Abstract:
A method and apparatus for high quality signal transmission, which utilizes normal-mode current flow and produces an audio output signal with suppression of normal-mode voltage amplitude, comprising transmitting an audio input signal source though (1) a buffer amplifier; (2) a modulated current source; (3) a pair of wires; (4) a current transformer; and (5) a receiver amplifier for said current transformer.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is a method of high quality analog or digital signal transmission by wire generally in the sub-audio to video range by means of normal-mode current flow with suppression of normal-mode voltage amplitude. 
         [0003]    2. Description of Prior Art 
         [0004]    The transmission of audio signals is generally in the range of 5 Hz-100 kHz between electronic equipment that process or otherwise employ the signal. Generally such transmission is done by originating the signal from a low impedance and receiving it at a high impedance, often referred to as “voltage matching” or “bridging”. In some (usually large) systems such as telephony the source and receiving impedances are matched. This is also true in voltage based video transmission. This is seldom done anywhere else in contemporary audio practice, either consumer or professional. In all such systems the signal voltage is impressed across the dielectric (insulation) of the conductors. As a result, the properties of the dielectric may strongly affect the quality of the resulting received signal. Specifically, the attribute of dielectric absorption seems to impart an audible degradation. 
         [0005]    Industrial control practice has for many years used another method for transmitting process signals over long distances. It is known as a current-loop. There are several variations of the technique, but the most common is known as a 420 mA current-loop. In this method of transmission, the signal is originated from a high impedance source and received by a low impedance receiver, essentially the opposite of voltage matching mentioned above. Generally the bandwidth of 4-20 mA loops is not high enough to support audio applications, but there is nothing inherent in the technique which precludes greater bandwidth. Industrial current-loops generally allow the reporting of a DC value and this is not necessary for audio or video, which are AC signals. This permits a different approach to current-based signal transmission. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is an apparatus for signal transmission comprising a transmitter and a receiver connected by a pair of wires. The transmitter is comprised of a non-inverting buffer amplifier, an inverting buffer amplifier and a pair of modulated current sources. The receiver is comprised of a current transformer and a receiver amplifier for the current transformer. The modulated current source can be single-ended or symmetrical with respect to the common terminal of the transmitter. The dielectric properties of the insulation of the pair of wires connecting the transmitter and receiver do not affect the transmission quality. The DC resistance of the pair of wires also does not affect the transmission quality. 
         [0007]    The receiver is comprised of a current transformer and a receiver amplifier for said current transformer. The receiver amplifier has essentially zero input impedance. 
         [0008]    The present invention is also a method of high quality signal transmission, which utilizes normal-mode current flow and produces an output signal with suppression of normal-mode voltage amplitude, comprising transmitting input signal source though (1) a buffer amplifier; (2) a modulated current source; (3) a pair of wires; (4) a current transformer; and (5) a receiver amplifier for said current transformer. The modulated current source is single-ended or is symmetrical with respect to the common terminal of the transmitter. The modulated current source comprises a transconductance amplifier or a voltage source, which is single-ended or is symmetrical with respect to the common terminal of the transmitter, and a sufficiently large series resistance. The dielectric properties of the insulation of the pair of wires do not affect the transmission quality. The DC resistance of the pair of wires also does not affect the transmission quality. The receiver amplifier has essentially zero input impedance. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0009]    In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments and structural changes may be made without departing from the scope of the invention. 
         [0010]    In one embodiment of the present invention, the incoming signal is fed to a modulated bilateral symmetrical current-source. A description of a current-source by Thevenin&#39;s Theorem is an ideal generator of current with infinite output impedance. This means that a specified output current will flow through a connected load irrespective of the impedance of that load as long as it is finite or zero. In practice, a current source can be represented as a source of voltage behind an extremely high resistance. The upper limit of the voltage is called the compliance voltage; and the range of approximately constant-current is a function of the ratio of the range of load impedance to the Thevenin generator resistance. A modulated bilateral current-source is one, which can either sink or source current to the load in response to an input voltage. Such a circuit or device is also known as a Transconducter and the ratio of output current to input voltage is known as the transconductance (G). Thus G=Iout/Vin, the unit of G (which stands for transconductance) in the S.I. system is Siemens. 
         [0011]    A symmetrical current-source is one in which the two terminals of the current port are identically electrically distant from the common reference, usually ground. There are numerous circuit topologies known to those skilled in the art for producing transconductance amplifiers. Any of several such circuits can be used to produce the high impedance source required by the invention. The receiver is another matter. A current-sourced signal may be accurately received in a summing amplifier at the node which is commonly known as the virtual ground. Such a circuit may be made symmetrical as well. The practical difficulty is that exposing virtual ground to the “outside world” causes stability problems is real circuit implementations. A far superior method is to regard the AC current coming from the source as a floating loop and galvanically sense the current in the loop. This is done by means of a device known as a current transformer. The advantages of this method are numerous. For example, the transformer galvanically isolates the source from the receiver. This eliminates the effects of common-mode differences in ground potentials between the equipments. Additionally, the input impedance of the receiver is essentially a short-circuit. This means that the signal voltage on the interconnecting cable is essentially zero. This eliminates the adverse influence of imperfect cable dielectrics because negligible voltage is impressed across said dielectric. Also, the symmetry of the transformer primary renders it immune to induced common mode or longitudinal currents on the interconnecting cable. Finally, the resistance of the cable has negligible influence on the results because the equivalent AC impedance of the current-source is extremely high in comparison to any possible cable resistance. 
         [0012]    A practical current source for the purpose at hand can be comprised of a differential-output voltage amplifier followed by a reasonable value of series output resistance. The amplifier must be able to drive the output resistance as its entire load, since the receiver will look like a short circuit. If this is done, the same output configuration can be used in the conventional way, i.e. bridged by the receiver, thus allowing compatibility with ordinary receivers in audio applications. In the embodiment shown in  FIG. 2 , on the left side (the transmitter) consists of an inverting and a non-inverting amplifier feeding out to the twisted-pair through fixed resistors. True modulated current sources can be used, but their complexity is not necessary to the example. Also, this topology illustrates the compatibility concept with ordinary receivers. 
         [0013]    On the right side of  FIG. 2 , the current transformer is typically embodied as a toroidal core with the secondary winding wound toroidally upon it and the primary consisting simply of a single conductor passing through the center of the toroid. What is important, however, is the application of the secondary winding directly to the summing nodes of the two amplifiers. This causes the secondary voltage across the transformer to be zero by the feedback action of the amplifiers. The current induced in the secondary still flows, and the resulting cancellation current in the feedback resistors of the amplifiers develops an output voltage. The two voltages are in opposite phase and are differenced by the third amplifier to produce a single-ended output. The elimination of secondary voltage on the transformer prevents magnetic core saturation which would otherwise limit the dynamic range of operation. The effective shorting of the secondary magnetizing inductance prevents its reflection to the primary which extends the time-constant of the transformer thus allowing better low-frequency response in relation to the core geometry. 
         [0014]    Turning to  FIG. 1 , in the idealized arrangement the source of the signal, 1 , is passed through a transmitter  9 , a pair of wires of arbitrary length  5 , a receiver  10  to produce an output signal  8 . The transmitter  9  is comprised of a non-inverting buffer amplifier  2 , an inverting buffer amplifier  3 , and a modulated current source  4 . The receiver  10  is comprised of a current transformer  6  and a receiver amplifier for the current transformer  7 . 
         [0015]    Tuning to  FIG. 2 , in a preferred embodiment the source of the signal  13 , is passed through a transmitter  11 , a pair of wires of arbitrary length  18 , a receiver  12  to produce an output signal  23 . The transmitter  11  is comprised of a non-inverting buffer amplifier  14 , an inverting buffer amplifier  15 , and Thevenin current-source resistances  16  and  17 . The receiver  12  is comprised of a current transformer  19 , virtual ground amplifiers  20  and  21 , a differential amplifier  22 . 
     
    
     BRIEF DESCRIPTION OF FIGURES 
       [0016]      FIG. 1  shows the idealized arrangement. 
         [0017]      FIG. 2  shows a preferred embodiment.