Dirac is a familiar name to many AVS Forum members. This Swedish company has created one of the most highly regarded room-compensation systems, which is implemented in products from Arcam, Datasat, Emotiva, miniDSP, NAD, and others. At VRLA 2018, the company demonstrated one of its latest developments—Dirac VR.
In virtual reality, audio is normally delivered via headphones. But in order for sounds to be convincingly immersive, they must appear to be coming from various directions outside your head. This is called virtualization, and there are several companies that offer it in one form or another. Dirac VR is a sophisticated step forward in headphone virtualization that promises to improve the VR experience dramatically.
At the core of all headphone-virtualization systems is something called a head-related transfer function (HRTF). It mathematically describes the aural effect of a person’s head on a sound coming from a particular direction. For example, a sound coming from directly in front of you sounds different than the same sound coming from over your left shoulder. Some of the important parameters in this process include the difference in arrival time, amplitude, phase, and spectrum of the sound at each ear. As a result, we have the ability to localize the direction from which various sounds originate.
In the most sophisticated—and expensive—virtualization systems, the HRTF of each person using it is measured and implemented separately. The result is a very accurate and believable 3D soundfield for that person, but a relatively poor soundfield for anyone else using the same HRTF. So, most VR applications use a generic HRTF that, hopefully, is good enough for most people.
Dirac VR uses a generic HRTF, but it is more sophisticated than most. Dirac refers to conventional generic HRTFs as “static”; they measure sound coming from only a few directions, and they assume that the head and torso move in tandem. Of course, the head and torso do not typically move in tandem in the real world. Dirac VR takes that into account, calling its solution a “dynamic” HRTF.
Also, Dirac has measured the HRTF of dozens of people with a higher degree of precision than other systems, taking directional measurements in 1° increments. These individual HRTFs were then compiled into a database, and a generic, dynamic HRTF was derived from all that information. The company acknowledges it’s still not as good as a correctly made individual HRTF, but it’s better than most generic HRTFs.
Another factor critical to VR audio is head tracking. In order to create a convincing aural landscape, the direction from which sounds appear to come must remain unchanged when the user moves their head. For example, in the real world, if you hear a bird above and to the right and you turn your head to face the sound, it is now coming from directly in front of you. It sounds different because of your natural HRTF.
To simulate this effect in VR, the headphones must have a head-tracking function. Without it, if a sound appears to come from directly in front of you and you turn your head to the side, that sound will still appear to be coming from directly in front of you. The source of the sound will appear to have shifted along with your head movement. Obviously, that would break the spell of being in another reality.
Without Dirac VR (or another virtualization system), the sound from headphones is entirely inside your head. When you turn on Dirac VR, sounds from, say, a 5.1 speaker system appear to come from outside your head. When you add head tracking, the apparent position of the speakers remains fixed as you move your head around.
Dirac VR also includes a 3D reverberation engine that simulates the effect of sounds in various acoustic environments. Another part of the system optimizes the performance of the headphones with frequency and time domain-correction algorithms to minimize any coloration attributable to the headphones themselves.
Like its room-compensation technology, Dirac licenses Dirac VR to product manufacturers. Its first customer is a Chinese company that makes gaming headphones. (The reps I spoke with wouldn’t reveal the name of that company.) The first product will be a pair of headphones that simulate a 2.0, 5.1, or 7.1 speaker system without head tracking, while the next product will be headphones with head tracking built in. The processing will be performed in the gamer’s PC.
Dirac presented two demos at VRLA 2018. The first used a Samsung Gear VR headset with a Samsung S8 smartphone and Sony MDR-1A headphones; the S8 provided the head tracking with its positional sensors. The virtual environment was commercial cinema with a 2-channel speaker system playing a clip from The Fate of the Furious. As I moved my head, the sound from the two “speakers” stayed put, just as it should. However, they did not appear to be very far outside my head.
The second demo was audio-only. In this case, a pair of Sennheiser HD 650 headphones had a tiny head tracker taped to the headband. Dirac obtained unmixed, multichannel music recordings and mixed them in 5.0.2. Here, the virtual speakers seemed to be farther away, and sounds from overhead were quite distinct. As before, apparent sound sources remained fixed in place as I moved my head around.
In both demos, the virtualization remained very stable as I moved my head around. The effect was very fluid and seamless. I look forward to checking out Dirac VR in commercial products when they become available.