Speech processor using controlled center clipping

Abstract

An echo suppressor for full duplex telephone circuits consists of a filter bank of contiguous band filters in the transmission path, with each filter output feeding a separate center clipper. Each clipping level is controlled by echo signal amplitude as attenuated by the trans-hybrid loss that occurs in the corresponding subband. The scheme permits "double-talking" while suppressing the echo component.

What is claimed is: 
     
       1. For a voice communications system containing imperfectly isolated transmit and receive paths, apparatus for reducing echo caused by leakage therebetween, comprising: means for dividing the transmit path signal into plural contiguous subbands,   means for producing, from an incoming voice signal in the receive path, a replica of the echo of said voice signal occurring in each of said transmit path subbands,   means for continuously detecting the peak level of each said echo replica,   means for center-clipping the signal in each transmit path subband a varying amount in response to the concurrent said peak level value, sufficient to .[.just.]. .Iadd.substantially .Iaddend.overcome said echo in the respective subband, and   means for combining the outputs of each center-clipping means for transmission.   
     
     
       2. Apparatus pursuant to claim 1, wherein said dividing means comprises a first set of contiguous band filters; and wherein said echo replica-producing means comprises a second set of contiguous band filters spanning the same subbands as said first set and being connected across said receive path, an attenuator connected to the output of each filter in said second set, and means for setting each attenuation level to obtain an output from each said attenuator having an amplitude comparable to said echo signal occurring in the corresponding transmit path subband. 
     
     
       3. Apparatus pursuant to claim 2, wherein said attenuation levels are fixed.  .[.4. Apparatus pursuant to claim 2, further comprising means for varying said attenuation levels, comprising: a white noise generator connected to said receive path,   means responsive to a selected initiating circuit condition for energizing said generator,   means for sensing the resulting echo signal level exiting from said center-clippers in each of said trasmit path subbands, and   means responsive to each said sensed level for adjusting the corresponding attenuator until no echo signal is sensed, and thereafter, for   
     
     
        deenergizing said generator..]. 5. Apparatus pursuant to claim 2, wherein said center-clipping means .[.includes means for returning.]. .Iadd.returns .Iaddend.said clipping levels to zero a prescribed time 
     
     
        after disappearance of an echo signal in that subband. 6. Apparatus pursuant to claim 2, wherein said center-clipping means .[.includes means for returning.]. .Iadd.returns .Iaddend.said clipping levels to a selected minimum value. .Iadd. 7. For a voice communications system containing imperfectly isolated transmit and receive paths, apparatus for reducing echo caused by coupling therebetween, comprising: means for producing, from an incoming signal in the receive path, a replica of the echo of said signal occurring in said transmit path,   means for continuously detecting the peak level of said echo replica,   means for center-clipping the signal in said transmit path a varying amount in response to the concurrent said peak level value, sufficient to substantially overcome said echo, and   means for applying the output of said center-clipping means to said   
     
     
        transmit path. .Iaddend..Iadd. 8.  Apparatus pursuant to claim 7, wherein said echo replica-producing means comprises an attenuator connected to said receive path and means for setting attenuation level to obtain an output from said attenuator having an amplitude comparable to said echo signal occurring in said transmit path. .Iaddend..Iadd. 9. Apparatus pursuant to claim 8, wherein said attenuation level is fixed. .Iaddend..Iadd. 10. Apparatus pursuant to claim 8, wherein said center-clipping means returns said clipping levels to zero a prescribed time after disappearance of said echo signal. .Iaddend..Iadd. 11. Apparatus pursuant to claim 8, wherein said center-clipping means returns said clipping levels to a selected minimum value. .Iaddend. .Iadd. 12. For a voice communications system containing a hands-free telephone station and imperfectly isolated transmit and receive paths, apparatus for reducing echo caused by coupling therebetween, comprising: means for producing, from an incoming signal in the receive path, a replica of the echo of said signal occurring in said transmit path,   means for continuously detecting the peak level of said echo replica,   means for center-clipping the signal in said transmit path a varying amount in response to the concurrent said peak level value, sufficient to overcome said echo, and   means for applying the output of said center-clipping means to said transmit path. .Iaddend. .Iadd. 13. Apparatus pursuant to claim 2, further comprising means for varying said attenuation levels, comprising:   a white noise generator for applying to said receive path a signal having substantial spectral components in the subbands of said contiguous band filters;   said white noise being received and filtered by said second set of contiguous band filters;   a monitor;   a connection between the output of each center clipper and said monitor; and   a corresponding number of connections between said monitor and each said variable attenuator;   signals received by said monitor from said center clippers altering the signal peaks received by said peak detectors and thereby in turn altering the clipping levels in said center clippers. .Iaddend.