GB2237704A

GB2237704A - Transceiver with noise immune voice operated transmission/reception switch

Info

Publication number: GB2237704A
Application number: GB8922852A
Authority: GB
Inventors: Colin Alexander Wilson
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1989-10-11
Filing date: 1989-10-11
Publication date: 1991-05-08
Anticipated expiration: 2009-10-11
Also published as: GB2237704B; GB8922852D0

Abstract

A mobile transmitter receiver intended for simplex radio communication via a base station with another mobile transmitter receiver, the mobiles being intended for use in vehicles, has a microphone 1 connected to envelope detectors 3 and 4. The latter receive the same signal A, but detector 3 has faster time constants than detector 4, so that the envelope B produced as output is representative not only of speech signals but also of noise signals in the vehicle. The envelope detector 4 is slower acting and does not respond to normal speech signals and consequently only represents the slower varying vehicle noise. When these signals are subtracted from each other an envelope D representative substantially only of speech is produced. This output can be used to trigger the mobile into transmit mode, and permits hands-free operation in a vehicle environment. <IMAGE>

Description

Radio Communication Apparatus This invention relates to radio communication apparatus, and especially to a mobile transmitter-receiver for simplex radio communication with another transmitter-receiver.

Mobile transmitter-receivers are typically located in vehicles which need to communicate with each other and/or with a fixed controller transmitter-receiver. The actual messages are usually relayed via a fixed base station with which all the mobile transmitter-receivers and any fixed transmitter-receiver are in communication.

Simplex communication means that the same communication channel is used by a mobile alternately for transmission and reception, each exchange customarily being terminated by the word 'over'. Each mobile is provided with a switch, and the user presses the switch when he desires to transmit, and releases it to receive.

It would be desirable if the transfer to the transmitting mode could be voice-triggered because this would then provide for desirable hands-free operation, but the problem is that vehicle noise can be considerable, and it is possible that the mobile could be triggered into the transmit mode by the vehicle noise in the middle of a receive period.

The invention provides a mobile transmitter-receiver for simplex radio communication with another mobile transmitter-receiver, comprising means for switching from a receive mode to a transmit mode in response to a signal derived from the rate of change of loudness of the sound level in the vicinity of the mobile.

The invention provides a mobile transmitter-receiver for simplex radio communication with another mobile transmitter-receiver, comprising means for switching from a receive mode to a transmit mode in response to a signal derived from a sound input reaching a pre-determined level, and means for producing a signal representing the difference between two signals derived from the same sound input, one of which is responsive to faster sound variation than the other.

The use of a signal representing the difference between a signal responsive to faster sound variations and one responsive only to slower sound variations enables the effect of (slower-varying) vehicle noise to be reduced when it is desired to trigger the transmitter-receiver to the transmit mode by (faster-varying) voice signals.

Amplitude envelope detectors may be used to produce the signals derived from the same sound source but responsive to faster, and slower loudness variations, respectively.

Radio communication apparatus in the form of mobile transmitter-receivers arranged for simplex operation constructed in accordance with the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a typical voltage signal at point A, together with a typical voltage signal at point B; Figure 2 shows a typical voltage signal at point C; Figure 3 shows a typical voltage signal at point D; Figure 4 shows in block form a part of the transmitter-receiver circuit; and Figure 5 shows in simplified form one form which the envelope detectors could take.

The radio communication apparatus consists of a base station and a number of mobile transmitter-receivers which communicate with each other via the base station.

The apparatus may operate in Radio Band III.

Communication between the mobiles is simplex, so that each mobile can only transmit or receive at any one time.

Each mobile has a circuit of the form shown in Figure 4 included in it. The mobiles are typically fitted in vehicles, and the microphone 1 of each mobile is therefore subject, when the vehicle is moving, to noise as well as to any speech to be transmitted.

The output of the microphone is fed to an automatic gain control 2, the time constant of which is relatively slow in order to ensure that the voltage at point A is within the operating range of the remainder of the circuit despite the wide dynamic range of signals being generated by the microphone. The use of a slow time constant results in the preservation of the rapid sound variations produces by speech. At point A the varying voltage could be of the rapidly varying form shown in Figure 1, and this corresponds to the microphone receiving speech signals as well as noise signals from the vehicle.

Envelope detector 3 with relatively fast attack and delay time constants produces the envelope signal shown in Figure 1, and this output appears at point B. Because of the relatively fast time constants, the envelope follows the variations of the peaks of the rapidly varying voltage signal faithfully.

The envelope detector 4, however, has slower time constants and therefore does not accurately reflect all the changes in the peaks of the rapidly varying voltage signal, but only records the general variation of that signal over a period of time. This produces a resultant profile shown in Figure 2 at point C in which only the slowly varying background is recorded and the rapidly varying speech part is not recorded.

The forcing circuit, 12 ensures that the voltage at C never falls below that at B. That is to say that during a period of loudness decay when the voltage at B is falling relatively quickly, the forcing circuit will ensure that B is at least equal to C at the beginning of the next attack period and subtraction of the two voltages gives a fair indication of the following speed of attack.

When these two signals are subtracted from each other in subtractor 5, the profile shown in Figure 3 is produced corresponding to point D, and this is representative of the difference in attack times of the two signals.

This difference signal is fed to a level detector 6, which sets a level 7 (Figure 3). When the voltage level 7 is exceeded for whatever length of time, the level detector 6 emits a corresponding logic signal, which is passed to a processor in the mobile. The effect of these signals is to switch the mobile from a receive state to a transmit state if the level 7 is exceeded for more than 100 milliseconds and to maintain the transmit state until the profile D falls below the level 7 for more than one second. In that situation, the mobile reverts to receive operation.

A suitable circuit for the envelope detectors may be based on the arrangement shown in Figure 5, in which a diode 8 feeds a series resistor 9 capacitor 10 arrangement, a resistor 11 being arranged in parallel with a capacitor 10. The diode 8 is responsible for the profiles shown in Figures 1 to 3 being positive-going only, and the resistor 9 and capacitor 10 are responsible for the attack or rise time constant of the respective envelope detectors, while the capacitor 10 and resistor 11 are responsible for the discharge time constant. Suitable values for the faster acting envelope detector 3 are 12 milliseconds and 68 milliseconds for the attack and discharge time constants, whereas a suitable value for the slower acting envelope detector 4 are 56 milliseconds and 320 milliseconds for the attack and discharge time constant, respectively.

The forcing circuit may consist of an active diode arrangement which discharges the slow detector capacitor in sympathy with the fast detector output voltage if it falls below that of the slow detector.

The invention enables the effect of vehicle noise to be reduced or substantially eliminated from the sound received at the microphone, and this enables the switching of the envelope detector to transmit mode to be voice-triggered.

Claims

1. A mobile transmitter-receiver for simplex radio communication with another mobile transmitter-receiver, comprising means for switching from a receive mode to a transmit mode in response to a signal derived from the rate of change of loudness of the sound level in the vicinity of the mobile.

2. A mobile transmitter-receiver for simplex radio communication with another mobile transmitter-receiver, comprising means for switching from a receive mode to a transmit mode in response to a signal derived from a sound input reaching a pre-determined level, and means for producing a signal representing the difference between two signals derived from the same sound input, one of which is responsive to faster sound variation than the other.

3. A mobile transmitter-receiver as claimed in Claim 2, in which the signal producing means is such that the signal represents the difference between two signals, one responsive to faster attack loudness variation than the other.

4. A mobile transmitter-receiver as claimed in Claim 2 or Claim 3, in which the signal producing means comprises a pair of amplitude envelope detectors, one of which has a slower time time constant than the other, and means to subtract the output of one from the other.

5. A mobile transmitter-receiver as claimed in Claim 4, including a level detector arranged to switch the transmitter-receiver to the transmit mode when the predetermined level has been exceeded for a predetermined length of time.

6. A mobile transmitter-receiver for simplex radio communication with another mobile transmitter-receiver substantially as herein before described with reference to the accompanying drawings.

7. A method of switching a mobile transmitter-receiver from a receive mode to a transmit mode, in response to a signal derived from the rate of change of loudness of the sound level in the vicinity of the mobile.