An air entrainment spa nozzle having a jet of water which passes over an air entrainment port with the air entrainment port having a downstream structure which enhances the air entrainment. The stent comprises a plurality of circumferential bands interconnected by rows of links. Of pulses included in the land pre-pit detection signal, those pulses having a small pulse width are regarded as noise pulses and removed thereby obtaining a corrected land pre-pit detection signal. A coil is coupled to the peripheral portion of the horizontal former.