IE41753B1 - Improvements in or relating to voice frequency receiver - Google Patents

Abstract

1479166 Voice frequency receivers STADABD TELEPHONES & CABLES Ltd 28 Oct 1975 [29 Oct 1974] 46745/74 Heading H4R During a persistence check in a V.F. receiver the rate of change of the envelope of an incoming tone is monitored and if the rate of change exceeds a predetermined limit the output of the receiver is inhibited. In a first embodiment the rate of change detection circuit Fig. 3 is applied to a known automatic gain control arrangement connected between a tone input and the frequency selective circuits, the rate of change detector monitoring the negative feedback voltage derived from the envelope of the received signal. In Fig. 3, the network C-R1 is such that a suitable voltage appears across R1 for a given rate of change of input voltage, voltage comparator 2 responding to a positive voltage across R1 exceeding a threshold level to inhibit the gate G and comparator 1 similarly responding to a negative voltage. The circuit of Fig. 3 may also be applied to a receiving arrangement as illustrated in Fig. 1 (not shown).

Description

This invention relates to speech immunity circuits for voice-frequency (VF) and multifrequency (MF) receivers, such as used in telephone exchanges.

An essential feature of all such receivers is the ability to accept certain frequencies and reject others, regardless of the amplitude of these frequencies, . within certain defined limits. One class of receiver obtains this independence from amplitude changes by using a limiting amplifier, which produces a squared waveform output of constant amplitude for all input amplitudes in excess of a threshold. This squared output is applied to one or more frequency-selective circuits, and these determine whether or not the frequency of the square wave . lies within acceptable limits. Receivers of this class are prone to accept speech signals as tone signals, since no speech immunity circuit is provided.

A second class of receiver, having a much greater immunity to speech signals, obtains the necessary independence from amplitude changes by deriving a steady potential from the input signal, and using this potential to maintain a constant . absolute output signal from the frequency selective circuits. This technique also operates as a speech immunity circuit, reducing the number of signal simulations obtained from speech, as will be seen later.

This invention has as its object the provision of improved speech-immunity, - 2 41753 especially in systems of the second type.

According to the present invention there is provided a speech immunity arrangement for a voice frequency or multi-frequency receivers which includes Means responsive to an alternating current input to the receiver to generate . a voltage whose amplitude is proportional to that of the input and varies with the variation of the input's amplitude, a circuit to which said voltage is applied and which responds to the rate of change of that voltage during the performance of a persistence check on an alternating current input, and means to inhibit the output of the receiver if this rate of change exceeds a . predetermined value.

An embodiment of the invention will now be described with reference to the drawings, in which Figures 1 and 2 show speech immunity arrangements of known type, while Figure 3 is an embodiment of the invention usable, inter alia with the circuit of Figure 1 or Figure 2.

. In Figure 1(a) the envelope detector ED produces a d.c. output proportional to the a.c. signal applied to it, and independent of its frequency. The a.c. signal is also applied to one or more frequency selective circuits (four are shown) and, if of an acceptable frequency, a magnified voltage at that frequency appears accross the appropriate inductor, this voltage being indicated by V^.

. The voltage V, which is measured with respect to system earth, is equal to VL minus Vg. Figure 1(b) shows how variations in VL are offset by equal and opposite variations in Vg, thus maintaining V constant. in Figure 2 a system of automatic gain control is used to maintain the output signal νθ constant and related to a reference voltage V regardless . of changes in the amplitude of the input signal.

Both these methods employ negative feedback and must respond fast enough to cater for the maximum rate at which bursts of tone may be received, but an upper limit to their rate of response is set by the time constants of the circuits - 3 41753 used. That is, there exists a limited bandwidth of frequencies over which the negative feedback can operate, and within that bandwidth it operates to remove amplitude variations in the input signal, regardless of frequency. Also, when two or more frequencies are simultaneously present, resulting in a modulation , envelope whose rate of change exceeds the bandwidth of the feedback system the latter responds to the average of this envelope, thus giving a greater d.c, feedback than would be obtained for one frequency alone and hence preventing recognition of the frequency. The new arrangement is an extra circuit to be added to an existing circuit of the form described in either Figure 1 or 2 and . which improves the signal immunity.

Note that the circuits of Figures 1 and 2 would not prevent simulation if a tone signal of acceptable frequency but varying amplitude were applied to the receiver provided the rate of amplitude variation lay within the bandwidth of the feedback system. Such a signal could be a true amplitude modulated . signal, consisting of a carrier frequency and symmetrically disposed sidebands, or it could consist of the linear addition of two or more sinewaves of arbitrary amplitude and frequency, since the latter waveform has an envelope that varies at a frequency corresponding to the difference between the frequencies of the two sinewaves. Speech consists of a large number of frequencies occurring simultan20. eously hence many difference frequencies occur in the resulting envelope, and many will be within the bandwidth of the feedback system.

All VF and MF receivers carry out a persistence check on an incoming signal. That is, once they have recognised a frequency as within the accepted range they time its duration'to ensure that it persists for at least some . minimum length of time, say 33 milliseconds. The circuit of Figure 3 measures the rate of change of feedback voltage occurring during this persistence check, and, if it exceeds a predetermined value, inhibits the receiver from accepting the signal. Thus, for example, an input signal consisting of an acceptable frequency but amplitude modulated would no longer be accepted as valid.

. In Figure 3, the time constant of the coupling network C-Rl is so chosen that a suitable voltage appears across resistor Rl for a given rate of change - 4 41753 of input voltage. Should this voltage across Rl reach a sufficiently positive value to overcome the bias provided by resistors R2 and R3 for Voltage Comparator 2, then the latter triggers, reversing its output potential from its maximum positive potential, e.g. + 12 volts, to its maximum negative potential, e.g. - 12 S. volts, and inhibiting the gate G and hence the output of the receiver. In a similar manner Voltage Comparator 1 operates when the voltage across Rl reaches a sufficiently negative value to overcome the bias on the comparator, and it also inhibits gate G. If adequate protection against signal imitation is obtained by sensing rates of change of input voltage in only one direction then one of the . voltage comparators and its associated components may be removed.

In the circuit shown in Figure 3, the resistance values are so chosen that R3=fi4 and R2=R5.

In the case of MF receivers which divide incoming frequencies into two bands, as for example is done in the case of the 2X(1 out of 4) code, then feedback . voltage rate sensing may be carried out on both bands, or only on one, depending on the degree of speech immunity required, since each band has a receiving system provided for it similar to that shown in either Figure 1 or Figure 2.

A normal tone signal causes a rectangular waveform to appear at point A of either Figure 1 or Figure 2. When this is applied to the network C-Rl, Figure . 3, voltages appear at its output of sufficient amplitude to trigger the comparators at the start and finish of the waveform due to the differentiating action of C-Rl. To ensure that a normal tone is not rejected, means is provided in the timing cycle of the persistence check to ignore the output of the comparators during the initial transient. A normal tone is always accepted before it ends, . assuming that it is long enough, so there is no need to provide means to ignore the end of waveform transient. In the circuit of Figure 1 the initial transient is always in the negative direction, while in Figure 2 it may be in either direction dependent on circuit details.

If only a single comparator is used which responds to the opposite polarity · from that, of the initial transient, then no provision is needed to ignore the initial transient.

Claims (8)

1. A speech immunity arrangement for a voice frequency or multi-frequency receiver, which includes means responsive to an alternating current input to the receiver to generate a voltage whose amplitude is proportional to that of the input and varies with the variation of the input's amplitude, a circuit to which 5. said voltage is applied and which responds to the rate of change of that voltage during the performance of a persistence check on an alternating current input, and means to inhibit the output of the receiver if this rate of change exceeds a predetermined value.

2. An arrangement as claimed in Claim 1, and in which the means to 10. inhibit the receiver output includes a differentiating Circuit to which the input is applied and which produces an output whose value depends on the rate of change of the input signal, and a voltage comparator to which said output is applied and gives an Output when said rate of change exceeds a preset value.

3. An arrangement as claimed in Claim 2, and in which two said voltage 15. comparators are provided, one of which is responsive to a rate of change in one sense while the other is responsive to a rate of change in the other sense.

4. An arrangement as claimed in claim 2 or 3, and in which each said voltage comparator is an operational amplifier having a first input to which the output of the differentiating circuit is applied and a second input to 20. which a voltage appropriate to the maximum permissible rate of change is applied. 25.

5. A speech immunity arrangement for a voice frequency or multi-frequency receiver, substantially as described with reference to Figure 3 of the accompanying drawings.

6. A voice frequency or multi-frequency receiver using an arrangement as claimed in any one of claims 1 to 5. - 6 41753

7. A voice-frequency or multi-frequency receiver substantially as described with reference to Figures 1 and 3 of the drawings.

8. A voice-frequency or multi-frequency receiver substantially as described with reference to Figures 2 and 3 of the drawings.