A first cartridge and a second cartridge are detachably mounted in a housing of the image-forming device. Any busy dies are skipped until they become ready again so that the system does not wait for busy dies to become ready. According to another aspect, features vectors extracted from a digital input signal of the hearing prosthesis and processed by the Hidden Markov Models represent substantially level andor absolute spectrum shape independent signal features of the digital input signal. A metal layer may be formed on the front side of the dice, covering the exposed portions of the metal plate and extending to side edges of the dice.