What does PlayStation VR's external processor unit actually do?

In the wake of its GDC 2016 announcements of price-point, release date and headline software, Sony has begun to reveal more of the inner workings of PlayStation VR, starting with key information on what tasks its mysterious external processor box actually carries out.

At a Sony presentation, senior staff engineer Chris Norden, spelled out the basics, beginning with what the processor unit doesn't provide in the wake of speculation following our December article on the device:

• It provides no extra GPU or CPU power.
• It's not any form of PS4 expansion or upgrade.
• It's not directly accessible by the developer in any way - code cannot be written to it.

"The PS4 is perfectably capable of 120Hz all on its own," said Norden, emphasising that the main game rendering and processing is all carried out by the main console. However, the fact is that the PU (processing unit, as Norden called it) is actively cooled, suggesting some measure of computational power. And it is capable of carrying out a number of functions, some of which we were not previously aware of. Some of these features are crucial to the experience, while others actually offer exciting, brand new gameplay opportunities.

So here's the breakdown on what tasks the device does actually carry out:

• It carries out object-based 3D audio processing ("really good and important to VR").
• it displays the social screen - undistorting the VR output for display on TV. Quality is lost in this process, so it scales the image up and crops it so you don't see edges.
• "Separate mode" - a completely separate audio and video stream you can send over to TV, as opposed to the mirrored social screen. It's sent compressed to the PU and then uncompressed by the device and sent to the screen. We're told that this was "an innovation that came quite late" in the development of the system.
• It displays PS4's system software interface in cinematic mode, handling the display of traditional 2D content.

In short, the second-gen PU we'll find in the final retail PSVR package carries over all of the features of the initial developer model - such as handling the social screen - but adds key functionality, such as 3D audio. It also allows developers to send a bespoke video output to the HDTV, entirely separate from what the HMD wearer sees, opening up new gameplay options. This appears to be using the PS4's hardware h.264 encoder used for streaming and Remote Play, running at 720p30. Obviously, creating another entirely separate framebuffer in addition to the already challenging VR viewpoint adds significantly to the computational load on PlayStation 4, so expect limited adoption. However, one of Sony's titles - Playroom VR, a free download for the system - has asymmetrical multiplayer for the VR player and external users using this functionality and is on show at GDC.

"It's a bit like Nintendo Land in that way," says Eurogamer editor Oli Welsh, who has played the demo on-site. "It's hilarious and great."