In previous studies two background characteristics of computer novices were consistently correlated with their success in learning to use a line-based computer text editor. Older people and those who scored low on a standard test of Spatial Memory had more difficulty than younger people and those with higher Spatial Memory test scores. In the present study, we observed computer novices as they learned to use a screen-based editor, which presumably reduced spatial memory load. Contrary to expectations, performance using a screen-based editor was again strongly correlated with Spatial Memory test scores. However, the correlation between performance and subjects' age was significantly reduced. Overall, subjects were able to perform the same text editing exercises almost twice as fast using the screen editor compared to subjects in previous experiments using the line editor. These results are discussed in terms of the different cognitive demands placed on users by line and screen text editors.

We present the criteria used in designing the human interface of the display text editor TED and explain how the design tradeoffs were made. Although discussed in the context of a text editor, these criteria are applicable to the design of human interfaces in general. We present a critical evaluation of the TED human interface in hindsight, and suggest modifications to overcome the shortcomings thus uncovered. The new interface is judged to be less obtrusive and more flexible than its earlier version. This process of design-implementation-evaluation is essential to learning how to build good human interfaces in the future. We describe the software architecture of TED and point out how the use of sound software engineering principles has yielded a program that is easy to modify and interface to a variety of I/O devices, namely, screens, keyboards, cursor pointing devices, and voice recognition units.

Two techniques for separating a composite NTSC signal into luminance and chrominance components are developed and compared. In the first technique, fixed filters, which are based upon minimum mean square error estimators, are designed. It is shown that, when samples from successive lines are used, these filters have a comb structure in the region of the frequency spectrum in which the luminance and chrominance signals overlap. In the second technique, the composite signal is filtered by several filters, and the outputs of the filters are selected dependent upon the local characteristics of the picture. Both recursive and nonrecursive filters are considered, the criterion for selecting between filters is investigated and the effect of delayed decisions is determined. The various filters are compared using subjective criterion and a number of quantitative criterion. The relationships between the quantitative and qualitative criterion are stated.

Visual thresholds play an important role in the process of incorporating properties of the human visual system in encoding picture signals. They tell us how much the picture signal can be perturbed without the perturbations being visible to human observers. We describe psychovisual experiments to determine the amplitude thresholds at a single edge having a given slope and then present methods to incorporate the visual threshold data directly into the design of quantizers for use in Differential Pulse Code Modulation (DPCM) systems. In the first class of methods, quantizer characteristics are obtained such that the quantization error is kept below the visual threshold as determined by the slope at a picture element and either (a) the number of quantizer levels, or (b) the entropy of the quantized output is minimized. In the second class of methods, different measures of the suprathreshold quantization error are minimized for a fixed number of levels, or for a given constraint on the entropy of the quantized signal. We present empirical relationships between the various distortion measures and the subjective quality of the pictures rated on a five point impairment scale. We then discuss the structure of the quantizers obtained by the above mentioned optimization methods, evaluate their performance on real pictures, and compare them with the ones described in the literature.