Abstract: This invention provides 3,9-bis[1,1-dimethyl-2-(4'-hydroxybenzoyloxy)ethyl]-2,4,8,10-tetraoxaspiro [5,5]-undecane represented by the formula: ##STR1## a process for preparing the above compound by a transesterification reaction of 3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5,5]undecane with alkyl 4-hydroxybenzoate.

Abstract: This invention provides 2,2-bis[4-(4'-hydroxybenzoyloxy)cyclohexyl]propane represented by the formula: ##STR1## a process for preparing the above compound by a transesterification reaction of 2,2-bis(4-hydroxycyclohexyl)propane with alkyl 4-hydroxybenzoate.

Abstract: In a multi-channel recording instrument, the reference signal of each channel is floating from the ground level. Therefore, one each isolated power supply means must be provided for each channel. However, when the error signal for each channel is transmitted through a photo-coupler, the circuit after the photo-coupler can be isolated from the biasing voltage of the reference signal, and a common power supply can be used for all the stages after the photo-couplers, including servo-motor drivers, of all the channels. This reduces the size and cost of the power supplies as a whole. The non-linear character of the photo-coupler does not cause a positioning error, since the photo-coupler is in the servo-loop having a sufficiently large gain.