 signal. Similarly summing element 1660 drives inverter 1681, and switch 1682 allows selection of the direct or inverted signal at point 1683. Switch 1685 allows selection of the signal 1683 or the input signal 1601 as the drive to attenuator 1686 which produces left channel output 1688.
Filter 1610, 1630 are identical, and one is shown in detail in FIG. 18B. A unity gain buffer 1611 receives the input signal 1601 and is capacitively coupled via capacitor 1612 to drive filter element 1613. Similar filter elements 1614 to 1618 are cascaded, and final filter element 1618 is coupled via capacitor 1619 and unity gain buffer 1620 to drive inverter 1621. Switch 1622 allows selection of either the output of buffer 1620 or of inverter 1621 at filter output 1623.
Filter elements 1613 through 1618 are identical and are shown in detail in FIG. 18C. They differ only in the value of their respective capacitor 1631. Input 1632 is connected to capacitor 1631 and resistor 1633 and resistor 1633 is coupled to the inverting input of operational amplifier 1634, output 1636 is the filter element output. Feedback resistor 1635 is connected to operational amplifier 1634 in the conventional fashion. The non-inverting input of operational amplifier 1634 is driven from the junction of capacitor 1631 and one of resistors 1637 to 1642, as selected by switch 1643. This filter is an all-pass filter with a phase shift that varies with frequency according to the setting of switch 1643.
Table 1 lists the values of capacitor 1631 used in each filter element 1613-1618, and Table 2 lists the resistor values selected by switch 1642; these resistor values are the same for all filter elements 1613-1618.
One embodiment of summing elements 1660, 1670 is shown in FIG. 18D, in which two inputs 1661, 1662 for summing in operational amplifier 1663 result in a single output 1664. The gains from input to output are determined by the resistors 1665, 1667 and feedback resistor 1666. In both cases input 1662 is driven from switch 1659, and input 1661 from joystick potentiometers 1651, 1652 respectively.
As examples of image placement, Table 3 shows settings and corresponding image positions to "fly" a sound image corresponding to a helicopter at positions well above the plane including the loudspeakers and the listener. To obtain the required monophonic signal for the process according to the present invention, the stereo tracks on the sound effects disc were summed. With the equipment shown set up as tabulated, realistic sound images are projected in space in such a manner that the listener perceives a helicopter at the locations tabulated.
TABLE 1______________________________________Filter # 1 2 3 4 5 6______________________________________Capacitor 1631 100 47 33 15 10 4.7Value, nF______________________________________
TABLE 2______________________________________Switch 1642Position # 1 2 3 4 5______________________________________Resistor # 1637 1638 1639 1640 1641Resistor 4700 1000 470 390 120value, Ohms______________________________________
TABLE 3______________________________________Filter 1630 element 1 switch pos. 5 5Filter 1630 element 2 switch pos. 5 5Filter 1630 element 3 switch pos. 5 5Filter 1630 element 4 switch pos. 5 5Filter 1630 element 5 switch pos. 5 5Filter 1630 inverting switch 1622 norm. norm.Potentiometer 1652 ratio 0.046 0.054Potentiometer 1654 ratio 0.90 0.76Potentiometer 1658 ratio 0.77 0.77Inverting switch 1691 position inv. inv.Selector switch 1685 position 1601 1601Output attenuator 1686 ratio 0.23 0.23Output attenuator 1687 ratio 1.0 1.0Image azimuth a, degrees -45 -30Image altitude b, degrees +21 +17Image range r remote remote______________________________________ Note to table 3: setting of reversing switch 1659 in both cases is such that signals from element 1657 drive element 1660, and those from element 1658 drive element 1670.
By addition of two extra elements to the above circuits, an extra facility for lateral shifting of the listening area is provided. It should be understood, however, that this is not essential to the creation