Inner Groove Distortion

Vinyl is not perfect, but then neither are digital recordings.  One of the inherent flaws in vinyl that, truthfully, I think may be reserved for the truly obsessive in “inner groove distortion”. 

Inner-groove distortion is caused by the progressive reduction of linear resolution as a record progresses. Simply put: there is more vinyl per second available at the large-diameter beginning of the record than exist at the smaller-diameter toward the end of each side. Subsequently, the wavelengths become gradually shorter and more compressed (like an accordion) as you get closer to the records center. These more condensed grooves are much harder for the stylus to track accurately.

The problem is most noticeable at higher recorded volumes (particularly if there’s a lot of high-frequency energy). Mastering engineers will attempt to mitigate end-of-side distortion by pressing quieter songs, with moderate bass and lower HF energy towards the center of each side. Alternatively, they will restrict the playing time, or spread an album over two discs to avoid inner grooves.

Even when taking linear resolution into account, there is still the issue of cartridge and stylus alignment. The machines used to cut records operate on a parallel basis, meaning the cutting stylus is perfectly aligned from start to finish. Turntables, on the other hand, use a pivoting tonearm, which can only line up correctly at two points across the record surface (it’s a fact of geometry). A properly configured, high-quality turntable with a minimum 10 inch tone arm can reduce tracking errors significantly, but tracking errors will always be most noticeable at the inner grooves – thus further encouraging end-of-side distortion.

With all of this in mind, it would be easy to dismiss vinyl as a flawed medium, which of course it is. But that doesn’t make vinyl any less relevant – far from it. In many cases, inherent distortion in the vinyl format is what draws people in; often referred to as “analog warmth,” what we’re actually experiencing is the analog medium imparting sonically pleasing distortion. The line at which pleasant, warming distortion becomes irritating distortion will be different for everyone, but some distortion is just part of the deal with vinyl.

One corrective approach is to employ a linear tracking turntable such as  Beogram 4000.  Technics, Philips, Harmon-Kardon and Pioneer all had linear tracking turntables.  But this approach only corrects for tracking error.
The reality is, with a record spinning at a constant velocity of 33 1/3 RMP, the linear distance will always be greater at the outer grooves and less as the stylus progresses to the inner grooves.  Think of it this way.  You have 10 inches of paper to write the alphabet.  Then I reduce that to 5 inches and then 2 1/5 inches.  Inevitably your printing is going to get more cramped and “distorted”.  It’s kind of the same thing with inner groove distortion.

An interesting side note is the approach taken for CDs.   With CDs the linear speed is kept constant, thus the CD spins faster when the laser is close to the center and slower at margins.


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