A decoding system for use with a drug delivery device having a drug dose dialing mode and a drug dose delivery mode is presented where the decoding system comprising has a first sensor configured to read encoded information from a first rotatable component of a drug delivery device, a second sensor configured to read encoded information from a second rotatable component of a drug delivery device, wherein the second sensor comprises an optical sensor configured to be directed at the second rotatable component, and a processor. The processor is configured to receive signals from the first and second sensors, and to determine from the received signals whether the drug delivery device is in a drug dose dialing mode or a drug dose delivery mode.