VRSG Multi-Channel Synchronization

The MetaVR ChannelSync™ cable is designed to work hand-in-hand with the MetaVR VRSG 3D visualization software. ChannelSync is a graphics card independent hardware device that enables multiple VRSG visual channels to intercommunicate such that the refresh rate of all the views is synchronized. The ChannelSync cable is used with MetaVR multi-channel VRSG systems so that multiple out-the-window views can  update in synchronization eliminating undesirable artifacts and flicker between adjacent views. The ChannelSync cable is designed to eliminate variable update rates that can result from uneven scene complexity across multiple out-the-window views.

Scene load management

The MetaVR PC IG controls the scene content across all channels to accomplish the following:

• Ensure that the image processing and storage capacity of the PC IG is efficiently utilized, but not exceeded, for all conditions.
• Ensure continuity of the scene and prevent "popping" of scene elements.
• Prevent and correct overload conditions.
• Ensure point lights are properly faded to prevent "popping".
• Eliminate scene details too small to contribute to visual cueing.
• Ensure scene detail is maintained within the FOV according to priority needs for the viewer.

VRSG implements scene management through level-of-detail (LOD) scaling. Moving models and terrain database features may employ LOD switches, which indicate the desired switch-in and switch-out ranges for the various levels of complexity. VRSG uses a control loop on an LOD scale, which modulates LOD ranges based on loading conditions. For example, under conditions of high load, LOD ranges may be scaled to smaller values so that a less complex version of a model may be selected sooner than it normally would. This is done to maintain a target frame rate, while maximizing image density. When the heavy load condition passes, the level of detail ranges return to their default values. Using this technique, the viewer is guaranteed to see every object in the scene regardless of load conditions. We feel that showing a viewer a lower complexity version of a model is preferred to completely eliminating the model from the scene under high load conditions.

Continuous image density

The MetaVR PCIG continuously monitors its load conditions to maximize image density. During each frame, the control loop on the LOD scale is adjusted upward if loading conditions warrant the ability to provide more detail, or downward if loading conditions threaten to violate the 30/60 Hz update rate. Each visual channel manages its scene density independently of the others.

Synchronization

The MetaVR PCIG provides a 30/60 Hz sync signal that is continuous at all times and has a maximum tolerance of +/- 0.1 Hz. This sync signal is provided by the host-side API.

To address the genlocking needs of multi-channel image generators, MetaVR has developed a graphics card-independent multi-channel synchronization technique named ChannelSync. ChannelSync is a hardware-based approach that allows computation of distributed consensus with microsecond order latency. Visual channels requiring synchronization (e.g. driver's vision blocks) are organized into synchronization groups. Visual channels belonging to a given synchronization group meet in time at the completion of their rendering cycle and perform their buffer swaps concurrently. The ChannelSync hardware informs each individual channel when all channels in the group are ready to perform the buffer swap at the completion of the rendering cycle. ChannelSync units were installed on each of the RWA simulators at the Ft. Rucker, Alabama, Aviation Testbed (AVTB) to synchronize five adjacent out-the-window views and are fielded at Ft. Knox as well.

Using the ChannelSync cable with a graphics card's genlock feature

The ChannelSync cable is beneficial to use in conjunction with a graphics card that has genlock. The ChannelSync cable and a card's genlock have complementary functions; genlock aligns the timing of the channels' vertical retraces, and the ChannelSync cable ensures all channels flip their buffers at the same time. ChannelSync ensures all channels run in sync; if one channel slows down, for example to a frame rate of 30 Hz, all the channels slow down. Genlock cannot achieve this synchronization alone. When synchronizing multiple channels that are not genlocked, VRSG performance typically drops to 30 Hz; the applications are out of phase with each other, because the software naturally aligns itself with the vsync signal. However, using the ChannelSync cable with a genlocked system has no performance impact. At present, the genlock card has a frame-lock feature which is not available for DirectX applications.

You can order this product directly from MetaVR.