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In Reply to: RE: Null points for 7" discs? posted by flood2 on July 08, 2017 at 02:42:06
> Consequently it is of greater importance to minimize the distortion at the innermost grooves
You used the plural form of "grooves" meaning multiple grooves. In other words, you seem to be talking about minimizing distortion across the inner groove area and not just at the innermost groove. You can accomplish this more effectively by placing the inner null-point in the middle of the inner groove area rather than at either end of it. This is what Lofgren's alignment does. This is why Lofgren's "A" alignment has the lowest distortion possible in-between a given modulated groove envelope whereas your alignment with the inner null-point placed exactly on the innermost groove will have greater RMS distortion throughout the inner groove area as well as throughout the entire modulated groove envelope.
On the other hand, it probably doesn't make that much difference since the modulated groove area on a 7-inch record is so small anyway. When you compare the distortion curves for a 12-inch record with a 7-inch record, you can see that a 7-inch record has much lower tracking error distortion as result of its smaller size. However, Lofgren's "A" solution for a 7-inch record still has slightly lower distortion than your solution.
Lofgren's "A" Tonearm Alignment for a 12-inch Record
Lofgren's "A" Tonearm Alignment for a 7-inch Record
Flood2's Recommended Alignment for a 7-inch Record
Edits: 07/08/17Follow Ups:
I think you will find the link interesting - this gentleman has done a very detailed analysis of the different standards and the screenshot comes from his page - I thought I had sent it to you during one of our previous discussions, but I can't remember.
"You can accomplish this more effectively by placing the inner null-point in the middle of the inner groove area rather than at either end of it. This is what Lofgren's alignment does."
This is exactly what I did! ;) Just to clarify, we don't disagree on LofgrenA as a method. We disagree on the Rmin and Rmax values to calculate a solution.
Your insistence that I am choosing the "innermost" radius (Rmin) = inner null (which used to be your original interpretation of Stevenson until we thrashed that one out) is predicated on an assumption that the 54mm Rmin value in the RIAA standard is to be considered a hard limit whereas it seems more of a recommended value which would be considered "good practice" for a cutting engineer to work to as opposed to a specific engineering requirement which would directly affect interchangeability/functionality which would otherwise make it a mandatory specification. The JIS standard as I pointed out specifies Rmin to be 53mm therefore the 54mm in the RIAA standard cannot be considered a hard limit and it is clearly not based on an engineering constraint. Just like for LPs, DIN and JIS specify much small radii than the 60.3mm you base your calculations on.
The clauses in the standard governing the number of turns possible in the lead out before reaching the label clearance zone and the finishing groove (which for a 7" is between 48.2 and 49.2mm) are the key and these therefore give rise to an uncertainty in the exact Rmin value to take. You will find that the Rmin value specified by Aardvark would be "the accepted value" which is intermediate to a range that meets all the requirements in the engineering specification. This would come down to cutting facility policy regarding the preferred clearance area in order to mark matrix codes, add funny messages or for the cutting engineer to sign his name! I don't believe it has anything to do with tracking error related distortion. If this were the case, then the novelty CD I saw on Youtube (look up Techmoan!) which had a playable surface for a turntable on the label side would be unsellable if compliance to the standard was mandatory. We aren't talking about electrical safety or medical devices here in which case the standard ARE mandatory!!
"...doesn't make that much difference since the modulated groove area on a 7-inch record is so small anyway"
I'm glad you finally agree on that! As I pointed out ages ago, the difference in our solutions was miniscule so our disagreement really only comes down to what value to take for Rmin.
So the optimal solution (as I stated previously) which we both agree on, is the one that matches an envelope that one is statistically most likely to encounter.
For exactly the reasons you point out, it would be foolish/inadvisable to set the inner null at Rmin (the extreme outlier value) and this is NOT what I have been advocating.
What I have been driving at from the beginning is that the calculations for distortion are over simplified (assuming a 10cm/s maximum velocity) and do not adequately model the rapid increase in the end of side distortion. Past the inner null, d(Angular error)/d(Radius) is an ever increasing value. However, the sensitivity of the distortion to small errors in SRA (for example) suggests that the cumulative level of distortion increases at a far greater rate. If the gradient were constrained within limits (as between the nulls), the distortion is less offensive unless the amplitude of the groove is sufficiently large in which case I can clearly hear changes in the coarsening of the sound across the record and between the nulls.
The resultant distortion is very tip profile alignment and design sensitive as the radius of curvature with respect to the groove wavelength becomes of a similar dimension. Therefore further gains are to be had (which I believe I achieve in practice with my modified Rmin and Rmax values... although you are perfectly at liberty to dismiss this as wishful thinking!) by biasing the choice of nulls/linear offset to favour a gradually reducing peak weighted distortion across the side. This is achieved by applying the standard Lofgren A equations to a slightly reduced Rmin and Rmax. As it happens Stevenson 1A does just that and he chose Rmax 145.3 and Rmin 57.9. When you apply this to your preferred envelope from 146.3 to 60.3 you get a gradually decreasing peak weighted distortion with a very small penalty in the maximum distortion at 146.3. However, the high groove velocity makes this largely imperceptible.
Stevenson 2 does the same for a 7" and my experiments with that alignment gave a preferable result to the standard application of LofgrenA to the "standard RIAA" envelope.
"Beauty is Truth, Truth Beauty.." Keats
We're good! Or at least I think we are ;)
This is just "creative dissent" (mental sparring) and John and I do this from time to time...
In all seriousness, the take home point for you is to choose the modulated envelope that you find most common in your collection.
Use John's formulae to calculate your null points. In the absence of any measurements, by all means go for John's recommendation.
My collection has a significant proportion cut further in and LOUD so my optimisation takes that into account and it is worth pointing out that the RIAA standard is not the only standard (there are country specific standards) and the minimum radius specified in John's optimisation is not an absolute hard limit based on an actual engineering constraint for functionality. In a standard, the wording is critical in determining a specific requirement vs a guideline. In some cases, the clauses are open to interpretation - the minimum radius being an example.
"Beauty is Truth, Truth Beauty.." Keats
We just have a difference of opinion regarding tonearm alignment. ;-)
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