2 WILL GET YOU 5.1

Oct 1, 2003 12:00 PM, By Scott Wilkinson

           
 

In EM's March 2001 “Tech Page,” I discussed a technology called Dolby Headphone, which convincingly simulates 5.1-channel surround sound in conventional stereo headphones. Now, the company is applying the same idea to audio systems with two speakers. Dubbed Dolby Virtual Speaker, the system accepts a 5.1-channel input and creates the illusion of having five speakers around the listening position.

Like Dolby Headphone, Dolby Virtual Speaker uses head-related transfer functions (HRTFs) derived from acoustic studies with actual five-speaker setups to re-create the sonic signature of each speaker, which depends in part on its location with respect to the listener and within the room. The Dolby HRTFs take into account both direct and reflected sound, and they allow the virtual speakers to be placed with pinpoint accuracy at 0°, ±30°, and ±105° at a reasonable distance from the listener. By contrast, most other surround-simulation systems use much simpler HRTFs based on the direct sound only, which create only a general sense of envelopment without precise speaker locations.

But Dolby Virtual Speaker must do even more, since the listener's ears are not isolated, as they are with headphones. To address this problem, the system uses a technique called crosstalk cancellation: sounds from the right speaker are also sent to the left speaker, filtered to mimic the response rolloff caused by the head partially blocking the higher frequencies, and delayed just enough to be out of phase with the right-speaker sounds at the listener's left ear

FIG. 1: In crosstalk cancellation, sounds from the right speaker are also sent to the left speaker (1) and delayed just enough to be out of phase with the right-speaker sounds at the listener's left ear (2), which means that only the right ear hears the sounds from the right speaker (3).

(see Fig. 1). As a result, the left ear doesn't hear sounds from the right speaker, and vice versa. Of course, using two speakers instead of headphones requires the listener's head to be in the sweet spot, equidistant from the speakers, for the simulation to be effective. However, the Dolby algorithms allow much more latitude in the listener's position along the centerline than other systems, thanks to the fact that the complex HRTFs allow lower-order crosstalk-cancellation filters to be used, which reduces audible phase artifacts.

All this takes some serious DSP power to accomplish in real time. While most surround-simulation schemes assume there will be little processing bandwidth available (constraining them to using simple HRTFs), Dolby looked at dedicating lots of processing to the system. For example, the acoustic path from each virtual speaker to each ear is simulated by a 7,000-tap, finite impulse-response (FIR) filter. That many large filters would normally require excessive processing power and introduce pronounced latency, but Dolby's convolution algorithms and the decreasing cost of DSP bandwidth eliminate these problems.

The Dolby Virtual Speaker algorithms have been implemented by all major IC manufacturers, including Analog Devices, Motorola, Texas Instruments, Cirrus Logic, and Sanyo. In addition, the algorithms can run as software on a PC with an Intel Pentium II or III or an AMD K6-II CPU running at a clock speed of at least 400 MHz. The specific DSP requirements vary from platform to platform, but in general, Dolby Virtual Speaker requires about 20 MIPS (million instructions per second) and 32 to 64 KB of coefficient storage to operate in real time.

The most obvious musical application is to use Dolby Virtual Speaker with computer audio systems, which would allow composers in cramped quarters to monitor multichannel mixes without having to set up an actual 5.1 system. In the consumer-electronics realm, Denon has announced it will include Dolby Virtual Speaker in some of its A/V receivers, including the new D-M71DVXP. Clearly, this technology has a lot of potential to bring the surround-sound experience to those who wouldn't otherwise be able to enjoy it.