Telephonic communications – Substation or terminal circuitry – For loudspeaking terminal
Patent
1994-03-23
1995-11-28
Hofsass, Jeffery A.
Telephonic communications
Substation or terminal circuitry
For loudspeaking terminal
379389, 379388, H04M 160
Patent
active
054715288
DESCRIPTION:
BRIEF SUMMARY
This invention relates to telephones, and more particularly to a handsfree circuit for use in a telephone set.
BACKGROUND OF THE INVENTION
A conventional telephone hand set operates in the full duplex mode, which means that the parties can transmit and receive at the same time. This is possible because a conventional hand set is designed to have minimal acoustic coupling between the speaker and microphone. A handsfree system, which allows the user to operate the telephone without a hand set, has difficultly operating in the full duplex mode because of the high degree of acoustic coupling between the microphone and loud speaker. Most handsfree systems are therefore operated in the half duplex mode, which means that some form of switching must be provided in the transmit and receive paths.
In a conventional handsfree circuit, variable complementary loss attenuators are inserted in the transmit and receive paths. These are controlled in accordance with the audio signals in the two paths. Level detectors determine the overall noise level in the respective paths and speech detectors distinguish between background noise, which is assumed to be a generally constant level, and speech, which is generally characterized by short bursts of higher level audio. One example of such a system is described in U.S. Pat. No. 4,490,582.
Prior art designs have generally required filters in the transmit and/or receive audio paths to band limit the speech path to the 400 to 3500 Hz region. These filters are required to reduce background noise other heavy machinery, but they create a "hollow" effect that degrades the quality of the transmitted sound.
A further problem with the prior art circuits stems from the fact that the speech detectors work by detecting short bursts of sound. Consequently, many prior art circuits will respond to intermittent background noise, such as typing, causing the attenuators to ramp up to the full transmit state when no speech is present.
Yet another problem with the prior art design is the frequent presence of echo signals due to trans-hybrid reflection or reverberation in the room. Peak detector decay rate is made long so as to prevent reverberation echo from causing the handsfree to switch states.
An object of the present invention is to alleviate the aforementioned problems with the prior art.
SUMMARY OF THE INVENTION
According to the present invention there is provided a handsfree telephone circuit with separate transmit and receive audio paths for connection to a microphone and speaker respectively, comprising a controllable attenuator in each of said transmit and receive paths; attenuator control means responsive to control signals to set the attenuation level of said attenuators; a separate monitoring circuit defining respective monitoring paths connected to each of said transmit and receive paths for sensing audio signals therein, each said monitoring circuit comprising a filter in the associated monitoring path for band limiting said audio signals, a rectifier, a peak level detector receiving said band limited signals, and a speech detector for distinguishing speech from background noise; and a switching comparator for comparing the outputs of the peak level detectors in the two monitoring circuits and producing directional control signals for said attenuator control means to determine which audio path has control thereof.
Preferably, a fast attack filter with a slow decay rate is provided in each of the input paths of the switching comparator. The filter in the non-controlling path can be bypassed so that the tendency of the switching comparator to switch on echo signals in the non-controlling path is reduced because the envelope from the filter in the controlling path extends beyond the echo signals in the non-controlling path.
The filter in the audio paths is preferably a second order biquad filter. The general 2.sup.nd order Z-domain transfer function is as follows: Z.sup.-1 -B.sub.2 Z.sup.-2) define the filter function.
The preferred values of G, A, A.sub.1, A.sub.2, B.sub.1, B.sub
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Hofsass Jeffery A.
Mitel Corporation
Weaver Scott L.
LandOfFree
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