GB1591869A - Alternating signal detector - Google Patents

Description

(54) ALTERNATING SIGNAL DETECTOR (71) We, PYE (ELECTRONIC. PRO DUCTS) LIMITED, a British company, of St. Andrew's Road, Cambridge, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a circuit arrangement for producing an output signal in response to an alternating input signal in a first frequency band.

Such a circuit may be part of a modem used in data transmission and its output signal may be used to activate the data decoding circuits. Typically the circuit uses a filter to select frequencies in the desired frequency band, i.e. that including the signalling frequencies and to reject other frequencies. However, when a filter is used to select the frequency band to be detected it is possible for lower frequency signals which have harmonics within the first frequency band to be erroneously detected. This may not be important in many cases since the encoded data can be coded so that the decoding circuits can detect errors in the received data. However, in a Viewdata terminal the presence of a receive carrier signal in the first frequency band is used to maintain the connection.Thus, if other tones, such as dial tones, which are used in the setting up of the connection are erroneously detected as a receive carrier signal the connection may be lost due to the normal cessation of that tone during the setting up procedure.

It is an object of the invention to provide a circuit arrangement for producing an output signal in response to an input alternating signal in a first frequency band which will not respond to harmonics of signals in a second, lower frequency, band.

The invention provides a circuit arrangement for producing an output signal in response to an alternating input signal in a first frequency band, the circuit arrangement comprising means for feeding an alternating signal from its input to the input of a filter and to the input of a detector, the filter having a pass band which includes the first frequency band and the detector being adapted to produce an output signal in response to alternating input signals having a fundamental frequency below the first frequency band, means for coupling the output of the filter to a first input of a signal generator, means for coupling the output of the detector to a second input of the signal generator, means for coupling the output of the signal generator to a second input of the detector and means for coupling the output of the signal generator to the output of the circuit arrangement wherein in operation the signal generator in the absence of a signal at its second input produce an output signal in response to a signal at its first input, the output of the detector being inhibited in the presence of a signal at its second input, and the output signal from the signal generator is the output signal of the circuit arrangement.

The detector may be responsive to signals in the frequency range of 200-750Hz while the filter may have a pass band which includes the frequency range 1200--2500Hz.

The invention further provides a line coupling unit for a Viewdata terminal including such a circuit arrangement.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a block schematic form a circuit arrangement according to the invention; Figure 2 is a circuit diagram of one form the detector shown in Figure 1 may take; Figure 3 is a circuit diagram of one form the signal generator shown in Figure 1 may take; and Figure 4 shows in block schematic form a line coupling unit for a Viewdata terminal including a circuit arrangement according to the invention.

As shown in Figure 1 the circuit arrangement has an input 1 which is connected via a line 2 to an input 3 of a filter 4 and via a line 5 to a first input 6 of a detector 7. An output 8 of the filter 4 is connected via a line 9 to a first input 10 of a signal generator 11 while an output 12 of the detector 7 is connected via a line 13 to a second input 14 of the signal generator 11. An output 15 of the signal generator 11 is connected via a line 16 to the output 17 of the circuit arrangement and via a line 18 to a second input 19 of the detector 7.

In operation alternating signals, which may be from a telephone line, are applied to input 1 and signals within a selected frequency band pass through the filter 4 to the first input of the signal generator 11 and cause the signal generator to emit an output signal which is available at output 17. Detector 7 is arranged to detect signals having a fundamental frequency below the pass band of filter 4, but which may have harmonics which lie within the pass band, and to produce an output signal in response to such signals at its input. The output from the detector 7 is fed to input 14 of the signal generator and is effective to inhibit the output of the signal generator.Thus the circuit arrangement will not respond to signals whose fundamental frequency is below the pass band of the filter 4 but which have harmonies of sufficient amplitude to trigger the signal generator in the absence of a signal on input 14.

The output 15 of the signal generator 11 is fed to the second input 19 of the detector 7 to inhibit its output. Thus once a signal having a fundamental frequency within the pass band of the filter 4 is detected without the simultaneous presence of a lower frequency signal the detector 7 is not effective until this signal and hence the signal generator output has ceased.

One application of this circuit arrange mcnt is in the detection of a carrier signal received in a Vicwdata terminal. The signal received from the computer via the telephone line has a frequency of either 1300 HZ or 2100he while that sent from the terminal to the computer has a frequency of 390No or 450Hz. Other signals which may occur on the telephone line such as dial tone, engaged tone, ringing tone etc. also have frequencies around 400Hz. It is important in a Viewdata terminal to detect when the received signal or carrier is present in order to provide appropriate signals to the decoding circuits.

However, it is equally important that harmonics of dial tone etc. are not detected as received carrier signals. The harmonics of these tones will lie within the pass band of filter 4 which has to accept the received carrier frequencies. In order to prevent the circuit arrangement giving a false output due to the harmonics of the lower frequency signals the detector 7 is provided to detect signals having a fundamental frequency below about 700Hz and produce a signal which inhibits the output of signal generator 11.

Once the connection has been set up the received carrier signal is continuously present but a transmit carrier signal is also generated and used to transmit information from the terminal to the computer. This signal is such that unless suitable precautions are taken it would cause the output of detector 7 to inhibit the output of the signal generator 11 and thus cause the connection to be iost as it would appear that the receive carrier had ceased. To prevent this happening the output of the signal generator is fed back to the second input of the detector te inhibit its output. Thus once a received carrier signal has been detected the detector 7 is inhibited until the received carrier signal ceases and does not cause the circuit arrangement to give a false output due to the transmit carrier signal.It will be noted that the transmit carrier signal is not generated until a receive carrier signal is detected.

The circuit diagram of the detector 7 of Figure 1 is shown in Figure 2. The input 6 of the detector 7 is connected to the positive input of a comparator GE whose output is connected to the junction of two resistors R1 and R2. A resistor R3 is connected between the positive and negative inputs of comparator G1 and the negative input of comparator G1 is connected via the parallel arrangement of a resistor R4 and a capacitor C1 to a negative supply rail. The other end of resistor R1 is connected to a positive supply rail while the other end of resistor R2 is connected via a capacitor C2 to the junction of the anode of a diode D1 and the emitter of a transistor T1.The cathode of diode D1 and the base of transistor T1 are connected to the negative supply rail. The series arrangement of a resistor R5 and a capacitor C3 is connected between the positive and negative supply rails, the junction of capacitor C3 and resistor R5 being connected to the collector of transistor T1 and the positive input of a comparator G2. The series arrangement of two resistors R6 and R7 is connected between the positive and negative supply rails with the junction of resistors R6 and R7 connected to the negative input of comparator G2, the positive input of a comparator G3 and the positive input of a comparator G4. The outputs of comparators G2 and G3 are commoned and connected to the negative input of comparator G4 and via a resistor R8 w the positive supply rail. The output of comparator G4 is connected to the output 12 -of the detector while the second input 19 of the detector is connected to the negative input of comparator G3.

A circuit diagram of the signal generator 11 of Figure 2 is shown in Figure 3. The series arrangement of three resistors R10, Ril and R12 is connected between the positive and negative supply rails with the junction of resistors R10 and Ril connected via -a resistor R13 to the positive input of a comparator G10 and the junction of resistors R11 and R12 connected via a resistor R14 to the negative input of comparator G10. The output of comparator G10 is connected via a resistor R15 to the positive supply rail and via a resistor R16 to the negative input of a comparator G11. A capacitor C10 is connected between the negative input of comparator G11 and the negative supply rail.

The series arrangement of two resistors R17 and R18 is connected between the positive and negative supply rails, the junction of resistors R17 and R18 being connected to the positive input of comparator G11. The output of comparator G11 is connected to the output 15 of the signal generator 11, via a resistor R19 to the positive supply rail and via a resistor R20 to its positive input. The first input 10 of the signal generator 11 is connected to the positive input of comparator G10 while the second input 14 is connected to the negative input of comparator G10 and via a resistor R21 to the positive input of comparator Gli. The comparators G1 to G4 G10 and G11 may be those manufactured by Signetics Corporation and sold under the type number LM339.These devices have an open collector output transistor which is switched OFF when the positive input is more positive than the negative input.

In the absence of a signal at input 1 comparator G1 is switched OFF, G2 is switched ON and hence G4 is switched OFF. Comparator G10 is switched OFF and hence comparator G11 is switched ON which causes comparator G4 to be switched OFF.

When a signal in the range 200-750Hz is applied to input 1 comparator G1 switches between the ON and OFF states at each zero crossing of the input signal and causes a current to be drawn through resistor R5 which is proportional to the input frequency. Thus at frequencies above a threshold value comparator G2 is switched OFF and hence comparator G4 switches ON. As the frequency increases and exceeds a second threshold value comparator G1 ceases to switch due to the action of capacitor C1. Hence comparator G4 switches ON when a signal having a frequency between the first and second threshold values is applied to input 6.

When comparator G4 is switched ON comparator G10 is held OFF whatever input signal is applied at input 10 and thus harmonics of the signal at input 1 which may be passed by the filter 4 will not cause a carrier detect signal to be generated. However, when no input signal or a signal having a frequency greater than the second threshold value occurs the output of comparator G4 is OFF and the output of comparator G10 will switch at the frequency of the signal applied to input 10. This will cause capacitor C10 to charge to a positive potential and thus cause comparator G11 to switch OFF and thus produce a carrier detect signal at output 15. This signal is also fed back to input 19 of the detector and causes comparator G3 to switch ON and thereby holds comparator G4 OFF.Thus the detector will not respond to the transmit carrier signal, which is within its detection range, while the receive carrier signal is present.

The line coupling unit shown in block schematic form in Figure 4 includes the circuit arrangement shown in Figure 1 for the detection of the receive carrier. The line coupling unit is provided with a 5-way plug and socket arrangement 40 through which it is connected to a telephone line 41 together with a telephone handset 42. A protection circuit 43 is provided to protect the unit from high voltages which may be induced on the telephone lines such as those that may be caused by lightening. The unit is further isolated from the line by an isolation circuit 44 which may take the form of that described in our co-pending application No. 25837/78 (Serial No. 1,591,870) (PP 1221) of even date or may be simply a transformer.The isolating circuit is connected to a loudspeaker amplifier 45, to input 3 of filter 4, and input 6 of the detector 7 and the output 8 of the filter 4 is fed to input 10 of the signal generator 11. The detector 7 and signal generator 11 form a carrier detector 20. Output 17 of the carrier detector 20 is fed to an output 46 of the line coupling unit and via a delay circuit 47 to the loudspeaker amplifier 45 the output of which is connected to a terminal 82. Output 8 of the filter 4 is also fed to a first input 48 of a modem 49 a first output 50 of which is fed to a terminal 51. A terminal 52 is connected to a second input 53 of the modem 49 a second output 54 of which is connected to the isolating circuit 44. A terminal 80 is connected to a third input 81 of the modem 49.A terminal 60 is connected to a first input 61 of an autodialler 62 while a terminal 63 is connected to a second input 64 of the autodialler 62 and to a first input 65 of a Viewdata latch circuit 66. An output 67 of the autodialler 62 controls the dial off normal (DON) relay contact 68 while an output 69 controls the inpulsing relay contact 70. A terminal 71 is connected to a second input 72 of the Viewdata latch circuit 66 while the output 73 of the Viewdata latch circuit 66 is connected to a terminal 74, the delay circuit 47 and a shared service control circuit 75. An output 76 of the shared service control circuit 75 controls relay contacts 77 and 78.

In operation a signal on terminal 71, which may be generated by a push button, denotes that a connection to the Viewdata computer is required. The Viewdata latch circuit 66 produces an output which is available on terminal 74 to give an indication that the coupling unit has received this instruction, to operate the shared service control circuit 75 which closes contacts 77 and 78, and to enable the loudspeaker through the delay circuit 47. A dial instruction is generated and fed to the autodialler 62 which causes the necessary dialling pulses to be sent along the telephone line to make the connection. The loudspeaker is enabled during this time so that the progress of the setting up of the connection can be monitored.As described hereinbefore the carrier detector 20 is designed so that the tones which occur on the line before the connection is completed are not detected as a receive carrier even through harmonics of their frequency lie within the pass band of the filter 4.

When the connection has been set up a receive carrier signal is received from the Viewdata computer and is detected by the carrier detector 20 and a signal is available at terminal 46 LO enable the decoding circuits of the terminal to decode the output from output 50 of the modem 49 which is available on terminal 51. A signal is generated in the terminal in response to the signal from terminal 46 and applied to the terminal 81 to enable the modem 49 to produce a transmit carrier signal at its output 54 in response to signals applied to input 53 via terminal 52. The output 17 of the carrier detector 20 is also fed via the delay circuit 47 to the loudspeaker amplifier 45 to mute the loudspeaker once the receive carrier has been detected, the delay circuit providing approximately two seconds delay after the receive carrier has been detected.When it is desired to disconnect from the Viewdata computer a signal is generated conveniently from a pushbutton, and fed to terminal 63. The signal resets the latch circuit 66 and the autodialler 62.

WHAT WE CLAIM IS:- 1. A circuit arrangement for producing an output signal in response to an alternating input signal in a first frequency band, the circuit arrangement comprising means for feeding an alternating signal from its input to the input of a filter and to the input of a detector. the filter having a pass band which includes the first frequency band and the detector being adapted to produce an output signal in response to alternating input signals having a fundamental frequency below the first frequency band, means for coupling the output of the filter to a first input of a signal generator, means for coupling the output of the detector to a second input of the signal generator, means for coupling the output of the signal generator to a second input of the detector, and means for coupling the output of the signal generator to the output of the circuit arrangement wherein in operation the signal generator produces in the absence of a signal at its second input an output signal in response to a signal at its first input, the output of the detector being inhibited in the presence of a signal at its second input, and the output signal from the signal generator is the output signal of the circuit arrangement.

2. A circuit arrangement as claimed in Claim 1 in which the detector responds to signals in the frequency range 200-700Hz.

3. A circuit arrangement as claimed in Claim 1 or 2 in which the pass band of the filter includes the range 1200--2500Hz.

4. A circuit arrangement substantially as described herein with reference to Figure 1 or to Figures 1, 2 and 3 of the accompanying drawings.

5. A line coupling unit for a Viewdata terminal including a circuit arrangement as claimed in any preceding claim.

6. A line coupling unit for a Viewdata terminal substantially as described herein with reference to Figure 4 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. coupling unit has received this instruction, to operate the shared service control circuit 75 which closes contacts 77 and 78, and to enable the loudspeaker through the delay circuit 47. A dial instruction is generated and fed to the autodialler 62 which causes the necessary dialling pulses to be sent along the telephone line to make the connection. The loudspeaker is enabled during this time so that the progress of the setting up of the connection can be monitored. As described hereinbefore the carrier detector 20 is designed so that the tones which occur on the line before the connection is completed are not detected as a receive carrier even through harmonics of their frequency lie within the pass band of the filter 4. When the connection has been set up a receive carrier signal is received from the Viewdata computer and is detected by the carrier detector 20 and a signal is available at terminal 46 LO enable the decoding circuits of the terminal to decode the output from output 50 of the modem 49 which is available on terminal 51. A signal is generated in the terminal in response to the signal from terminal 46 and applied to the terminal 81 to enable the modem 49 to produce a transmit carrier signal at its output 54 in response to signals applied to input 53 via terminal 52. The output 17 of the carrier detector 20 is also fed via the delay circuit 47 to the loudspeaker amplifier 45 to mute the loudspeaker once the receive carrier has been detected, the delay circuit providing approximately two seconds delay after the receive carrier has been detected.When it is desired to disconnect from the Viewdata computer a signal is generated conveniently from a pushbutton, and fed to terminal 63. The signal resets the latch circuit 66 and the autodialler 62. WHAT WE CLAIM IS:-

1. A circuit arrangement for producing an output signal in response to an alternating input signal in a first frequency band, the circuit arrangement comprising means for feeding an alternating signal from its input to the input of a filter and to the input of a detector. the filter having a pass band which includes the first frequency band and the detector being adapted to produce an output signal in response to alternating input signals having a fundamental frequency below the first frequency band, means for coupling the output of the filter to a first input of a signal generator, means for coupling the output of the detector to a second input of the signal generator, means for coupling the output of the signal generator to a second input of the detector, and means for coupling the output of the signal generator to the output of the circuit arrangement wherein in operation the signal generator produces in the absence of a signal at its second input an output signal in response to a signal at its first input, the output of the detector being inhibited in the presence of a signal at its second input, and the output signal from the signal generator is the output signal of the circuit arrangement.

2. A circuit arrangement as claimed in Claim 1 in which the detector responds to signals in the frequency range 200-700Hz.

3. A circuit arrangement as claimed in Claim 1 or 2 in which the pass band of the filter includes the range 1200--2500Hz.

4. A circuit arrangement substantially as described herein with reference to Figure 1 or to Figures 1, 2 and 3 of the accompanying drawings.

5. A line coupling unit for a Viewdata terminal including a circuit arrangement as claimed in any preceding claim.

6. A line coupling unit for a Viewdata terminal substantially as described herein with reference to Figure 4 of the accompanying drawings.