Speech Controlled Switching Arrangement

Abstract

To differentiate speech on an incoming line from control tones, such as engaged tone signals, busy signals, or other non-speech frequencies, so that associated recording apparatus will not be activated by non-speech frequencies but will be activated by speech frequencies, the incoming signal is fed to a discriminator which discriminates on the basis of time dependent frequency variations (i.e. in the intervals between changes in frequency which in the case of speech are different and much shorter than in the case of control tone signals), rather than on the conventional basis of control tones and speech being of different frequencies. Control tones, since they are of substantially constant frequency, even though intermittent and separated by pauses, will not activate an associated switching device to start the operation of the associated recorder. On the other hand, signals of varying frequency or modulated frequency, as in speech, will activate the switching device. This is accomplished in one form of system by passing the incoming signal successively through an amplitude limiter, a discriminator, a separator unit, an amplifier, and a comparator stage, the output of which activates a switching device.

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 206,798, filed Dec. 10, 1971 now abandoned.

Description

BACKGROUND OF THE INVENTION

This invention relates to a speech-controlled switching arrangement for recording apparatus, and in particular for telephone answering systems, which discriminates between speech frequencies on the one hand, and predetermined signal frequencies (for example, busy signals, or engaged tones on telephone lines) on the other hand hereinafter sometimes called control tones.

In speech controlled arrangements of this kind, as used for example in telephone answering systems, difficulties may arise because, in addition to the speech frequencies, various signal frequencies may occur in the range of speech frequencies. For this reason, it may happen, in a telephone answering system, that the signal frequencies trigger the same effect as the speech frequencies. Thus a signal frequency, for example, an incoming engaged tone may be misinterpreted by an automatic telephone answering system as incoming speech, and in some circumstances may use up all or a substantial part of the available storage capacity of the recording device, recording thereon merely the engaged tone or other useless information.

From German Offenlegungsschrift No. 1,180,789 (a German patent application which is available for public inspection) it is known to apply the incoming signal in a recording appliance to the input of a monostable multivibrator which is connected in series with a differentiator, a rectifier, and a further monostable multivibrator, the output signal from the second monostable multivibrator being fed to an integrating unit after it has been rectified. A voltage proportional to the frequency of the incoming signal is set up in this integrating unit. An appropriate switch is operated, or not operated, depending on whether this voltage lies above or below a predetermined threshold value.

In spite of the considerable outlay required in a speech-controlled arrangement of this character, it is not always possible to achieve a satisfactory solution to the problems which arise. In the first place, this is attributable to the fact that reaching and surpassing the predetermined threshold only takes place very gradually as the frequency increases, so that the critical switching point is not always absolutely clear. The main disadvantage of this known arrangement is, however, the fact that it is found impossible to suppress undesirable effects arising from signal frequencies which lie in the medium or upper range of speech frequency.

In accordance with the present invention, the difficulties and drawbacks of the prior art are overcome by using a discriminator arrangement which is responsive to time-dependent variations in the frequency of speech signals, which are present in the case but not for control tones. The output signal from such discriminator is used as the criterion for operation or non-operation of a switching device. By discriminating on the basis of time variations rather than merely on the basis of different frequencies, the system insures satisfactory and reliable discrimination between speech and signals, regardless of whether the signal frequencies lie in the upper, medium, or lower range of the transmitted speech frequency band. By the dependent frequency variations meant variations in the intervals between the instants of time when the frequency of a signal changes, which variations are not of constant frequency and thus not independent of time. In speech signals, the frequency variations are time dependent in that they are of a random nature and can occur in rapid succession.

Control tones, such for example as an engaged tone, are usually constant frequencies and do not change in frequency (i.e. do not exhibit the time dependent frequency variations of speech signals). Thus, according to the present invention, no output voltage, or only an output voltage which is very small in comparison to that produced by speech frequencies, occurs at the output of the discriminator. On the other hand, speech signals continuously change in frequency and have an adequate time-dependent frequency variation to which, the discriminator is responsive to provide a large output voltage which results when the speech signals are present, but not when the control tones occur.

In transmission networks in which very severe fluctuations in level may occur between the speech signals on the one hand and the control tones (such for example as engaged tones) on the other hand, it is advantageous to connect an amplitude limiter before the discriminator in the comparator connections.

The arrangement or system as a whole may be referred to broadly as a comparator system or comparator arrangement.

In a preferred embodiment of the invention, the arrangement is so devised that at least two alternately conductive and blocking switching stages are provided in the comparator system, and they only permit operation of the switching device when they are simultaneously conductive.

In another embodiment of the invention, the arrangement is so devised that only impulses at very short intervals resulting from the speech signals will produce the simultaneously conductive condition of the switching stages for operation of the switching device.

A simple and economical version of the comparator system is obtained, in accordance with a further feature of the invention, by providing each of the switching stages of the comparator system with at least one transistor, the time constants of these transistors being of different value.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention are described in more detail below, with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of the complete connections of the apparatus according to one embodiment of the invention;

FIG. 2 is a similar block diagram of a second embodiment of the invention;

FIG. 3 is a wiring diagram of the comparator system; and

FIGS. 4 and 5 are pulse diagrams referring to the comparator system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the first embodiment of the invention as illustrated schematically in the block diagram of FIG. 1, the first element of the monitoring and comparator system is an amplitude limiter 4 which brings the speech signals S and the control tones B to the same amplitude. At the output of the amplitude limiter 4, the speech signals have the form indicated at SL, while the engaged tones or signals have the form indicated at BL. It will be noted that the amplitude limited signals SL are subject to time-dependent variations in that the instants of time when the frequency thereof changes occur randomly, as shown. In contrast, the limited engaged tones BL are not subject to time-dependent variations in that the tones are of a single frequency only.

Connected after the amplitude limiter 4 there is a discriminator indicated in general at 5, which converts the time of frequency change of the limited speech signals SL into a signal which varies in amplitude level (i.e., has a significant A.C. component) while the limited control tones BL, because of their constant frequency, give no output voltage or only a very small output voltage which varies in amplitude (i.e. is essentially D.C. without any significant A.C. component). After passing through a rectifier stage in the discriminator 5, and a separator unit 6, the speech signals take the form indicated at SL', and exhibit a large amplitude variation with relatively high voltage peaks. In contrast, the controlled tones, after passing through the separator unit 6, are of the form indicated at BL' (for interrupted engaged tones) or BL" (for continuous engaged tones). Apart from transient phenomena, the levels of signals BL' and BL" are practically constant, or only very minor extent. In other words the difference between the amplitudes of the SL' signals as compared to the BL' or BL" signals is significant.

The output of the unit 6 is passed through an amplifier 7 to a comparator stage 8 having a threshold level such that the presence of speech signals will be recognized when the threshold level is exceeded, and the presence of control tones, such as include tones and other signalling, will be indicated below this threshold level. The disturbing effect of temporary voltage peaks, which may occur for instance at the start of an engaged tone, may be inhibited by providing a delay circuit which will not permit emission of a signal to trigger the recording means when the voltage threshold in the comparator stage 8 is only exceeded temporarily.

The threshold level is preferably adjustable. Depending upon whether the threshold level in the comparator stage 8 is reached (by input of a speech signal) or is not reached (by input only of a control tone), the recording means to which the present apparatus is connected will be brought into operation by a switching device or switching unit 9 when there is a speech signal but will be cut out where there is only a control tone, and will then be prepared for a fresh incoming call.

Referring now in more detail to the previously mentioned discriminator unit 5, the output signal from the limiter 4, is applied to a differentiating circuit consisting of a series capacitor C9 and shunt resistor R10 which differentiates the limited output (SL or BL) into a signal the amplitude of which will fluctuate with time. These amplitude fluctuations (in other words the positive peaks of the differentiated limited signals SL and BL) are rectified by the diode D4 and applied to a smoothing capacitor C10. An adjustable resistance or potentiometer P4 is connected across the capacitor C10 which provides the output of the discriminator 5. This output is applied to the coupling capacitor C11 in the separator unit 6.

The system thus far described in connection with FIG. 1 operates as follows: If speech signals are fed into the system, the changes in frequency and thus the changes in amplitude of the limited speech signals SL occur at intervals which vary in duration (see waveform SL as compared to wave form BL). The thereof voltage produced across the capacitor C9, and thus across the resistance R10 correspondingly occurs at intervals which vary in duration. This voltage is rectified through the diode D4, permitting successive charging of the capacitor C10 via the diode D4 and of capacitor C10 across the potentiometer P4. The level of the voltage across the capacitor C10 and across the potentiometer P4 varies correspondingly (i.e. has a significant A.C. component). Depending upon the regulation of sensitivity by the setting of the adjustable resistance or adjusting potentiometer 14, a correspondingly varying proportion (A.C. component) of this voltage at the capacitor C10 passes through the coupling capacitor C11 in the separator unit 6, to the input of the amplifier 7 and of the threshold level switch 8.

If, in contrast, individual signals of a constant frequency arrive due to control tones, for example in the form of an engaged tone, the frequency of the voltage due to the capacitor C9 which appears across the resistor R10 is regularly repetitive so that, after rectification through the diode D4, there is a voltage at capacitor C10 which remains constant in amplitude level with relation to time (viz. is essentially D.C.). Practically no input voltage will be applied to the threshold level switch 8, in view of the interposed coupling capacitor C11 and because there is no change in voltage. In other words the coupling capacitor C11 functions as such and blocks D.C. levels due to the control tones while passing A.C. levels due to the speech signals. Consequently this switch 8 will not be operated by control tones but will be operated by speech signals for control tones. Thus there is no input voltage, or only a voltage which is very slight in comparison with the threshold at which there is to be a response, and thus there will be no voltage at the switching device 9. The missing signal criterion will thus be met, so that the switching device wil forthwith switch off the recording applicance, because no speech signal is present, but only control tones.

At times, it may be desired to use a symmetrical push-pull discriminator, which may be arranged so that the output voltage after demodulation becomes zero for superimposed markings or engaged tones, if the frequency thereof is applied to the middle of the discriminator characteristic line as a constant disturbance frequency to be suppressed.

Also, it may be advantageous in many instances to shunt off a second transmission line after demodulation and rectification. FIG. 1 shows schematically the use of such a second transmission line, branching off between the elements 5 and 6 of the first transmission line. The second line incorporates an integrating member 10, advantageously in the form of an RC member (resistance capacitance member), along with an amplifier 11 and a relay 12. When such a second transmission line is used, it may be advantageous to connect the output end of this second line to the switching device 9 which is connected next after the comparator stage 8, and make this switch device operable by signals in the second transmission line which are in addition to and independent of the signals of the first transmission line. This may lead to the facility for suppressing disturbances.

Another embodiment of the invention is illustrated schematically in FIG. 2. Many parts of this embodiment are similar to those already described in connection with FIG. 1, and require little or no further discussion.

The first element of the system in FIG. 2 is the amplitude limiter 4 which brings the speech signals S and the control tones which include interrupted engaged tones or other signalling B to the same amplitude. At the output of the amplitude limiter 4, the speech signals have the shape indicated at SL and the engaged tones signals have the shape illustrated at BL. It will be noted as explained above that the speech signals SL are subject to frequency variations which charge randomly with times, that is, are subject to a kind of frequency modulation. In contrast, the control tones of constant or single frequency and are not subject to such variations.

The output of the amplitude limiter 4 constitutes the input of the discriminator 5, which converts the output level which changes in speech signals SL into an amplitude. Because of the absence of any frequency variation i.e., its a significant A.C. component, limited control tones BL, the discriminator 5 produces an output level which is essentially constant in amplitude (i.e. D.C.) After passing through a rectifier stage in the discriminator 5, and through the separator unit 6, the speech signals acquire the shape indicated at SL' in FIG. 5; that is, they have large amplitude excursions and relatively high voltage peaks. On the other hand, the control tones have the shape indicated at BL' in FIG. 4, at the output of the separator unit 6. Apart from voltage peaks at the beginning and at the end, the signals BL' exhibit no amplitude excursion, or only a very slight.

In the discriminator 5, the output signal from the limiter 4 is differentiated by the capacitor C9, and resistor R10: diode D4 and applied to a snoothing capacitor C10. These differentiated output signals are rectified by the capacitor C10 is connected across the adjustable resistance or potentiometer P4 which is connected to the coupling capacitor C11 in the separator unit 6. It will be noted that these elements operate in the same way as the corresponding elements in FIG. 1.

The output of the separator unit 6 is fed into the amplifier unit 7, and the output of the unit 7 is fed into the comparator unit 8, shown in greater detail in FIG. 3. This comparator unit or stage 8 includes two switch stages consituted by the transistors T4 and T5, and corresponding doubling capacitors C14/C16 and C12 which, with the base-emitter resistance of the transistor T4, give the different time constants of the two switch stages. The diodes D5 and D6 are included for rectification and voltage duplication doubling. Resistances R18, R19 and the base of transistor T4 control the discharge time of the capacitor C16. The switching device 9 is connected to the comparator stage 8 at K1 and K2. The switching device operates (i.e., turns on or off) the recording appliance A, for example a magnetic tape recorder. A transistor T3, representing the amplifier unit 7, is disposed in advance of the comparator stage 8, as seen in FIG. 3.

The system shown schematically in FIGS. 2 and 3 operates as follows: When speech signals come in the capacitor C9 and resistance R10 provide a signal which varies in amplitude and has amplitude excursions which are time variant in accordance with the frequency changes in the speech signals. These signals (the limited, differentiated speech signals) thus vary randomly in their occurrence with time. These signals are rectified by the diode D4. The capacitor C10 is charged through the diode D4 and discharged via the potentiometer P4. Depending on the adjustment of sensitivity by the adjustable or regulatable resistance or potentiometer P4, a correspondingly (A.C.) part of this voltage is applied to the coupling capacitor C11 which presents an output signal corresponding to the varying or A.C. part of the voltage from the potentiometer P4 (see FIG. 5) to the input of the amplifier 7, i.e., of the transistor T3.

On the other hand, when the control tones, e.g. due to an engaged tone signal arrive, a positive impulse is produced at the input of the amplifier-transistor T3 at the beginning of the input of such signal frequencies, and a negative impulse at the end. These impulses are conditioned by the voltage peaks already mentioned, and have a pulse length of about 20 millliseconds. When, for instance, there is an engaged tone signal of 250 ms. duration and a succeeding pause of about 250 ms. duration, the pulse diagram is that illustrated in FIG. 4. Positive impulses are applied at intervals of 500 ms. at the input of amplifier 7, or its transistor T3, and negative impulses, with a phase shift of 250 ms., are likewise applied at intervals of 500 ms., these being correspondingly amplified and transmitted to the succeeding comparator stage 8.

A positive impulse arriving at the comparator stage 8 illustrated in FIG. 3 causes a charging of the capacitor C16, and the transistor T4 becomes conductive for a short period of about 50 to 100 milliseconds after the threshold level of the transistor T4 is reached. This conductive condition of the transistor T4 is illustrated in broken lines in the diagram of FIG. 4. In contrast, a succeeding negative impulse causes a discharging of the capacitor C12, and the transistor T5 becomes conductive for a period of 10-20 ms. when the threshold level is reached. The conductive condition of this transistor T5 is illustrated with dot-dash lines in FIG. 4.

The switch 9 for operating (i.e., for starting) the recording appliance A, which is connected to the terminals K1 and K2 of the comparator stage 8, becomes active only when the two transistors stages T4 and T5 of the comparator stage 8 are simultaneously conductive. As clearly shown in FIG. 4, this is not the case when the input is the signal frequency of an engaged tone signal, because the conductive conditions of transistors T4 and T5 are produced at intervals one from another, and the two transistors do not become simultaneously conductive. When speech signals are received, however, the positive and negative impulses follow one another in quick succession timewise, and at intervals of only about 10-20 milliseconds. The result of this is that shortly after the commencement of a speech frequency signal, there is a simultaneous conductive condition of the two transistors T4 and T5 of the comparator stage 8, as illustrated in FIG. 5, where it is seen that the broken line impulse and the dot-dash line impulse overlap each other. As a result of the simultaneous conductivity of the transistors T4 and T5, the switch 9 responds, and the recording appliance A is turned on to receive the spoken message.

It is to be understood that the invention is not limited to the particular system of the comparator stage 8 which has been described above as an example, but that various forms of this stage are feasible. For example, the time constants of the two switching stages T4 and T5 might even be kept the same, if the impulses of the two stages are of sufficient duration.

Better results are obtained if the discriminator 5 is a discriminator of the type known in the art as a flank discriminator or edge-steepness discriminator. This is a special kind of discriminator (known per se) which measures only the speed of change of an electric pulse, i.e., the steepness of the edge or flank of the pulse, rather than measuring the height or absolute value of the pulse.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Speech-operated switching apparatus for controlling a switch unit adapted to control a recording appliance in a telephone answering and message recording system, which apparatus is responsive to input speech signals and to other input signals of substantially constant frequency arriving from a telephone line, said apparatus comprising:

a. means responsive to said input signals for converting said input signals into signals which change in amplitude,

b. means responsive to the signals from said last named means for providing output signals which exceed a certain treshold when the frequency of said input signals changes at a rate corresponding to the rate of change of the frequency of said speech signals, and

c. means for operating said switch unit when the interval between successive ones of said output signals is less than a certain interval which is characteristic of time dependent variations in the frequency of said speech signals.

2. The invention as set forth in claim 1 wherein said means for operating said switch unit comprises a comparator stage including first and second transistor stages connected in series, said first and second stages having R/C input circuits each having a different charging time constant, and means for applying said output signals simultaneously to said input circuits.

3. The invention as set forth in claim 2 wherein said first transistor stage includes a PNP transistor and said second transistor stage includes an NPN transistor, the collector emitter paths of said transistors being connected in series, means for connecting said series connected paths to said switch unit, a voltage doubler circuit being connected to the base of the transistor in one of said first and second stages, and a capacitor being connected to the base of the transistor in the other of said transistor stages, said output signal applying means being connected to the input of said voltage doubler circuit and to said last-named capacitor.

4. The invention as set forth in claim 1 wherein said converting means comprises differentiating means, and wherein said means for providing output signals includes:

i. means responsive to the signals from said differentiating means for providing an output signal level which varies in accordance with the time variations in the occurrence of said signals from said differentiating means, and

ii. coupling means responsive to said output signal level for transmitting signals which change in amplitude, said coupling means providing said output signals.

5. The invention as set forth in claim 4 wherein said differentiating means is a flank discriminator.

6. The invention as set forth in claim 4 wherein said differentiating means comprises a series capacitor and a shunt resistor across which said input signals are applied; and wherein said output signal level providing means comprises a rectifying circuit connected to the junction of said resistor and capacitor; and wherein said coupling means is a coupling capacitor.

7. The invention as set forth in claim 6 including an amplitude limiter for converting said input signals into square waves repetitive at the frequency of said input signals for applying said input signals across said series capacitor and shunt resistor.

8. The invention as set forth in claim 7 wherein said rectifying-circuit comprises a rectifier connected to a smoothing capacitor, said coupling capacitor being connected to said smoothing capacitor.

9. The invention as set forth in claim 8 wherein said means for operating said switch unit comprises a comparator stage including first and second transistor stages connected in series, said first and second stages having R/C input circuits each having a different charging time constant, and means for applying said output signals simultaneously to said input circuits.

10. The invention as set forth in claim 9, wherein said first transistor stage includes a PNP transistor, the collector emitter paths of said transistors being connected in series, means for connecting said series connected paths to said switch unit, a voltage doubler circuit being connected to the base of the transistor in one of said first and second stages, and a capacitor being connected to the base of the transistor in the other of said transistor stages, said output signal applying means being connected to the input of said voltage doubler circuit and to said last-named capacitor.