Audio signal transmission line with a low pass filter

An audio signal transmission line comprising an additional inductance inserted in series with the line and/or coupled in parallel therewith near the load end of the line for providing a low pass filter which overcomes parasitic and dielectric capacitance of the line so as to reduce audio frequency noise generated in the line by low level and low frequency audio signals. The magnitude of the inductance used may vary widely, e.g. from 20 microhenries to 1 millihenry, depending on the length of the line and the space available.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to audio signal transmission systems in 
general and in particular to an audio signal transmission line having a 
low pass filter circuit incorporated therein. 
2. Description of the Prior Art 
An audio signal transmission system comprises an audio signal generator and 
a load coupled to the generator by means of an audio signal transmission 
line. For example, the generator may comprise an acoustic transducer, such 
as a microphone, or an amplifier; the load may comprise an amplifier or a 
speaker; and the transmission line may comprise a pair of twisted or 
untwisted, single or multistrand wires or a coaxial cable. 
Even in systems comprising expensive and high quality components, it has 
been found that signals generated in the audio frequency range for 
transmission to the load can generate noise in the audio frequency 
spectrum on conventional transmission lines and cables which can in turn 
result in a significant and detectable distortion of the audio signals 
being transmitted thereon. 
SUMMARY OF THE INVENTION 
The generation of noise in the audio frequency spectrum by audio signals 
placed on an audio transmission line or cable has been traced to 
transitions in the potential placed on the line. It has been found that 
when a low level, low frequency potential, such as a potential in the 
millivolt or microvolt range having a frequency of approximately 20 to 
1500 Hz, is applied to an audio signal transmission line, particularly one 
coupled to a high impedance load, and goes from a negative potential to a 
positive potential, there is an instantaneous storage of charge in the 
distributed capacitance of the line. The capacitance may comprise the 
mutual or parasitic capacitance of multistrand wire as well as the 
capacitance due to dielectrics used in the line. When the charge 
discharges, high frequency noise, e.g. above 1 MHz, is generated. The high 
frequency noise, when it decays, produces a sustained low level, low 
frequency oscillation in the audio frequency spectrum on the line. To 
eliminate the undesired low level, low frequency oscillation, it has been 
found that by adding a compensating inductive element or network to the 
line, preferably on the load end thereof, the noise producing effect of 
the distributed capacitance is reduced if not substantially eliminated.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed to an audio signal transmission line with 
low pass filtering for eliminating undesired low level, low frequency 
oscillation in an audio signal transmission system. 
Referring to FIG. 1, there is provided in accordance with the present 
invention an audio signal transmission system designated generally as 1. 
In the system 1 there is provided a source of audio signals 2 and a load 3 
which is coupled to the source 2 by means of a pair of transmission lines 
designated generally as 4. The source 2 may be, for example, a microphone 
or other acoustic transducer, a preamplifier, an amplifier, or the like. 
The load 3 may be a preamplifier, an amplifier, a speaker, or the like. 
The transmission lines 4 may comprise a pair of twisted or untwisted, 
single or multistrand wires or a coaxial cable. As is conventional, the 
signal source 2 and the load 3 comprise a positive terminal and a negative 
terminal as shown by positive and negative signs, respectively. The 
transmission lines 4 comprise a first conductor or line 5 coupled between 
the positive terminals of the source 2 and load 3 and a negative conductor 
or return line 6 coupled between the negative terminals of the source 2 
and the load 3. As thus far described, the source 2, load 3 and 
transmission lines 4 are conventional. 
In accordance with the present invention there is provided in the system 1 
an inductor L1 coupled in series with the line 5 between the source 2 and 
the load 3 and preferably near the end of the line 5 coupled to the load 
3. For transmission lines 4 comprising lengths of approximately 3 meters, 
the inductance of inductor L1 is from 10-30 microhenries. In a working 
embodiment of the present invention, the inductor L1 comprised 20-22 
microhenries. As will be further described below, the actual magnitude of 
the inductor L1 depends on the parasitic and distributed capacitance of 
the lines 4 which, for a 3 meter transmission line, is extremely small, 
e.g. 35 to 100 picofarads (pf) per foot. 
Referring to FIG. 2 in another embodiment of the present invention, the 
inductor L1 is located on the load end of the transmission line 6. 
Referring to FIG. 3 there is provided in another embodiment of the present 
invention an inductor L2 which is coupled in parallel with the 
transmission line 5 near the load end thereof. The inductor L2 comprises 
an inductance of from 0.5 to 1.5 millihenries and may be as large as space 
will permit. In a working embodiment of the invention of FIG. 3, the 
inductor L2 comprised 1 millihenry. 
Referring to FIG. 4, there is provided in still another embodiment of the 
present invention an inductor L3 which is coupled in series with the line 
5 between the source 2 and the load 3 preferably near the load end 
thereof, and an inductor L4 which is coupled in parallel with the line 5 
and the inductor L3 near the load end thereof. In this embodiment of the 
present invention, the inductor L4 comprises an inductance of 
approximately 1 millihenry and the inductor L3 comprises an inductance of 
approximately 20 microhenries. In practice, the magnitude of the inductor 
L4 can be as large as space will permit. 
As is well known, capacitance in series with a low frequency signal 
provides a large impedance to the signal whereas capacitance in series 
with a high frequency signal provides a low impedance to the signal. The 
present invention is concerned with reducing the impedance to low 
frequency signals caused by the series capacitance in the transmission 
lines 4, especially in the positive leg thereof, line 5 and in the case of 
the apparatus of FIG. 2, line 6. 
In practice, it has been found that the inductors L present a low impedance 
path to current flowing through the transmission line 5 until current 
builds up in the inductors. As the current increases in the inductors, the 
effects of the capacitance of the cable are reduced to such a level that 
the cable no longer appears capacitive but rather appears inductive and 
acts like a low pass filter. This current buildup is seen to occur as low 
level applied voltage, e.g. voltage in the millivolt and microvolt ranges, 
goes from a negative potential to a positive potential in the low audio 
frequency range, e.g. 20 to 1500 Hz Under these conditions, there is an 
instantaneous storage of charge in the distributed capacitance of the line 
which causes distortion in the audio frequency range when the charge is 
dissipated due to the above-mentioned low level, low frequency 
oscillation. The addition of inductance in the line as described above, 
provides a low pass filter and thereby reduces the storage of charge in 
the parasitic and dielectric capacitance of the lines. The reduction of 
this charge serves to reduce, if not eliminate, the noise which would 
otherwise be produced. 
While preferred embodiments of the present invention are described above, 
it is contemplated that various modifications may be made thereto without 
departing from the spirit and scope of the present invention. For example, 
while the placement of the inductors L in the positive transmission line 5 
near the load end of the line is preferred, placement of inductors L in 
the return line 6 and at other positions in the lines may provide some 
noise suppression. In addition, noise suppression in the audio frequency 
spectrum can be achieved by placing the inductor L in both the positive 
and the return lines. Accordingly, it is intended that the embodiments 
described be considered only as illustrative of the present invention and 
that the scope thereof should not be limited thereto but be determined by 
reference to the claims hereinafter provided and their equivalents.