drive in an exploded view briefly the disc drive 10 includes a housing base 11 and a top cover 12 which engage a gasket 13 to form a sealed housing that maintains a clean environment therein a plurality of discs 14 is mounted for rotation on a spindle motor hub 15 a plurality of transducer heads 16 is mounted to an actuator body 17 the actuator body 17 is adapted for pivotal motion under control of a voice coil motor vcm including a voice coil 18 and magnets 19 to controllably move a head 16 to a desired track 20 along an arcuate path 21 signals used to control the vcm and the heads 16 pass via a flex circuit 22 and a connector 23 to and from electronic circuitry on controller board 24 the controller board 24 includes a controller not shown a channel interface 25 a serial port connector 26 and a spindle connector 27 fig2 1 shows a read write head 16 operably connected to a source 28 of test data the test data may comprise a 2t pattern in the form of 1 1 u2212 1 u2212 1 as shown in fig3 the 2t pattern is simply a sinusoid in actual fact the form of 1 1 u2212 1 u2212 1 is only achievable with pr4 partial response 4 th order where the sampling is done in this way as can be seen in fig6 a the pattern can also be in the form 2 2 u2212 2 u2212 2 fig2 u2014 2 shows apparatus including a preamplifier 29 operably connected to read write head 16 the analog output from preamplifier 29 passes to variable gain amplifier vga 30.