lementation. We must restrict the specification to use only negative powers of z, since these corresponds to positive delays. A positive power of z would correspond to a negative delay, that is a response before a stimulus was applied.
With these notations in hand we may described equipment to allow placement of images of broad and sounds such as speech and music. For these purposes the sound processor of the present invention, for example, processor 802 of FIG. 8, may be embodied as a variable two-path analog filter with variable path coupling attenuators as in Fig. 18A.
In FIG. 18A, a monophonic or monaural input signal 1601 is input to two filters 1610, 1630 and also to two potentiometers 1651, 1652. The outputs from filters 1610, 1630 are connected to potentiometers 1653, 1654. The four potentiometers 1651-1654 are arranged as a so-called joystick control such that they act differentially. One joystick axis allows control of potentiometers 1651, 1652; as one moves such as to pass a greater proportion of its input to its output, the other is mechanically reversed and passes a smaller proportion of its input to its output. Potentiometers 1653, 1654 are similarly differentially operated on a second, independent joystick axis. Output signals from potentiometers 1653, 1654 are passed to unity gain buffers 1655, 1656 respectively, which in turn drive potentiometers 1657, 1658, respectively, that are coupled to act together; they increase or decrease the proportion of input passed to the output in step. The output signals from potentiometers 1657, 1658 pass to a reversing switch 1659, which allows the filter signals to be fed directly or interchanged, to first inputs of summing elements 1660, 1670.
Each responsive summing element 1660, 1670 receives at its second input an output from potentiometers 1651, 1652. Summing element 1670 drives inverter 1690, and switch 1691 allows selection of the direct or inverted signal to drive input 1684 of attenuator 1689. The output of attenuator 1689 is the so-called right-channel 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 joyst