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 2 responses by tonywinsc

The linear velocity of the groove at the stylus starts out at about 20.1 inches/second and finishes at about 8.7 inches/second. That is based upon the following: 33.3 rpm, outer groove 5.75" from center and inner groove 2.5" from center. It is a simple formula: V=2Pi*r*33.3333/60 (rev/sec) r is in inches so you get inches/second. Cartridges have a rated output voltage at a given lateral velocity. That is defining the amplitude of the stylus motion. The output isn't really affected by groove speed, but by the amplitude and velocity of the stylus motion. The stylus is coupled to a tiny generator and the voltage output is an AC signal that varies in frequency and amplitude based on the stylus motion which is driven by the record grooves. So a 1 kHz waveform, for example, (the squiggles in the vinyl) at the outer groove of the record will be longer than a 1kHz waveform at the inner groove. The musical information is essentially packed tighter at the inner grooves of the record because the linear velocity is lower. 2.3 times tighter to be exact.
You can us the math to help understand it. Take a 1kHz waveform again for an example. At the outer groove, the velocity is 20.1"/s. So 1000 (cycles/sec)/20.1 (inches/sec)= 49.8 cycles/inch. Now at the inner groove the velocity is 8.7"/s so the same 1kHz waveform is generated by 114.5 cycles/inch. The amplitude of the cycles is the same at the two locations of the record for the same volume level because the excursion of the stylus must be the same. That makes the inner grooves more demanding because the stylus has to track these 2.3 times more dense undulations in the grooves.