The arrival of fast and cheap digital electronics and sensory devices opens new pathways to the development of sophisticated equipment to overcome limitations of the human senses. This paper addresses the technical feasibility of replacing human vision by human hearing through equipment that translates images into sounds, which could one day become relevant for the visually impaired. It should be emphasized here, that this paper only proves the feasibility of a low-cost system that preserves much visual information in the generated sound patterns. Conclusive results about the actual capability of humans to successfully interpret these sound patterns, and the advantages and disadvantages encountered in practical daily use, are beyond the scope of this paper, and still require much investigation. Even though these points are not evaluated in this paper, anticipations about several of their aspects need to be considered during the early design phases of any prototype system, in order to increase the probability of approaching a practical solution. In the following sections, some of these considerations are discussed, partly based on several overviews and discussions of previous work that can be found in the references [4, 9, 10, 14, 17].
A successful visual substitution system would have to meet many rather stringent requirements, which, taken together, may seem insurmountable. The system must be cheap, portable and low-power, while taking into account the limited capabilities of a human user and the typical characteristics of his dynamic environment. If the system interferes with other senses, it should be well worth it, by offering an easy-to-learn high resolution mapping of the visual environment. The system must also operate in real-time, giving an up-to-date representation of a changing environment, which is essential under mobility conditions. Still other requirements would become relevant at later stages of system development. Additional ergonomic aspects, including aesthetics, must be dealt with, in order to obtain final acceptance by the visually impaired user. Finally, it should be noticed, that visual impairment can arise due to many different causes, including many kinds of brain damage. The nature of malfunctioning can have great consequences for the options for a vision substitution system. For simplicity and clarity of discussion, it is assumed here that we need only consider the malfunctioning of the eyes, or their direct neural connections to the brain, as the single cause of impairment. In that case, the problem of finding alternatives to normal vision reduces to the problem of finding other ways of accessing the healthy brain and providing it with useful visual information.