Telephonic communications – Substation or terminal circuitry
Reexamination Certificate
1998-06-09
2001-10-16
Kuntz, Curtis (Department: 2643)
Telephonic communications
Substation or terminal circuitry
C379S392000, C379S395000
Reexamination Certificate
active
06304654
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to the field of telephone set features and apparatus, and specifically those directed at reducing or eliminating handset feedback.
2. Description of the Related Art
It is not uncommon for currently available telephone sets to be equipped with volume controls for increasing or decreasing the amount of gain applied to the receiver circuit of the telephone handset. Such controls are added to accommodate the hearing impaired, to overcome excessive background noise, as well as to compensate for variances in transmission quality from one telephone call to the next. They are generally limited to adding 12 db or less of gain, so as to minimize the chance that the signal fed to the handset receiver will be introduced to the handset transmitter, resulting in the creation of a loud and unpleasant tone being sent through the receiver. This tone, caused by a feedback loop established between the receiver and transmitter of the handset, is commonly referred to as singing, but has also been referred to as howling, squealing, screeching, or simply feedback. However, even at the presently practiced limit of 12 db of receiver gain the potential for singing exists, depending on the impedance of the telephone line and the acoustic environment surrounding the handset.
As is well known in the art, the feedback loop which causes singing is created in part by acoustic coupling between the handset receiver in the handset earpiece and the handset microphone in the handset mouthpiece. The feedback loop is completed by electrical coupling between the transmit and receive circuits in the telephone hybrid.
As is well known in the art, telephone signals to and from a telephone are carried through two wires, long known in the telephony field as tip and ring. Within the telephone set these two wires connect to the telephone hybrid. The hybrid splits the tip and ring into a four wire circuit, two wires of which pass the far end caller's signal from the hybrid to the handset receiver as a receive signal. The remaining two wires carry a transmit signal from the handset transmitter to the hybrid for transmission over the tip and ring to the far end caller.
A perfect hybrid would result in no signal component leakage between the transmit and receive circuits of the hybrid, i.e., perfect sidetone cancellation or infinite trans-hybrid loss. In practice, trans-hybrid loss is never infinite and, instead, depends on how close the hybrid impedance network matches the actual line impedance of the phone line to which the telephone apparatus or set is connected. Since, in general, telephone line impedance is complex and is not known a priori by the telephone set, trans-hybrid loss is usually only about 6 db to 10 db. This means that a reduced amplitude version of the transmit signal will always be present in the receive signal. If the sum of the acoustic coupling, electrical coupling through the hybrid, and circuit gains exceed unity, the telephone will sing or howl.
For example, if the line impedance of the telephone line to which the phone is connected represents a significant impedance mismatch for the hybrid, and the handset is placed on a hard, acoustically reflective surface such as a smooth desk or countertop, the gain in this loop may exceed unity at some frequencies, causing singing to occur.
Moreover, this problem cannot be overcome by simply reducing the gain of the transmit signal when the receive signal gain is increased, which would merely reduce the volume of the speech heard at the far end by the other caller.
The present invention is directed at overcoming the aforementioned shortcomings found in presently known telephone sets.
SUMMARY OF THE INVENTION
The invention provides a handset interface apparatus for placement in a telephone set between the telephone hybrid and the telephone handset.
The inventive handset interface is preferably constructed as an electronic circuit controlled by a microprocessor. The handset interface contains a volume control circuit which allows the telephone user to vary up or down the amount of gain applied to the receive and transmit signals passing between the handset and the hybrid, thus affecting the amplitude of the signals which ultimately pass through tip and ring between the telephone user and a caller on the far end of the call.
The volume control is by way of example selectably adjustable by the user to one of eight volume set points. Each set point represents a particular amount of gain to be applied to the transmit and receive signals. All gains are gains relative to the nominal handset level, i.e. to no gain on the transmit or receive signal. The gain amounts for each volume set point are configured such that the overall gain introduced into the handset/hybrid loop preferably do not exceeds 6.6 db.
However, the application of negative amounts of transmit gain during the entire conversation would hamper the overall call quality, since the transmit signal would be attenuated while the telephone user is speaking. This would make the user's voice sound unnaturally low to the far end caller. To overcome this potential problem, a further feature is included in the handset interface. Specifically, the handset interface detects the presence of the transmit signal, and measures its amplitude at a point in the interface circuitry before the gain reduction is applied. If the pre-gain transmit signal amplitude exceeds a predetermined threshold value, and the user has selected any one of volume set points four through seven, the volume set point is changed by the handset interface to set point three, a set point which represents 0 db of gain being applied to the transmit signal. Thus, if the telephone user selects high volume settings, the handset interface automatically removes any transmit signal gain reduction while the user is speaking. The set point is then reset to the user selected set point when the user stops speaking.
For improved performance, the transmit signal may be filtered prior to its comparison with the threshold values to prevent self-switching of the handset interface, in the absence of user speech, as a result of acoustic coupling of high frequency components of the sound emanating from the handset receiver.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.
REFERENCES:
patent: 4715063 (1987-12-01), Haddad et al.
patent: 4773088 (1988-09-01), Matheny
patent: 5170430 (1992-12-01), Schuh
patent: 5867573 (1999-02-01), Wittman
Eng George
Kuntz Curtis
Lucent Technologies - Inc.
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