Potentials and limitations of pen-based computers
Authors: Wayne Citrin, Dan Halbert, Carl Hewitt, Norm Meyrowitz, Ben Shneiderman
Venue: Proceedings of the 1993 ACM conference on Computer science (1993)

There are four possible genres of input devices that can be attached to personal workstations; keyboard, mouse, pen, and voice. For investigating potentials and limitations of pen-based computers, we propose to compare those four categories as different types of man-machine communication channel. Even though arrow keys allow a limited scope of 2D capability in keyboard usage, the primary use of a keyboard is typing. Typing generates a linear sequence of discrete characters. The maximum speed of typing is roughly 10 characters per second, hence the bandwidth of 100 (10&times;10) bps. A mouse is used primarily for pointing (menu item/object selection) and then for dragging (moving and re-sizing objects). Mouse pointing generates a discrete information of a point location on a 2D (planer) plane with the maximum speed of 2 clicking per second. Each pointing may generate 20 bits of information with the bandwidth of 40 (2&times;20) bps. Mouse dragging generates a continuous geometric pattern in 2D. Assuming the maximum rate of 40 selections per second in the eight (3 bits) possible directions of dragging, the bandwidth of mouse dragging peaks at 120 (40&times;3) bps. The usage of pen on a planer flat surface of LCD unit can be divided into scribing and tapping. By scribing we mean a generation of continuous pen strokes forming a character, a gesture, or a picture. Scribing includes drawing and gesturing. Tapping corresponds to a mouse clicking. Tapping may be considered as a special kind of gesture as in the PenPoint operating system. The bandwidth of scribing can be calculated in the same manner as for mouse dragging. With the maximum rate of 100 selections of direction per second for pen, scribing may produce strokes with the speed of 300 (100&times;3) bps. The bandwidth of pen tapping is almost the same as that of a mouse clicking except that a selection of a point is easier by a pen (3 tappings per second) than by a mouse (2 clickings per second). Talking through a microphone generates a linear sequence of continuous speech with a high degree of redundancy. Using the CELP speech compression algorithm, the bandwidth of normal speech can be reduced to 4800 bps. By vocal signaling we mean a generation of a sequence of discrete messages each of which consists of different pitches and loudness levels. The maximum rate of signaling could be 5 messages per second with 10 differentiable pitches and 10 levels of loudness producing the bandwidth of 35 (5&times;7) bps. Note that mouse dragging, pen scribing, and voice talking, each produces continuous data objects. Only after quantization by sampling, the data objects can be represented by discrete data structure. By precision we mean the degree of ease in duplicating the identical information using the same input technology. Keyboard is a high precision device because there is no difficulty in generating the same character over and over again. Drawing by a mouse is more difficult than drawing by a pen, because pen is easier to control than mouse. Precision of voice is low because it is difficult to duplicate the sound of the same pitch and the same volume. By latency we mean the set-up time necessary to start generating a stream of information. The latency of using keyboard and mouse is larger than the latency of using pen and voice. By translation we mean a process of converting the information generated by the input device into a sequence of discrete symbols, i.e., a transduction of a continuous data type to a discrete data type. Translation for a keyboard is not necessary. Translation of a mouse click on a menu item requires a finite table look-up which is rather simple operation. Translation of pen scribing and mouse dragging involves a handwriting recognition algorithm which is still a difficult problem at present time. Voice recognition is a very difficult problem. With the assumption that real-time translation is feasible for handwriting recognition and speech recognition, the efficiency of input device for text entry can be measured by how many characters can be entered in a second (cps). A simulated keyboard on CRT is used for entering text by a mouse. Long handwriting on a pen computer is used for a pen. When personal workstations become down-sized, the physical dimension of an input/output device becomes a dominant factor for the mobility of workstations. Keyboard and mouse are portable but intrusive. Wireless pen is mobile and less intrusive. Voice can be ubiquitous but intrusive. One conclusion we can draw from the above analysis is that pen is mightier than mouse. A pen can replace a mouse any time any place. However, keyboard, pen, and voice have different strong points and weak points. They compensate with each other. Therefore, we predict that future workstations will carry multi-modal user-interface with any combination of keyboard, pen, and voice.