Speed of groove.


We all know that record spins at (let's take at this point) 33.5 rpm and while the rotational speed is the same the speed of the actual groove is not.
Our cartridges are specked at output voltage for a specific speed of the grove(cm/c). What happens when
a)the speed is higher than specified at outer grooves
b)the speed is lower than specified at inner grooves
c)the record spins at 45rpm or 78rpm
128x128marakanetz

Showing 5 responses by almarg

Interesting question. Since the variation of tangential velocity that occurs across the record is presumably the same when the record is played back as when it is cut, and since one period of a given signal frequency will extend over a physically longer distance if it is located towards the outer edge of the record than if it is located towards the inner edge, I believe that it all cancels out. Signal frequency will be reproduced identically at the two locations, because the time required to traverse the distance corresponding to one period will be the same in the two cases. Signal amplitude at any instant of time, which is proportional to groove velocity at that instant, will also be maintained for the two locations, because as tangential velocity decreases towards the inner part of the record, a given groove excursion will occur over a correspondingly shorter distance, and therefore in the same amount of time, and therefore with the same velocity.

Regards,
-- Al
12-29-11: Marakanetz
At some point of time on this post I saw response from Al (Almarg) but it dissapeared.
???
I deleted it, and posted the revised version you can see above. My first submittal wasn't quite right :-)

Regards,
-- Al
To me, if one would need to correlate the output voltage to a speed, it would make more sense that that speed would be the speed of the stylus relative to the cartridge itself (*vertical* movement) as that's the movement that induces the voltage to begin with.
Yes, the output voltage is a function of the velocity with which stylus deflection occurs, relative to the cartridge body, the deflection being lateral for the mono component of the signal, and vertical for the stereo component of the signal.

However, for a given physical excursion of the groove, laterally or vertically, the speed of that stylus deflection relative to the cartridge body will vary depending on the tangential velocity of the groove, which gets smaller towards the inner part of the record. Therefore for a given music signal the physical distance over which that excursion occurs has to be smaller at the inner part of the record compared to the outer part of the record, in order to result in the same stylus deflection speed. Tony put it well: "The musical information is essentially packed tighter at the inner grooves of the record because the linear velocity is lower."

Regards,
-- Al
The linear velocity was "abstracted away" by the "length of the bump".
Precisely! That's all we are saying. As you correctly put it, "at the outer edge of the record, the bump will be 'elongated' whereas towards the center of the record it will be 'shortened.'" That shortening or lengthening results in the velocity of the stylus deflection being the same for a given signal at different points on the record, despite the fact that tangential ("linear") velocity is not the same.

Regards,
-- Al
Mihaitaa, I understood the OP as asking what the effects of the differences in tangential ("linear") velocity at different points on the record would be on the output of the cartridge, given that the cartridge output is spec'd based on velocity. The answer, as you realize, is that there is no effect, because what is relevant is stylus deflection velocity, not tangential groove velocity, and for a given signal stylus deflection velocity does not change as a function of position on the record.
12-30-11: Marakanetz
Would you consider a decreased performance on the inner groves normal in some of records especially fully "packed"?
Inner groove distortion is certainly recognized as a potential issue, but my analog experience is not extensive enough to make a general comment on how pervasive a problem it might be. You might want to start a separate thread on that subject, that will probably catch the eye of some of the analog experts we have here.

Regards,
-- Al