Active noise suppression system for telephone handsets and method

System for use by a caller and a recipient of a telephone call for suppressing environmental noise in the vicinity of a telephone comprising a telephone handset forming a part of the telephone. The telephone handset has a human voice sensor for picking up human sounds generally in one direction, and creating a first electrical signal. A speaker capable of producing substantially directional human voice sounds is provided. A second sound sensor is carried by the handset and has sound pick-up capabilities from a direction which is generally opposite said one direction for picking up external environmental noise in the vicinity of the handset and producing a second electrical signal having a frequency and amplitude. Electrical circuitry is provided for processing the second electrical signal to provide a third electrical signal of the same frequency as the second electrical signal but of opposite amplitude and sign and for combining the third electrical signal with the first electrical signal to suppress environmental noise present in the first electrical signal so that the recipient will receive an electrical signal in which the environmental noise in the vicinity of the caller has been suppressed.

This invention relates to an active noise suppression system for telephone 
handsets and method for suppressing environmental noise. 
There has been continued difficulty in utilizing telephone handsets in 
noisy environments because of background noise. For example, in making 
telephone calls in airplanes, it has been difficult for the user on the 
airplane and also the recipient to carry on normal conversations because 
of the background noise, as for example the noise from the jet engines 
utilized to propel the aircraft. Similarly, for cellular telephone users 
in automobiles, the noise from freeway traffic has also been troubling. 
There are many other situations where the ambient noise makes it difficult 
to use telephone handsets. This can include construction sites, airport 
sites, street sites and the like. The use of telephone booths for 
shielding has proved to be inadequate in many situations to overcome this 
problem. 
In general, it is an object of the present invention to provide an active 
noise suppression system and method for telephone handsets. 
Another object of the invention is to provide such a system and method 
which is particularly suitable for noisy environments. 
Another object of the invention is to provide a system and method of the 
above character in which environmental noise is suppressed so that it does 
not have an appreciable affect on telephone conversations.

In general, the system of the present invention is for use by a caller and 
a recipient for suppressing environmental noises in the vicinity of a 
telephone handset of the caller having a directional voice sensor for 
picking up human sounds, generally in one direction, and creating a first 
electrical signal. The handset also has a speaker capable of producing 
directional human voice sounds. A second sound sensor is carried by the 
handset for picking up external environmental noise in the vicinity of the 
handset and faces in a direction which is generally opposite said one 
direction and produces a second electrical signal having a frequency and 
amplitude. Means is provided for processing the second electrical signal 
to provide a third electrical signal of the same frequency as the second 
electrical signal but of opposite amplitude and sign. Means is provided 
for combining the third electrical signal with the first electrical signal 
to suppress any environmental noise present in the first electrical signal 
so that the recipient will receive a combined electrical signal which is 
substantially free of the environmental noise in the vicinity of the 
caller. 
More specifically, as shown in FIG. 1 of the drawings, there is provided a 
telephone system 11 in a caller location 12 in a noisy environment of a 
type hereinbefore described, as for example in an airplane, automobile and 
other locations and a recipient location 13 in a conventional home or 
office environment. The caller location 12 is provided with a telephone 
station 16 of a suitable type, as for example a car phone which typically 
includes a console 17 and a handset 18 connected to the console 18 by a 
cord 19. Similarly, the recipient is provided with a telephone station 21 
which consists of a console 22 of a conventional type that is provided 
with a handset 23 connected by a cord 24 to the console 22. The two 
stations 16 and 21 can be interconnected in a conventional manner such as 
by the use of a hard wire, radio frequency, microwave frequencies and 
fiber optics which are indicated by a microwave link 26. 
The handset 18 of the telephone station 16 which is located in the noisy 
environment is shown in more detail in FIG. 2 and consists of a housing 31 
which is provided with two enlarged end portions 31a and 31b facing 
generally in the same direction with an intermediate portion 31c which is 
adapted to be grasped by the human hand to hold the portion 31a to the 
users ear while the other portion 31b can be held in close proximity to 
the mouth of the user. The end portion 31b carries a first sound sensor 36 
in the form of a conventional electromechanical device such as a 
microphone which is utilized for converting voice signals to an electrical 
signal having a frequency and amplitude. As shown, the first voice sensor 
36 is positioned so that it is substantially directional, i.e. preferably 
receiving a sound in a direction which is perpendicular to the face of the 
first sound sensor 36. 
The handset 18 also includes a speaker 37 which is provided in the portion 
31a. It is also an electromechanical device which converts electrical 
signals to sound signals. As shown in FIG. 2, the speaker 37 faces 
generally in a direction which is perpendicular to the axis of the 
intermediate portion 31c whereas the portion 31a faces in a direction 
which is inclined slightly from the intermediate portion 31c so that it is 
adapted to be placed in front of the mouth of the user. The portions of 
the handset 18 thus far described are conventional. 
In accordance with the present invention, a second sound sensor 41 is 
provided in the handset 18 in the vicinity of the first voice sensor 36 
but facing generally in an opposite direction than that of the first sound 
sensor 36. The second sound sensor 41 is also an electromechanical device 
and can be in the form of a microphone for converting sound signals to an 
electrical signal. 
Also in accordance with the present invention, a third sound sensor 46 can 
be provided on the handset 18 and is located in the portion 31a and faces 
in the same direction as the speaker 37. Thus, as shown in FIG. 2, it can 
be centrally disposed within the confines of the speaker 37 so that it is 
in close proximity to the speaker 37 and is oriented so that it faces in a 
direction very similar to the speaker 37 for picking up any environmental 
noise which may be present in the region immediately surrounding the space 
forward of the speaker 37. 
Operation of the first and second sound sensors 41 and 46 in connection 
with the present invention in utilizing the handset 18 in a noisy location 
may now be briefly described as follows in conjunction with the block 
diagrams as shown in FIGS. 3 and 4. Let it be assumed that the caller is 
in an automobile and has a car telephone and wishes to make a call during 
very noisy conditions, as for example passing truck traffic on a busy 
freeway or alternatively on an airplane wishing to make a call on the 
telephone on the plane. The caller at location 12 picks up the handset 18 
and utilizes the same in a conventional manner to place a call to the 
recipient at the recipient location 13. As soon as the circuit is 
established between the caller 12 and the recipient 13, the circuitry 
which is shown in FIG. 3 is operable. Assuming that the caller is 
speaking, the caller's voice is picked up by the microphone 36 which 
converts the voice signal into an electrical signal having a frequency and 
amplitude corresponding to the voice signal. The electrical signal created 
is supplied through an amplifier 51 to a summer 52. At the same time that 
the caller's voice is being picked up by the first sound sensor 36 in the 
form of a microphone, the externally generated actual environmental noise 
surrounding the handset 18 is also being picked up by the second sound 
sensor or microphone 41. Since this sound sensor 41 is directed in a 
generally opposite direction of the microphone 36 it will not pick up any 
substantial sounds from the voice of the caller but will principally pick 
up the actual environmental sounds which are surrounding the handset 18. 
This actual environmental noise is converted to a second electrical signal 
having a frequency and amplitude or phase corresponding to the actual 
environmental noise and is supplied to an adaptive filter 56 of the type 
described in co-pending application, Ser. No. 08/206,464, filed Mar. 4, 
1994, which converts the second electrical signal into a third electrical 
signal of the same frequency but of opposite amplitude and sign or 
180.degree. out of phase of with the second electrical signal. The third 
electrical signal is supplied to the summer 52 to in effect suppress the 
environmental noise which may have been picked up by the first sound 
sensor 36 to provide a combined output which is supplied to the link 26 
connecting the caller to the recipient so that the recipient receives an 
electrical signal which carries the caller's voice but in which the 
ambient environmental noise present at the handset 18 of the caller is 
largely suppressed or minimized so that the recipient can hear the 
caller's voice with clarity. If the environmental noise has not been 
suppressed to a desired level, the error in the suppression is picked up 
and supplied by the feedback loop 57 to the filter 56 so that an 
appropriate correction can be made by the adaptive filter 56 to supply a 
corrected third electrical signal to the summer 52. This procedure 
continues until sufficient correction has been made to bring the combined 
signal provided on the link 26 is such so that the level of environmental 
noise present in the signal received by the recipient is below a desired 
threshold level. 
In order to prevent the adaptive algorithm which is utilized by the 
adaptive filter 56 from being operative both during the time a voice 
signal is being received from the first sound sensor 36 and external 
environmental noise is being received from the second sound sensor 41, a 
voice activity detector 61 is connected to the first and second sound 
sensors 36 and 41 and to the adaptive filter 56. The voice activity 
detector 61 can be of a type well known to those skilled in the art. It 
can be implemented in the same chip used for the adaptive filter 56 by 
incorporating computational algorithms that monitor the performance by 
making use of exponential or sliding window-based estimators. With the 
voice activity detector 61 in use, the adaptive filter 56 is continually 
operative utilizing the signal received from the external noise sound 
sensor 41 and supplies a signal utilizing the adaptive algorithm to 
provide the third electrical signal of the same frequency but of opposite 
amplitude and sign or 180.degree. out of phase to the summer 52. As soon 
as the caller speaks this is picked up by the microphone 36 and is sensed 
by the voice activity detector to freeze the adaptive algorithm being 
utilized by the adaptive filter 56 so the adaptive algorithm thereafter 
will not be changed to further affect the speech which is picked up by the 
first sound sensor 36. In this way the processing which is being performed 
by the adaptive filter 56 using the adaptive algorithm is frozen so that 
it does not change as long as voice is being picked up by the sound sensor 
36. As soon as there is a pause or there is no voice being picked up by 
the first sound source 36, the adaptive filter 56 is again permitted to 
vary the adaptive algorithm to correspond to the environmental noise being 
received on the second sound sensor 41 to provide an appropriate signal to 
suppress the environmental noise on the communication link 26. This 
freezing of the adaptive algorithm in the adaptive filter 56 during the 
time when speech is taking place does not appreciably affect the noise 
cancellation characteristics of the present invention because the 
quietness occurring during speech typically is not of long duration. Thus, 
in quiet periods during speech by the caller's voice, the adaptive filter 
56 is placed in operation, as for example during a few milliseconds or 
seconds to provide the necessary correction, if any to the adaptive 
algorithm being utilized by the adaptive filter. Thus it can be seen that 
appropriate noise suppression can be achieved without affecting the voice 
of the caller. 
The circuitry which is shown in FIG. 3 will not have any affect upon the 
incoming voice from the recipient and is supplied to the loud speaker 37 
in a conventional manner. To overcome ambient environmental noise, it is 
necessary for the user to press the portion 31a close to the ear to block 
out surrounding environmental noise and at the same time to cover the 
other ear with the other hand of the user so that the external 
environmental noises cannot be heard. 
However, if desired, additional means can be provided to provide a zone of 
quietness around the portion 31a so that the caller can hear even in the 
presence of environmental noises. Such circuitry is shown in FIG. 4 and as 
explained previously includes a third sound sensor 46 which is positioned 
in very close proximity to the speaker 37. The signal from the third sound 
sensor or error microphone 46 is supplied to another adaptive filter 66 
which also receives an input from the second sound source 41. The adaptive 
filter 66 utilizing this information from the sound sensors 41 and 46 and 
utilizing an adaptive algorithm in the manner described in co-pending 
application, Ser. No. 08/206,464, filed Mar. 4, 1994, supplies an output 
signal to a summer 71. The recipient's voice is received on the link 26 
and supplied through an amplifier 72 to the summer 71. The signal which 
carries the recipient's voice is summed with the external noise signal 
from the adaptive filter 66 and is supplied through an amplifier 73 to the 
speaker 37 to provide a sound signal at the output of the speaker 37 which 
carries the recipient's voice and a signal to suppress the external noise 
present in the vicinity of the speaker 37 so that there is provided a zone 
of quietness or noise suppression around the speaker 37 permitting the 
caller in the noisy environmental location to clearly hear the words being 
spoken by the recipient. If the noise has not been suppressed to the 
desired threshold level, this noise above this level is ascertained by the 
error microphone 46 and an error correction signal is supplied to the 
adaptive filter 66 so that the adaptive filter 66 through its adaptive 
algorithm provides a noise suppression signal to the summer 71 that is 
adequate to suppress the environmental noise at the output of the speaker 
37 to a level which is below a desired threshold level. 
Thus, with the circuitry shown in FIGS. 3 and 4 it can be seen that the 
caller in the noisy location, the voice of the caller can be clearly heard 
by the recipient and similarly the recipient's voice can be clearly heard 
by the caller even though the caller may be in a very noisy environment. 
However, for the caller in the noisy environment to clearly hear the 
recipient's voice free of the external environmental noise it will be 
necessary for the caller to close off his other ear with his other hand. 
In FIG. 5, there is shown a graph in which the ordinate is in volts and the 
abscissa is in time and in which the external noise shown in a waveform 76 
ranging from 0-100 millivolts is attenuated between 30-40 dB within one 
second. Such attenuation is more than adequate to bring the external noise 
down to an acceptable threshold level so that the recipient can clearly 
hear the caller and that the caller can clearly hear the recipient. 
In connection with the present invention it can be seen that the noise 
suppression is being carried out electronically and internally of the 
system. 
From the foregoing it can be seen that there has been provided an active 
electronic noise suppression system particularly useful for telephone 
handsets and a method which makes it possible for a caller at a noisy 
location to be clearly heard by the recipient and similarly for the caller 
to clearly hear the recipient. Although the present invention has been 
described principally with a single noisy location, it should be 
appreciated that the same principles can be utilized when both the caller 
and recipient are located in noisy locations. It is merely necessary that 
both locations be provided with the circuitry of the present invention. 
Also from the foregoing it can be seen that the present invention can be 
utilized in many different locations where environmental noise is a 
problem. Thus, in addition to being useful in car phones and air phones, 
it can be useful in connection with pay telephones at noisy locations.