each image and the maximum frame rate for each eye. The disadvantage of dual video inputs is that it requires separate video outputs and cables from the device generating the content. Time-based multiplexing preserves the full resolution per each image, but reduces the frame rate by half. For example, if the signal is presented at 60 Hz, each eye is receiving just 30 Hz updates. This may become an issue with accurately presenting fast-moving images. Side-by-side and top-bottom multiplexing provide full-rate updates to each eye, but reduce the resolution presented to each eye. Many 3D broadcasts, such as ESPN, chose to provide side-by-side 3D which saves the need to allocate extra transmission bandwidth and is more suitable to fast-paced sports action relative to time-based multiplexing methods. Not all HMDs provide depth perception. Some lower-end modules are essentially bi-ocular devices where both eyes are presented with the same image. 3D video players sometimes allow maximum compatibility with HMDs by providing the user with a choice of the 3D format to be used. Peripherals The most rudimentary HMDs simply project an image or symbology on a wearer's visor or reticle. The image is not bound to the real world, i.e., the image does not change based on the wearer's head position. More sophisticated HMDs incorporate a positioning system that tracks the wearer's head position and angle, so that the picture or symbol displayed is congruent with the outside world using see-through imagery. Head tracking – Binding the imagery. Head-mounted displays may also be used with tracking sensors that detect changes of angle and orientation. When such data is available in the system computer, it can be used to generate the appropriate computer-generated imagery (CGI) for the angle-of-look at the particular time. This allows the user to look around a virtual reality environment simply by moving the head without the need for a separate controller to change the angle of the imagery. In radio-based systems (compared to wires), the wearer may move about within the tracking limits of the system. Eye tracking – Eye trackers measure the point of gaze, allowing a computer to sense where the user is looking. This information is useful in a variety of contexts such as user interface navigation: By sensing the user's gaze, a computer can change the information displayed on a screen, bring added details to attention, etc. Hand tracking – tracking hand movement from the