GB2137055A - Voice Communication System - Google Patents

Abstract

A voice communication system is provided with a voice operated switch 7 which is rendered conductive when a voice signal originating at a microphone 1 or the mouth piece of a telephone is detected by a detector 9. A delay line 8 having a delay of about 5 mS is positioned ahead of the switch 7, to prevent first syllable clipping of a voice signal. This greatly improves the intelligibility of a message but does not introduce an unacceptably long delay into a two-way conversation or disturb the person speaking who can hear the delayed version of his own voice. <IMAGE>

Description

SPECIFICATION Voice Communication System This invention relates to voice communication systems and is particularly concerned with such systems which include a voice operated switch.

Switches of this kind are often included in systems in which voice signals originate from microphones located in a noisy environment, or which include automatic volume controls. In such systems it is desirable to render the voice communication channel inoperative until a person begins to speak so that a listener is not subjected to excessive noise. One such voice communication system which includes a voice operated switch is described in our previous U.K.

patent 1506510. This invention seeks to provide an improved voice communication system.

According to this invention a voice communication system includes means for detecting the presence of a voice signal on a signal path; means for utilising the detection of a voice signal to render conductive a switch in said path; and delay means connected in said signal path so as to delay said signal for a predetermined period of between 3 mS and 1 5 mS before it is applied to said switch.

The length of the delay period is particularly critical if the signal path is to be used as part of a system carrying a two way conversation. The delay must not be so short as to result in clipping of the initial syllable of a voice sound as this could result in impaired intelligibility. Many previously known voice communication systems suffer from this disadvantage, as no intended delay is provided. The-patent referred to above falls into this category. On the other hand, it is not possible to provide a sufficiently long delay which is of the order of the duration of a syllable, as this would result in a su'biecively-unacceptabJy long delay in a two way conversation or where the speaker himself can hear the delayed signal.Very long dela.ys are acceptable for. certain applications, for example in those cases where an available signal path is seized by the first voice signal which appears on the line. In such a case the voice detector operates to allocate the signal path and to prevent signals originating at a second source from interferring with the existing voice signals. In this case the length of delay is almost immaterial and relatively complex signal processing can be employed, such as is described in the previous patent application'number G.B. 2061 576.:A. The delay of about one second referred in thqt document would be quite unacceptable for a twq way conversation or if the speaker can hear the delayed version.

Particular-problems arise when a person speaks into a telephone or microphone, and the resulting voice signal fed through a voice operated switch onto a signal highway whidh is coupled to his dwri headphones or the ear piece of a telephon.e. It has been found that under such circumstances the provision of a signal delay which is apparent to persons using the system is subjectively unacceptable. However the distortion or impaired intelligibility which is present if no delay of any kind is provided, can be removed to a remarkable extent by providing a delay line having a relatively short delay which is much less than the duration of a syllable.A delay within the range 3 mS to 1 5 mS is quite acceptable although it is preferred to make the delay of the order of 5 mS.

The invention is further described by way of example with reference to the accompanying drawing which illustrates a voice communication system.

Referring to the drawing, the voice communication system illustrated is particularly suitable for use on board an aircraft in which the pilot uses a microphone 1 or telephone so that his voice can be heard via a number of separate electro-acoustic speakers 2, 3, and 4 which typically are the ear pieces of telephones, connected to a signal highway 5. At the same time he must be able to hear any other conversation originating elsewhere in the aircraft and which are fed onto the signal highway 5, e.g.

by the mouth pieces of the telephones associated with the speakers 2, 3 and 4. He is able to monitor voice signals present on the highway 5 by means of headphones 6, or by the ear piece of his telephone, as the case may be.

Because the environment within an aircraft cockpit is likely to be very noisy indeed, it is desirable to close the signal path whenever possible by means of a switch 7 so that the crew who are able to use the speakers 2, 3 and 4 are not subjected to unnecessary and excessive noise.

This difficulty is enhanced in a voice communication system having an automatic volume control, as in the absence of voice signals, noise on the line is greatly amplified. The signal generated at the microphone 1 is fed via a delay line 8 to the switch 7 and thence onto the highway 5. The output of the microphone 1 (and any associated amplifiers, which are not separately shown) is monitored by means of a voice detector 9. The voice detector is one which operates relatively rapidly, and it may conveniently take the form illustrated in our U.K.

patent 1506510.

When a voice signal is detected by the detector 9, the normally open switch 7 is operated so as to be conductive, thereby allowing the voice signal onto the highway 5. Because the voice detector takes a finite time ta operate, even though this time is quite short, the first syllable of the voice signal would be distorted were it not for the presence of the delay line 8 and syllable clipping of this kind results in impaired intelligibility. The duration of the delay provided by the delay line 8 is very critical, because any significant and discernible delay:occuring between the generation of signal at the microphone 1 and the time at which it -is heard via the headphone 6 would be extremely disconserting for the person speaking and canbt-be permitted.

Conveniently, the delay sne 8 is a multiple bit shift register in the form of an analogue device having a cascaded sequence of charge cells. Such a device is well known and is sometimes called a bucket brigade delay line.

It is found that a short delay of about five milliseconds is quite sufficient to permit operation of the switch 7 without causing unacceptable distortion or loss of intelligibility, and at the same time the delay is so short as to be almost unnoticeable by the speaker. Thus the delay is made approximately equal to the turn-on time of the switch, and the processing circuits which control it. Even though the delay of 5 mS is very short as compared with the typical duration of a syllable, which typically can be up to 60 mS, it is found that it contributes very significantly to the intelligibility of a voice message.

Claims (5)

1. A voice communication system including means for detecting the presence of a voice signal on a signal path; means for utilising the detection of a voice signal to render conductive a switch in said path; and delay means connected in said signal path so as to delay said signal for a predetermined period of between 3 mS and 1 5 mS before it is applied to said switch.

2. A system as claimed in claim 1 and wherein said delay is of the order of 5 mS.

3. A system as claimed in claim 1 and 2 and wherein means are provided for feeding the signals which are allowed to pass the switch, onto a signal highway having a plurality of electroacoustic speakers connected to it.

4. A system as claimed in claim 3 and wherein at least one of said speakers is audible to a person whose voice constitutes said voice signal.

5. A voice communication system substantially as illustrated in and described with reference to the accompanying drawing.