Building Wireless VR Goggles

Wireless is good. Most people prefer cordless or mobile phones over corded ones; a wireless network connection over a wired one; wireless game controllers; wireless speakers; wireless charging. You get the point.

Would a wireless goggle - one that does not require cables to connect to a PC - be desirable? I certainly think so. With a wireless goggle, you could:

• Use the goggle in the living room even if the computer is on your desk
• Have multiple people use multiple goggles in the same space - such as an arcade - without tripping over each other's wires
• Avoid the risk of wrapping the goggle's umbilical cord around you as you turn
• Have greater freedom of movement

While wireless solutions already exist for professional-grade goggles (see this 2012 video of the Sensics zSight to the right), let's examine what it would take to do this for a consumer-grade product such as the OSVR HDK (Hacker Developer Kit)

For a wireless solution, one would need to consider three key components: video transmission (how you get the video to the goggles), wireless tracking (how the PC gets information about where the user is) and power.

Video Transmission

Not all wireless video solutions are created the same, because not all aim to solve the same problem. Here is what's important to look for:

• Low latency. Streaming a movie or a sports event to your TV does not require tight control over latency. If the movie is shown with a 1-second delay, that is perfectly acceptable. Streaming a game to a goggle with a 1-second delay is completely unacceptable. Sometimes, the need to control latency also dictates whether compression can be used. Compressed video saves on bandwidth but takes time to compress at the transmitter and decompress at the receiver.
• No line-of-sight requirement. Some wireless video solutions require there the transmitter can "see" the receiver, which is typically called 'line of sight'. Those solutions typically don't work for VR goggles because the whole premise of wireless goggles is to allow the user to move around as well as turn. Such turning will inadvertently result in breaking the line of sight from transmitter to receiver and thus dropping the connection. Other use cases, such as having the PC in one room and using the goggles in another probably don't even have line of sight to begin with. This requirement that the wireless signal can go through objects or walls also influences the wireless transmission frequency. Higher frequency transmission (e.g. 60 GHz) results in shorter wavelength which in turn results in greater difficulty in going through walls or humans. Lower frequency bands such as 5 GHz does a better job in penetrating through obstacles and is thus more suitable for wireless goggles.
• Support the right resolution. Wireless video solutions were originally designed with home entertainment in mind, and thus focus on supporting standard home video resolutions of 1080p (1920x1080) or 720p (1280x720). However, many consumer goggles use smartphone displays that have a native resolution of 1080x1920 - portrait mode - as opposed to the traditional 1920x1080 landscape mode. Many wireless video links do not support 1080x1920. One solution, that is now available as an option with the OSVR HDK is to use on-goggle FPGA to perform the video rotation, so that the wireless link still carries the standard 1920x1080 resolution but it is rotated in real-time to 1080x1920. This enables using wireless video links with such products (note: see the 'no free lunch' section below)
• Ability to support multiple independent video links. While this is not required if there is just one video link from the PC to the goggles, it might be required if 1) there are multiple goggles in the same space, and they each want to operate independently; 2) there is a need to carry video, such as from a video camera, from the goggles back to the PC (see this wireless camera) or 3) if the goggles require multiple HD1080p signals such as those goggles that have dual screens

Tracking

Goggles are interactive and thus very often require head orientation and sometimes head position tracking to be sent back to the controlling application. Thus, cutting the video cable is not enough and one needs to find a solution for wireless tracking.

Some wireless transmission technologies - such as WHDI - support an in-band downstream data link. If the transmission of video is considered 'upstream' - from video transmitter to video receiver - a downstream data downlink is sending information from the video receiver (where the goggles are) to the video transmitter (where the PC is). These data downlinks can often carry USB HID (Human Interface Device) messages such as those coming from game controllers and provide a reasonable bandwidth of about 100 kbps. If on-board orientation tracking is formatted into a compatible message, head tracking can be carried to the PC on the same link as the video. This is how tracking is performed in the 2012 zSight video above. The downside of this approach is the update rate. Assuming 60 FPS video, WHDI transmits the downstream data during the blanking periods inbetween frames. Thus, USB HID transmission is also provided at 60 Hz, which is sometimes not interactive enough.

Another method is to use or embed a wireless tracker such as the YEI 3-space sensor. Adding an out-of-band data link ((not on the same link as the video transmission) to a tracker can provide high-rate tracking information without needing a cable.

Other methods are to use trackers that have some kind of base station, such as the Sixense STEM. In this case, the tracker base is connected to the PC. An advantage of this approach is that beyond the head, other parts of the body can also be tracked, as is also the case with the PrioVR suit.

Power

Cutting the cord means also providing local power to the goggles as well as other local consumers of energy - wireless receiver, wireless tracker, on-board camera where applicable and so forth. Including the goggle and wireless receiver, one could expect a power draw of 5-10 Watts (1-2 Amps on +5V). This can typically be satisfied with a 1 or 2 high-current battery such as those being marketed to charge phones and tablets. These batteries are rated in mAh (milliampere-hour) or Ah (ampere-hour). An Ampere-hour means that the battery can theoretically provide 1 ampere of current for an hour. For instance, a 10000 mAh battery that outputs +5V can theoretically provide 10 Watts (2 Amps x 5V) for 5 hours. A typical AA battery provides 2-3Ah. A typical external battery for smartphones can provide 10Ah and a typical car battery can provide about 50Ah (though it would be quite heavy). However, because of many factors - such as the drop in voltage when the battery drains - these figures are a someone optimistic. However, one can often plan for 1-2 hours between charges on an external smartphone battery.

There's no free lunch

What are the downsides of a wireless VR solution?

• It costs more. One would need to factor in the price of the wireless video link, a potential price increase in the tracking costs, and the costs of a battery
• There may be a price to pay in video latency. If you use an on-board FPGA to rotate the image from 1920x1080 to 1080x1920, you add 1 frame's worth of latency, or 16mSec. This is because you have the store the entire image in memory before you can start outputting the rotated image.
• You are often limited to 60 FPS video. Because most wireless video solutions were designed for home entertainment, they provide 1080p @ 60 FPS. Newer solutions that aim to provide higher resolutions might be able to address this issue.
• You have to carry the battery and the receiver. This is can be done in a beltpack or small backpack.
• You have to recharge or change the battery from time to time.

What about using a phone for your HMD?

One way to completely overcome this issue is not use a PC. Phone-based HMDs such as Google Cardboard perform all the processing and display on the local phone, thus eliminating the need for wireless video transmission. However, one could probably safely assume that a powerful PC would always have more computing power, more graphics power and access to a wider range of peripherals than a phone, and thus some PC-based experiences could not be replicated on a phone.

Should you do it?

Having discussed the advantages and disadvantages, should you use a wireless HMD? At the very least, you will want to experience it. Having no cables can be quite liberating and is certainly worth a try. Let me know what you think!

The post "Building Wireless VR Goggles" first appeared on the VRguy's blog at www.vrguy.net