the second optical spot f 2 while the second sampling phases 62 i are scanned by the first optical spot f 1 in order to be able to clearly distinguish optical read signals sr 1 obtained by the first optical spot f 1 from optical read signals sr 2 obtained by the second optical spot f 2 the optical system 30 preferably comprises two independent optical detectors 135 and 235 wherein the first optical detector 135 receives the light reflected from the first optical spot f 1 and wherein the second optical detector 235 receives the light reflected from the second optical spot f 2 as illustrated in fig9 in view of the tangential distance between the two optical spots f 1 and f 2 the timing relationship between the readout signals regarding the two sampling phases is shifted in the illustrated example the second optical spot f 2 is ahead of the first optical spot f 1 hence first optical read signals sr 1 obtained by the first optical spot f 1 lag with respect to the second optical read signals sr 2 obtained by the second optical spot f 2 in order to eliminate this timing difference the second optical read signals sr 2 may be delayed in a.