Electrical audio signal processing systems and devices – Binaural and stereophonic – Pseudo stereophonic
Patent
1995-02-21
1997-09-23
Kuntz, Curtis
Electrical audio signal processing systems and devices
Binaural and stereophonic
Pseudo stereophonic
381 18, 381 20, 381 1, 381 97, 381 22, H04R 500
Patent
active
056712871
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND TO THE INVENTION
This invention relates to directional sound production and reproduction systems wherein it is desired to provide sound source signals with a desired directional dispersion or angular spread of signal components.
In many applications, it is undesirable that the reproduced image of a sound source in a directional reproduction system should be absolutely sharp. Actual sounds subtend a finite angular width at a listener, and it is often desired to simulate such a natural angular size. Additionally, it is often desired to take monophonic material, such as historical monophonic recordings or the monophonic "surround" channel of a film surround soundtrack and to provide reproduction having a wide angular spread.
Methods of providing such angular spread or dispersion for individual sound source signals are often termed "pseudostereo" methods. Pseudostereo methods are well known in the prior art. For example, see R. Orban "A Rational Technique for Synthesizing Pseudo-Stereo from Monophonic Sources", Journal of the Audio Engineering Society, vol. 18 no. 2 pages 157 to (February 1970), and M. R. Schroeder "An Artificial Stereophonic Effect Obtained from a Single Audio Signal" Journal of the Audio Engineering Society, vol. 6 no. 2 pages 74 to 79 (April 1958).
However, prior art pseudostereo methods have numerous defects. Most prior art pseudostereo methods work by providing a dual filter arrangement whereby a monophonic source signal is fed to a left and a right stereo channel with complementary filter characteristics, whereby frequency components that are cut on one channel are boosted on the other. However, prior art filter arrangements such as those described by Orban in the cited reference generally cause unpleasant phase differences between the two speaker signals, producing an unpleasant subjective sensation often termed "phasiness". While in the cited reference Schroeder describes a dual filter arrangement that avoids phasiness, the arrangement suggested has a total reproduced energy response, measured as a function of frequency, that is not flat, but which has variations of 3 dB. Such variations in the reproduced total energy response are undesirable, as they can cause audible colouration effects.
Phasiness and unflat reproduced energy response are not the only problems with prior art pseudostereo methods. It is not difficult to degrade the sharp localisation quality of stereophonic images by introducing irregular amplitude and/or phase differences between the stereo channels, and/or adding delayed simulated early reflections. However, in the desired applications of pseudostereo, it is desired to avoid unnatural side effects that cause listening fatigue. Such side effects can arise from different auditory localization cues giving mutually contradictory results. For example, the ears tend to localise transient and continuous sounds by different mechanisms, and methods of pseudostereo relying on the use of time delays, especially those in excess of about 1 or 2 milliseconds, tend to provide contradictory cues by these two mechanisms, resulting in an audible splitting of the directionality of transient and continuous sound components.
Another cause of audible splitting of the directional effect caused by dual filter arrangements is when different frequency components of a single sound are heard as being sharply localised in different directions. Sometimes such frequency splitting is found to be desirable, as in the case where the different frequency components correspond to different sound sources within a monophonic mix, in which case the splitting can be used to provide different stereo directions for different sound sources, but in other cases such splitting is undesirable, such as when the different frequency components should have the same localisation quality.
Besides these problems, prior art pseudostereo methods are also only applicable to separate monophonic source signals, whereas it is often desired to be able to take a pre-mixed stereo sound source with sharp sound images,
REFERENCES:
patent: 3670106 (1972-06-01), Orban
patent: 4653096 (1987-03-01), Yokoyama
patent: 5208860 (1993-05-01), Lowe et al.
Kuntz Curtis
Nguyen Duc
Trifield Productions Limited
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