An image forming apparatus includes an electrophotographic developing device, a photosensitive body, a precharger for precharging the photosensitive body, and exposing units of two optical systems with different light amount distributions, which are provided for one photosensitive body for forming electrostatic latent images on the photosensitive body. A state transition record is determined, based on the measurement sequence. The second address phase generating circuit is adapted to carry out a write operation when the write data bus is idle and the read data bus is busy, and vice versa. The receivers may be addressed individually, in units or in groups for the purpose of receiving information on the basis of one or more pre-selected criteria. A voice detection module defines an identification number for each of the signal channels in sequence and detects whether the DVR receives an analog voice signal, and determines whether all of the signal channels have been tested.