TT speed

holmz 

I am the OP, so it was my fantasy not @dover .

It bears repeating. This was very cool discussion to me. It begs the observation of even daily life. We see what we see, but do we?

  Thanks so much for keeping civil a topic that was entertaining and informative.

   Cheers, Greg >>>>>>>> P.S. Don't forget to think.

What you’re describing is tangency, I don’t know why you’re not comfortable with the word commonly used to describe this aspect of pickup arm geometry. Tangency has zero effect on freqeuncy (pitch). But I’m done with this conversation - the question you’ve raised is easily answered several ways, as I’ve already pointed out.

@cleeds I am not comfortable with the word, because

1. It is the wrong word.
2. Tangency does not affect pitch.

According to audio technical what we are talking about is not tangency:

The term “translocation” was used earlier, and that at least is not able to be easily confused with the stylus being parallel to the track, or tangent with the track… which is what “tangency” commonly refers to.

The OP is correct and his numbers are spot on target. The math to prove it is not difficult.

If the position of the cartridge changes by 20° from beginning to end of the LP, it will lengthen the time of the play back by 100mS: 20°/360°=1/18th of a rev; at exactly 33 1/3 RPM, 1 rev=1.8seconds x 1/18=100mS. The 100mS change in play back takes place over 500 revs (15 min) or 200µS/rev. So each and every revolution will experience a longer playback time of 200µS. 200µS/1.8S (expected time per rev @ 33 1/3)=0.00011111 or 0.0111% longer. Another way to look at it, the playback of each revolution will take 1.800200 seconds instead of 1.8S so the effective speed would be 33.3296 RPM (60Sec/1.80020). This will change a 1kHz tone to 999.888 Hz, assuming the record was cut with a linear tracking cutting head and the speed was held constant at 33 1/3 RPM.

The shift in frequency is insignificant and not audible even to someone with perfect pitch. It is not immeasurable or beyond calculation. It has nothing to do with tangency or FFT algorithms. It is purely a timing issue.

Another way to conceptualize this: Imagine a stylus is playing a 1kHz tone in a locked groove for 15 minutes (500 revs). Over the course of that time, the tone arm is moved in such a way that the tracking error stays within the expected range, but the stylus location finishes at a point that is 20° CW from its starting point in the locked groove. The effect would be the same as what is described by the OP.

My tonearm is 10" effective length. I placed a piece of masking tape on the platter to form a radial "spoke" on the platter.  If I align the stylus tip with the tape at the outer groove radius (146mm) then rotate the arm to the inner groove radius (60mm), it is slightly more than 3/4" CW of the stationary spoke of tape.  At 60 mm radius, 20° represents 0.824" of circumference.  So a 20° shift in location is a reasonable amount.