A method of character input for the user interface with a low degree of freedom

In recent times, smart devices equipped with small touch panels have become very popular. Many such smart devices use a software keyboard for character input. Unfortunately, software keyboards have a limitation: the number of buttons and the input degrees of freedom remain the same, because a button and an input value correspond one-to-one. Thus, if we reduce the screen size while the button size remains the same, the number of buttons must decrease. Alternatively, if we maintain the number of buttons and reduce the screen size, the size of the button decreases, making input increasingly difficult. In this study, we investigate a new character input method that is specifically adapted for use on small screens. The proposed input interface has 4 × 2 operational degrees of freedom and consists of four buttons and two actions. By handling two operations as one input, 64 input options are secured. Additionally, we experimentally evaluate the proposed character input user interface deployed on a smart device. The proposed method enables an input of approximately 25 characters per minute, and it shows robust input performance on a small screen compared to previous software methods. Thus, the proposed method is better suited to small screens than were previous methods.