 is a schematic in block diagram form of equipment for demonstrating the present invention;
FIG. 14 is a waveform diagram of a test signal plotted as voltage against time;
FIG. 15 tabulates data representing a transfer function according to an embodiment of the present invention;
FIG. 16 is a schematic in block diagram form of a sound image location system according to an embodiment of the present invention;
FIGS. 17A and 17B are graphical representations of typical transfer functions employed in the sound processors of FIG. 16;
FIG. 18A-18C are schematic block diagrams of a circuit embodying the present invention; and
FIG. 19 is a schematic block diagram of additional circuitry which further embodies the present invention.
In order to define terms that will allow an unambiguous description of the auditory imaging process according to the present invention, FIGS. 1-4 show some dimensions and angles involved.
FIG. 1 is a plan view of a stereo listening situation, showing left and right loudspeakers 101 and 102, respectively, a listener 103, and a sound image position 104 that is apparent to listener 103. For purposes of definition only, the listener is shown situated on a line 105 perpendicular to a line 106 joining loudspeakers 101 and 102, and erected at the midpoint of line 106. This listener position will be referred to as the reference listener position, but with this invention the listener is not confined to this position. From the reference listener position an image azimuth angle (a) is measured counterclockwise from line 105 to a line 107 between listener 103 and image position 104. Similarly, the image slant range (r) is defined as the distance from listener 103 to image position 104. This range is the true range measured in three-dimensional space, not the projected range as measured on the plan or other orthogonal view.
In the present invention the possibility arises of images substantially out of the plane of the speakers. Accordingly, in FIG. 2 an altitude angle (b) for the image is defined. A listener position 201 corresponds with position 103 and an image position 202 corresponds with image position 104 in FIG. 1. Image altitude angle (b) is measured upwardly from a horizontal line 203 through the head of listener 103 to a line 204 joining the listener's head to image position 202. It should be noted that loudspeakers 101, 102 do not necessarily lie on line 203.
Having defined th image positional parameters with respect to a reference listening configuration, we proceed to define parameters for possible variations in the listening configuration. Referring to FIG. 3, loudspeakers 301 and 302, and lines 304 and 305 correspond respectively to items 101, 102, 106, and 105 in FIG. 1. A loudspeaker spacing distance (s) is measured along line 304, and a listener distance (d) is measured along line 305. In the case that a listener is arranged parallel to line 304 along line 306 to position 307, we define a lateral displacement (e) measured along line 306. For each loudspeaker 301 and 302 we define respective azimuth angles (p) and (q) as measured counterclockwise from a line through loudspeakers 301, 302 and perpendicular to a line joining them, in a direction toward the listener. Similarly for the listener we define an azimuth angle (m) counterclockwise from line 305 in the direction the listener is facing.
In FIG. 4, a loudspeaker height (h) is measured upward from the horizontal line 401 through the head of the listener 303 to the vertical centerline of loudspeaker 302.
The parameters as defined allow more than one description of a given geometry. For example, an image position may be described as (180,0,x) or (0,180,x) with complete equivalence.
In conventional stereophonic reproduction the image is confined to lie along line 106 in FIG. 1, whereas the image produced by the present invention may be placed freely in space: azimuth angle (a) may range from 0-360 degrees, and range (r) is not restricted to distances commensurate with (s) or (d). An image may be formed very close to the listener, at a small fracti