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Frequency Response of Vinyl, Split from "To get into vinyl, or not?" (TOS #5)
almostmitch
post Dec 3 2012, 20:15
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QUOTE (botface @ Dec 3 2012, 13:49) *
I guess you can expect a factory installed arm/cartridge to be set up properly. I'd still want to check though even if only for reassurance that things haven't gone "off" in transit.


This was something I considered. I will most likely double check things, because as you implied, you never know what kind of beating the box took in transit.
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BFG
post Dec 4 2012, 00:16
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This whole conversation has me intrigued about the technical limitations of vinyl. For example, is there a minimum or maximum frequency that can be "encoded" in vinyl (akin to the ~20Hz and 44.1kHz cutoffs on digital CDs)? Does this differ for 33s, 45s, 78s? Is there a way (based on the needle tip and equipment setup) for a knowledgeable person to precalculate the bands that the most noise interference will likely be in, and thus minimize or remove it? Etc.

It's particularly pertinent as I'm planning to attempt a vinyl-to-FLAC encoding soon.

This post has been edited by BFG: Dec 4 2012, 00:17
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CherylJosie
post Dec 6 2012, 08:09
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QUOTE (BFG @ Dec 3 2012, 15:16) *
This whole conversation has me intrigued about the technical limitations of vinyl. For example, is there a minimum or maximum frequency that can be "encoded" in vinyl (akin to the ~20Hz and 44.1kHz cutoffs on digital CDs)? Does this differ for 33s, 45s, 78s? Is there a way (based on the needle tip and equipment setup) for a knowledgeable person to precalculate the bands that the most noise interference will likely be in, and thus minimize or remove it? Etc.

It's particularly pertinent as I'm planning to attempt a vinyl-to-FLAC encoding soon.


Since you still have not received a reply to this question, I will give it a shot, but truthfully you should be asking someone who is currently mastering vinyl and has the technical background to answer in practical rather than theoretical terms.

The information you have been given so far is mostly wrong. Infinite frequency can only be recorded with an infinitely small stylus, which does not exist in practical terms. Other factors, such as the grain size of the vinyl plastic molecules, and the tensile strength of the vinyl at high playing temperature for any given thickness of groove wall also enter into the equation in a way I cannot address, but keep in mind that as frequency goes up, the groove wall shifts orientation from side to side to front to back. It goes through a 90 degree rotation, and thins out too because the waves crowd closer together. All of these mathematically inescapable physical limitations play a part in the upper frequency limit. Complex geometrical calculations are required to determine the absolute theoretical limits under any given condition such as stylus geometry and groove velocity.

This example below assumes the viewpoint of cutting the master, because that is where the ultimate limits of the technology lie. Duplication errors reduce the performance from the master theoretical limit. Once the master is cut there is absolutely no way to improve the result on playback. Turntables are already optimized for playback at baseband.

The groove velocity (linear velocity of the groove as it traverses across the stylus from front to back) and the dimension of the stylus that lies along the groove (rather than across it) pretty much dictate the upper frequency limit. The reason can be summed up like this. If the frequency is infinite, the groove is essentially motionless, and the stylus retraces its path across groove that has already been cut on cycle n as it cuts cycle n+1.

Stated alternatively, for a fixed groove velocity and stylus dimension (length, let's call it, and keep in mind the length is longer at the top of the groove than at the bottom because the stylus is pointy) the maximum frequency that can be cut is the highest frequency for which the groove displacement is larger than the wavelength of the audio plus the stylus length. If the groove displacement is smaller than the wavelength plus the stylus length, the stylus will recut material that has already been cut, destroying the features of the cut groove.

Remember, as the wavelength gets shorter, so does the groove displacement, because the frequency goes up, shortening the time between peaks. At some limit the wavelength will shorten until it is equal to the length of the stylus, and at that point the leading edge of the stylus cannot cut any waves into the groove because it has already cut that part of the groove on the previous cycle with the trailing edge of the stylus.

Long before that point is reached, the amplitude of the wave that is cut into the vinyl will begin to go down because the stylus starts partially (as opposed to completely) retracing the cut of the wave into the groove when any part of the prior wavelength is still in contact with the stylus. This means that in addition to the absolute high frequency cutoff point where no signal at all can be recorded, there is also a rolloff point where the amplitude begins to decline at lower frequencies too, and a somewhat linear trace connects the two points together (in the dB world, anyway), assuming that the rolloff follows some usual curve. Then again, this particular rolloff is not due to resonance, but rather to overlapping waves, so who knows what it really looks like...

The dimension of the stylus must be added to the wavelength because half the stylus sticks out past time zero on the leading and trailing edge of the cut, so the leading edge of the stylus adds to the length of the trailing edge of the cut wave, and the trailing edge of the stylus adds to the length of the leading edge of the cut wave.

This is why shorter stylus of the elliptical or shibata profile, where the stylus is wide to contact the groove, but short to contact in as small a profile as possible, is the preferred stylus. It allows a stylus to trace higher frequency waves without mistracking or obliterating detail. Conical stylus essentially runs through your record like a snowplow, permanently destroying the high frequency content on the very first pass. Shibato or elliptical stylus concentrates the contact force into a smaller area, causing more rapid wear on the stylus that also damages grooves, so checking and replacing the stylus regularly is the single most important factor (after cleaning stylus and record, that is) to preserving fidelity.

This is why elliptical and Shibata stylus is preferable to conical. Shortening the stylus from front to back allows it to track a higher frequency accurately.

As already stated elsewhere on this forum, 70KHz may be the upper limit of vinyl, but no one I know specified either the stylus dimension or the groove velocity. Remember, single masters used full 12 inch LP form factor running at 78 rpm to maximize fidelity. I presume that is the format that will get you 70KHz but I do not know if that is measured at the outer edge where groove velocity is maximum, or at the inner edge where groove velocity is minimum. The average 33 1/3 rpm LP has noticeable high frequency rolloff and distortion on the innermost track fresh out of the wrapper, but at the outer edge where the groove velocity is higher, the noise has higher frequency components also, giving it a nice breezy whooshing character.

As far as filtering a commercially released LP is concerned, I would expect that your sound equipment already filters out anything outside the audible range of 20Hz to 20KHz and as such you should not need to add any additional filtering. Your preamp will handle it for you. The only additional filtering I would recommend would be a lowpass at 10KHz or 15KHz to denoise a particularly damaged LP, or a highpass at 60Hz or so to denoise a particularly badly warped LP that is making your woofers fly out of their baskets and thumping your pictures off the walls. However, I would apply the filtering at playback rather than at recording, because you might change your preferences depending on the listening environment. If you filter your recordings there is no way to go backward to the original sound...


There are a dozen or so other factors to consider also such as cartridge resonances and cutting amplifier limitations that also limit the frequency response. Take it from me, an engineer, what engineers do is start with an impossible specification in terms of time, budget, performance, and price, and massage it all until it sort of fits into the design space, then ship it quick before the market window closes. If vinyl does 70KHz in the best case, and 45KHz for quadraphonic frequency modulation, then we can assume that it just barely fills the frequency range of perfect human hearing, namely 20Hz to 22KHz, with some degree of linearity, and from what I recall, almost every cartridge specification except the very most expensive fail that specification in either the low or high end, or both, by more than 6db, when actually installed in a turntable. If you get 15KHz out of old vinyl you should consider yourself lucky. If you are over the age of 40 you should also consider yourself lucky if you can still hear 15KHz, especially if you are an audio junkie that warms up your rack of amplifiers every evening with a 6 pack of beer and a pizza and your best friend.

Good luck.
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