible for the groove to cutover or break through the wall of an adjacent groove, of for the grooves to overlap which is called twinning.  The former is likely to cause the record to skip when played.  The latter causes either distortion of the signal or an echo of a signal in the adjacent groove, due to the deformation of one groove wall by the information cut into the next.  Groove echo can also occur even if the walls do not touch.  It is a function of groove width, pitch, and level, and it decreases as the signal frequency increases.  In addition, high-frequency echoes decrease in level as the groove diameter decreases [3].     These cutting problems can be eliminated by either reducing the cutting level or by cutting fewer lines per inch.  A conflict arises here because in comparison with a softer record, a louder one sounds brighter, punchier, more present, and fuller.  As a result, record companies and producers are concerned about the competitive level of their discs relative to those cut by others, so they do not want to reduce the cutting level.  However, reducing the pitch shortens the playing time of the record.     The solution to these problems is to vary the pitch, cutting more lines per inch during soft passages and fewer lines per inch during loud passages.  This automatic pitch control is achieved by adding an additional preview playback head to the tape machine feeding the lathe.  This head is positioned at a distance ahead of the regular playback head on the machine.  This gives the pitch/depth control computer in the lathe, which determines the pitch required for each portion of the program and caries the speed of the lead screw motor, time to change the pitch as required.  Since left channel signals are cut in the inside groove wall and therefor run no danger of cutting into an existing groove, a pitch change is not needed for loud left channel signals until the moment they are cut.  At that point, the pitch is decreased and the grooves expanded so that the following groove will be far enough away that a cutover cannot occur.  Since the pitch correction is not needed until the signal is cut, the computer derives its level information from the left program channel.  Right-channel signals, however, are cut in the outside wall of the groove, so loud right-channel signals require that the pitch change occur before the signal is cut, to make room for the new groove so that is does not cutover into the preceding groove.  In order to provide the computer with right-channel signal level information before the signal is cut, a preview playback head and its associated playback electronics must be added to the tape machine.  This head is positioned at a distance ahead of the program playback head of the machine (16.5 inches for a 33 1/3-rpm disc and 15-ips tape).  While pitch control requires preview information only for the right channel, depth control, to be described later in this chapter, requires preview information from both the left and right channels, so a stereo preview head is used.  When cutting a mono disc from a stereo tape, the sum of the left and right preview channels is used for pitch control information.  The computer samples the left program and the right preview signal level information every one-quarter revolution of the turntable and adjusts the pitch to the value required by the highest of the current and previous two level samples.     Pitch is divided into two categories; coarse which refers to between 96 and 150 lpi, and microgroove which is between 200 and 300 or more lpi.  Microgroove records have less surface noise, wider frequency range, less distortion, and greater dynamic range than coarse-pitch recordings.  They can also be tracked with lower stylus pressure, resulting in longer life.  This lower tracking force, however, makes the stylus more likely to skate across the record if the turntable is not level.  The playback stylus for a microgroove record must have a tip radius of 0.7 mil or less, as compared to 2.5 mils + or – 0.1 for coarse-groove records.  Early 33 1/3-rpm and 78-rpm records were recorded with a coarse-pitch.  Virtually all current records are microgroove with 265 lpi being an average pitch.  At maximum pitch, the playing time of one side of a 12-inch disc with no modulation in the grooves is 45 minutes.  The duration of modulated 12-inch discs cut at average levels is 23 to 26 minutes per side when they are cut with variable-pitch lathe.

Depth Control

In mono cutting, the depth of the groove remains constant since there is no difference between what would be left and right channels of A stereo disc, and thus there is no vertical information. The depth of the groove in a stereo disc varies with the vertical excursions of the cutting stylus and is measured from the surface of the disc to the bottom of the groove. If the depth is too great, the stylus will cut through the lacquer surface into the metal base of the recording disc, causing distortion and possible damage to the stylus. If the depth is too shallow, the cutting stylus could rise off the disc during a highly modulated passage and the groove would stop. If the disc were played, the playback stylus would skip at the point the groove stopped and would jump either to another groove or off the disc completely.     Ideally, groove depth should not go below two mils for reliable tracking on all turntables. One mil is a standard compromise for minimum depth to cut louder records. Grooves less than ¾ mil deep are considered too shallow or light to provide reliable tracking and are likely to cause skipping.     The problem of light grooves can be eliminated by either decreasing the separation between the channels when using a constant depth- lathe or by using a lathe with automatic depth control. This automatic control is achieved through the same depth / pitch control computer and preview playback head described earlier.     For depth control, the preview head outputs are added together out of phase to produce a signal equal to the upcoming information to be cut vertically. This signal is then applied to the depth control amplifier. Since the stylus point forms an approximately 90 degree angle, deepening the groove also makes it considerably wider, so the pitch- and depth-control amplifiers are interconnected to expand the distance between the grooves when the cut is deepened. Thus, when strong out-of-phase or random phase signals are present, the depth-control amplifier receives a greater signal than usual and deepens the cut to prevent the groove from becoming too light.

Recording Discs

The recording discs used on the lathe are very flat aluminum discs coated with a film of lacquer, dried under controlled temperatures, coated with a second film, and dried again. The flatness and the smoothness of the aluminum base determine the quality of these discs, called lacquers, any irregularities in its surface such as holes or bumps will cause similar defects in the lacquer coating. The disc flatness is achieved by stretching the aluminum. This can produce a cosmetic effect of two flashes of reflected light per revolution of the disc because the lacquer is not completely opaque. The presence of this effect does not degrade the quality or recording capability of the lacquer.     The lacquers for mastering (cutting the lacquer to be sent out to the plating plant) are always larger in diameter than the final record, making it easy to handle the master without damaging the grooves. A 12-inch album is cut on a 14-inch lacquer, while a 7-inch single is cut on a 10-inch or 12-inch lacquer. Producers often cut a reference lacquer to hear how the master tape will sound after being transferred to disc. Long play references are cut on normal sized discs because most turntables cannot handle a disc more than 12 inches in diameter. The lacquers used for references are noisier and of poorer quality than those use to cut masters.     The recorded disc consists of several distinct sections as shown in Fig. 12-9: (A) the starting spiral, (B) the lead-in grooves, (C) the program, (D) the lead-out groove, (E) the spiral out, (F) the locked groove.     The starting spiral is cut at a very low pitch of 6 to 10 lpi; serves to catch the playback stylus as he stylus is lowered onto the record and feeds the stylus to the lead-in groove. Between one and three spiral-in grooves are recommended. This standard is especially important for a record changer since its tone arm falls at a preset distance from the center pin and the stylus must land in a spiral-in groove. If an insufficient number of spiral-in grooves are cut, the tone arm could fall outside of the groove, either failing to feed into it or jumping off of the disc.     The lead-in groove is unmodulated and is cut at the pitch preset as the maximum for the program material. This preset pitch, together with the starting spiral, which feeds the stylus into the lead-in groove, stabilizes the tone arm motion. The lead-in groove must be at least one complete revolution long. The first modulated groove is cut at a diameter no less than 11 7/16 inches for a 12-inch disc and 9 9/16 inches for a 7-inch disc. The last modulated groove is cut at a diameter no less than 4 ¾ inches for a 33 1/3-rpm record and no less than 4 ¼ inches for a 45-rpm record. The limitation on outer groove diameter helps standardize the location of the starting spirals. The inner groove limitation is a combination of standardization for the lift-off function of record changers as well as prevention against severe high-frequency losses and distortion, which result from the low groove velocities at small diameters.     Inner spirals or “bands” are often cut between sections of the program to facilitate finding different selections by reading the selection number on the disc label and counting the sections of program between the spirals. The spiral grooves are cut, at least one unmodulated lead-out groove is cut before the spiral-out action begins. The final spiral is used to start automatic lift off and change cycle of record changers. This spiral leads into the last groove on the disc called the locked groove. The locked groove leads back into itself and holds the stylus at the same groove diameter until the record changer cycle begins. If this groove were not locked, the stylus might continue towards the center of the disc, jump up on label and perhaps even across it, and damage the stylus. On a manual turntable, the locked groove holds the stylus until, someone lifts the tone arm from the record.     The lathe can be programmed via plug-in modules to produce any desired parameters such as lead-in and end groove diameter; lead-in, spiral, and lead-out pitch; and cutter lift-off delay after reaching end groove diameter. These parameters are achieved through control of the lead-in screw drive motor, in conjunction with the position sensing of the sled in the lathe bed and solenoid which lifts and lowers the cutting head onto the disc.

Page Two page two contains more great information on the vinyl record making process

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