Dirac Live - Technical Description

What is Dirac Live?
Dirac Live® is a state-of-the-art digital room correction technology which optimizes the sound system both in terms of the impulse response as well as the magnitude frequency response. The result is a substantially improved musical staging, clarity, voice intelligibility, and a deeper and tighter bass, not just in a small sweet spot but in the entire listening volume.

What is the time-domain and why is it important?
At Dirac, we believe that the reproduction of transients is critical for a truly natural and transparent sound. For example, the stereo image is critically dependent on the time-domain differences and similarities between the sound at the left ear and the right ear. That's why a working impulse response correction improves the depth, the positioning and distinction of individual voices and instruments.
In the academic literature, a measure of clarity is based on the impulse response's decay time.

Mixed-phase technology

• By using a look-ahead buffer it becomes possible to perform impulse response correction

• Minimum-phase and linear-phase room correction filters can't physically optimize the acoustic impulse response in a room. They only try to minimize the additional smearing caused by the filtering

• Room-acoustic responses are non-minimum-phase. This is why mixed-phase correction is used by Dirac Live

• Dirac Live is unique in improving the impulse response not just in a single point

Multiple measurements
Dirac Live uses several measurement positions (9 typically) in the listening room.This way, it is possible to find consistent acoustic problems. Only these should be corrected.
 

In terms of the impulse response, this means the direct wave and very early reflections. Generally, at higher frequencies there is less consistent time-domain behavior. Dirac Live automatically finds consistent problems in a frequency-dependent way and corrects for them. After finding consistent time-domain behavior, there may still be some late reverberation in the room that causes a gentle coloration. This is corrected in the frequency domain with just the right resolution.

Impulse response correction
The impulse response of a loudspeaker affects clarity, tightness and all spatial aspects of the sound, such as location and distinctness. Dirac Live is unique in actually correcting the impulse response in a large listening area, not just a single point. By focusing on consistent problems across the measurement positions, and correcting only these, a faster decay time is achieved. Typically, the power ratio between the direct wave and the tail is improved by 6 dB or more, representing a vastly tighter sound.

Frequency response correction
Why is it that the same frequency response can sound so different in different rooms? The answer lies to a large extent in the time-domain aspects of the measured frequency response and to some part by differences in the spatial (angular) domain. By addressing time-domain problems as such, and not as minimum-phase spectral problems, our frequency response correction also becomes more effective. We treat the spectral coloration in a room after we have performed a time-domain correction. We also make sure not to overcompensate. This is done by paying attention to the variations across different measurement positions, not just the average.

Implementation for best accuracy

• Traditional EQ uses IIR filters for lowest processor usage

• Many room correction filters use FIR filters for ease of implementation and optimization

• IIR and FIR filters both have their pros and cons. At Dirac we use a proprietary filter structure. In essence, maximum performance at much less processor usage than regular FIR filters. This filter structure also has great numerical properties, not causing any digital noise.

Choosing a target response
A flat frequency response does not necessarily sound neutral. It has to do with the ratio between direct and reverberant sound and the frequency-variable propagation loss. The late reflections cannot be removed by Dirac Live, because they change too much between different positions. Therefore, a slight roll-off is often appropriate if you are listening at a distance. The Dirac Live algorithm suggests a target response appropriate for your listening environment and speakers. Of course, you can still adjust it to your taste using our simple graphical interface. 

Advanced, yet very simple to use
Don't worry, Dirac Live is aimed at normal people, not signal processing or computer experts. All these aspects that go into the making of a world-class room correction are automatically handled by our software. As a user, you only worry about positioning the microphone and pressing the Play button. A software wizard lets you select the sound system setup (stereo,  5.1, etc.), calibrate sound levels, and shows where the microphone should be placed. The test signals are played and recorded by the software. You just follow the instructions. The impulse and frequency responses are shown for your reference along with a suggested target response. If you are an advanced user, you can edit the target response graphically if you wish to. Otherwise, just press "Optimize" and you're ready to get face-to-face with your favorite artists!

Further reading
Dirac Live technology is used in the most demanding environments. The reason is our vast body of knowledge built up through R&D over tens of years. Our list of publications and patents is long, but here are some references:

• Spatially Robust  Audio Compensation Based on SIMO Feedforward Control, Brännmark, Ahlén, IEEE Trans Signal Processing, May 2009.

• On Room Correction and Equalization of Sound Systems, Johansson, invited talk at panel discussion on "FIR or IIR? That is the question!", 123rd AES Convention, NY, USA, 2007

• Room Correction and Equalization of Cinema Sound Systems, Johansson, invited talk at ICTA Annual Seminar, LA, USA, 2010