US 2830117 A
Description (OCR text may contain errors)
April 8, 1958 A. w. BLOW 2,830,117
- PUBLIC ADDRESSSYSTEM Filed Dec. 7, 1953 FIG. 2
ANGUS W. BLOW AGENT nite States acin PUBLIC ADDRESS SYSTEM Angus W. Blow, Rochester, N. Y., assignor, by mesne assignments, to General Dynamics Corporation, a corporation of Delaware Application December 7, 1953, Serial No. 396,667 6 Claims. c1. 179-1 Nice Referring now to Fig. 1, there are provided, as in a conventional public address system, amplifying means. 1 and an output transducer, such as loudspeaker 2. Those skilled in the art can readily appreciate that any conventional amplifier system may be used, and may accommodate more than the single output transducer shown through well-known circuit configurations. a
Line 3 is the line mentioned above. Input transducers 4 may be located at various points in the public address system, but may be connected to line 3 by suitable selective connecting means, such as switch means 5. Again, those skilled in the art can appreciate that a greater or lesser number of input transducers than shown may be employed according to my invention.
In accordance with my invention, I connect, in the I embodimentof Fig. l, non-linear impedance means 6.in
such as microphones or loudspeakers used as microphones, may be selectively connected to an amplifier input line at will for paging over an output transducer, such as a loudspeaker. p r
It most often occurs in arrangements of this type that the line for connecting the input transducers to the input of the amplifying means is so long that whenever the line is floating, that is, when no input transducer is connected to the line for paging purposes, a variety of spurious and stray interference noises and signals, such as induced hum, static and crosstalk, for example, may be picked up by the line and conveyed to the amplifier. As a result, all within hearing of the output transducer are bothered by annoying sounds. The noises and extraneous signals are particularly bothersome if the amplifier is operated at a high level of gain, which is usually the case.
series with line 3. Non-linear impedance means 6 is preferably substantially resistive in character. Thus, means 6 may be a varistor, thermistor or other nonlinear impedance means having a high internal impedance at low voltages and a low impedance when the current is high. Non-linear impedance means 6 is preferably included in each side of line 3, although beneficial results will be obtained if only one side of line 3 has non-linear impedance means 6 placed in series with it.
There is preferably provided in the embodiment of V Fig. 1 transforming means 7 for coupling non-linear im- Prior to my invention, the only ways of preventing inter- I ference of this type have been relatively expensive, since they generally require the use of a relay to ground the amplifier input, etc.
It is accordingly an object of my invention to provide new and useful means for public address systems for limiting extraneous noises pickedup by an amplifier input line to which, upon occasion, no input transducer is connected.
It is also an object of my invention to provide in a public address system, simple, cheap and easy to install extraneous-noise-reducing means.
In general, I accomplish these and. other'objects of my invention by providing a circuit for connecting the input transducer to the amplifying means, and by providing nonlinear impedance means and a source of inaudible current connected in the circuit such that current from the source flows through the non-linear impedance means only when an input transducer is connected to the line. The amplifying means, upon looking back into the line, consequently sees a very. high impedance when there is no transducer connected to the line, but sees a low impedance when a transducer is connected to the line. Signals from an input transducer connected to the input line therefore reach the amplifier input with a negligible amount of attenuation.
Further objects and advantages of my invention will become apparent as the following description proceeds and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.
For a better understanding of my invention, reference may be had to the accompanying drawing in which:
Fig. 1 is a schematic wiring diagram of one preferred embodiment of my invention; and
Fig. 2 is a schematic wiring diagram of another preferred embodiment of my invention.
is. posite those connected to source 11.
pedance means 6 to the input of amplifying means 1. Transforming means 7 may have a pair of primaries 8 and a secondary 9. The latter may be connected to the input of amplifier 1. Resistor 10 may be shunted across the leads of secondary 9 to provide a load for transforming means 7. This enables the non-linear impedance means to work into an impedance comparable to its own when current from source 11 is flowing therethrough.
Further in accordance with my invention, I provide-a source of inaudible current 11, by which I mean that the frequency of the current is outside the range of audible reproducibility by transducer 2 or other output transducers. The source preferably furnishes direct current 1: and may be a battery as indicated, although other sources,
such as a supersonic oscillator, might be employed. In any case, primary windings 8 are preferably connected in series through source 11, the non-linear impedance means being connected to the sides of primary windings 8 op-- The embodiment of my invention shown in Fig. 1 operates as follows: When no transducer 4 is connected to line 3, no current flows through nonlinear impedance means 6 because the circuit for source 11 is incomplete. Non-linear impedance means 6 therefore has a high value of impedance, and as a result, any voltage picked up by line 3 is prevented from reaching transforming means 7. When one of switch means 5 is closed, however, an obvious circuit is established for current source 11 to flow through non-linear impedance means 6. Because of the resultant current flow, non-linear impedance means 6 reaches an appreciably lower value of impedance, and audio-frequency currents generated at the particular input transducer 4 connected to line 3 may thereupon reach input transforming means 7 with a minimum of attenuation. Thus, in accordance with my invention, I am enabled to block undesired noises from amplifier 1 when no paging is taking place from any input transducer 4.
In the embodiment of Fig. 2, there is likewise provided amplifying means 1 and one or more output transducers of which only loudspeaker 2 is shown. There are also one or more input transducers 4, each selectively connectiole by its own switch means 5 to line 2.
In accordance with my invention, I connect to the arrangement ofFig. 2 a source of inaudible current 11 as in the case of Fig. 1, together with non-linear impedance means 12 "The latter may be tungsten-filament lamps,
which in one pilot lamp version exhibit a hot-to-cold resistance ratio of 14 to 1. Signals may be transferred from linefi to theinppt of amplifying means 1 by suitable means suchas coupling capacitorslm Suitable imped anc emeans such as linear resistors 14 are preferably connected in series with line 3. The impedance of the nonlinear means is preferably comparable to that of nonlinear impedance means 12 when no current from source 11 is flowing therethrough.
The embodiment of Fig. 2 operates as follows: when no input transducer is connected to line 3, lamps I2 are cold,and therefore have a low resistance. Extraneous voltages picked upon line 3 are therefore shunted to ground, represented by the symbol 15. Under these cit cumstances, the extraneous noises cannot reach the input of amplifier 1. When an input transducer 4 is connected to line 3 through the closure of a switch 5, however, an obvious circuit is completed for causing the flow of current from inaudible current source'll through lamps 12. The current flow raises the resistance of the lamps 12., thereby decreasing the shunting eifect of lamps '12 to ground on the voice currents transmitted over line 3 from input transducer 4. Paging may then take Place.
Non-linear impedance means 6 or 12, of course, may be for coupling the input of said amplifying means to said series circuit, 'an input transducer and means for selectively connecting said transducer across said series'D. C. circuit whereby the current from said source flows through said impedance means upon the selective completion of said series circuit thereby changing the value of said impedance means. p
3. In a public address system, the combination of a line, transformingmeans, amplifying means, means for connecting the input of said amplifying means to the output of said transforming means, impedance means having selectively a first or a second value of impedance, said impedance means coupling said line to the input of said transforming means, an input transducer, and means for selectively connecting said transducer to said line, and means responsive to the establishment of said connection for selectively causing said impedance means to assume its second value of impedance.
' 4. In a public address system, the combination of a line, a transformer having a pair of primaries and a secondary, amplifying means, means forconnecting the input of said amplifying means to the secondary of said transformer, impedance means responsive to current flow to have selectively different values of impedance, a
source of current, means for serially connecting said impedance means, an input transducer, and means for selectively connecting said transducer across said series circuit to cause current flow' through said impedance selectively a high and a low value of impedance and tions will readily occur to those skilled in the art. I
do not, therefore, desire my invention to be limited to the specific. arrangement shown and described, and I intend in the appended claims to cover all modifications within the spirit and scope of my invention.
-.What I claim is:
1. Ina public address system, the combination of a line, amplifying means, means for coupling said line to the input of said amplifying means, said coupling means having selectively a high or a low level of coupling, an input transducer, means for selectively connecting said transducer tosaid line, and means to cause said coupling means to selectively assume its high level of coupling in response to the connection of said transducer to said line. i
1 2. In a public address system, the combination of a series D. C. circuit including a line, non-linear impedance means and a source of current, amplifying means, means.
means to thereby change its value of impedance.
5 The combination of claim 1 in which said coupling means includes 1 non-linear impedance means having means to connect said impedance in shunt with the input of saidamplifying means and in series with said line.
6. The combination of claim 1 in which said coupling means includes non-linear impedance means having selectively a high and a low value of impedance, said means to cause the coupling means to selectively assume its high level of coupling includes a source of current and said coupling means further including means for serially connecting said source and. said impedance,
means in series with said line and in shunt with the in- 45 put of said amplifying means.
References Cited in the file of this patent UNITED STATES PATENTS