GB2179822A - Two-way speech communication system - Google Patents

Abstract

A two-way communication system for communicating speech between stations provides a transmission channel (14) and a receiving channel (18) each including a howl detector (18) arranged to detect any signal in the channel indicating acoustic feedback. A variable gain device (20) is provided in one channel (18) and arranged to reduce the gain and avoid acoustic feedback in response to operation of both howl detectors (18) simultaneously. <IMAGE>

Description

SPECIFICATION Two-way speech communication system The invention relates to a two-way speech communication system. It is particularly applicable to systems employing telephones together with loudspeaking devices including for example hands-free telephone systems.

Speech communication systems for permitting two-way communication between remote stations commonly involve telephones. One or both of the stations may incorporate a loudspeaking device so that the message received at that station can be heard without holding an earpiece close to the operator's ear. Such loudspeaking devices are particularly applicable to telephone systems known as hands-free systems which do not involve the operator in holding a microphone or earpiece at the time of sending or receiving a message. A common difficulty with such systems is caused by acoustic feedback where sound from the loudspeaking device is picked up by a microphone or similar audio/electric transducer. The acoustic gain can become greater than unity causing acoustic feedback and a condition known as howlround.

It is an object of the present invention to provide a two-way communication system for communicating speech between stations in which the problem of howlround is reduced.

The present invention provides a two-way communication system for communicating speech between two stations, at least one of the stations having a transmission channel for transmitting signals to the other station, the transmission channel having an audio electric transducer for generating electrical signals representing speech which is input at said station, and a receiving channel for receiving signals from the other station, the receiving channel including a further transducer for generating audible signals from electrical signals received by the station, each of said channels including howl detecting means arranged to detect in the respective channel an electrical signal indicating acoustic feedback, together with variable gain means in one of the channels and control means responsive to each of the howl detecting means to reduce the gain of the variable gain means when the howl detecting means detect signals indicating acoustic feedback in both channels.

Preferably said control means includes gate means operable to reduce the gain only when howl is detected in both channels.

Preferably the variable gain means is located in said receiving channel. In the case of a hands-free telephone system, the reduction in gain necessary in the receiving channel to avoid howl need not necessarily cause a significant reduction in the audible level of the output of the loudspeaker. If however the reduction in gain is applied to the transmission channel, it may present difficulties in picking up speech by a microphone where the speaker is not particularly close to the microphone and may for example be in a noisy environment such as a motor car.

Preferably the control means includes holding means to maintain the gain at a reduced level once howl has been detected in both channels together with means to reset the holding means independently of the output of the howl detecting means. In this way the reduction in gain which has been necessary to avoid howlround is maintained even though the howlround condition has been removed by the reduction in gain. This reduces the likelihood of the howlround being resumed during that message. The holding means may be reset by effecting termination of the message transmission.

Preferably the control means and variable gain means are arranged to effect successive reductions in gain if after a first reduction in gain to avoid acoustic feedback howl is again detected in both channels. In this way the gain may be successively reduced if howlround reoccurs after a previous reduction in gain.

Conveniently the audio electric transducer is a microphone. The said further transducer may be a loudspeaker.

The invention is particularly applicable to a communication system in which the or each station includes a telephone having a loudspeaker. The telephone may be a hands-free telephone.

Preferably each howl detecting means comprises electrical circuit means arranged to respond to the peak to average amplitude ratio of the signal in the channel.

Preferably the electrical circuit means is arranged to provide a control signal when the peak to average amplitude ratio of the signal in the channel. Preferably the electrical circuit means is arranged to provide a control signal when the peak to average amplitude ratio is less than 2:1.

Conveniently each howl detecting means includes means for generating a waveform having a mark to space ratio dependent on the peak to average amplitude ratio of the audio signals in the channel. The howl detecting means may include integrating means for integrating the said waveform to provide a signal indicating a howl condition in the channel.

Preferably the integrating means of each howl detecting means are connected to a common gating circuit arranged to provide a control signal to a variable gain amplifier.

The howl detecting means may examine other characteristics of the signal in the channel in order to detect acoustic feedback. For example each howl detecting means may be arranged to detect the occurrence of a single sinusoidal waveform indicating tone generated by acoustic feedback.

The invention also provides a method of preventing a howlround condition in speech communication between two remote stations at least one of which has a transmission channel for generating electrical signals representing speech input at the station and transmitting signals to the other station, and a receiving channel for receiving signals from the other station and generating audible output representing the electrical signals received by the station, which method comprises examining electrical signals in both the transmitting and receiving channels of the station to detect signal forms representing acoustic feedback and reducing signal gain in one of the channels when acoustic feedback is detected in both said channels.

One embodiment of the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows schematically an embodiment of the invention comprising a two-way speech communication system between two stations interconnected by a telephone line, Figure 2 illustrates in more detail the construction of a transmission channel and a receiving channel at one station of the arrangement of Fig. 1, Figures 3A and 3B illustrate signal waveforms which may be found in the system, and Figures 4A, 4B and 4C illustrate waveforms encountered in the howl detecting circuit used in Fig. 2.

This embodiment comprises a communication network for permitting two-way speech communication between remote stations 11 and 12 interconnected via a telephone line 13.

Each station has a transmission channel 14 for transmitting along the telephone line 13 speech which is input at the station, and a receiving channel 15 for providing an audible output for speech which is received at the station via the telephone line 13. In this particular example each station has a hands-free telephone which includes a microphone 16 and a loudspeaker 17 which may be located at a distance from the mouth and ears of the person communicating through the station so that the equipment can be used without the telephone equipment being held in the hand.

The telephone equipment at each of the stations is identical and only one will be described in detail.

Each of the transmission channel 14 and receiving channel 15 includes a howl detector circuit 18 and the detector in the transmission channel 14 is arranged to provide a signal on line 19 to the receiving channel 15. The receiving channel 15 incorporates a variable gain device 20 for reducing the gain of the signal fed to the loudspeaker 17 when both the howl detector circuits 18 in each of the transmission channel and receiving channel indicate a howl condition. The howl condition is caused by acoustic feedback from the loudspeaker 17 into the microphone 16 causing an acoustic gain of greater than unity.

The howl detector 18 in the transmission channel 14 is arranged to examine the audio signal received from the microphone 16 and the howl detector in the receiving channel 15 is arranged to examine the audio signal which is fed to the loudspeaker 17. The difference between a speech signal and a single sinusoidal tone signal generated by howlround can be seen by reference to Figs. 3A and 3B.

Whereas Fig. 3B indicates the type of waveform generated by a speech signal, Fig. 3A indicates the sinusoidal form generated by tone due to howlround or acoustic feedback.

As can be seen from Figs. 3A and 3B, the peak to average amplitude ratio of a signal during speech is of the order of 8:1 while in a howlround condition the tone represented by a single sinusoidal wave has a much lower peak to average amplitude ratio and this may well reduce to less than 2:1.

In accordance with the present example, each howl detector is arranged to sense the peak to average amplitude ratio and when a howlround condition is detected in the transmission channel and simultaneously in the receiving channel, the variable gain device 20 in the receiving channel is operated to lower the gain until the howlround condition disappears.

Fig. 2 shows more detail of the transmission channel 14 and receiving channel 18 of one station. The microphone 16 generates an electrical audio signal which is fed through a microphone amplifier 22, a band pass filter 23 and a line sending amplifier 24 to the telephone line 13. A signal on line 25 is taken to a howl detecting circuit 18 which consists of an envelope detector 26, a pulse shaper 27, and an integrator 28.

The receiving channel 15 receives signals from the telephone line 13 which are fed through a buffer 30 to a variable gain amplifier 20 which is a voltage controlled amplifier so that the gain is dependent on the voltage applied on line 31 to the amplifier. The amplified signal is then fed on line 32 through a power amplifier 33 to the loudspeaker 17. The nature of the signal on line 32 is examined by a howl detector 18 which is connected via line 34 to the line 32. The detector 18 is of the same construction in both the transmission and receiving channels and so will be described in detail with reference to the transmission channel only.

The signal which is input on line 25 to the howl detector (and similarly the signal which is input on line 34 in the receiving channel) will normally have a waveform of the general type shown in Fig. 3B but in the event of howl condition being established, a sinusoidal waveform of the type shown in Fig. 3A may occur. The envelope detector 26 consists of a rectifier 37 whose output is held at a fixed potential due to a resistor 38 connected between the output of the rectifier and a line 39 which provides a negative potential set at ap proximately -0.6 volts. The effect of the envelope detector can be seen by reference to Fig. 4. The envelope detector cuts out the signal indicated below the line 40 shown in Fig. 4 and permits transmission of the signal above the line.The pulse shaping circuit 13 consists of two successive NAND gates 41 and 42 which convert the waveform into rectangular waves of the type shown in Fig. 4B in which the mark to space ratio is a representation of the peak to average amplitude ratio of the audio signal. The rectangular wave is then fed to the integrator circuit 28 which consists of a resistor 43 connected in series with a rectifier 44. A capacitor 45 and parallel connected resistor 46 are connected to the output of the rectifier 44. This transforms the rectangular wave into the integrated curve of the type shown in Fig. 4C. This is used as one input 47 to a further NAND gate 48. The other input to the NAND gate is derived from line 49 which forms an output of the howl detector 18 in the receiving channel.When the peak to amplitude ratio of the signal in either of the channels falls to a level indicating the presence of a sinusoidal waveform representing a howlround condition, the signal level on the associated line 47 or 49 will reach a level indicating howlround. When this occurs simultaneously in both channels, both input lies 47 and 49 operate to cause the NAND gate 48 to provide an output to reduce the gain in the receiving channel. The output of the gate 48 is fed through a rectifier 50 and resistor 51 to a sample and hold circuit 52.

This consists of two transistors 53 and 54 in an emitter follower arrangement together with a capacitor 55. Once howlround has been dectected in both channels, the output of the transistor 54 is held at a voltage level which is fed on line 31 to the variable gain amplifier 20 so as to reduce the gain of the audio signal fed to the loudspeaker 17. The voltage supplied to the variable gain amplifier 20 is held at a level corresponding to that reduced gain even through the howlround condition is caused to disappear as a result of the reduction in gain. Should the circumstances change such that howlround is reestablished and redetected in both channels then a further voltage reduction will be caused on line 31 causing the amplifier 20 to cause yet a further successive reduction in the gain of the audio signal fed to the loudspeaker 17.This may happen successively should howlround be reestablished after a previous reduction in amplifier gain. The voltage level output by the sample and circuit 52 is not reset until the message has been terminated. Once the signal levels have been removed from lines 24 and 34, the transistor 54 resets to a level corresponding to the normally required gain of the amplifier 20.

The rectifier 50 and resistor 51 act as an error filter which removes short-term spurious pulses which may be output from the NAND gate 48.

It will therefore be seen that the above example provides a hands-free telephone system in which the transmission and receiving channel both examine the waveform to detect whether any acoustic feedback is occurring and when, but only when, it is detected in both channels, the gain in the signal supplied to the loudspeaker is reduced to remove the acoustic feedback. By testing both channels for the existence of any signal corresponding to acoustic feedback the risk of causing an erroneous reduction in gain is reduced. It is possible that the transmission channel or receiving channel may intentionally carry a signal which has a high sinusoidal content such as for example a whistle sound. This may cause the howl detector in one channel to generate a signal requiring a reduction in gain but unless this is detected in both channels no gain alteration will be effected. It is unlikely that a howl type signal will be detected in both channels simultaneously unless acoustic feedback is occurring. Furthermore the system described above permits both channels at each station to remain open all the time that a message is occurring. In other words the detection of any howl condition does not cause temporary closing of either channel. In this way it is possible to avoid any clipping of syllables due to closing of either channel should howlround be detected.

The invention is not limited to the details of the foregoing example.

Claims (18)

1. A two-way communication system for communicating speech between two stations, at least one of the stations having: a transmission channel for transmitting sig nals to the other station, the transmission channel having an audio electric transducer for generating electrical signals representing speech which is input at said station, and a receiving channel for receiving signals from the other station, the receiving channel including a further transducer for generating audible signals from electrical signals re ceived by the station, each of said channels including howl detect ing means arranged to detect in the respec tive channel an electrical signal indicating acoustic feedback, together with variable gain means in one of the channels and con trol means responsive to each of the howl detecting means to reduce the gain of the variable gain means when the howl detect ing means detect signals indicating acoustic feedback in both channels.

2. A two-way communication system ac cording to claim f in which the said control means includes gate means operable to reduce the gain only when howl is detected in both channels.

3. A two-way communication system according to claim 1 or claim 2 in which the variable gain means is located in said receiving channel.

4. A communication system according to any one of the preceding claims in which the control means includes holding means to maintain the gain at a reduced level once howl has been detected in both channels together with means to reset the holding means independently of the output of the howl detecting means.

5. A communication system according to claim 4 in which the control means and variable gain means are arranged to effect successive reductions in gain if after a first reduction in gain to avoid acoustic feedback howl is again detected in both channels.

6. A communication system according to any one of the preceding claims in which the audio electric transducer is a microphone.

7. A communication system according to any one of the preceding claims in which the said further transducer is a loudspeaker.

8. A communication system according to any one of the preceding claims in which the or each station includes a telephone having a loudspeaker.

9. A communication system according to claim 8 in which the or each telephone is a hands-free telephone.

10. A communication system according to any one the preceding claims in which each howl detecting means comprises electrical circuit means arranged to respond to the peak to average amplitude ratio of the signal in the channel.

11. A communication system according to claim 10 in which the electrical circuit means is arranged to provide a control signal when the peak to average amplitude ratio of the signal in the channel.

12. A communication system according to claim 11 in which the electrical circuit means is arranged to provide a control signal when the peak to average amplitude ratio is less than 2:1.

13. A communication system according to claim 11 or claim 12 in which each howl detecting means includes means for generating a waveform having a mark to space ratio dependent on the peak to average amplitude ratio of the audio signals in the channel.

14. A communication system according to claim 13 in which the howl detecting means includes integrating means for integrating the said waveform to provide a signal indicating a howl condition in the channel.

15. A communication system according to claim 14 in which the integrating means of each howl detecting means are connected to a common gating circuit arranged to provide a control signal to a variable gain amplifier.

16. A communication system according to any one of the preceding claims in which each howl detecting means is arranged to detect the occurrence of a single sinusoidal waveform indicating tone generated by acoustic feedback.

17. A method of preventing a howlround condition in speech communication between two remote stations at least one of which has a transmission channel for generating electrical signals representing speech input at the station and transmitting signals to the other station, and a receiving channel for receiving signals from the other station and generating audible output representing the electrical signals received by the station, which method comprises examining electrical signals in both the transmitting and receiving channels of the station to detect signal forms representing acoustic feedback and reducing signal gain in one of the channels when acoustic feedback is detected in both said channels.

18. A two-way speech communication system substantially as hereinbefore described with reference to the accompanying drawings.