Experimental studies of the use of phase lead filters to compensate lags in head-coupled visual displays

Display lags degrade performance when using the head to track a target presented on a helmet-mounted display. These lags originate from delays in measuring the position of the head and the time required to generate the image of the target. This paper presents two laboratory studies on the use of phase lead filters to improve head tracking performance in the presence of display lags. In the preliminary study, the benefits of lag compensation by a phase lead filter were impeded by associated changes in filter gain. The frequency responses of two phase lead filters were then optimized to have near unity gain at frequencies below 0.7 Hz where there was most head motion. The main study showed that these optimized filters significantly improved head tracking performance with a system having a total lag of 140 ms. At frequencies above about 0.7 Hz, a greater than unity filter gain caused jittery image movement. Although this jittering degraded head tracking performance it was removed by an alternative lag compensation technique involving 'image deflection'. This deflection shifted the displayed image to its correct horizontal and vertical position relative to the head. Image deflection, combined with the phase lead filters, produced a tracking performance unaffected by lag.