Speed of turntable

[Walter]

How is the speed calculated?
At outside,middle or inner diameter.

[Craig Maier]

Turntable speed is a constant angular velocity. Thus, it's angular velocity or speed is the same wherever you measure it, be it outside, middle or inner diameter. If it's speed was given in tangential velocity (as with a CD), it's speed would vary given a constant data rate. CDs have constant data rates, thus their rotational speed starts off fast (they start playing near the center of the disc) and become slower in terms of rpm at the laser approaches the outer edge of the media.