The use of B-format measurements, recordings and playback in the provision of more ideal acoustic reproductions which capture part of the spatial characteristics of an audio reproduction are well known.
In the case of conversion of B-format signals to multiple loudspeakers in a loudspeaker array, there is a well recognized problem due to the spreading of individual virtual sound sources over a large number of playback speaker elements. In the case of binaural playback of B-format signals, the approximations inherent in the B-format sound field can lead to less precise localization of sound sources, and a loss of the out-of-head sensation that is an important part of the binaural playback experience.
U.S. Pat. No. 6,259,795 by Lake DSP Pty Ltd. describes a method for applying HRTFs to a B-format signal which is particularly efficient when the signal is intended to be distributed to several listeners who require different rotations of the auditory scene. However, that invention does not address issues related to the precision of localization or other aspects of sound reproduction quality.
WO 00/19415 by Creative Technology Ltd. addresses the issue of sound reproduction quality and proposes to improve this by using two separate B-format signals, one associated with each ear. That invention does not introduce technology applicable to the case where only one B-format signal is available.
U.S. Pat. No. 6,628,787 by Lake Technology Ltd. describes a specific method for creating a multi-channel or binaural signal from a B-format sound field signal. The sound field signal is split into frequency bands, and in each band a direction factor is determined. Based on the direction factor, speaker drive signals are computed for each band by panning the signals to drive the nearest speakers. In addition, residual signal components are apportioned to the speaker signals by means of known decoding techniques.
The problem with these methods is that the direction estimate is generally incorrect in the case where more than a single sound source emits sound at the same time and within the same frequency band. This leads to imprecise or incorrect localization when there is more than one sound source is present and when echoes interfere with the direct sound from a single source.