GB2213686A

GB2213686A - Radio telephone terminal unit

Info

Publication number: GB2213686A
Application number: GB8728758A
Authority: GB
Inventors: Richard Barkey
Original assignee: Racal Communications Equipment Ltd
Current assignee: Racal Communications Equipment Ltd
Priority date: 1987-12-09
Filing date: 1987-12-09
Publication date: 1989-08-16
Also published as: GB8728758D0

Abstract

An RTTU 2 uses an electronic hybrid 18 to connect a two-wire telephone line 4 to a digital circuit including a receive line and a transmit line connected to a receiver 6 and a transmitter 8 of a radio telephone channel. An echo canceller 26 is connected between the receive and transmit lines in order to subtract from the signal in the transmit line any echo signal related to that present in the receive line. The coefficients of the echo canceller are updated by feedback from the transmit line downstream of the echo canceller. The coefficients are frozen when there is a real signal from the telephone line. <IMAGE>

Description

RADIO TELEPHONE TERMINAL UNIT The present invention relates to radio telephone terminal units (RTTU) which are used to interconnect a four-wire equivalent radio telephone channel and a two-wire telephone circuit.

In order to convert between a four-wire channel and a two-wire line, it has been customary to use a hybrid transformer or an electronic hybrid. Impedence mismatches between the line and the two to four wire conversion hybrid give rise to echo effects, which are particularly troublesome on long distance radio links where the delay is significant. It is possible to suppress such echoes by preventing transmission of a signal in both directions at once. However, this prevents full duplex working of the link. Although such a link can be used in half-duplex and simplex modes, this is unreliable for voice communications and not acceptable for data communications, because signals reflected by the two to four wire converter can confuse the control circuitry and cause erroneous transmitter keying and suppression of signals.Other proposals have suggested incorporating control tones in transmitted signals so that any echo returning to the transmitter can be identified in order to enable an echo suppressor to be switched into the circuit. However, if such a control tone is used, it is necessary to incorporate a notch filter in order to remove the control tone before it reaches the intended receiver. Such a notch filter makes this sytem entirely unsuitable for the transmission of data by tones where, for example, MFSK is used, and also prevents use with encryption devices.

The problem of echoes is particularly prevalent where the radio channel uses HF band frequencies. Complex echo effects may also arise where a communications link incorporates two RTTUs arranged back to back with an intermediate radio telephone channel, as arises frequently in military applications.

In accordance with the present invention, the technical problem of echo effects arising in an RTTU is solved by providing a radio telephone terminal unit comprising a two-wire to four-wire converter for connection to a two-wire telephone line, and a digital circuit defining a receive line and a transmit line connected between the converter and a radio telephone channel, the digital circuit comprising an echo canceller having an input connected to the receive line and an output which is connected to a summer arranged to subtract the output signal from the echo canceller from the signal in the transmit line, a control input to modify at least one coefficient of the echo canceller being provided by the output of the summer only when no information-carrying signal is present in the transmit line.

The use of an echo canceller in a digital circuit allows echo problems to be overcome while ensuring that the RTTU is still effective for full duplex data and voice communications including encrypted signals.

Preferably, the digital circuit also comprises other functional blocks of the RTTU, for example a Voice Operated Switch (VOX) used to key a transmitter connected to the output of the transmit line from the digital circuit. This is only able to operate reliably as a consequence of the good separation between the receive and transmit channels provided by the echo canceller.

In addition, the digital implementation permits the inclusion of sophisticated call-handling facilities.

These include the encoding, detection and processing of telephone dialling information transmitted over the radio link using FSK tones, which allows remote dialling in a back-to-back RTTU system.

Although echo cancellers with settable coefficients are known per se and have been used in telecommunications equipment (especially in ISDN), such an echo canceller has not previously been proposed for use with an HF radio link in an RTTU. RTTU have always been analogue in nature and operated in half-duplex mode, relying on crude, once-only manual tuning of a transformer hybrid with a consequential lack of reliability of operation and flexibility of use.

Since the RTTU comprises a digital circuit, it is readily possible to exploit the channel separation provided by the canceller by the inclusion of additional facilities. These may include more reliable radio signalling, semi-automatie call handling, and the use of multiplexers to enable an operator to be connected to both the receive and transmit lines so that he can speak to both parties to a call on the link in order to set up a desired call manually.

In a preferred embodiment, the digital circuit may comprise a general purpose digital signal processor in conjunction with appropriate software to perform the functions of the various components described. For example, appropriate algorithms for implementing the echo canceller on such a general purpose digital signal processor are described in the report of the IEE colloquium on Adaptive Filters, digest number 1985/76.

A digital RTTU embodying the invention will now be described, by way of example only, with reference to the accompanying block diagram illustrating the functional components.

The RTTU is connected between an HF radio link and a two-wire telephone line 4. The radio link is terminated by a radio receiver 6 and a radio transmitter 8. The received signal is fed into a receive line of a digital circuit 2 of the RTTU via an analogue-to-digital converter (ADC) 10. The output from a transmit line of the RTTU digital circuit to the transmitter 8 is via a digital-to- analogue converter (DAC) 12. Similarly, the output from the receive line of the digital circuit to the telephone line is via DAC 14 and the input from the telephone line to the transmit line of the RTTU digital circuit is via ADC 16.Since the analogue output produced and the analogue input required by the telephone line are in two-wire form but the analogue signal output by the DAC 14 and required by the ADC 16 is in four-wire form, an electronic hybrid 18 providing simple two to four-wire conversion connected between the telephone line 2 and the output of DAC 14 and the input of ADC 16.

In the digital circuit 2 of the RTTU the signal received from the telephone line is fed through a summer 20 to a multiplexer 22. The output of the summer 20 provides the control input 24 to an echo canceller chip 26. The echo canceller has an input connected to the receive line. The output of the echo canceller is connected to an inverting input of the summer 20. The function of the echo canceller is to remove from the transmit line an estimate of the reflected proportion of the signal which is present in the receive line i.e. the echo signal. This proportion is defined by a set of coefficients which are variable in response to the control input. The control input varies the coefficients so that the output from the summer is null when there is no information-carrying signal from the telephone line. The adapted coefficients then continue to remove the correct proportion of the receive line signal from the transmit line, even when there is a real information-carrying signal in the transmit line, (in this case, the control input is disabled as described below).

The echo canceller has a further input 28 which disables the control input and freezes the coefficients at their current values when the input changes to a specified logic state. The operation of the control input 24 is enabled again when the input 28 changes state again.

The input 28 is connected to a digital comparator 30, which compares the power level at the input to scaling block 52 with the power level at the output from the ADC 16 (this connection is not shown in the accompanying drawing for the sake of clarity). If the power level in the input from the telephone line 4 is greater than half of that in the receive line, indicating that a signal is being received from the telephone line, the updating of coefficients is disabled.

The multiplexer 22 in the transmit line of the RTTU also receives inputs from a radio ring generator 34 and from the microphone input of an operator headset 36 via an ADC 38. The output of the multiplexer is connected to a transmitter AGC circuit 39 which is connected both to a VOX detector 40 and a delay line 42. The output of the VOX detector, which is produced in response to detection of a signal to be transmitted, provides a signal to power up the transmitter 8. The function of the delay line 42 is to ensure that no signal data is lost before the transmitter is operational. The output of the delay line 42 is fed via a scaling circuit 44 to DAC 12. The use of such a delay line would result in extreme signal degradation for an RTTU implemented in analogue.

Therefore digital implementation has provided a solution to another technical problem of the existing proposals.

The receive line of the RTTU includes a multiplexer 46 which has one input connected to the output of ADC 10 and another input connected to the output of ADC 38.

The output of the multiplexer is fed via a receiver AGC circuit 50, which is connected via an output scaling circuit 52 to the input of DAC 14. The input to the echo canceller 26 is taken from the output of the receiver AGC circuit 50.

The operator headset 36 includes an earphone 56 which is connected via a DAC 58 to both the receive and transmit lines by connections not shown for the sake of clarity, so that the operator can listen to both sides of the communication. Equally, since his microphone amplifier is connected to multiplexers 22 and 46, he can transmit to both parties selectively using the communications link. This enables him to set up a call by responding to a radio or telephone ring signal, detected by detector 60 connected to the output of the ADC 16, or a telephone ring detector (not shown), respectively. The output of the detectors are also connected to a buzzer at a front panel of the RTTU.

Claims

1. A radio telephone terminal unit comprising a digital signal processor and an electronic hybrid, the processor being programmed to cancel the echo from the hybrid interface.

2. A radio telephone terminal unit comprising a two-wire to four-wire converter for connection to a two-wire telephone line, and a digital circuit defining a receive line and a transmit line connected between the converter and a radio telephone channel, the digital circuit comprising an echo canceller having an input connected to the receive line and an output which is connected to a summer arranged to subtract the output signal from the echo canceller from the signal in the transmit line, a control input to modify at least one coefficient of the echo canceller being provided by the output of the summer only when no information-carrying signal is present in the transmit line.