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a virtual image to the headset. This image presented, that is, what the user
sees, is based on the user’s most recent position and direction within the
defined virtual space. As the user changes head or body position, there is a
need to update this positional information to the host, or through the back
channel, so that future video frames present an updated virtual image based on
the user’s most recent position. The round-trip latency between updating
positional changes from the user to receiving the correctly updated image must
be sufficiently low to eliminate motion sickness. This limit is empirically
established as roughly 20 milliseconds.
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1
Jerald, J. (2009). Scene-Motion- and Latency-Perception Thresholds for
Head-Mounted Displays. Department of Computer Science, University of North
Carolina at Chapel Hill.
Figure 1: AR example of forward and backchannel transmission.
Bandwidth Requirements
The overall bandwidth requirements for forward channel and back channel are
steadily increasing. For the example use case above, for a simple, single wire
connection between headset and end user system, the bandwidth requirements
for a headset running approximately 2k per eye resolution at 90 frames per
second (fps) is over 30 Gbps. This is equivalent to using all four lanes of
Display 1.4 alt-mode overall a USB-C connection. In the near future, as screen
resolution increases at the same frame rate, the forward channel transmission
bandwidth would exceed this limit.
Likewise, if this same headset supported high definition video capture, possible
with depth capture, the backchannel requirements could exceed 10 Gbps,
equivalent to USB 3.2, independent of supporting tracking and other control
data.
Latency Model
The example above represents an extreme case of round-trip latency limits,
where a hard, physical limit must be met or an immediate adverse effect, i.e.
motion sickness, occurs. Additional techniques, such as predictive rendering,
are used in addition to compression and transmission optimizations to mitigate
this constraint, but the requirement must be met, even as bit rate requirements
increase driven by the need for higher resolution imaging or image sizes
increasing to support a wider user field of view.