It's interesting that so many people are interested in this topic.
What stpa and GurraG have to say is also most interesting. When I was working in the audio business during the introduction of CD4 one of the problems was that while many elliptical styli and even some spherical styli could play back a CD4 record once or twice, they could also ruin these discs too.
CBS labs had once issued a set of photos (this is not the CBS Labs associated with Stereo Review magazine but the lab associated with CBS Records who would actually run tests like: How many times can a stylus made by XXX play back a specific record at 2 grams, and then a similar test at 3 grams etc. They took photos of the grooves quite often every 25th time a record was played and so on. These projects were used to determine the best lubricants to add to vinyl formulations, the best pigments (vinyl is not naturally black), etc.
The most interesting photos to me described the "infinite" amount of force applied to the groove by the spherical and elliptical stylus designs. These designs effectively rest on 2 round contact points about 80%-90% of the way down into the groove. Since the contact points are rounded, the contact area is infinitely small, so no matter what the tracking force, the tracking pressure at the point of contact is effectively infinite. Combine this with the fact that vinyl is relatively hard and somewhat abrasive, and the fact that most metal "mothers" are not fully deburred after plating and you can have some roughness in the groove to start. (Back in-the-day at least 10% re-used vinyl was added back to each batch from crushed "defectives and returns").
After a record has been played a few times with elliptical or spherical shaped styli, all the records micro-photographed showed what looked similar to the wave paterns you see behind slow moving motorboats (nicely spaced light ripples) all along the travel area of the contact points between the stylus tip and the groove.
CBS determined that the burrs in the groove and the ripples 80%-90% of the way down the groove only created high frequency noise. This noise had no audible effect directly because it was in the 30KHz - 90 KHz range (the burrs being random and while the ripples were constant and repetative and dependant on the vinyl formulation), they might interfere with CD4 demodulation. In addition there were some calls to change the stylus tip shapes because some of this high frequency ripple noise excited tip resonances which caused "chatter mistracking" that was really obnoxious in the high frequencies.
The stylus tip design submitted by Shibata/JVC solved many of these problems. The Shibata stylus really is a kind of Line Contact stylus and not really of the type I'm worried about in this thread (nice photos by the way). But the Shibata rode right on the bottom of the groove. It could be made with a high frequency response out to and tip resonance of about 53KHz. The key factor that record people took note of was that this stylus didn't produce the vinyl motorboat ripples on the downside edge of the grooves of stereo-only records. In fact the contact surface area was actually measurable as something other than "infinitely small" for the first time with the Shibata stylus. Other line contact stylus designs came along, and then VdH made their big leap forward. It is the group of post VdH1 styli I am concerned about.
Getting back to the ripples caused by spherical and elliptical styli near the bottom of grooves, there seem to be 2 forces at work. The stylus tip gets warm on the two contact surfaces after only a few seconds playing. At that point you have a hot stylus pressing infinitely hard against vinyl and this can easily create substantial softening (melting is not required, in fact melting is not the mechanism that creates the ripples). After the stylus has passed any groove location the after-effect of the stylus bouncing merrily along in the groove causes the rippling. The groove begins to re-harden very quickly because the vinyl disc is a large heat sink. But there are so many stretching, bouncing, musical, pushing/pulling pressure waves being generated, some driven into the disc and some being generated as the stylus tries to drag the vinyl along with it, that as the soft vinyl hardens again, it reacts in a resonant fashion.
All liquids have excitation resonant frequencies. Input some frequencies into a glass of water and you make a few ripples. Input other frequencies with the same energy level and at resonance, the water wants to jump right out of the glass. The range of frequencies being input into softened vinyl is limited, but each vinyl formulation responds to this energy input at its softest locations by creating raised ripples in the groove. The next time the styli drag over the top of these ripples, the excitations of the energy input increase. The ripples represent stored mechanical energy. Eventually the ripples can have so much stored energy that either cracks form on the vinyl surface, or the stylus can actually skip out of the groove for no "obvious" reason.
The Shibata stylus design can prolong record life 200% in these circumstances. Perhaps other line contact stylus designs can prolong record life as well. The most advanced line contact designs like the VdH1, Gyger1, MR, ML, etc. also distribute the tracking pressures over a really measuable contact surface area, and that seems to be the secret as to why record life is extended by these designs. Sadly I've seen some JVC photos showing that using a stylus design that makes full groove contact, top-to-bottom eventually causes the oputbreak of little blister-like structures up and down the entire groove. These structures exist in the 75KHz-to-125KHz excitation range when the noise they create is laser analyzed, but they do show vinyl deterioration of some kind. We know vinyl is imperfect so we shouldn't be surprised by any of this.
I'm not really concerned that some LC and Shibata styli are mounted on lightweight bushings (bushings can be good heat sinks, but the boundry adhesive layer between the stylus and bushing is an insulator so you win some and you lose some). Mounting an elliptical or spherical stylus on a bushing though seems like asking for damage to vinyl. But I'm still concerned that users don't always realize how much more prone to damage the thin LC surfaces of the most advanced stylus tip designs really are. It is a question of how much trouble we are willing to go through to check our stylus tips every few days vs the amount of slicing damage a VdH1, Gyger1, ML, MR or other simlar stylus can cause.
I've been looking microscopically at the scraped up crud I get when I play some of my records with my VdH1 and ML styli. Some is air pollution, some is dust and some are vinyl bits as these cartridges scrape the tops off of the vinyl ripples, leaving smoothed out pits where the ripples were in the record groove. The pits are harder to even see being black-on-black, and my microscope isn't set up to look at vinyl records, but the stylus tip residue is easily checked. This groove scraping may be good for the sound of the disc if I play that disc with a less "aggressive" LC stylus, and even a spherical or elliptical stylus run through those grooves may sound better because there will be less high frequency chatter at the tip contact area. But it also shows how easily a high end stylus design can rip bits of vinyl off of a groove (even though it could be for-the-good). If that same stylus developed a chip on the edge, who knows how much ripping damage it could do?
The less aggressive LC designs are much less likely to chip, they are less aggressive "scrapers" to start, and they don't need to be checked so often. I have sadly noticed that while most of my Shibata styli have tip resonances way up in the 50KHz region, they also show small dips in the 20KHz response. This makes those cartridges sound less "exciting" compared to my AT 440ML or other. The highs that are recorded alone are super clear and low distortion using Shibata styli playback but they are generally -2db below the level they produce with VdH1, ML or Gyger1 styli (I don't have any more MR styli). So you do give up a bit with Shibata styli. Other LC styli I have give other varied results, unlike the "super-stylus-designs " that I''ve listed before which can sound quite similar to each other. I find Stereohedron styli to be particularly "crisp". But now I'm most concerned about my irreplaceable record collection. So perhaps I'll use my most advanced stylus sdesigns less.
