INTERFACE CIRCUIT

The invention relates to an interface circuit for coding analogue signals for digital transmission and decoding digital signals for analogue transmission.

The interface circuit is of the type comprising a processing unit, an analogue port having a pair of terminals for connection to an analogue loop, a digital output port and a digital input port, said processing unit including an input path between the digital input port and the analogue port arranged to convert digital input signls to analogue output signals, an output path between the analogue port and the digital output port arranged to convert analogue input signals to digital output signals and a cross-over path for supplying digital signals derived from analogue input signals to the input path wherein the processing unit is arranged to synthesise a component of a terminating impedance across the analogue terminals.

An interface circuit of the aforesaid type is shown in United Kingdom Patent Application 2 086 196 which discloses a circuit for coding an analogue signal from a telephone line. The resulting digital signal is processing in combination with an incoming digital signal by means of a digital filter. The output from this filter is then decoded and supplied to the telephone line via a transconductance amplifier. A problem with this circuit is that it may become unstable due to the delay introduced by the analogue to digital converter, the digital filter and the digital to analogue converter.

Another interface circuit is disclosed in the Ericsson Review of 1983, No 3, page 192 in which a digital filter synthesises a terminating impedance in combination with a

resistor placed across the analogue terminals. This arrangement is suitable for providing a fixed terminating impedance but does not solve the problem of providing a variable terminating impedance particularly if a range of values are desired for which both the real and imaginary components are to be changed. A problem is that in attempting to provide stability for such a circuit it is difficult to maintain the necessary frequency response for each of the available terminating impedances. Thus even if a relatively small number of terminating impedances are available, complex filters must be added to maintain the necessary frequency response.

An object of the present invention is to provide an interface circuit of the aforesaid type providing a plurality of terminating impedances while providing a satisfactory frequency response.

According to the present invention there is provided an interface circuit of the aforesaid type characterised by a transmission path having variable resistance electrically connected across the analogue ports and means for varying both the resistance of the transmission path and characteristics of the processing unit to change the value of the impedance across the terminals of the analogue port.

In a preferred embodiment the characteristics of the processing unit are determined by a single variable (K) and the resistance of the transmission path is also varied in relation to the value of said variable. The advantage of this embodiment is that a whole range of terminating impedances may be provided which is only restricted by the resolution of the digital circuitry.

Preferably the transfer function of the digital filter has realisable negative delay and may be implemented as a finite impulse response digital filter. Preferably a digital scaling means having a numerical value equal to the

resistive transmission path is in parallel with the finite impulse response filter.

The interface circuit may be employed in a full duplex transmission system. According to a second aspect of the present invention there is provided a transmission system comprising an analogue loop, means for coding analogue signals from said loop to provide a digital output, means for decoding a digital input to supply analogue signals to said loop, and a digital filter for supplying a portion of the digital input to synthesise a terminating impedance for the analogue loop: characterised by a variable resistive transmission path terminating the analogue loop, and means for varying the resistance of said transmission path and for varying characteristics of the digital filter to change the value of the synthesised impedance.

Preferably the variable resistance is a variable gain amplifier.

According to a third aspect of the present invention there is provided a method of interfacing a subscribers analogue loop to a digital input line and a digital output line wherein an analogue impedance is simulated by Supplying a proportion of the digital output to the digital input via a digital filter: characterised by terminating the analogue loop with a variable resistance, and varying said resistance and varying the characteristics of the digital filter to change the value of the simulated impedance.

The invention will now be described by way of example only with reference to the accompanying drawings of which: Figure 1 shows a schematic representation of an improved interface circuit, and

Figure 2 to 12 show the steps taken from an ideal system to a working embodiment.

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An interface circuit 10 is shown in Figure 1, fabricated as an integrated circuit, including a processing unit 11. The circuit 10 also includes an analogue port having a pair of terminals 12 which are connected to an analogue loop 14. Analogue signals are received from the loop 14 and coded into digital signals which are in turn supplied to a digital output port 15. Similarly digital input signals supplied to a digital input port 16 are decoded and supplied to the analogue port.

In addition to an output path 17, supplying an analogue signal to the analogue port in response to digital input signals, and an input path 18, supplying a digital output signal to the digital output port 15 in response to analogue input signals, a cross-over path 19 supplies digital signals derived from an analogue input signal to the output path. The purpose of this feedback loop is to synthesise the imaginary component of the terminating impedance Zt for the analogue line. difference amplifier 20 has input terminals connected across the analogue terminals 12 and provides an input voltage V. An analogue output signal is in the form of current generated by a transdulance amplifier 21 in response to an output voltage identified as I. The real component of the terminating impedance Zt is provided by a resistive transmission path 22 which provides a feedback path between the input V and the output I. The resistance of path 22 is controlled by a digital signal from the processing unit 11 and the circuit 10 is provided with means for varying both the resistance of the transmission path 22 and characteristics of the processing unit 11 to implement changes to the desired terminating impedance seen across the terminals of the analogue port.

An ideal interface circuit in which a terminating impedance is synthesised is shown is Figure 2. The analogue to digital and digital to analogue converters are not present because they are assumed ideal and have a transfer function of 1. The output signal Tx is supplied to an adder 50 via an inverter 51. The adder 50 also receives the input signal Rx and supplies an output to a sealer 52. Sealer 52 has a transfer function of k/ZT - where Zt is the terminating impedance. However in practice the converters introduce delay and the aforesaid converters have transfer functions of Z ^"n and Z ^" * ¹¹ . The configuration of Figure 2 would not respond quickly enough and would therefore be unstable.

The arrangement of Figure 1 provides a resistive path on the analogue side of the converters while retaining the overall transfer function thus ensuring that the required frequency response is retained without adding compensating filters.

The detailed processing algorithm for the circuit shown in Figure 1 may be derived from the ideal arrangement of Figure 2 as follows:-

1. Let K/R be the zero delay component of K/Zt. The other component is then

K/Zt - K/R = K(R-Zt)/RZt and the resulting transfer alogorithm is shown in Figure 3. The sealer 52 has been divided into two sealers 53 and 54 the outputs from which are combined by an adder 55.

2. The K/R block 53 ^' is moved to the other side of the adder thus necessitating another K/R block 56 -

Figure 4.

3. The input of K/R block 56 is moved to the other side of the inverter 51 so that it becomes -K/R - Figure 5.

_. ₆ _

4. The output from sealer 53 is removed from adder 55 by using another adder 57 - Figure 6.

5. The delays due to the converters 14 and 16 are assumed to have a transfer function of Z and are

5 inserted to the left of the -K/R block 56 along with negative delays 58, 59 having a transfer function of Z.

6. The negative delays are moved in order to incorporate them into blocks having non zero-delay components as shown in Figures 8 and then 9. In

¹⁰ " Figure 10 a double delay and two negative delays are added which may be manipulated as shown in Figure 11.

7. In Figure 11 delays Dl and D2 which are in series with the digital input and the digital output are ignored.

2

-3 8. The double negative delay Z is combined with the K(R-Zt)/RZt block 54 if Z ^"1 represents a sufficiently small delay.

A generalised version is shown in Figure 12 and if n+m equal 3 then K(R-Zt)/R-Zt can be realised by a finite

²⁰ impulse respone filter having the following co-efficients:

C1=C2=C3 = 0

C4 = -0.65625

C5 = +0.296875

C6 = -0.078125

-0 C7 = +0.234375

C8 = -0.265625

C9 = +0.046875

The structure of Figure 1 does not require extra filters in order to achieve the required frequency ³⁰ response, the transfer functions of the block only require one variable to be changed (K) in order to change Zt while ensuring that the circuit has the required frequency response and remains stable.