COMBINED ANALOG/DIGITAL DATA TRANSMISSION SYSTEM

Abstract: A system and method for transmitting digital data (24) and/or control signals on the same pairs of wires which carry analog signals (22) is proposed. The current mode transmission of the digital/analog mixed signals (16, 18) is used instead of conventional voltage mode transmission, and effectively eliminates design difficulties traditionally associated with the voltage mode transmission.

Description: COMBINED ANALOG/DIGITAL DATA TRANSMISSION SYSTEM Cross Reference to Related Application This application claims priority based upon United States provisional patent application Serial No. 60/085,820 filed May 18, 1998. Technical Field The invention relates generally to data transmission systems and, more particularly, to a data transmission system which enables digital signals such as data and control signals to be transmitted together with analog signals such as voice and/or music information. Background A variety of techniques may be used to represent information. Two such techniques are the use of analog signals and digital data signals. Analog signals are used most often to represent audio, video or other types of information characterized by a continuously variable amplitude. Digital data signals, on the other hand, are used to represent information using two discrete states. As a result of this distinction, devices such as televisions and stereo speakers which reproduce information from analog signals remained apart from devices such as computers which reproduce information from digital data signals. In recent years, these seemingly disparate technologies have begun to converge, particularly in multi-media applications such as a digital computer equipped with a sound reproduction system or a stereo system equipped to handle digital control signals originating at the audio signal source. Similarly, many televisions can generate teletext using digital data originating at the analog video source. The digital data used by televisions to generate a teletext display is typically injected into the horizontal and/or vertical blanking pulses contained in all analog video signals, a portion of the video signal where no video data is carried. As a result, it has been a relatively simple task to equip a television with a special decoder which extracts digital data out of this portion of the analog video signal. Analog audio signals, on the other hand, lack the blanking pulse which has enabled the injection of digital data into analog video signals. Accordingly, to transmit an analog audio and digital data simultaneously, a voltage signal source has been used to superimpose a data signal across the two wires of the cable pair carrying the analog audio signal. By modulating the data signal onto a carrier frequency which lies above the upper bound of the audio frequency band, typically, around 20 KHz, the superimposed digital data signal will have no effect on the analog audio signal. However, in a typical audio reproduction system, the driving impedance at the power amplifier is very low, in effect, a short circuit while the load impedance at the speaker is often as low as 4 ohms. As a result, a series of inductors are mandatory at each end of the system that allow the lower frequency audio signals to pass with little attenuation while presenting a high impedance to the carrier frequency at which the digital data has been modulated. The problem associated with this arrangement is that the amplifier's damping factor will hardly be preserved unless the coils for the inductors have an extremely low impedance, ideally below 0.1 ohms, at audio frequencies. In order to achieve this, the inductors must have relatively heavy gauge wire and relatively large, high permeability cores. The physical size of the cores has to be big enough to prevent saturation with audio frequency current surges of over 1 Ampere, e.g., often 5 to 10 Amperes. This presents a difficult design problem both for size of the circuit and the cost of the components. It is, therefore, an object of the disclosure to provide an efficient design of a mixed signal communication system, suitable for use in audio and other applications, which, like the afore described data transmission systems, carry digital data and/or control signals on the same pair of wires used to carry analog signals. Summary In one embodiment, a mixed signal communication system for transmitting digital signals on the same pair of wires which carry analog signals is proposed. An inexpensive transmitter circuit adapted to transmit the digital signals is disclosed to use a transformer for performing current mode transmission. The current mode transmission of the digital signals eliminates design difficulties conventionally associated with voltage mode transmission. Further, a low-cost digital signal generator, such as a switch circuit or a serial pulse stream generator, can be integrated to output a desired digital signal. And in order to accommodate situations where a DC power is not available at or around the transmission portion, a DC power extraction circuit adds the feature of obtaining the power from the same transmission wires and supplying the power needed for the digital signal generator or other components of the transmitter portion. At the receiver portion of the system, a current sense circuit is also installed to download the transmitted mix signal from the transmission wires, and send the signal to a decoding circuit to output digital signals originally encoded at the transmitter portion. In another embodiment, the transmitter and receiver portion of the system can be designed in such a way that the whole system is capable of exchanging digital information bi- directionally. One example application is an analog audio system with an audio and data transmission capability, such as a personal computer or home stereo, and an output device such as a speaker. In this example, it is desired to not only send the analog audio signals from the audio system to the speakers, but to transmit data signals back and forth using the same pair of wires. The present invention facilitates this desire in an efficient and economical manner. Brief Description of the Drawings Fig. 1 is a circuit diagram of a transmitter portion of a mixed signal communication system constructed in accordance with one embodiment of the present invention. Fig. 2 is a circuit diagram of a receiver portion of the mixed signal communication system of Fig. 1. Fig. 3 is a circuit diagram of a transmitter portion of an alternate embodiment of the mixed signal communication system of Figs. 1-2. Fig. 3A is a power extraction circuit in accordance with the teachings of one embodiment of the present invention. Fig. 3B is a detailed switch circuit in accordance with the teachings of one embodiment of the present invention. Fig. 3C is an oscillator circuit in accordance with the teachings of one embodiment of the present invention. Fig. 3D is a transmitter circuit in accordance with the teachings of one embodiment of the present invention. Fig. 4 is a circuit diagram of a receiver portion of the mixed signal communication system of Fig. 3. Fig. 5 is a block diagram of a bidirectional mixed signal communication system constructed in accordance with the teachings of one embodiment of the present invention. Fig. 6 is an expanded block diagram of a first transceiver of the bidirectional mixed signal communication system of Fig. 5. Fig. 7 is an expanded block diagram of a second transceiver of the bidirectional mixed signal communication system of Fig. 5. Fig. 8 is a circuit diagram of line interface units used in Figs. 6 and 7. Detailed Description Turning now to the drawings, in Fig. 1, the reference numeral 10 illustrates a data transmitter portion of a mixed signal communication system. As disclosed herein, the mixed signal communication system is uni- directional, i.e., analog and digital information are transmitted to respective target devices for use thereby. However, the present invention is equally suitable for use in bi-directional systems such as those embodiments of the invention to be described with respect to the figures below. Similarly, while the mixed signal communication system is disclosed as transmitting an analog audio signal to an audio signal reproduction system, again, the present disclosure is equally suitable for use in conjunction with other types of analog and/or digital information signals, for example, an analog video signal such as that received at an outlet for a cable TV distribution system. As may now be seen, the data transmitter 10 includes an audio signal generator 22 and a digital signal generator 24 for generating analog audio and digital signals, respectively. The audio signal generator 22 may be physically incorporated into the data transmission portion 10 of the combined analog/digital data transmission system or, as illustrated herein, be externally located, relative to the data transmission portion 10, and placed across the line of the data transmitter portion 10 by coupling the signal output lines of the audio signal generator 22 to terminals 12 and 14 of the data transmitter 10. As previously mentioned, the present disclosure is directed to a system and method for adding digital information, such as data or control signals, to an analog signal. The resulting signal carries the digital signals to the data receiver portion where the digital signals are extracted by a digital data output device such as a tone detector. Furthermore, because the digital information is added to the analog audio signal in the form of a varying current, difficulties traditionally associated with the addition of digital information to an analog audio signal using a superimposed voltage signal are avoided. As previously set forth, the digital signal is generated by the digital signal generator 24. As is well known in the art, the digital signal is a binary signal which, by varying between logical "0" and logical "1" states, conveys information to a device. It should be clearly understood that, while Fig. 1 shows the digital signal as being produced by the digital signal generator 24, it is fully contemplated that the digital signal may be produced by a processor subsystem of a personal computer or other programmable device. Alternately, the digital signal may be produced by a manually controllable switch. A carrier signal generator 26 generates a carrier signal at a selected frequency. While it is fully contemplated that the disclosure is suitable for use with carrier signals at various frequencies, a carrier signal having a frequency of about 400 KHz has been found to be suitable for the uses contemplated herein. In alternate configurations, thereof, a ceramic resonator, for example, a model EFO- A400K048 ceramic resonator manufactured by Panasonic, or an RC oscillator may be used to generate the carrier signal. After being buffered by a NAND gate 28, the carrier signal produced by the signal generator 26 and the digital signal generated by the digital signal generator 24 are provided as first and second inputs to a NAND gate 29. The NAND gate 29 modulates the digital signal onto the carrier signal by generating, as its output, an integrated signal. The NAND gate 29 drives the integrated signal to a bandpass filter consisting of a resistor 30, a capacitor 31, and an inductance of a primary winding of a transformer 32. This bandpass filter attenuates the higher harmonics of the carrier signal, changing it from a square wave to a sine wave, and also attenuates hash down in the audio frequencies. Having between 18 and 100 turns on the primary winding and between 1 and 2 turns on the secondary winding, the transformer 32 acts as a current mode transformer by stepping up the amplitude level of the current for embedding the integrated signal. Since there are only one or two windings on the primary side, the voltage generated is negligible while the current changes are preserved. The current mode transformer 32 can be very small and inexpensive, and may include a toroidal core with 30 gauge magnet wire as the primary winding. The secondary winding uses a relatively heavy wire passing through the center of the toroidal core, presenting a negligible impedance to the audio signal. Further, a small and inexpensive capacitor, typically a 0.1 μF or less ceramic bead, is used at each end of the transmission line to bypass the current around the power amplifier output and speaker. However, the impedances of the power amplifier output and the speaker are often low enough to make the bypass capacitor unnecessary in some applications. Referring now to Fig. 2, the wire pair carries the digital information to a receiver end 11 of the mixed signal communication system. Provided along the output line of the receiver end 11 is a current sense transformer 34 having between 1 and 2 turns on the primary winding and between 18 and 100 turns on the secondary winding. Because the primary winding has a very small inductance, a small voltage is induced across the secondary winding because of the current changes across the primary winding. The voltage induced in the secondary winding of the current sense transformer 34 is applied to an input, e.g., pin 3 of a tone detector 36 in this case, selected to detect and extract the digital signal from the combined signal. In this manner, the tone detector 36 extracts the digital signal from the received integrated signal. The tone detector 36 drives the extracted signal to a digital data output device 38 where the digital signal is decoded and any instructions contained within the signal are executed. In some cases, the digital signal may convey information related to the audio signal transmitted to the audio signal reproduction system 40. For example, the information may be text information associated with the audio signal and the digital data output device 38 may be a display. Thus, in this example, the audio signal reproduction device 40, typically, a low (e.g., 4 ohm) impedance loudspeaker, would produce audible sound from the received analog audio signal while the digital data output device 38 would display information associated with the produced sound. Of course, it is fully contemplated that the analog signal may contain information other than audio information and that the digital signal need not necessarily carry information related to the analog signal. In addition, if desired, the respective analog and digital signals may be used to drive unrelated devices. Referring next to Fig. 3, an alternate embodiment of the transmitter portion 10 of Fig. 1, herein referenced as transmitter portion 10', may now be seen. In situations when a DC power is not available in a remote place, it is known in the art to add additional circuitry to the transmission wires so that a DC power extraction circuit 42, as one proposed by this disclosure, can serve as a power supply to the transmitter circuit 44 or any digital signal generator. For example, a plurality of switch circuits 46 and an oscillator circuit 48 will generate a digital on and off signal at different frequencies. The oscillator circuit 48, along with a selected switch circuit, will drive a signal 50 at a certain frequency to the transmitter circuit 44. In another embodiment, a simple remote control IC encoder can be used to generate a digital pulse stream as the digital signal input. In Fig. 3A, more details about the DC power extraction circuit are shown, whereas an RC filter and a voltage regulating zener diode 52, or alternatively a series of regulators, can be arranged to output a DC voltage supply 54. In Fig. 3B, one of the switch circuits 46 is illustrated. Each switch circuit includes a switch 56, a CMOS device 58, and a resistor 60 with a resistance R. A debounce circuit is also provided between the switch 56 and the CMOS device 58 to reduce transient noises. The resistance R for each switch is different, thereby providing a uniquely identifiable frequency for each switch. When the switch 56 is closed, the CMOS device 58 turns on. As a result, the signal 50 connects to the oscillator 48 (described below) through the resister 60. In Fig. 3C, an exemplary version of the oscillator circuit 48 is shown. In essence, a frequency generated from the oscillator circuit 48 depends on the resistance and capacitance connected to an oscillator 61. The frequency is supplied to the switch circuits 46 and the transmitter circuit 44, as discussed above. In Fig. 3D, an embodiment of the transmitter circuit 44 is shown. The transmitter circuit 44 includes a voltage-to-current transformer 62 and a CMOS switch 63. When the CMOS switch 63 is switched on, the signal 50 from the aforementioned oscillator circuit 48 and switch circuits 46 is transmitted through the voltage-to-current transformer 62 in accordance with the teaching of the present disclosure. Referring now to Fig. 4, an alternate embodiment of the receiver portion 11 of Fig. 2, herein referenced as a receiver portion 11', may now be seen. The receiver portion 11' includes a current sensing receiver circuit 64 for taking the digital signal off the integrated signal on the transmission wire 16. In the present embodiment, the receiver circuit feeds the digital signal into one or more tone detector 66 for matching the particular frequency of each signal in the digital signal. The tone detector 66 provides the signals to an output device. Here, as an example, six light emitting diodes (LEDs) 68 turn on and off according to each signal. Although not shown, different variations can be made to the receiver portion 11'. In one embodiment, the receiver circuit can send the received digital signal to a computer processing unit and let the decoding be done by software. In another embodiment, a matching remote control decoder can decode the digital signal pulse stream generated by a remote control encoder in the transmitter portion. Referring now to Fig. 5, a system 100 represents yet another embodiment of the present invention. The system 100 includes two nodes 102, 104 connected by a transmission medium 106. The transmission medium may be a pair of wires, such as speaker or telephone wires, a computer bus, a power line, a trunk, or other type of link well known in the art. To clarify the description, reference will continue to be made to the example described above, it being understood that many different applications can also benefit from the present invention. In the present example, the transmission medium 106 will be a line consisting of a balanced paired audio cable typical of that used in recording studios. The first node 102 includes an audio signal generation system 108 for producing an audio signal 110a and a data processing system 112 for sending and receiving digital data signals 114a, 116a, respectively. The audio and data signals 110a, 114a, 116a are also connected to a transceiver 120 which combines the audio signal 110a with the data signal 114a for transmission on the line 106 and receives the data signal 116a from the line. The second node 104 includes an audio signal reproduction system 122, such as a speaker, for receiving an audio signal 110b and a data input/output device 126 for sending and receiving digital data signals 116b, 114b, respectively. The audio signal 110a is essentially identical to the audio signal 110b, and the digital data signals 114a, 116a are essentially identical to the digital data signals 114b, 116b. The audio and data signals 110b, 114b, 116b are also connected to a transceiver 130 which separates the audio signal 110b from the data signal 114b for reception from the line 106 and transmits the data signal 116b on the line. Referring now to Fig. 6, the transceiver 120 receives the digital data signal 114a into a frequency shift keying (FSK) modulator 140a. It is understood, of course, that different types of modulators, or no modulator at all, can be used. To continue with the above example, the digital data signal 114a is a non-return-to- zero (NRZ) serial data input at TTL logic levels (0V-5V) at a data rate of 19.2kbs. The FSK modulator 140a transforms the digital data signal 114a into an FSK signal 142 with carrier frequencies of 137 kHz (for a space) and 167 kHz (for a mark). The FSK signal 142 is then provided to a filter 144a and the filtered signal is then provided to a line interface unit 146a. The line interface unit 146a also receives the audio signal 110a and combines it with the FSK signal 142. These combined signals are then driven onto the line 106. In the present embodiment, the line interface unit 146a also receives a modulated digital signal 148 from the line 106 and provides it to a second filter 150a. In one embodiment, these functions of the line interface unit 146a may be performed by the current sense transformer 34 and the tone detector 36, all of Fig. 2. The second filter provides the modulated digital signal 148 to an FSK demodulator 152a. The FSK demodulator demodulates the signal, thereby providing the data signal 114a. In continuance of the present example, the data signal 114a is a NRZ serial data output at TTL logic levels (0V-5V) and at a data rate of 19.2kbs. Referring now to Fig. 7, the transceiver 130 is in many respects similar to the transceiver 120 of Fig. 6. However, component values may be different to accommodate different carrier frequencies for each direction of transmission. The transceiver 130 receives the digital data signal 114b into an FSK modulator 140b. The FSK modulator 140b transforms the digital data signal 114b into an FSK signal 142b with carrier frequencies of 91 kHz (for a space) and 100 kHz (for a mark). The FSK signal 142b is then filtered by a filter 144b and filtered signal is then provided to a line interface unit 146b which drives the signal onto the line 106. The line interface unit 146 also receives the audio signal 110a and a modulated digital signal 148b from the line 106. The line interface unit 146 provides the modulated digital signal 148b to a second filter 150b, which filters and provides the modulated digital signal to an FSK demodulator 152b. The FSK demodulator 152b demodulates the signal, thereby providing the data signal 116b. Referring now to Fig. 8, one embodiment for the two line interface units 146a, 146b is shown in extended detail. In continuance with the above example, the line interface unit 146a serves an amplifier or a mixer, and will hereinafter be referred to as the hub node. The line interface unit 146b serves an output device such as a powered speaker, and will hereinafter be referred to as the speaker node. Each of the line interfaces units 146a, 146b are identical, and therefore use identical reference numerals. The suffix of "a" or "b" are appended to the components of the line interfaces units 146a, 146b, respectively. An example of operation from the line interface unit 146a to the line interface unit 146b will illustrate the functionality of all the components of both units. The line interface unit 146a includes a pair of series resonant circuits 160a, 161a across a set of terminals 162a, 164a. The terminals 162a, 164a selectively connect with the audio signal 110a of the hub node. In the present embodiment, the series resonant circuit 160a includes a capacitor in series with an inductor for providing a resonant frequency that matches the mean between the 91 kHz and 110 kHz carrier frequencies and the series resonant circuit 160b includes a capacitor in series with an inductor for providing a resonant frequency that is the mean between the 137 kHz and 167 kHz carrier frequencies. It is understood, however, that the carrier frequencies may be different for different applications, and that one of ordinary skill in the art can correctly choose and compensate for a desired frequency. As a result, the terminals 162a, 164a will have a consistent impedance (a short circuit) at the carrier frequencies, regardless of whether or not any other components are connected to the terminals. A current mode transformer 166a, similar to the transformer 32 of Fig. 1, is placed serially in-line with the audio signal 110a and across the input digital signal 114a. The number of turns on the primary of the transformer 166a (the side connected to the input digital signal 114a) is significantly greater than the number of turns on the secondary. For the sake of example, the primary-to-secondary ratio of the transformer 166a may be 18:4. As a result, the transformer 166a converts the voltage variations on the input digital signal 114a with only a negligible voltage drop on the audio signal 110a. However, the transformer 166a modifies the current on the audio signal 110a, at the specified frequency (from the FSK modulator), in response to the input digital signal 114a. The audio signal 110a, as modified by the transformer 166a, is then connected across the line 106 to the terminals 162b, 164b of the line interface unit 146b. The terminals 162b, 164b can connect the audio signal 110b to a speaker, or may be simply left open. The series resonant circuits 160b, 161b ensure that a consistent impedance (a relative short) appears across the terminals at the desired frequencies. The input digital signal 114a can be received by the line interface unit 146b through a transformer 168b. The transformer 168b is part of a resistor- capacitor-inductor peaked high pass circuit 170b across the terminals 162b, 164b. The transformer 168b senses the FSK carrier frequency voltage across the secondaries of the transformer 166b for the output FSK data signal 114b. Although illustrative embodiments, or examples, of the invention have been shown and described, other modifications, changes, and substitutions are intended in the foregoing disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.

Claims: 1. A system for transmitting digital data on a pair of wires which carry analog signals, comprising: a signal transmitter adapted to transmit said digital data utilizing current mode transmission; a receiver for receiving the transmission; and a circuit to decode said digital data from the received transmission. 2. The system of claim 1 wherein the transmitter comprises: a carrier signal generator to generate a carrier signal of a first frequency; a gate to integrate said digital data into the carrier signal; and a transformer winding in series with the pair of wires to transmit the integrated signal with the analog audio signal. 3. The system of claim 2 wherein the transformer winding encodes the digital integrated signal into a varying current. 4. The system of claim 1 wherein the receiver comprises a current sense circuit and a tone detector. 5. The system of claim 4 wherein the current sense circuit receives the encoded integrated signal from said pair of wires. 6. The system of claim 4 wherein the tone detector separates the digital data from the integrated signal. 7. A device for transmitting a digital signal on a transmission medium, comprising: a source for generating a carrier frequency; a switch for controlling the carrier frequency in response to the digital signal; and an output circuit including a current sense transformer for converting the switch-controlled carrier frequency to a current mode transmission signal so that the digital signal is transmitted as a current mode transmission signal on the transmission medium. 8. The device of claim 7 further comprising: means for combining an analog signal with the current mode transmission signal. 9. The device of claim 8 wherein the analog signal includes an audio signal. 10. The device of claim 8 wherein the analog signal includes a power signal. 11. The device of claim 7 wherein the transmission medium has a very low impedance. 12. The device of claim 7 wherein the transmission medium connects to a receiver with a very low impedance. 13. The device of claim 7 wherein the digital signal is separable into a plurality of control signals from a plurality of sources, the device further comprising: a circuit for receiving the plurality of control signals and combining the control signals into a single digital serial pulse stream to form the digital signal so that the switch controls the carrier frequency in response to the single digital serial pulse stream. 14. The device of claim 7 further comprising: a circuit for extracting power from the transmission medium in order to operate at least a portion of the device. 15. A device for receiving a digital signal, comprising: means for receiving a current mode transmission signal; a current sense transformer for converting the current mode transmission signal to a voltage mode signal; and a tone detector for converting the voltage mode signal to the digital signal. 16. The device of claim 15 wherein the receiving means also receives an analog signal. 17. The device of claim 16 wherein the receiving means includes two terminals with a very low impedance. 18. The device of claim 16 wherein the analog signal includes an audio signal. 19. The device of claim 16 wherein the analog signal includes a power signal. 20. A communication system for transmitting an audio signal and a digital signal between nodes connected by a transmission medium, the system comprising: means for driving the audio signal onto the transmission medium; a source for generating a carrier frequency, the carrier frequency being outside of a predetermined frequency range for the audio signal; a switch for controlling the carrier frequency in response to the digital signal; an output circuit including a current sense transformer serially connected to the transmission medium for converting the switch-controlled carrier frequency to a current mode transmission signal; means at a first node for transmitting the current mode transmission signal on the transmission medium; means at a second node for receiving the current mode transmission signal; a current sense transformer for converting the current mode transmission signal to a voltage mode signal; and a tone detector for converting the voltage mode signal back to the digital signal. 21. The system of claim 20 wherein the transmission medium has a very low impedance. 22. The system of claim 20 wherein at least one of the two nodes has a very low impedance. 23. The system of claim 20 further comprising: a circuit for receiving a plurality of control signals and combining the control signals into the digital signal as a digital serial pulse stream. 24. The system of claim 20 further comprising: a circuit for extracting power from the transmission medium in order to operate at least a portion of the system. 25. The system of claim 20 wherein the tone detector includes a circuit for receiving the digital signal as the digital serial pulse stream and converting the digital serial pulse stream into a plurality of control outputs. 26. The system of claim 20 further comprising: means responsive to the plurality of control outputs. 27. The system of claim 20 wherein there system is a multi-node, multi- directional system. 28. A line interface unit comprising: a terminal for supplying an audio signal; a first series resonant circuit connected across the terminal; a current mode transformer connected in series with the audio signal for converting a first voltage variant signal to a first current variant signal on the audio signal. 29. The line interface unit of claim 27 further comprising: a second series resonant circuit connected across the terminal, wherein the second series resonant circuit is tuned to a different frequency than the first series resonant circuit. 30. The line interface unit of claim 29 further comprising: a second transformer connected in parallel across the audio signal for converting a second current variant signal to a second voltage variant signal on the audio signal. 31. The line interface unit of claim 29 wherein the current mode transformer has relatively few turns on the audio signal. 32. The line interface unit of claim 29 wherein the series resonant circuits provide a relative short across the terminal at desired frequencies.

Inventors: MOSES DONALD W, MOSES ROBERT W, MOSES, DONALD, W., MOSES, ROBERT, W.
Applicants: PAVO LABS L L C, PAVO LABS, L.L.C.
Application year: 1999
Main classification: H04B3
IPC codes: G10L19, H04B3, H04J99, H04L27, H04N7