Nandric, you said:
why are we not free to choose the zero points on the record radius independant of the tonearm used?
You are, of course, as Dertonarm says, free to choose whatever null points you wish. The origin of the null points for the geometries used are based on the inner and outer radii of a record which give a constant, the linear offset, which in turn gives an offset angle and effective length. To be strictly correct, any calculation is only accurate for one record, and is, to a greater or lesser extent, a compromise for all others.
So, a designer of tonearms has to decide first and foremost what are these inner and outer radii. Are they IEC or DIN, or the median of his own collection, or of a surmised potential buyer? In the past, 7" singles had to be considered so the range of possible radii was extended if they had to be accommodated as well as LPs.
In my case, the Odyssey arm used nulls similar to Lofgren A/Baerwald DIN. That was a design choice, as was the nominal effective length of around 230mm. If I was designing an arm today, I would make it have an adjustable offset angle in addition to an adjustable base, thereby allowing the user to choose the alignment. Also, the effective length used to be governed by the space available on turntables. This is less so now, so longer arms are more common, and they have advantages in reducing tracking error and skating forces.
(Though, as an aside, an 8" arm plays a 7" single with less distortion than a 12" arm playing an LP)
All these features in an arm are merely for facilitating setup, they do not define the arm except in the single case of the hypothetical standard LP, whatever it may have been.
Some corollaries of this are:
1. Some LPs may be more useful than others at showing the effects of particular alignments.
2. When evaluating an alignment choose a track at a distortion maximum not a null.
3. When comparing alignments you shouldn't use the same LP, as it will probably suit one alignment better than another unless you wish to use that LP as your standard.
4. You will have LPs with loud passages at the beginning of a side which may sound worse with certain alignments which favour end of side.
5. The cartridge offset angle need not be the same as the tonearm headshell offset, nor the mounting distance be the same as the specified tonearm alignment should another be used.
In all this it is illustrative to play with the fantastic calculators on Vinyl Engine which allow a reverse engineering of the geometry to try non-Lofgren alignments and look at how small are the variations in distortion with small changes in nulls. And how quickly the distortion rises on the inner grooves with LofgrenB. All a lot easier than calculating in the old way...
John
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