WO2001035640A2

WO2001035640A2 - Signal switching device and method

Info

Publication number: WO2001035640A2
Application number: PCT/US2000/030976
Authority: WO
Inventors: Donald Berg; Dale R. Johnson; Leonid B. Volfson
Original assignee: Inetcam, Inc.
Priority date: 1999-11-12
Filing date: 2000-11-10
Publication date: 2001-05-17
Also published as: AU1917001A; WO2001035640A3

Abstract

A signal switching device (2) has a switch assembly (14) with a plurality of inputs (4, 6, 8, 10) for connection to respective signal sources and at least one signal output (16) for connection to an output device. The switch assembly has a plurality of passive switches each connected at one end to a respective one of the signal inputs and at the other end to an output connected to the signal output. A micro-controller (24) is connected to the switch assembly for turning on the switches sequentially in a predetermined order and for predetermined intervals of time, such that only one switch is on at any one time, and the input signals are repeatedly provided in a predetermined sequence to the signal output.

Description

SIGNAL SWITCHING DEVICE AND METHOD

BACKGROUND OF THE INVENTION

The present invention relates generally to switching devices, and more specifically to switching devices for connecting a number of signal sources, such as video, audio, or the like, to a smaller number of output destinations.

There are many applications where it is desirable to connect a plurality of different signal sources to a single output device or a smaller number of output devices than signal sources. For example, in security monitoring applications, it is often the case that there are many more cameras (or other video sources) than there are monitors or recorders (or other video destinations) . Video switchers are known which comprise a system of coordinated switches to connect a number of video sources to a smaller number of video destinations. Similar switching devices are known for connecting other types of input signals to a smaller number of output devices. However, known switching devices are typically limited to switching only one type of signal, and have inputs and outputs dedicated specifically to video or specifically to audio, for example. Known video switchers typically have high power consumption and require separate power supply connections to the video sources, such as cameras. Additionally, they are normally relatively expensive to manufacture and large in size. U.S. Patent No. 5,264,929 of Yamaguchi describes a video signal switching apparatus in which video signals from a plurality of cameras are provided sequentially to a single output monitor.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a new and improved signal switching device and method for connecting a number of input signals to a smaller number of outputs. According to one aspect of the present invention, a signal switching device is provided which comprises a switch assembly having a plurality of signal inputs for connection to respective signal sources and at least one signal output for connection to an output device, the switch assembly comprising a plurality of passive switches each having an input connected to a respective one of the signal inputs and an output connected, to the signal output, and a microcontroller connected to the switch assembly for turning on the switches sequentially in a predetermined order and for predetermined intervals of time, so that the input signals are connected sequentially to the single output. Passive switches are simply connections which are of low resistance when turned on and high resistance when turned off, and are of low cost when compared to active switch devices or amplifiers. In addition, a passive video switch does not need to consume power to drive the destination video signal. Instead, the existing video driver in the source is used to drive the destination. Such switches have not been used in the past for video switching, because passive switches cannot accept signals which are negative with respect to ground. The sync portion of a video signal is negative with respect to ground. Therefore, in a preferred embodiment of the invention, the signal switching device is adapted to handle video signals by providing a power control circuit having an input connected to a power supply, the power control circuit being connected to the ground reference of the switch assembly to shift the ground to a predetermined negative voltage.

The switch device of the exemplary embodiment accepts a variety of other signals in addition to video, such as audio or other analog and digital signals. In one embodiment of the invention, the switch device has four inputs for selective connection to four signal sources, including video and audio. This switch device is referred to as a four to one multiplexer ("4 to 1 mux") . Another embodiment of the invention comprises a switch device having eight inputs with two outputs ("dual 4 to 1 mux") or one output ("8 to 1 mux") . Preferably, each switch device is capable of switching any signal having a bandwidth less than 10 MHz, an impedance between 75 and 600 ohms, and a voltage between -1 and + 2 volts. The capability to switch a variety of signals is an improvement over prior art switch devices which typically are dedicated specifically to one type of signal, such as video or audio.

In one embodiment of the invention, the switch device has a power input for connection to a serial port of a personal computer (PC) . Thus, no additional power is required to operate the device. Alternatively, the switch device may have an input for connection to an external power supply. The 4 to 1 mux switch device will have sufficiently low power requirements to be operated entirely from the power available on a PC serial port, while the 8 to 1 mux switch device requires a separate power supply.

The switch device, when used as a video and/or audio switch, can be connected directly to the outputs of several video cameras and the output can then be connected directly to any video monitor, such as a conventional TV screen. However, in a preferred embodiment of the invention, the output of the switch device is connected to a video capture device in a personal computer, and the video stream is displayed in a window on the PC screen. The computer is controlled by software to scan all the serial ports and to discover the presence of a switch device connected to the computer, without disturbing any other serial devices that may be connected to the computer. The switch device then sends an identifying code to the computer and indicates that it is ready to send a video stream output.

In the simplest case, the internal controller in the switch device operates to sequentially turn the switches on and off so that the output signals are supplied sequentially to the output at predetermined intervals. However, the switch device may alternatively be controlled by software residing on a personal computer connected to the switch device so that the personal computer may be used to control which video signal is connected to the computer at any one time.

Multiple switch devices may be connected together if a greater number of signals is to be connected to the output device. When multiple units are connected, the outputs of the units are all connected to a single output line, and a controller is provided to control which of the switch devices or units is active at any one time. This provides the capability to connect a very large number of separate signal sources, such as video and/or audio signals, to a single output device. Preferably, a switch mode display is provided on the switch device and is visible to a user to indicate the currently selected video source.

In one embodiment of the invention, the switch device is powered from the wall by an external power supply module of the type commonly used by modems, and a single, ethernet-type cable is used to connect each video source to the switch device. The cable from the switch device to each video source carries both the power to operate the respective video source and the video signal itself. Thus, no separate power cable to the video source is required, unlike prior arrangements. Additionally, both video and audio signals from a video camera may be received by the switch device over a single ethernet-type cable.

According to another aspect of the present invention, a method of connecting a plurality of signal sources to at least one output device is provided, which comprises the steps of connecting the output of each signal source to a respective passive switch in the switch device, connecting each switch output to a single signal output of the switch device, and controlling the switches to be switched on sequentially for predetermined time intervals such that only one switch is on during any time interval and the signals are provided sequentially on the signal output for connection to an output device. In a preferred embodiment of the invention, the method comprises connecting the outputs of at least four video sources successively to a single video output, and includes the step of shifting the ground reference of the passive switches to a negative voltage so that the switches can accept video signals which are negative with respect to ground. The microcontroller uses the same shifted ground reference as the switches. Preferably, the output is connected to a PC video capture device, and displayed on the video monitor of the PC.

The method may further comprise connecting several switch devices in parallel to a single output, to allow more signal sources to be connected sequentially to the output. An external controller is connected to the switch devices to control which of the switch devices is active at any one time. A single switch device preferably has inputs for connection to four to eight separate signal sources, such as video cameras.

The switch device and switching method of this invention is less expensive than the prior art and uses less power, since it utilizes passive switches rather than active switch elements. It also has the capability of handling several different types of input signal, not just video signals, and both video and audio signals may be provided simultaneously to the same switch device or unit. The ability to connect multiple switch devices together in parallel significantly increases the number of signals which can be controlled for sequential connection to one or more individual destinations.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood from the following detailed description of some preferred embodiments of the invention, taken in conjunction with the accompanying drawings in which like reference numerals refer to like parts and in which:

Figure 1 is a block diagram of a switch device according to a first embodiment of the invention; Figure 2 is a block diagram illustrating a switch device according to a second embodiment of the invention;

Figure 3a is a diagram illustrating the switch device connecting plural video sources to a personal computer;

Figure 3b is a modified diagram illustrating the switch device output of Figure 3a connected to a TV monitor;

Figure 4 is a block diagram illustrating three switch devices connected in parallel to a single output;

Figure 5 is a flow diagram illustrating a software program for controlling communication between a personal computer and the switch device of Figure 1 or Figure 2;

Figure 6 is a flow diagram illustrating a software program in the switch device controller for controlling communication with an external control device;

Figure 7 is a schematic illustrating the power circuitry of the switch device of Figure 1 utilizing signal lines from the serial port of a computer; Figures 8a illustrates switching modes for a first embodiment; Figure 8b illustrates switching modes for a second embodiment; and Figure 9 is a schematic illustrating the power circuitry of the switch device of Figure 2. DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 1 is a block diagram of a signal switching device 2 of a first embodiment of the present invention. The signal switching device 2 includes source inputs 4,6,8, 10 connected to a switching circuit 1 4 through source signal lines 42,44,46,47. The switching circuit 14 selects one or none of the source inputs 4,6,8, 1 0 for connection to a output jack 1 6. The switching device 2 further includes a microcontroller 24 for controlling a source selection mode of the switching circuit 14, and for controlling the output of the mode display 28. A serial port 26 connects to the microcontroller 24 through serial port lines 30 to provide external control of the switching device 2. Power is supplied to the microcontroller 24 and switching circuit 1 4 through a power circuit 20 that derives power from signal lines 22 of the serial port 26.

The switching circuit 1 4 of the first embodiment is referred to as a four to one multiplexer ("4 to 1 mux"). The switching circuit selects one or none of the source inputs 4, 6,8, 1 0 for connection to the output jack 1 6. Selection of a particular source input 4,6,8, 1 0 is controlled by a microcontroller 24 through select and enable control lines 32. The combined "hi" or "low" states of the select and enable control lines 32 define switch modes as illustrated in Figure 8a. For example, a switch circuit mode of " 1 a" connects a first source input 4 to the output 1 6. Similarly, switch mode "4a" switches a fourth source input 10 to the output 1 6. Switch mode "0" corresponds to a high impedance mode wherein the output signal line 34 is set to a high impedance state.

The switching circuit 1 4 of the preferred embodiment utilizes a passive switch element. Switches may be classified as either active or passive. Active switches are amplifiers, also referred to as buffers, that consume power to drive their outputs. For example, a video output may have a voltage level as high as one volt with a destination load of 75-ohm. Thus, an active switch must produce a current of 1 3 mA to drive the video output. In contrast, a passive switch element is simply a low resistance connection when turned on, and a high resistance connection when turned off. A passive switch does not consume power to drive the destination video signal. It simply connects the video source inputs 4,6,8, 1 0 to the destination output through output jack 1 6. The power to drive the destination output is produced by an existing video driver that is input to the switching element 2 via one of the source input jacks 4, 6,8, 10. Thus, the power required by the switching device 2 of the first embodiment is minimal.

The switching circuit 14 of the first preferred embodiment, as shown in Figure 1 , is passive semiconductor multiplexer that utilizes NMOS pass transistors for each switched path. The passive semiconductor cannot accept signals that are negative with respect to a ground level. This presents a problem for source input signals 4, 6,8, 1 0 such as video signals that include a sync portion that is negative with respect to a ground level. A method of handling the negative portion of such signals is to use a technique called "capacitive coupling" as is known in the art. But capacitive coupling, in conjunction with a passive switch, results in a video signal of degraded quality, i.e. the video picture is fuzzy. Thus, the preferred embodiment of the present invention utilizes direct current (DC) coupling to reproduce low frequency components of the video signal.

DC coupling is accomplished in the present invention by shifting the ground reference of the switch circuit 1 4 with respect to the ground reference of a video signal received from the source inputs 4,6,8, 10. The preferred embodiment utilizes a reference shift of approximately 1 .3 volts. Other embodiments may utilize other voltage shift levels depending upon the type of sources that are switched through the switching device 2 for a particular system application. For an application of the switching device 2 to a system having video sources, video ground is utilized as the zero volt reference. Thus, it is desired to set the ground of the switch circuit 14 at -1 .3 volts. The positive power voltage to the switch circuit 14 is set at 5.5 volts above the switch ground reference, which is the maximum permitted for the semiconductor of the preferred embodiment, in order to allow the video signal to have the largest voltage swing possible. Thus, the positive power voltage for the switching circuit 14 is + 4.0 volts. A negative voltage, VSS, and a positive power voltage, VDD, are supplied by the power circuit.

Although the primary intended application of the signal switching device 2 is to switch video, the exemplary embodiment of the present invention may be utilized to switch a variety of analog and digital signals. Specifically, the signal switching device 2 of the preferred embodiment can switch any signals that have a bandwidth less than 10 MHz, an impedance between 75 and 600 ohms, and a voltage between - 1 and + 2 volts. The capability to switch a variety of source inputs 4,6, 8, 10 is an improvement over prior art switchers that have inputs and outputs that are dedicated specifically to video or specifically to audio. A microcontroller 24 for controlling the mode of the switching circuit 1 4 is connected through the select and control lines 32 to the switching circuit 1 4. The microcontroller 14 of the preferred embodiment receives mode instructions from an external control device connected to a serial port 26. The external control device may be a personal computer (PC) 302 as shown in Figure 3a utilizing an available RS-232 serial port connection for control communication between the PC 302 and the switching device 362, and a video capture card in the PC 302 for accepting the video output from the switching device 362. In an alternate embodiment, the microcontroller 24 operates independent of external control as illustrated in Figure 3b. Switching device 362 provides jacks for an audio cable 366 and a video cable 364 to be connected directly to a monitor or television 350. Sources 352, 354, 358, 360, which are connect to the switching device 362 through cables 356, are switched to the outputs 364,366 in a predetermined sequence, so that each source is displayed on the monitor/television for a set time period.

As illustrated in Figures 3a and 3b, the embodiments of the present invention provide simultaneous monitoring of multiple source inputs such as cameras. Figures 3a illustrates an application utilizing a switching device 300 operated in conjunction with software and video capture hardware on a personal computer 302. Video sources 310,312,31 4, 31 6 are connected with cables 320 to the "four to one" switching device 300. The switch output is connected to video capture hardware of the computer 302 through cable 306. Cable 322 connects the RS232 serial port of the computer 302 to the serial connector of the switching device 300. The computer sends RS232 commands to the switching device 300 to select a particular video source S 1 through S4. The image stream from the selected video is displayed in a window on the computer screen 304. The window may be sized as desired by a user. It is possible to display all sources on the computer screen at once in separate windows that may be sized and positioned as desired. The video displayed in the forward, or active screen is streamed to the window through the switching device 300.

As shown in Figure 3b, a switching device 362 may be utilized in conjunction with a monitor or television 350. In this system, video sources 352,354,358,360 are connected to a switching device 362. The switching device 362 shown in figure 3b includes an audio output 366 and a video output 364. Images from the video sources S1 through S4 are switched in a pre-determined sequence for display on the monitor 350. Each image remains on the monitor for a set time. Alternatively, the monitor screen 350 may be split into four fixed and equal quadrants for displaying video from a different camera in each quadrant of the monitor screen 350. The images are updated as the switching device 362 sequences through the source inputs 352,354,358,360. Displaying multiple images on a screen is often referred to as "quadrature". Referring back to Figure 1 , the microcontroller 24 of the preferred embodiment is a programmable interrupt controller ("PIC") which is an 8-bit EPROM/ROM based CMOS microcontroller having low power consumption. One such microcontroller is the PIC 1 6C5X microcontroller series manufactured by Microchip. However, any microcontroller having low power consumption may be utilized by the present invention. The microcontroller 24 of the present invention is configured to have two bidirectional inputs for communicating to an external control device via the serial port 26. At least one display output 50 of the microcontroller is utilized to control the mode display 28. The microcontroller 24 also includes control and enable outputs for controlling and enabling the switching circuit 1 4 via the control and enable lines 32. Control inputs to the microcontroller include a reset line connected to the serial port 26 and oscillator inputs connected to an oscillator circuit 52. The microcontroller 24 must drive logic signals sharing a common ground reference with the switching circuit 1 4. Thus, the microcontroller 24 utilizes the same shifted ground reference, VSS, as the switching circuit 1 4. It is also convenient to supply the microcontroller 24 with the same positive power voltage, VDD, as used by the switching circuit 14. The microcontroller 24 of the preferred embodiment is is supplied with a VDD and a VSS from the power circuit 20. In the preferred embodiment, the switching device 2 communicates with a personal computer (PC) as shown in Figure 3a over a serial port. The serial port 26 is an RS-232 port having receive data (RXD) and transmit data (TXD) signal lines connected by data lines 30 to the microcontroller 24. The serial port 26 also includes a Data Terminal Ready (DTR) signal line that is connected to the reset input of the microcontroller 24. The voltages used for signals on the PC RS232 serial port are symmetric positive and negative voltages with respect to the ground level of the PC. However, ordinary digital logic used in components such as the microcontroller 24 is positive with respect to a logic ground. Thus, the signals inputs from the serial port 26 are incompatible with the levels that are utilized by the control circuitry, i.e. microcontroller 24. Prior art controllers utilize expensive driver/receiver chips to convert between RS-232 signal levels and ordinary digital logic signal levels.

The switching device 2 of the present invention dispenses with the need for an RS232 level converter by connecting the serial port signal lines 30 directly to the microcontroller 24. Since the negative level, VSS, is offset from the true ground, that is video ground and PC ground, the microcontroller 24 power rails at - 1 .3V and + 4.0 volts with respect to the PC and serial port ground. The logic signals produced by the microcontroller 24 swing between these two limits. The VSS to VDD voltage range is sufficient to make the microcontroller 24 logic signals directly compatible with RS232 logic signals. Thus, the switching device 2 of a preferred embodiment as shown in Figure 1 offers improvements over the prior art by providing a smaller unit with a lower power consumption, and a lower cost of manufacture.

Referring to figure 3a, to further insure compatibility between the switching device 300 and the RS232 signals, the preferred embodiment utilizes a low RS232 data transmission rate. In addition, a short RS232 signal cable 322 is used to connect the switching device 300 to the serial port of the PC 302.

A positive voltage of an RS232 signal is a logic zero, and a negative voltage is a logic one. In contrast, digital logic one is the more positive voltage, and the more negative voltage is a logic zero. Thus, there is a logic sense inversion between the RS232 signals and the digital logic of the microcontroller 24 as shown in Figure 1 . The logic sense inversion is handled by the microcontroller 24 by the microcontroller firmware. It is also possible that the RS232 signals sent from the PC to the serial port 26 of the switching device 2 may have a voltage swing that goes above the positive power rail or below the negative power rail of the microcontroller 24. The microcontroller 24 of the preferred embodiment includes internal protection diodes that become forward-biased and will begin to conduct under the above circumstance. As shown in Figure 7, the preferred embodiment includes series resistors R5, R6, R7 to limit the current that flows in the protection diodes. An RS232 communication protocol is asynchronous. Therefore, both the switching device 2 of the preferred embodiment and the PC must supply its own accurate time reference in order to accurately send and receive the RS232 data bits. At a communication rate of 2400 Baud with a PC, the microcontroller 24 consumes only microamps of current. This ultra low power mode is necessary to avoid overloading the negative power voltage, VSS. The switching device 2 of the preferred embodiment illustrated in Figure 1 utilizes a quartz crystal in the microcontroller oscillator circuit 52. An oscillator having a frequency output below 200KHz provides a convenient multiple of the baud rate. The preferred embodiment uses a commercially available 76.8KHz crystal as the lowest cost crystal below the 200kHz frequency.

The switching device 2 utilizes a mode display 28 to indicate the current switching mode. The mode display of the preferred embodiment includes Light Emitting Diodes (LEDs) connected to mode display outputs 50 of the microcontroller 24. It is common practice in prior art systems to drive LEDs by connecting a first LED terminal to a positive power voltage and a second LED terminal to an output pin of a driver such as the microcontroller 24. The second LED terminal is either driven low to turn an LED on, or driven high to turn an LED off. However, in the present invention, this configuration of the mode display would cause all of the LED current to flow through the microcontroller ground and the negative power source (from the TXD signal) which cannot supply sufficient current. The preferred embodiment of the present invention uses an alternative approach to illuminate each LED. One side of each LED is connected to the true ground, i.e. PC and video ground, and the other sides of each LED is connected to an output of the microcontroller 24. The microcontroller output lines 50 are driven high to turn a LED on, and driven low to turn an LED off. Thus, the LEDs consume power only from the positive power voltage, VSS, and do not place a load on the negative power voltage. The LEDs of the mode display 28, although not necessary for the operation of the switching device 2, provide an interface for the user to identify the switched source available at the output jack 1 6, and a means to verify that the switching device 2 is operational. Other embodiments of the present invention may include other types of displays that require low power. The switching device 2 of a preferred embodiment, as shown in

Figure 1 , consumes less than 1 mA and operates solely by taking power from the RS232 signal lines of a serial port 26. Power consumption is minimalized by the use of a passive switching element in the switching circuitry 1 4. The power required by an active switch as used in prior art switchers cannot be supplied from the limited resource of a PC serial port.

The power circuit 20 is shown in detail in Figure 7. An RS232 connector 702 includes connections for Data Terminal Ready (DTR),

Request to Send (RTS), Transmit Data (TXD), Receive Data (RXD), and

Signal Ground. The connector 702 of the preferred embodiment is a DB-9 pin connector. The power circuit 700 utilizes signal lines 704,706,71 2 from the serial port 702 to derive the required operating power. In the preferred embodiment of the present invention, DTR line 704 and the RTS line 706 each supply a current of approximately 5ma, for a total of 1 0mA. These signals are input through diodes D3 to a voltage regulator 708 having a low power requirement. The output voltage 71 0 of the voltage regulator 708 is approximately 4 volts which is utilized as the positive power voltage VDD for the switching circuit 2 and microcontroller 24 as shown in Figure 1 . The DTR signal line 704 is also connected to the microcontroller of the switching device through resistor R5, and is used as a reset signal. Referring again to Figure 7, the voltage regulator of the preferred embodiment is a TC55 series low dropout positive voltage regulator manufactured by TelCom Semiconductor, Inc. As is known in the art, voltage regulators are typically used for both the positive power voltage and the negative power voltage, particularly for circuits having varying power levels. In the preferred embodiment of the present invention, the load on the positive power voltage VDD varies greatly depending on the number of LEDs that are turned on, and thus, voltage regulator 708 is used to maintain a constant voltage output 71 0. However, a voltage regulator is not necessary for the negative power voltage since the load on the negative power voltage VSS of the preferred embodiment is relatively constant. Elimination of this circuit component reduces the physical size and the cost to manufacture the switching device.

A negative power voltage VSS is derived from the TXD signal line 71 2. Since the TXD signal line 71 2 pulses high each time the PC sends data, it is necessary to use a diode D2 and a capacitor C8 to extract only a negative voltage from the TXD signal and to avoid unwanted positive voltage levels typically ranging to 12 volts that are present when the PC sends data. The current available at the output of negative power voltage circuitry 71 8 output is less than 1 mA at approximately - 1 .3 volts. The TXD signal line 71 2 is also used connected to the microcontroller of the switching device through resistor R7.

The DTR signal line 704 is utilized by the power circuit as a current source and is connected through resistor R5 to the microcontroller for use as a reset signal. The remaining RS232 signal line RXD 714 is connected to the microcontroller through resistor R6 for receiving data from the PC.

As illustrated in Figure 4, multiple switching devices 402,404,406 of the exemplary embodiment may be connected together by connecting the switcher outputs to a single destination 41 4. Each added switching device provides four additional sources 408,410,41 2. For example, two switching devices 402,404 provide an eight input to one output capability, and three switching devices provides a twelve input to one output capability. A particular source of any one switching device is chosen by setting all but one unit to a high impedance mode "0" as shown in Figures 8a and 8b utilizing the switching circuit control lines. Thus, multiple switching devices can display video images from a large number of camera sources. In a preferred embodiment as shown in Figure 3a, each camera source 31 0, 31 2,31 4,31 6 is displayed in its own window on a PC screen 304. Each window can be sized and positioned independently.

Figure 2 illustrates a second embodiment of a switching device 200 of the present invention. Switching device 200 provides connection of any one of four pairs of sources 202,204,206,208 to a single pair of destinations 222, 224. The switching device 200 of this embodiment is referred to as a dual four to one multiplexer ("dual 4 to 1 mux"). If the pair of destination outputs 222,224 are connected together, then any one of eight sources may be connected to a single destination. This configuration of the switching device 200 is referred to as an eight to one multiplexer ("8 to 1 mux"). Multiple eight to one multiplexers may be connected together in a variety of configurations. For example, one unit provides a switching capability for a dual four to one multiplexer or an eight to one multiplexer. Two units provide a dual eight to one switching capability or a sixteen to one switching capability. Likewise, three units provide a dual twelve to one or a twenty-four to one switching capability.

In a dual four to one configuration of the second embodiment, the signal switching device 200 of the second embodiment utilizes Ethernet- type connectors 202,204,206,208, such as RJ45 connectors, for connecting four video sources 250 to the switching device 200 through cables 252. The received source signals, such as the video and audio, are switched as a set onto output lines 21 8,220. The cables 252 to each video source 250 carry both the power to operate the video source 250, and the signals from the video source 250 to the switching device 200. Video switchers of the prior art require two separate cables for each video source. A first cable carries power to operate the camera, and a second cable, typically an expensive coaxial cable, carries the video signal from the camera to the switcher. The switching circuitry 21 4 and microcontroller 232 of the second embodiment operate the same manner as the four to one switching device 2 illustrated in Figure 1 . Control and enables lines 242 connect the microcontroller 232 to the switching circuitry 214. A source selection mode, as shown in Figure 8b, is determined by command signals sent from a PC through a serial port 236. The selected source mode is displayed on a mode display 234. The serial port 236 of the preferred second embodiment accepts RS232 signals that are connected to the microcontroller 232 through data signal lines 246. In a preferred embodiment of the switching device 200 of Figure 2, RS232 communications between the PC and the switching device 200 may take place at rates of up to 38,400 bits per second (38.4 kilobaud). The oscillator circuit 248 of the preferred second embodiment uses a 3.6864MHz quartz crystal. The oscillator frequency is chosen as the lowest cost crystal less than 4MHz, which is a convenient multiple of the desired RS232 bit rate. The power circuit 230 of the second embodiment is derived from a standard alternating current ("AC") source connected to an external power supply module 238 of the type commonly used by modems. The power supply module 238 is connected to switching device 200 utilizing a power cable 244 having a plug that mates with a DC power jack J2 on an exterior casing of the switching device 200. DC power from the jack J2 is connected to the RJ45 source connectors 202,204,206,208 for use by sources 250 via power line 226. A fuse F1 , as shown in Figure 9, protects the video sources from power surges. The power circuit 230 of Figure 2 is illustrated in detail in Figure 9.

Power circuit 900 includes a voltage regulator 902 for maintaining the positive power voltage VDD at + 4.0 volts. A negative voltage converter 904, often called a charge pump, produces a negative power voltage VSS of - 1 .3 volts. The power voltages VSS and VDD are used to power the passive switch element of the switch circuit and the microcontroller 908. A supervisory circuit 906 device holds the microcontroller 908 in a reset condition until the supply voltage VDD reaches a stable operating level.

Figure 8b illustrates the possible switch modes of the dual 4 to 1 or 8 to 1 switch device of Figure 2. For example, in switch mode 1 AB, the output of camera or video source S1 A is provided to video destination DA, while the output of video source S1 B is provided to video destination DB. If outputs DA and DB are coupled together, an eight to one switch is provided.

The preferred embodiments of the present invention, when used in conjunction with a personal computer 302, provide non-intrusive "plug-and- play" auto-configurations. That is, the PC is able to scan all the serial ports on the PC and discover the presence of a switcher without disturbing any other serial devices that may be connected to the PC. A firmware controlled micro-controller in the switcher communicates with the PC via a PC serial port. The PC software, as illustrated in Figure 5, manipulates the Data Terminal Ready (DTR) signal on the serial port 500 to stimulate the micro-controller to transmit a model-dependent identifying code 502, 504 to the PC. Thus, discovering the presence of a switching device does not require the PC to send any actual serial data from the PC to the serial port. This process eliminates possible undesirable action from other devices that are connected to the serial port. The model number message can also be sent to the PC from the switching device without solicitation, for example, when power is turned on to the switching device of the preferred embodiment. The message is equivalent to the statement "I am a model VSxxx switcher and my power was just turned on."

Referring back to Figure 5, the PC controls switching of the signal sources utilizing simple commands that are sent from the PC serial port to the switching device to tell the switching device which mode to assume 506. The switching device responds to the PC serial port to confirm that each command was received and obeyed 508. For example, when the switching device receives a command from the PC of "assume mode 1 A now," the switching device replies "I have assumed mode 1 A." Thus, simple handshaking is sufficient for effective control of the signal switching device. If the user desires to change video sources 51 0, a new mode command is sent to the switching device 506. A reset state 51 4 is sent to the switching device whenever the DTR signal is driven low by the PC. When DTR is then driven high, the microcontroller begins execution at the beginning of its program as illustrated in Figure 6. The microcontroller 232 of the second preferred embodiment, as shown in Figure 2, is also held in a reset condition when a voltage from the external power supply is below normal operating levels. When the power supply voltage rises above the minimum operating level, the microcontroller 232 begins execution at the beginning of its program.

Figure 6 illustrates a basic software communication between the switching device of the present invention and the personal computer. The microcontroller is held in reset until the DTR signal is toggled "hi" 600. The microcontroller begins execution after a reset by sending a message to the PC over the serial port 602. This message is equivalent to the statement "I am a model VSxxx switcher and my power was just turned on." The switching device then waits to receive a mode 604. Upon receiving a valid mode, the microcontroller sends a message back to the PC verifying receipt of the mode 604. The microcontroller sets the mode display 608 to indicate the desired source. Control and enable commands are sent to the switch circuit 61 0 to switch the desired source to the destination output. The switching device maintains its current mode until it receives a mode change 61 2 or a reset 61 4 from the personal computer.-

The signal switching devices and methods as described above provide significant advantages, particularly when used for the switching of video and/or audio signals. The switch devices of Figures 1 and 2 have significantly reduced power consumption over known video switchers that utilize active switches. The devices are also more compact and less expensive than prior art alternatives, and are able to handle a variety of different analog and digital signals. Although a preferred embodiment of the invention has been described above by way of example only, it will be understood by those skilled in the field that modifications may be made to the disclosed embodiment without departing from the scope of the invention, which is defined by the appended claims.

Claims

1 . A signal switching device, comprising: a switch assembly having a plurality of signal inputs for connection to respective signal sources and at least one signal output for connection to an output device; the switch assembly comprising a plurality of passive switches each having an input connected to a respective one of said signal inputs and an output connected to said signal output; and a micro-controller connected to said switch assembly for turning on the switches sequentially in a predetermined order and for predetermined intervals of time, such that only one switch is on at any one time, and the input signals are repeatedly provided in a predetermined sequence to said signal output.

2. The device as claimed in claim 1 , further comprising a power control circuit having an input connected to a power supply, the power control circuit being connected to a ground reference of the switch assembly to shift the ground to a predetermined negative voltage, whereby video signals may be connected to the switch assembly inputs.

3. The device as claimed in claim 1 , wherein the signal sources comprise video sources.

4. The device as claimed in claim 1 , wherein the signal sources comprise video and audio sources.

5. The device as claimed in claim 1 , wherein the signal sources comprise a plurality of different types of signals.

6. The device as claimed in claim 1 , wherein each signal source has a bandwidth less than 10 MHz, an impedance between 75 and 600 ohms, and a voltage between - 1 and + 2 volts.

7. The device as claimed in claim 1 , wherein the switch assembly has four switches and four inputs for selective connection to four signal sources.

8. The device as claimed in claim 1 , wherein the switch assembly has eight switches and eight inputs for selective connection to eight signal sources.

9. The device as claimed in claim 8, wherein the switch assembly has two outputs.

1 0. The device as claimed in claim 1 , including a power input for connection to a serial port of a personal computer to provide power for operating the device.

1 1 . A video system, comprising: a plurality of video cameras; a video switching device having a plurality of inputs, each input connected to a respective one of said video cameras to receive at least video signals from said camera; a plurality of passive switches in said switching device, each switch connected to a respective one of said inputs; the video switching device having at least one output connected to the switches for receiving a video signal from a respective video camera when the switch associated with the camera is on; a controller in the video switching device linked to the switches for controlling the condition of the switches such that only one switch is on at any one time to provide a single video signal to the output, and turning each switch on in sequence for a predetermined time period such that video signals are provided repeatedly in sequence to said output; and at least one video monitor connected to the switching device output for displaying the video signals.

1 2. The system as claimed in claim 1 1 , further comprising a plurality of switching devices connected in parallel, the outputs of the switching devices being connected together to provide at least one video output, and a control device for controlling the switch devices such that only one switching device is active and the outputs of all the other switching devices are zero at any one time.

1 3. The system as claimed in claim 1 1 , further comprising a computer having a serial port and a video capture device, the switching device output being connected to the video capture device, and the video monitor comprising a computer monitor.

1 4. The system as claimed in claim 1 3, wherein the switching device has a power input connected to the computer serial port to provide power for the switching device from the computer.

1 5. The system as claimed in claim 1 1 , including a plurality of ethernet-type cables, each cable connecting a respective one of the video cameras to the respective switching device input, and the cable both supplying power to the camera from the switching device and providing signal communication from the camera to the switching device.

1 6. The system as claimed in claim 1 1 , wherein the computer includes control means for scanning all serial ports to detect the presence of a video switching device, and for initiating communication between the switching device and the computer if a switching device is detected.

1 7. A method of connecting a plurality of signal sources to at least one output device, comprising the steps of: connecting the output of each signal source to a respective passive switch in a switch device; connecting each switch output to a single signal output of the switch device; and controlling the switches to be switched on sequentially for predetermined time intervals such that only one switch is on during any time interval and the signals are repeatedly provided sequentially on the signal output for connection to an output device.

1 8. The method as claimed in claim 1 7, wherein the step of connecting a plurality of signal sources to the switch device comprises connecting the outputs of at least four video sources successively to a single video output, the method further including the step of shifting the ground reference of the passive switches to a negative voltage so that the switches can accept video signals which are negative with respect to ground.

1 9. The method as claimed in claim 1 8, including the step of connecting the switch device output to a serial port of a personal computer.

20. The method as claimed in claim 1 9, including the step of using power from the computer to operate the switch device.

21 . The method as claimed in claim 1 7, further comprising connecting several switch devices in parallel with their outputs connected to a single signal output, and controlling the switch devices such the outputs of all but one switch device are zero at any one time and the switch devices are activated sequentially to provide signals to the single output.

22. A signal switching device comprising: a switching circuit comprising a plurality of signal inputs, at least one signal output, and a plurality of passive switching elements, the switching circuit configured to selectively connect said signal inputs to said at least one signal output in response to a control signal; a power circuit coupled to the switching circuit and configured to provide a ground reference which is shifted to a predetermined negative voltage; and a micro-controller coupled to said switching circuit and configured to provide the control signal to the switching circuit.

23. The signal switching device of claim 22, wherein the switching circuit comprises a passive semiconductor multiplexer.

24. The signal switching device of claim 22, wherein the power circuit is configured to couple to an RS-232 connector.

25. A signal switching device for switching signals received from a plurality of devices and for providing power to those devices, the signal switching device comprising: a plurality of signal input connectors, each of said plurality of input connectors having an input configured to receive signals from a device and to provide electrical power to the device and having an output configured to output a received signal; a switching circuit comprising a plurality of signal inputs, at least one signal output, and a plurality of switching elements, each of the signal inputs coupled to the output of one of the plurality of signal input connectors, the switching circuit configured to selectively connect said signal inputs to said at least one signal output in response to a control signal; and a micro-controller coupled to the switching circuit and configured to provide the control signal to the switching circuit.

26. The signal switching device of claim 25, wherein the switching plurality of switching elements comprise a plurality of passive switching elements.

27. The signal switching device of claim 26, further comprising a power circuit coupled to the switching circuit and configured to provide a ground reference which is shifted to a predetermined negative voltage.

28. A video signal switching device comprising: a switching circuit comprising a plurality of video signal inputs, at least one video signal output, and a plurality of passive switching elements, the switching circuit configured to selectively connect said signal inputs to said at least one signal output in response to a control signal; a power circuit coupled to the switching circuit and configured to provide a ground reference which is shifted to a predetermined negative voltage with respect to the ground reference of a received video signal; and a micro-controller coupled to said switching circuit and configured to provide the control signal to the switching circuit.

29. A multi-source switching system comprising: a switching circuit comprising a plurality of signal inputs, at least one signal output, and a plurality of passive switching elements, the switching circuit configured to selectively connect said signal inputs to said at least one signal output in response to a control signal; a plurality of sources, each source having a source signal coupled to one of the signal inputs of the switching circuit; a power circuit coupled to the switching circuit and configured to provide a ground reference which is shifted to a predetermined negative voltage; and a micro-controller coupled to said switching circuit and configured to provide the control signal to the switching circuit.

30. The system of claim 29 further wherein at least one of said plurality of sources is a video camera.

31. The system of claim 30 further comprising at least one video monitor coupled to the at least one signal output of the switching circuit.

32. A multi-source switching system comprising: a plurality of signal input connectors, each of said plurality of input connectors having an input configured to receive signals from a device and to provide electrical power to the device and having an output configured to output a received signal; a plurality of signal sources, each signal source coupled to one of the plurality of signal input connectors to thereby receive power from the signal input connector and to provide a signal to the signal input connector; a switching circuit comprising a plurality of signal inputs, at least one signal output, and a plurality of switching elements, each of the signal inputs coupled to the output of one of the plurality of signal input connectors, the switching circuit configured to selectively connect said signal inputs to said at least one signal output in response to a control signal; and a micro-controller coupled to said switching circuit and configured to provide the control signal to the switching circuit.

33. The system of claim 32 further wherein at least one of said plurality of sources is a video camera.

34. The system of claim 33 further comprising at least one video monitor coupled to the at least one signal output of the switching circuit.