There are many applications where monitoring on headphones is the only practical solution — for example, recording on location, or mixing at home late at night. It's not really ideal, but it is possible, with a little practice and experience, to become accustomed to the essential differences between headphone and loudspeaker auditioning. If you want to know more about mixing on headphones, Martin Walker's article in SOS January 2007 is a good starting point.
This is all very well for stereo mixing. However, if you want to work in surround sound and loudspeakers aren't a viable option, what can you do? Until now the only solution was to monitor in conventional stereo and hope for the best — but help is now at hand!
Beyerdynamic's new Headzone Pro is a system comprising a compact digital processor box and a pair of modified, semi-open DT880 PRO HT headphones. The box accepts 5.1 signal sources (either via unbalanced analogue inputs or a computer Firewire connection) and uses modelling software to reproduce a convincing surround sound image on stereo headphones, using binaural sound processing techniques.
The thing that makes the system credible is the headtracker technology. This provides the ability to turn your head while the virtual sound sources appear to stay in the correct spatial positions, just as would happen if you were in a room listening to separate speakers. This headtracking function is achieved with a pair of ultrasonic transmitters mounted on the headband of the headphones, and a pair of receivers mounted at some convenient point in front of the user. This is essentially the same kind of technology as used in car burglar alarms to detect intruders.
As the head is turned, the changing level and phase relationships of the two ultrasonic signals allows the processor to work out the angle of the listener's head relative to the transmitter, and the modelling parameters are adjusted accordingly. Turn the headtracking off and, although there is still a surround sound image, the whole thing instantly becomes quite artificial sounding!
The heart of the system is the control unit, which is housed in a half rack-width, 1U-high case with blue moulded sides. The front-panel facilities comprise a large rotary volume control, a source select button (analogue or digital) with associated indicator LEDs, a bypass button, and a headtracker status LED. The headphones are connected via a conventional TRS socket on the front panel, and they can be used with normal headphone outlets on other equipment if required. Equally, alternative headphones can be used with the Headzone Pro. In this case, though, while you'll still hear a surround sound-stage effect, you won't have the benefit of the headtracking, which means that the image rotates with your head, rather than appearing to be fixed
in space around you. The bypass button forces the unit to become a simple, but high quality, stereo headphone amplifier.
The rear panel has a three-pin socket for the static ultrasonic receivers, six phono (RCA) sockets for the unbalanced 5.1 analogue inputs (along with a slide switch to select +4dBu or -10dBV sensitivities), and a coaxial power inlet socket. The power supply is an in-line universal type that provides 5V DC. A push-button provides the on-off control and a standard six-pin Firewire socket provides the link to a computer. This carries both digital audio inputs and allows remote control and configuration of the unit.
Sadly, a Firewire cable isn't included in the impressive kit of parts — presumably because some people will require four-pin connectors for laptops and others might need six-pin or eight-pin connectors for desktop machines. It would be frustrating, though, to buy the Headzone Pro and find you have to wait to use it because you don't have a suitable Firewire cable available!
That omission aside, the kit includes a very well-designed soft nylon case roughly the size of a large briefcase, with foam cut-outs to support the headphones on one side and the control unit on the other. The power supply, cables and headtracker bar stow neatly underneath. Further pockets are provided for the installation CD and handbook, making a very neat and portable package.
A neat little bracket is also included that allows the headtracker receiver to be mounted on the top of a laptop screen, or on a wall or shelf. It can also be mounted on a mic stand, if required.
The ultrasonic sensor block on the headphones is fitted neatly and securely, and a tiny push-button forces the software to realign the nominal centre point. The headphones themselves are, naturally, very comfortable and provide excellent sound quality, as you would expect.
Alternative Surround Sound Headphone Systems
The idea of monitoring surround sound via headphones isn't new. Basic binaural recordings can produce reasonably convincing surround sound effects for many people, and all of the more advanced systems have been built on that premise.
In 1997, Sennheiser produced a compact handheld processing box called Lucas, that accepted a stereo Dolby Pro-logic signal and reconstructed a binaural surround sound effect in the headphones. It cost a modest £280 at the time and was groundbreaking in its use of a DSP chip to manipulate the two channel inputs with sophisticated 'head related transfer functions' (HRTF) — something they called 'Toltec Processing' — to create the virtual surround effect. Fifteen different HRTF sets were provided and the user had to go through a configuration process to select the set that most closely matched their own physiognomy — an essential step in creating a stable image. The Lucas system worked surprisingly well if you kept your head still, but any movement tended to shatter the illusion.
The first time I found a system that addressed this specific problem was at a Studer demonstration in 2000. The Swiss boffins had come up with 'BRS' (Binaural Room Scanning), in which the impulse responses of an existing 5.1 monitoring environment were 'captured' using a binaural dummy head, positioned where the mix engineer would normally sit. Impulses were captured for each ear of the dummy head from all five surround channels, and these impulse responses were then used to control an elaborate convolution processor which replicated the precise surround sound environment for the listener in the stereo headphones. Rather than modelling the HRTF data, as the Headzone Pro does, it used accurate in-room measurements.
However, the really clever part of the BRS system was that additional impulse responses were taken with the dummy head in a series of small rotational increments left and right. A sophisticated headtracker system fitted to the headphones informed the processor of the user's head position, and thus which set of impulse responses to use at any moment.
The results were truly stunning, with utterly convincing and completely stable surround sound images that really did sound as though you were in the measured room. The feedback on the prototype systems was very positive but, sadly, the vast amount of real-time hardware convolution processing and the sophisticated headtracking sensor made the system extremely expensive, and it wasn't put into production.
The supplied software CD loaded onto my laptop without any trouble, detecting the Headzone unit and loading the appropriate drivers automatically. ASIO and WDM are included for PCs, and the Headzone is compatible with Apple Core Audio for users of Mac OS X (10.4 and higher) systems.
With everything plugged up and the appropriate drivers selected in my DAW, I was able to audition the 5.1 test material supplied on the setup CD and start playing with the software configuration tools.
The software display has four tabbed pages, the first of which shows the layout of virtual speakers, together with a table of their relative angles. You can alter the default setup either by dragging the speakers around on the graph or by entering new angle values in the chart. Settings can be stored and recalled as presets if required.
Also on this page are three sliders that set the virtual room size, speaker distance, and amount of ambience. The default values of 50, 50 and 30, respectively, provide a good starting point. Reducing the levels makes the sound unnaturally dry and closed in, while high settings make it very reflective and lively. The idea is to find a setting that sounds like speakers in a decent control room, with just enough ambience and space to sound realistic, without swamping the acoustics of the sound you are monitoring. Naturally, all three controls interact, but it's not too hard to reach a good subjective compromise.
The second tabbed page provides seven sliders to adjust the relative volumes of each of the 5.1 input channels, and the binaural output. There are also tick boxes to mute each input and to configure the LFE channel, with overload lights and bar-graph meters for each channel.
The LFE options include mute, 'full range' and 'polarity inversion.' The first and third are obvious enough, and the manual suggests that the polarity inversion is included because some DVDs are mixed this way! The full-range mode disables the default 24dB/octave filter removing everything above 120Hz. There are two reasons why you might want to do this: the first is because you have already applied the filter when you produced the LFE track, and the second is to allow the LFE input to serve as a talkback channel to the person wearing the headphones. This would be very useful for, say, a TV sound mixer in an outside broadcast truck producing a 5.0 surround mix.
The Control/Status tabbed window carries tick boxes to select the required input and to bypass all processing or disable the headtracker. These facilities reflect the front panel status of the control box, and allow remote control at the same time. There is also a set of status lights showing which input is in use, whether the signal processing is active, and whether the headtracker is in range and working correctly.
More usefully, a graphical display shows the current headtracker axis, with numerical readouts of angle and distance. The system is optimised to work over a ±60-degree range, with the 60-90 degrees area on each side becoming progressively less accurate. A 'Scale' slider adjusts the sensitivity of the headtracker from very sharp and precise at the top to pretty slow and loose at the bottom. I found my preference was for a fairly high setting, around 75-80 but, again, this is a matter of personal preference. When set incorrectly it feels as if the virtual speakers aren't bolted down! Instead they tend to move towards you as you turn your head if the control is too high, and continue moving away if it's too low.
Finally, the Advanced tab window provides software and firmware version numbers and ASIO driver settings, plus the facility to load new firmware to the DSP. The review model was running V1.2 DSP firmware, with V1.1 GUI software.
I experimented with remixing some familiar surround material on the Headzone headphones and was able to create good mixes that worked exactly as expected on a full 5.1 speaker system. Listening to commercial SACD and DVD-As late at night via the analogue inputs was a joy as well. However, I found that the system added an air of artificiality to the overall sound, simply because of the room-modelling artifacts — although, with careful adjustment of the relevant parameters, this can be minimised. This roomy characteristic is always present when the unit is creating the binaural surround sound stage, but I learned quickly to ignore it and it didn't affect my ability to mix or, for that matter, to enjoy commercial releases.
The Headzone Pro works remarkably well, provided you take the trouble to optimise the various settings properly. When I first heard this system at a trade show I wasn't impressed with its portrayal of a surround environment, largely because it didn't seem very stable. However, having played with the unit over an extended period, and having configured it carefully for my own ears, I am impressed with how well it works. It is perhaps not quite as precise or realistic as Studer's prototype BRS system (see the box above), but it's not far off — and the Headzone Pro is affordable and in production!
As far as I am aware, there is nothing currently on the market that competes directly with the Headzone and its headtracker technology.