In HiFi, Virtual Reality Might Be Better Than Reality

As I write this, I’m listening to a recording of Joss Stone. Her voice sounds completely natural, hovering in the air just a few meters in front of me, placed distinctly at the center of my sound system, remaining there regardless of how I move my head. I can almost touch the ambience of the recording. The low frequency extension is great, the room modes are extremely well controlled. The listening room is remarkably well treated, with just the right amount of air and sense of space, and without the annoyance of comb filters or spectral coloration. It’s treated so well, I don’t need digital room correction. This is an experience you can’t get without a HiFi and room treatment budget of at least $100,000 USD.

The funny fact is this: I’m getting this experience with a pair of headphones. And the sound system I’m referring to? It’s a virtual one. 

We’ve succeeded in creating a virtual HiFi system using a new approach to head-related transfer functions (denoted dynamic HRTFs), a head tracker, a new 3D reverberation engine, and some other processing techniques. I will explain in more detail below, but first let’s consider the problem being addressed.

The problem of lackluster headphone sound

At Dirac, we have worked more than 15 years combatting the detrimental effects the listening room has on your HiFi experience, and we have come up with a number of solutions, including Dirac Live and, more recently, Dirac Unison. Step by step, we have removed more and more of the acoustical problems through the use of increasingly advanced digital signal processing. We can’t break physics, however. Some room acoustic and speaker-related problems are not possible to solve in practice. 

Another problem we have always wanted to solve has to do with headphone listening.  Headphones are different from anything else we listen with. Without headphones, whatever you listen to, you’ll always hear part of a sound with your right ear and another part with your left ear. Headphone listening, on the other hand, is inherently unnatural. With headphones, your ears are blocked, so the sound is prevented from entering both ears in a natural way. You see, stereo music is recorded in such a way so as to create a sound image that spans from about -30 to +30 degrees in front of you. When an instrument is hard-panned to the right, it is only played through the right speaker, and the sound is 30 degrees to the right of you if the speaker is placed at that angle. With headphones, the sound is at 90 degrees, and, not only that, it is completely blocked from your left ear, which, in reality, no sound at 90 degrees would be. This is why it can sound so annoying, and why you get listening fatigue much sooner with headphones than with a HiFi system. 

One solution is to use cross-feed. That is, you send a little bit of that right signal to your left ear as well. But if you do this the obvious way, that is, by adding an attenuated version of the right signal to the left signal and vice versa—and there is more to the music mix than just that hard-panned instrument—you’ll also introduce artifacts such as comb filtering. 

What if the solution has nothing to do with the reality of the sound, but could instead be achieved with a virtual HiFi system? A perfect listening room and ideal loudspeakers. Wouldn’t that be cool? But how could we achieve that? After all, in order to listen to music we would need some form of transducer, which invariably has some coloration.

Here was our thinking: If we can manage to design the ideal custom listening space and speakers in the virtual domain, well, then we have also succeeded in creating the ultimate room correction solution—replacing the real room with a virtual room, decked out to provide the best possible listening environment!

And so we set out to do just that.

Designing HiFi in the virtual domain

In order to simulate a sound coming from a particular direction, such as a loudspeaker playing at 30 degrees to the right of a listener, we needed to precisely simulate how that sound would propagate through the listening room and enter the listener’s ears, and then play back that signal through a set of finely optimized, high-quality headphones. In essence, we had to model the direct wave, the early reflections, diffuse reverberation, and the impact made on the sound by the body, head, and ears. This has taken some time, but now we’re ready to show the world the results—and they are exciting!