The player will not pick up bits from the CD layer when playing the SACD layer.
First, the laser (a shorter wavelength one, at that) is not focused on the CD layer. Second, the bits are not stored on the disc in a linear fashion like music in the grooves of an LP. They are grouped together into bundles (long story here) which are each protected by an Error Correcting Code (ECC) calculated during the mastering process. The player uses the ECC code to detect all read errors and to exactly reconstruct missing data from most read errors.
Further, these bundles are arranged out of sequence on the disc so that a scratch or defect is less likely to cause uncorrectable damage. The player rearranges them in the right sequence before decoding.
Remember, the CD and SACD standards assume that each disc will have manufacturing errors, and go to great lengths to provide robust, error-free recovery of a bitstream exactly matching the original.
First, the laser (a shorter wavelength one, at that) is not focused on the CD layer. Second, the bits are not stored on the disc in a linear fashion like music in the grooves of an LP. They are grouped together into bundles (long story here) which are each protected by an Error Correcting Code (ECC) calculated during the mastering process. The player uses the ECC code to detect all read errors and to exactly reconstruct missing data from most read errors.
Further, these bundles are arranged out of sequence on the disc so that a scratch or defect is less likely to cause uncorrectable damage. The player rearranges them in the right sequence before decoding.
Remember, the CD and SACD standards assume that each disc will have manufacturing errors, and go to great lengths to provide robust, error-free recovery of a bitstream exactly matching the original.