MULTIPLE INPUT SILENT AUDIO SWITCH

Abstract: A silent audio switch provides switching between multiple audio sources while eliminating the "pop" caused by switching spikes. Two internal buses (66, 68) are used for each visible external bus, whereby a new audio signal is first switched to an off-air bus opposite an on-air bus, any switching spike is allowed to subside, and then a fast mix is made between the two buses to derive the new audio signal as the switch output signal.

Description: The present invention relates to means for switching between multiple signal sources, and in particular to a silent audio switch employing large switch arrays with built-in decoders in combination with a fast mixer circuit, to provide audio switching without causing audible pops. In general, typical analog switchers such as those employed in video switcher apparatus to switch between as many as 32, or more, audio signal sources, generate switching spikes which can overload subsequent audio stages, causing the well-known problem of switching noise, or "pops". The switching spikes are caused by switching during non-zero values of the input signal waveform, for example, by switching between two high quality audio signals of different magnitudes, and by the coupling of the switch's drive signal through the switch's capacitance to the output. Since very high quality stereo audio is becoming increasingly popular in the television field, switching noise generated, for example, by switcher apparatus is totally unacceptable. Typical of present schemes for eliminating such transient spikes when switching between high quality audio signals is the technique of increasing the switch driver's transition time to reduce the transient current through the switch capacitance. Thus, in switches where the input of the switch element is available, the transition time of the driver may be adjusted to any value, making silent audio switching possible. Such switches generally employ a field effect transistor (FET) as the switching element in the respective audio signal path, and provide a relatively slow switch control signal, i.e., a ramp signal, to the FET. However, such schemes generally require a prohibitive amount of discrete circuitry since large switch array devices usually do not allow access to the control signal input of the respective switching element. A more sophisticated scheme presently in use employs a matched pair of FETs for receiving respective audio signals, which are coupled together at their outputs to define a fast FET signal mixer configuration which mixes the two audio signals during a switching time interval. The switch drive voltage fed to the FETs is selectively controlled to extend the switching time interval over milliseconds, to eliminate capacitive feedthrough while performing what is hereinafter termed a "fast mix" between the audio signals. It follows that one signal is replaced by another signal without generating the undesirable transient spike. However, in a system such as used in switcher apparatus wherein as many as 16 stereo high quality audio signal inputs are common, this latter scheme is relatively cumbersome due to the fact that each of the corresponding large number of FET control signals are ramps of both increasing and decreasing magnitudes. This, in turn, necessitates the attendant complexity of multiplexing a large number of ramp control signals. Further, the FETs must be matched to provide the optimum performance required of the mixer device. In addition, such scheme does not allow the advantageous use of present commercially available large switch matrices with built in decoding and drivers. US-A-3654394 describes a switching tree in which each stage comprises a field effect transistor. US-A-3757139 describes a switching matrix in which each analog signal can be selected by an analog switch comprising a balanced pair of junction FET switches. DE-A-2841314 describes a digitally controlled cross-fade mixer in which two digital ramps obtained from programmable read-only memories are inversely applied to control two attenuators each of which couples a respective analog signal to an adder, one of the analog signals gradually supplanting the other in the final output. The invention provides a circuit for switching between multiple analog audio signals, comprising an analog switching stage for switching an output signal between a first analog audio signal and a second analog audio signal, which comprises two analog switches each for receiving a respective one of the first and second analog audio signals and control means for applying a respective analog ramp signal to a control input of each analog switch, so as to cause the switching stage to steadily reduce an output of the first analog audio signal to zero and to increase an output of the second analog audio signal from zero whereby the output of the second analog audio signal gradually supplants the output of the first analog audio signal; and a preceding input switching stage for selecting the second analog audio signal from a multiplicity of possible audio input signals and switching the second analog audio signal to a line or bus leading to the second switch at a preselected time before the said control means begins to effect the changeover between the first and second analog audio signals. According to a further aspect, the invention provides a method for switching between multiple analog audio input signals while eliminating audible transient switching spikes, adapted to such a circuit, comprising: supplying a first of the multiple analog audio input signals as an on-air signal; selecting a second of the multiple analog audio input signals and coupling it to an off-air bus; and after a preselected time effecting an analog cross-mixing, controlled by analog ramp signals, of the first analog audio signal with the second analog audio signal from the off-air bus to supplant the first analog audio signal by the second analog audio signal. In its preferred forms the present invention circumvents the problems and disadvantages generally found in typical analog audio switches while allowing the use of commercially available large switch matrices with decoding and switch drive facilities. This provides the added advantage of a relatively simple switching control circuit in systems employing large numbers of high quality audio input signals, particularly in a microprocessor controlled system. Thus the control circuitry is simplified while still retaining the silent switching provided, for example, by the soft switch technique obtained by use of a fast FET signal mixing circuit. To this end, to obtain the full advantages of the invention, at least two switch arrays are used to provide two identical matrices, with each array coupled to the multiple audio input signals. The arrays include multiple crosspoints, the number of which is dictated by the number of audio inputs and the array configuration selected. The outputs of the arrays are coupled to respective buses and thence to respective FET switches of a fast FET signal mixing circuit. The FET outputs, in turn, are connected together to provide the switch output. Thus, it may be seen that two internal buses are used for each visible external bus. The invention contemplates switching crosspoints onto an "off-air", or "phantom", bus opposite an "on-air" bus, waiting for the transient spike to subside and then performing a fast mix between the two internal buses. In a microprocessor controlled scheme, the microprocessor remembers which internal bus was last written to, and which crosspoint was last selected on the opposite off-air bus. Thus, the invention contemplates a two-stage switching technique, utilising the phantom array/bus on the off-air array/bus and, at a given time interval later dictated generally by the switching speeds of the various components, is selected as the on-air array/bus via the FET signal mixing circuit. The prior on-air array/bus simultaneously is placed off-air. The use of a phantom bus in combination with an on-air bus, to provide a two stage switching process which allows a transient spike and thus any associated "pop" to subside, further reduces the switch control complexity while minimising the number of discrete components in the system. The technique also may be used to switch signals other than audio signals, i.e., video, control line, etc., signals, wherein switching transients are unacceptable. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a simplified schematic diagram depicting a known fast FET signal mixing circuit. FIGURES 2A-2E illustrate the waveforms generated by the signal mixing circuit of FIGURE 1. FIGURE 3 is a simplified schematic diagram of an embodiment of the invention. FIGURE 4 is a schematic diagram depicting an implementation of the switch array circuit of FIGURE 3 with attendant switch control signals. FIGURE 5 is a partial schematic diagram depicting an implementation of the associated FET signal mixing circuit of FIGURE 3 with attendant switch control signals. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGURES 1 and 2, a generally known analog audio switching circuit includes a pair of FETs 12 and 14 coupled via respective inputs 16 and 18 to input signals 1 and 2 illustrated in FIGURES 2A and 2B. A corresponding pair of switch control signals 1 and 2 are supplied to the respective gate or control elements of the FETs via control lines 20, 22, and are illustrated in FIGURES 2C and 2D as simultaneously increasing and decreasing voltage ramps, respectively. The outputs of the FETs 12, 14 correspond to decreasing and increasing signal envelopes (FIGURE 2E), and are coupled together at a common junction 24 to provide an output signal at an amplifier 28 and output 26 which corresponds to the input signals 1 and/or 2 as determined by the state of the FETs. During the switching interval, FET 12 ramps off while FET 14 ramps on, whereby input signal 1 is replaced by signal 2 by means of a known fast mix technique. The switching interval is of the order of five milliseconds, during which interval the magnitude of the input signal 1 decreases as the magnitude of signal 1 increases, as depicted in FIGURE 2E. In a system wherein a large plurality of input signals are to be switched, an additional FET stage is provided for each input signal. Thus, as depicted in FIGURE 1, an additional FET with a respective switch control signal must be included for each input, as indicated by numerals 30, 32, 34 and dashed line 36. Since the control signals are ramps such as depicted in FIGURE 2C, 2D and since both ramps are supplied to each FET, it can readily be seen that multiplexing such control signals in a system having, for example, 16 stereo audio signals (32 channels of high quality audio) becomes extremely complex and cumbersome. The complexity of the added FETs and associated control lines is eliminated in the embodiment shown in FIGURE 3, wherein multiple audio input signals 1, 2, 3 and N are supplied, via inputs 38, 40, 42 and 44 respectively, to first and second switch arrays 46 and 48. The latter arrays provide identical switch matrices wherein, in a simple configuration, such as illustrated in FIGURE 3, the number of switch crosspoints 50, 52, 54, 56 and 58, 60, 62, 64 of the arrays 46, 48 respectively, are dictated by the number of input signals, and the number of switch control signals, as well as the configuration of the array selected. Each matrix then includes, and is part of, first and second buses 66, 68 respectively, coupled to respective FETs 70, 72 which form a fast signal mixing circuit 74 similar to that of FIGURE 1. An output signal is provided via an output 76 and a common connection of the FET outputs. Although the signal mixing circuit 74 is not a true mixer circuit, it is hereinafter termed a FET mixer circuit (74) to more readily identify and thus distinguish it from the switch arrays (46, 48). Switch control signals in the form of digital words are provided to the switch arrays 46, 48 and particularly to the associated individual crosspoints 50-56 and 58-64 thereof, via a switch control bus 78. In turn the switch control signals are supplied, for example, by a microprocessor 80 as a digital address word whose value is indicative of the corresponding crosspoints of the arrays 46, 48. The number of bits of the address word is dictated by the size of the array, i.e., the number of crosspoints. The microprocessor further supplies first and second strobes on lines 82, 84 to the arrays 46, 48, to select the array whose crosspoint is affected. The microprocessor 80 also supplies ramp control signals via lines 86 extending to a ramp control and generator circuit 88. The latter supplies first and second FET control ramps to the FETs 70, 72 via FET control lines 90, 92. The various switch control signals supplied by the microprocessor 80 are further illustrated in FIGURES 4 and 5 below. In operation, it is assumed that audio input signal 1 is on-air and corresponds to an output signal on output 76, supplied via the respective crosspoint in switch array 46, the bus 66 and the FET 70, in response to the switch control signal on control bus 78, the first strobe on line 82, and the FET control ramp on line 90. Thus, array 48, second bus 68 and the FET 72 define the off-air signal path, i.e., the phantom bus. At such time as input signal 1 is to be replaced by another audio signal as, for example, input signal 3, the switch control signal on control bus 78 and the second strobe on line 84 switch the crosspoint to place input signal 3 on the off-air signal path. Simultaneously, the FET control ramps on lines 90, 92 turn the FET 72 on while turning the FET 70 off, to initiate the fast mix process which replaces signal 1 with the signal 3. Since the ramps take several milliseconds to switch the FETs, and the crosspoints of the arrays are switched within microseconds, there is a sufficient time interval between the two switching stages to allow any transient spike to subside before the input signal 3 is coupled to the output 76, i.e., is placed on-air. At such time the array 48, bus 68 and FET 72 provide the on-air path, and the array 46, bus 66 and FET 70 provide an off-air path which is available for the next switch between input signals. FIGURES 4 and 5 illustrate one implementation of the circuit of FIGURE 3, wherein FIGURE 4 includes four dual switch arrays 94, 96, 98 and 100. The top half of the arrays receive 16 stereo audio signals corresponding to the first, or left, channel of stereo and herein labelled AUD IN-L0 through AUD IN-L15 respectively. The lower half of the switch arrays receive 16 complementary stereo audio signals AUD IN-R0 through AUD IN-R15 corresponding to the second, or right, channel of stereo. The arrays provide four stereo signals on a bus 102, with one channel of the four signals termed AUD-LA, LA', LB, LB' and with the four complementary signals of the second channel termed AUD RA, RA', RB, RB'. The prime symbol indicates the instantaneous off-air, or phantom, bus. By the way of example only, the arrays employed herein are 4 by 4 by 2 switch arrays, commercially available as a CD22101 array manufactured by RCA Corporation. In this particular embodiment, the upper half of each array receives four audio signals corresponding to the left channel of four sterio signals, and each lower half receives the four audio signals corresponding to the right channels, of the four stereo signals. With the array depicted, the switch control signal is a 4-bit digital address word supplied on the switch control bus 78. The 32 crosspoints are switched in respective channel pairs in response to the digital word, which equal the successive values zero through 15 for the respective 16 audio signals in one-half of the array, and the complementary audio signals of the other array half. Thus crosspoint XO-Y0 in each of the left channel arrays, and the corresponding crosspoint XO-Y0 in each of the right channel arrays, are addressed simultaneously by the digital value zero (all inputs are "0"). Likewise, all the crosspoint pairs of both channels of the arrays are addressed simultaneously by respective digital values. To complete the switch control, an input D0 on a line 104 from the microprocessor 80 supplies a high logic level to the arays in which crosspoints are to be switched, while the selected array is strobed via a respective strobe line 106, 108, 110 or 112, to enable the crosspoint of the specific switch array. Since the operation and microprocessor control of large switch arrays as in FIGURE 4 is generally known to those skilled in the art, no further description thereof is believed required herein. FIGURE 5 includes, also by way of example only, four fast FET mixer circuits 114, 116, 118, 120 similar to the signal mixer circuit 74 of FIGURE 3, wherein each mixer circuit receives a respective pair of audio signals from the arrays of FIGURE 4. Thus signals AUD-LA, LA', RA and RA' are supplied on respective lines 102 to the FETs in mixer circuits 114 and 116, while the signals AUD-LB, LB', RB and RB' are fed to the FETs of mixer circuits 118 and 120, as shown in FIGURE 5. The FETs are selectively switched via microprocessor-generated ramp control signals supplied to a ramp control circuit 122 which is coupled in turn to a ramp generator circuit 124. The ramp control signals fed to the ramp control circuit 122 are logic level and include a REG/PRIME signal on a line 126, a RAMP STROBE signal on a line 128 and an A/B BUS signal on a line 130. The REG/PRIME signal selects either the regular (on-air) bus or the phantom (off-air) bus, the RAMP STROBE signal is a logic level for initiating the generation of the ramps by the ramp generator 124, and the A/B BUS signal selects either the A or B bus. The control signals are provided by the microprocessor 80 as depicted in FIGURE 3. The circuit 122 comprises in essence a pair of flip-flops (not shown), and provides opposite logic states on lines 132, 134 coupled to the ramp generator circuit 124. The latter circuit 124 supplies FET control ramps of opposite slopes to the mixer circuits 114, 116 on respective FET control lines 136, 138. Likewise, the ramp generator circuit supplies a second pair of opposite FET control ramps to the mixer circuits 118, 120 via FET control lines 140, 142. The mixer circuit 114 is shown schematically to illustrate an implementation of the mixer circuits 114-120 which are generally conventional in design, and accordingly are not further described herein. The stereo audio output signals AUD OUT-LA, RA, LB and RB are selectively supplied on the outputs 144, 146, 148, 150, respectively, as previously described for the single audio output signal of FIGURE 3. Although the FIGURES 4 and 5 depict the switching of stereo audio signals, the invention may be employed to handle various pluralities and types of audio signals, as well as other signals such as, for example, video signals, control line signals, etc. Also, various other switch array matrices and associated fast signal mixing circuits may be used as dictated by the plurality and type of signals being switched.

Claims: A circuit for switching between multiple analog audio signals comprising an analog switching stage for switching an output signal between a first analog audio signal and a second analog audio signal, which comprises two analog switches (70,72) each for receiving a respective one of the first and second analog audio signals and control means (88) for applying a respective analog ramp signal to a control input of each analog switch, so as to cause the switching stage to steadily reduce an output of the first analog audio signal to zero and to increase an output of the second analog audio signal from zero whereby the output of the second analog audio signal gradually supplants the output of the first analog audio signal; and a preceding input switching stage (48) for selecting the second analog audio signal from a multiplicity of possible audio input signals and switching the second analog audio signal to a line or bus (68) leading to the second switch at a preselected time before the said control means (88) begins to effect the changeover between the first and second analog audio signals. A circuit according to claim 1 in which an input switching stage (46) for the selection of an audio input signal from a multiplicity of input audio signals is also provided for the said first analog audio signal, each of the input switching stages (46, 48) being arranged to select an audio input signal and couple it to a respective bus leading to the analog switching stage (70, 72) before that stage is controlled to increase its output of the selected audio input signal from zero. A circuit according to claim 1 or 2 in which the or each input switching stage (46, 48) is a digitally addressed switching matrix. A circuit according to any foregoing claim in which each analog switch (70, 72) is a FET switch and the control input is a gate terminal of the FET. A method for switching between multiple analog audio input signals while eliminating audible transient switching spikes, adapted to a circuit as claimed in Claims 1 to 4, comprising: supplying a first of the multiple analog audio input signals as an on-air signal; selecting a second of the multiple analog audio input signals and coupling it to an off-air bus; and after a preselected time effecting an analog cross-mixing, controlled by analog ramp signals, of the first analog audio signal with the second analog audio signal from the off-air bus to supplant the first analog audio signal by the second analog audio signal. A method according to claim 5, wherein each of the first and second analog audio signals is selected from a multiplicity of possible audio input signals at different times by means of a respective digital switch control signal and is switched to to a respective off-air bus a preselected time before it supplants in an output signal a previous signal by means of analog cross-mixing controlled by analog ramp signals.

Inventors: FLORA, JAY L., HAYCOCK, DAVID A.
Applicants: AMPEX CORPORATION
Application year: 1986
Main classification: H03K17
IPC codes: H03K17, H04R3