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Apr 18 2011

Pique the Geek 20110417: Vinyl Records

This is sort of a companion piece to Friday’s Popular Culture piece about eight track tapes.  There was quite a response to that piece, and several commentators suggested that we talk about vinyl records tonight.  I have a great respect for my readers, so I am happy to oblige.

On the surface, records seem to be quite simple things indeed.  In practice, few things are further from the truth.  While the concept behind records is fairly simple, the technology is extremely complex to attain high fidelity, defined as sound reproduced with high enough quality as to give the impression that actual performers are playing at the time.  In other words, artifacts of recording and playback should be so small as barely to be noticed.

Recording sound for playback has been done for a long, long time.  Thomas Alva Edison is generally credited for inventing the first real system for both recording and reproducing recognizable sound in 1877 , but sound had been recorded earlier.  There was just way to play it back (until recent computer technology which has actually decoded some sound recordings made as early as 1860.  For practical purposes, Edison’s Phonograph was the first.

From 1877 onwards, until the invention of magnetic tape, sound was recorded using exactly the same concept:  the waveforms being recorded are somehow etched mechanically into a medium soft enough for a vibrating stylus to deform it, but rigid enough for another stylus, usually of lower force, to in turn be caused to vibrate based on the etchings in the medium, thus reproducing the original sounds.

On the surface, that sounds very easy.  In practice, it is exceedingly difficult to provide high fidelity sound with system, and true high fidelity did not really come to market until well over five decades had passed.  We are not going to cover all of the evolutionary steps in this process, but will note some in passing where they are of interest to the larger story.

For many years, the largest problem with records was low capacity.  The old 78 RPM records (made of ground slate, paper pulp, with shellac as a binder) are only good for about four minutes per side, and at what we would consider today as very low fidelity.  By the way, the old saw the DJs sometimes still use, “…stacks and stacks of hot shellacs…”, is a reference to the shellac binder.  These records were heavy, very brittle, and had a inherently high surface noise because of the nature of the medium itself.

If you think about it for a minute, there are three ways to increase the capacity of a record.  The first way is to slow down the angular velocity of the record, thus allowing more information to be laid down in a given length of groove. This works, but in common with almost all analogue signal recording schemes, the more densely the information is recorded, the lower the quality of the resulting playback.  This appears to be a fundamental limitation of analogue recording technology.  If anyone knows any exceptions, please let me know.

The second way to increase the playback time is simply to make a record larger.  This produces logistical problems, since a modern 12 inch LP is about as large as is practical.  In addition, making a record larger requires more raw material, so the expense increases.  However, back in the old days some professional formats used records up to 15 or 16 inches in diameter.

The third way to increase the playback time is to make the grooves more closely spaced.  This is a little different than increasing the information density along a given length of groove, and is the predominant mechanism to get around 25 minutes or so from each side of a modern vinyl LP.  However, even this has its problems.  Let us do a thought experiment about how records are made and played back.  For the purposes of this exercise, we shall assume that all of the records are directly cut with a recording stylus (although most all LPs are now pressed from metal master discs, which are in turn made from metal mother discs, in a process very similar to which modern coins are manufactured).

To record sound, the cutting stylus is applied to the surface of a blank disc of recording material, a turntable holding the disc started at whatever RPM value that is desired, and the sound to be recorded is in one way or another impressed onto the stylus.  This causes the stylus to vibrate in a manner that represents the vector sum of all of the sound at a given instant.  This cutting stylus then distorts the recording medium in a manner that preserves these vector sums of all of the waveforms as it travels across the blank medium.  For recording, some sort of control is necessary to define the groove spacing.  For playback, the drag of the reproduction stylus in the groove is sufficient to take care of that.

As an aside, back in the 1950s and early 1960s it was possible to go to many record stores that were equipped with direct to vinyl recording equipment.  Thus it was possible literally to cut one’s own record.

Now, and you knew that we would, we get Geeky.  For the moment, let us consider the limited case for monaural recording, where only the side-to-side motion of the stylus is important for recording or playback.  Remember that we are recording the vector sums of all of the waveforms of the sound at a given instant, and now remember that low frequencies (bass) are much more slowly moving than high frequencies (treble).  They also tend to be of higher amplitude than the middle and high frequencies.  Thus, when recording, say, bass drums, the cutting stylus is moving very slowly, but at the same time having a lot of travel.  This high travel requires a fairly wide groove compared to the middle and high frequencies.  This provides a fundamental limit as to the minimum groove width.

Without some sort of compensatory measure, a standard 12 inch LP would have a capacity of around 10 to 15 minutes per side, far short of the nominal 25 minutes on most modern records.  Enter the famous (or infamous, depending on your viewpoint) RIAA curve.  The RIAA curve is standard that reduces the level (volume) of low frequencies during recording, whilst boosting the volume of high frequencies.  During playback, the preamplifier in the playback system is programmed to reverse this distortion, in theory restoring the reproduced music to the original balance before anything happened.  It works really pretty well, and also has the advantage of lowering the apparent volume of high frequency noise from dust and debris on the record, since the high frequencies are deemphasized by the preamplifier whilst the low ones are emphasized by it.  Thus, around 25 minutes per side is the norm.

During playback, exactly the reverse occurs.  Instead of the groove being impressed with the vibrations from the cutting stylus, the playback stylus in impressed instead by the waveform as represented by the groove.  These vibrations are converted into electrical impulses that reproduce the waveform and these go the the amplifier to boost the extremely feeble electrical signal to a level that will drive loudspeakers.  In the old, mechanical ones the vibrations were directly fed to a horn, with the loss of almost all of the bass, giving the old Victrola type players a very tinny quality.

In old playback systems, a ceramic cartridge was used to generate the signal.  This was a piezoelectric material directly coupled to the playback stylus, and they gave quite a high output, making a preamplifier unnecessary, since the signal was large enough to go directly to the power amplifier.  However, since the preamplifier decodes the RIAA curve, they were not really high fidelity devices and were mostly used for children’s toy record players.

Now, let us tackle the problem of stereo recording and playback.  In this case, the side-to-side motion of the stylus provides only one channel, but stereo requires two.  To overcome this limitation, the only solution is to use the last physical dimension, the up-and-down motion of the stylus.  Remember, we already have the x-axis being used to move the stylus down the groove, the y-axis for the side-to-side motion of the stylus, so only the z-axis is left.  This greatly increases the technical requirements.

It had long been noted that vertical displacement of the stylus produced a more true reproduction of sound, but for various technical reasons it is much easier to use the y-axis instead.  Because of those technical difficulties, the so called 45/45 system was developed.  Instead of using discrete horizontal and vertical groove architecture, a “compromise” was hit upon where the horizontal and vertical motions were hybridized, thus reducing the mechanical difficulty of using the pure vertical motions.  This is the system that is still in use.  Obviously, it is more difficult to accomplish that from a mechanical point than pure horizontal, but those problems are surmountable.

One development that made this possible was the invention of the light weight pickup, which greatly reduces the force with which the stylus contacts the groove.  Not only did this increase fidelity, it also increased record life for reasons that we shall go into in a little bit.  Most modern pickups have a diamond stylus coupled with a magnetic pickup.  The magnetic pickup (there are several designs) does not provide nearly as large a signal as the piezoelectric ones, but reproduces sound much more faithfully, since the stylus is not mechanically coupled directly to the pickup as it is in a ceramic one.  In those, the stylus directly distorts the pickup material, whilst in the magnetic ones the stylus motion is indirectly sensed by the pickup.

There are other multiplex designs, such as quadrophonic (there are multiple standards), but they really never caught on very much.  For music, stereo is usually adequate, because rarely does one hear a performance except as one faces the performers (there are some exceptions, certainly) so right to left discrimination is much more important than front to back discrimination.  For theater it is a different situation, but the modern digital formats do a much better job of front to back discrimination that the quadrophonic records did.

In vinyl, stereo separation is not perfect.  In most cases, the right and left channels carry around 20%, give or take, of the signal from the other channel.  With analogue tape complete separation is possible, and it also is possible for digital formats.  In reality, that does not matter that much because in a live venue the left ear hears for the most part the same thing as does the right ear.  Thus, 80% separation of channels is fine.

Now for the sad part.  The very act of playing a vinyl record irreversibly damages the medium every time that the stylus passes along the groove.  With very well maintained and very high quality equipment, the damage is minimal, but it is quite real.  As a matter of fact, the RIAA curve is nominally from 20 Hz to 20,000 Hz, and RIAA specifies that after only one play that the upper frequency cutoff is reduced to only 18,000 Hz.  It only gets worse with multiple plays.  The stories about “wearing out” LPs are quite true.  To be fair, dirt and dust do a lot of the damage, and with excellent systems the wear is much lower than the RIAA acceptance range.  However, it is real.

Not counting dust and dirt, the main problem has to do with mechanical distortion of the grooves (this is why it is worse with the high frequencies, since the modulation is much faster at high frequencies and the stylus requires time to move, thus dragging through other parts of the groove before it recovers.  The second is heat, because the stylus does encounter some friction with the groove.  Unless that heat is dissipated, local softening of the vinyl can occur.  Heat is also generated when the stylus drags against the groove as mention at the start of the paragraph.

This is one of the reasons that diamond is the stylus material of choice.  Sapphire is probably hard enough for the purpose, and certainly is cheaper and easier to form into styli, but it has one huge disadvantage compared to diamond:  low thermal conductivity.  I could not find the exact figure for sapphire, but assuming it is close to that of glass, diamond has a thermal conductivity of around 2000 greater.  If you remember in the piece about carbon, diamond has the highest thermal conductivity of any known substance.  This allows heat to be rapidly conducted away from the grooves before doing too much damage.

Now, just a few words about how commercial records are produced.  Except for a relative few stylus cut ones, they are produced by pressing, actually a molding process.  In this process, these steps occur.  First, a “soft” master (usually in some sort of plastic) is stylus cut and coated with an extremely thin layer of silver to make it conductive (it may be that modern materials are inherently conductive, but I can find no reference to that).  It is then nickel plated (nickel is good because it is very hard, plates well, and does not corrode in the atmosphere) to a thickness to be structurally useful.  This is separated from the soft master and becomes a the primary master.  Since it is a negative image of the soft master, it can be used directly to press vinyl.   However, they do not last forever, so the process usually is modified by the following steps.

This negative master is treated by electroplating with more nickel (a proprietary part of this process how the negative master is separated from the new nickel), forming another, this time positive, master.  This process can be repeated as needed.  From the second, positive, master, new negative, secondary masters can be produced, and those are the ones used to press most mass produced vinyl.  You have to remember that this is an analogue process, so each time a copy is made some loss of information is experienced.  In addition, the working masters deteriorate with each pressing, so the first records pressed by a given working master is “better” than subsequent ones.  This is uncannily like how US coins are made.

I think that I shall stop here, but would like your input about what recording format that you prefer.  Many people prefer vinyl, and others digital.  Please chime in with your comment.

Well, you have done it again!  You have wasted many einsteins of photons reading this analogue article.  I usually end with a joke, but have a special comment in the next paragraph instead.  Please keep those comments coming, because I always learn much more than I could possibly hope to teach by writing this series.  Comments, questions, corrections (especially), and other thoughts are valued.  No scientific or technology issue is off topic here.

I have to debunk the Fox “News” Channel for their medical team once again.  This afternoon whilst I was writing this one of their contributors, Dr. Cynara Coomer, was babbling about gluten free diets.  She made a few points with which I agreed, such as people with celiac disease having to stay away from gluten, and that a gluten free diet having no particular value unless one’s individual situation indicated it.  Then she made the jaw droppingly incredibly statement that follows.  One of the stupid news readers mentioned wheat beer, and she correctly pointed out that not only wheat beer need to be considered, but regular beer as well since it is made from barley.  Fair enough.  Then she went on to make herself look incredibly stupid by saying that vodka distilled from potatoes is fine since potatoes do not contain gluten.  Fine, as far as the potatoes.  But any freshman chemistry student knows that gluten, a huge molecule of  many thousands of daltons in weight, and it does not distill over when making vodka, whiskey, or anything else!  What is it with Fox and facts?

Warmest regards,

Doc

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