June 1, 2013
High Quality Stereo with Center
An economic and thus appealing setup.
Introduction
While our Immersive Audio Processor supports multi-channel surround loudspeaker setups with up to 16 loudspeakers, an economic and thus appealing setup is stereo with an additional center loudspeaker.
Stereo music is reproduced with a center loudspeaker with the goals of providing:
– highest quality
– a vastly improved listening experience for listeners that are not in the sweet spot
In the following, we are describing our strategy and challenges we faced for achieving these goals.
Stereo Sounds Great – In the Sweet Spot
High quality music, reproduced with a high quality stereo system, properly setup and assuming a good acoustic, enable a spatially remarkably faithful and rich listening experience. Phantom sources are perceived between the left and right loudspeakers and the listener is to a good degree immersed into the room/environment contained in the recording.
Much Is Lost – Outside of the Sweet Spot
If a listener is not in the sweet spot, phantom sources are perceived from the nearer loudspeaker. Immersion vanishes, making way to noticing being in the local room and “listening to loudspeakers” as opposed to instruments.
If a listener is in the sweet spot, but turns his head to the side, immersion vanishes, too.
Goal: Great Sound Everywhere
The goal is to enable a great listening experience for everyone, that is, not only for the person in the sweet spot, by using a center loudspeaker. The key to high quality is how the three channels (left, right, center) are generated from the given stereo music. In the following, we describe the three challenges we faced when developing this technology:
Extracting a Center Channel
Timbre of Center
Spaciousness of Center
Extracting a Center Channel
The goal is to extract a signal from the stereo signal, corresponding to the signal a listener in the sweet spot hears as a center phantom source. Localized sound is dry. Thus, a first step is to extract from the given stereo signal its dry part. The next step is to analyze the dry stereo signal and extract dry sound localized in the center. The so-obtained center signal is (scaled and) subtracted from the original left and right channels.
Timbre of Center
A listener’s head and body act as a filter to the arriving sound. This filter is different for sound from the side and center. Therefore, the same signal reproduced as stereo phantom center versus a center loudspeaker has a different timbre. For high quality stereo reproduction using a center channel, the center timbre has to be corrected such that it is similar to the original phantom center timbre.
Spaciousness of Center
A stereo phantom center is perceived as blurred. In contrast, a signal from a center loudspeaker is perceived very compactly. It is as if you “hear the loudspeaker” as opposed to a more spacious object like a stereo phantom center.
A phantom center corresponds to the same signal at the left and right loudspeakers. From these, not only direct sound reaches the listener, but also left and right early reflections. In contrast, early reflections from a center loudspeaker come mostly only from the center direction (back wall, floor, and ceiling reflections). Thus, a center loudspeaker is perceived as more focused than a phantom center.
Additionally, especially at high frequencies, the localization information available at the left and right ears is not precise and consistent for the phantom source, further adding spaciousness to its perception.
To add spaciousness to the center signal, center signal early reflections are added to left and right. The difficulty is to do this without impairing timbre and sound quality. Furthermore, at high frequencies, the energy of the center signal is spread to left and right.
Putting It All Together
The use of techniques for center signal extraction, timbre correction, and increase of spaciousness enables high quality reproduction of stereo signals using a center channel.
Disclaimer
Note that the functionality and functioning of Illusonic technologies and products may be modified at any time without notice. The algorithm descriptions above are for illustrative purpose only.