GB2201864A

GB2201864A - Signal converter unit

Info

Publication number: GB2201864A
Application number: GB08803172A
Authority: GB
Inventors: Robert Cheetham; Michael James Sayer
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1987-02-11
Filing date: 1988-02-11
Publication date: 1988-09-07
Anticipated expiration: 2008-02-11
Also published as: GB2201864B; GB8703066D0; GB8803172D0

Abstract

The invention concerns a converter which can extract the High Frequency Subscriber's Private Meters Signal from a Subscriber's line pair, lower the frequency of the signal and reinject it into the line pair so that a subscriber with low frequency equipment can utilise the latest current High Frequency signalling without additional bulky equipment. <IMAGE>

Description

Signal Converter Unit The present invention concerns a converter for use with telephone exchange equipment. Traditionally telephone exchanges have provided subscribers with a Subscriber's Private Meter (SPM) using a 50 Hz signal. This signal is generated at the exchange with the meter being located at the subscriber's premises.

However, 50 Hz generation and signalling equipment is relatively bulky so that recently High Frequency Signalling has been adopted. this is known as HFSPM and the signals are typically at 12 kHz and 16 kHz.

When analogue multiplexers are provided the requirement for 50 Hz SPM necessitates extra equipment such as the SPM source transformers and auxiliary control. This both takes up a lot of space and uses power. The use of HFSPM makes this additional 50 Hz equipment unnecessary. However, if the subscriber does not have HFSPM receivers the benefits of HFSPM cannot be gained.

The present invention is concerned with a solution to this problem.

Accordingly the present invention consists in a converter for use with a telephone subscriber's private meter system, the converter comprising means for extracting a High Frequency Subscriber's Private Meter (HFSPM) signal, converting the high frequency signal into a lower frequency signal, and re-injecting the lower frequency signal into the subscriber's line pair.

The converter may be located at the exchange or at the subscribers premises.

Preferably the converter includes means for detecting the HFSPM signal and blocking its further passage, for stretching the pulses of the detected HFSPM signal, and for re-injecting the SPM signal at a lower frequency into the subsciber's line pair. The lower frequency will preferably be 50 Hz. The 50 Hz signal may be derived from mains power supply.

In order that the invention may be more clearly understood, an embodiment thereof will now be described by way of example and with reference to the accompanying drawings1 in which: Figure 1 is a schematic diagram of a known low frequency SPM system, Figure 2 is a similar diagram of a system incorporating a converter according to the present invention, Figure 3 is a block diagram of one embodiment of a SPM converter according to the present invention, and Figure 4 is a circuit diagram of the converter of Figure 3.

Referring now to Figure 1 of the drawings this shows a subscriber's telephone 10 associated with a Low Frequency Subscriber's Private Meter (LFSPM) 11. This equipment is connected via a subscriber's pair 12 to an exchange generally indicated at 13. The exchange 13 includes SPM auxiliary equipment 14 and a line circuit 15.

Figure 2 of the drawings shows a system incorporating a LFSPM converter 16 according to the present invention.

In this case the exchange equipment sends its signals at 16 kflz and incorporates an analogue MUX. The converter 16 is shown located at the premises of the subscriber. The converter 16 receives the 16 kHz SPM signal from the exchange and converts it to 50 Hz for reception by the subscriber's existing SPM equipment. The SPM converter 16 is located next to the existing SPM equipment in series with the subscriber's pair. It is possible for the SPM converter 16 to be located at the analogue multiplexer or exchange. This is of benefit only if one or two lines are required.

Referring now to Figures 3 and 4 of the drawings, these respectively are a block diagram and a circuit diagram of the converter 16.

The SPM signals from the exchange at, for example, 16 kXz are carried on lines indicated at 22 and received by a protection circuit 23. In this circuit protection against overvoltage is provided, as shown in Figure 4, by a gas tube 24, 10 ohm thermistors 25 and a semi-conductor crowbar device. From protection circuit 23 the signal lines are connected to a blocking circuit 30 which provides attenuation of the 16 kHz signal. The blocking circuit comprises a filter formed by a parallel tuned transformer 31 with zener diodes 32 to protect a capacitor 33 from overvoltage. Protection fros: fault voltages from the subscriber is provided at 34.

The line output of protection circuit 23 is also supplied to a filter 40. This is a 4th order bandpass filter with two tuned circuits formed by capacitors 41, 42, inductance 43 and a transformer 44. The two tuned circuits are individually tuned to 16 kHz and the filter is designed to work from a 200 ohm source. The output of filter circuit 40 is next supplied to a detection and pulse stretching circuit 50.

In circuit 50, diodes 51, 52 provide protection against overvoltage whilst the operational amplifier 53 provides a high impedance buffer to avoid loading the filter. The operational amplifier 54 acts as an inverting half-wave rectifier with its output averaged by resistor 55 and capacitor 56. Finally operational amplifier 57 acts as a threshold detector with the threshold set by resistors 58, 59, 60 and voltage VN2 derived from power supply 80.

The output of circuit 50 is supplied via a 100K resistor 61 to an integrated logic circuit 62 and an operational amplifier 63. These two circuits determine the make and break persistance times. Thus the break time is set to approximately 50 msecs longer than the make time so the output pulse of the circuit is 50 msec longer than the input pulse. In this way a 200 msec 16 kHz pulse is stretched to 250 msecs.

The timer values for the pulse stretching circuit 50 are set by resistors 64, 65, 66 and 67 and a capacitor 68.

The resistor 65 sets the make persistance time, whilst resistor 67 sets the make time reset speed. Resistor 66 sets the break persistance time and resistor 64 sets the break timer reset speed. Capacitor 100 adds hysteresis to give a clean switching action.

The effect of the pulse stretching circuit can be best appreciated from the waveforms shown in Figure 5. Thus waveform (a) is an undetected 200 msec pulse of the 16 kHz SPM signal. Waveform (b) is the result of detecting this pulse and appears at the output of operational amplifier 57. The detected pulse passes through integrated circuit 62 at its output at X with the waveform shown at (C) in Figure 4. The waveform (d) shows the output of operational amplifier 63 from which it can be seen that the ramps of waveform (C) have been replaced by sharp leading and trailing edges and that the previous 200 msec pulse has been stretched by 50 msecs.

The stretched pulses which form the output of operational amplifier 63 are supplied to an injection circuit 90 so that the detected and stretched SPM signal, originally at 16 kHz, can be injected back into the subscriber's line pair 101 at 50 Hz. The injection is carried out via a pair of relay contacts 70,71. These are driven by a 50 Hz signal derived from a power supply circuit 80.

Circuit 80 is a fused mains power supply having a transformer 81.

The 50 Hz signal is taken from the ends A,B of a coil of the transformer 81 which also provides regulated rails at +11.3, -5.1 and -10.2 volts respectively via a chain of diodes 82, 83 and 84.

The injection circuit 90 includes an injection transformer having windings 91, 92 driven by a parallel tuned circuit which includes protecting resistors 93, 94. The tuning is provided by capacitors 95, 96 and a resistor 97 and is intended to minimise the current required.

The entire system completed by a standard coil relay circuit generally indicated at 100.

Claims

1. A converter for use with a telephone subscribers' private meter system, the converter comprising means for extracting a High Frequency Subscribers Private Meter (HFSPM) signal from the subscriber's line pair, means for converting the HFSPM signal into a lower frequency signal, and means fbr reinjecting the lower frequency signal into the subscriber's line pair.

2. A converter as claimed in Claim 1, including means for blocking the further passage of the HFSPM signal.

3. A converter as claimed in Claim 2, wherein the converting means comprise means for stretching the extracted HFSPM signal.

4. A converter as claimed in Claim 3, wherein the stretching means include an operational amplifier acting as an inverting half-wave rectifier connected to a second operational amplifier acting as a threshold detector.

5. A converter as claimed in Claim 4, wherein the output of said second operational amplifer is connected to an integrated logic circuit and a third operational amplifier operative to set the make and break persistance times with the break time set longer than the make time.

6. A converter as claimed in any one of the preceding claims, wherein the injection means comprise an injection transformer having windings driven by a parallel tuned circuit.

7. A converter for use with a telephone subscriber's private meter system substantially as herebefore described with reference to any one of Figures 2 to 5 of the accompanying drawings.