Telecommunication Apparatus

Abstract

In telecommunications apparatus for transmitting a speech signal and at least one digital data signal in different frequency ranges over a common channel, the amplitude of the data signals is made to follow the mean amplitude of the speech signal. An amplitude limiter circuit may be provided in the speech signal path; the amplitude-limiting action of this circuit is preferably controlled by a signal derived from a sample or simulation of the intermodulation, distortion and noise-producing effects of the common channel. A controllable amplitude-limiter circuit is described.

Parent Case Text

This application is a continuation of application Ser. No. 661,877, filed Aug. 21, 1967, and now abandoned.

Description

The present invention relates to apparatus for telecommunications systems of the type wherein a speech signal and a data signal (or data signals) are transmitted in different frequency bands over a common telecommunications channel. In a commonly used system of this type, a narrow band of frequencies in the speech signal is suppressed and telegraph signals are transmitted within this narrow frequency band, being separated out by filters at the receiving end of the channel. When the speech signal is an analogue signal and the bit rate of the data (e.g. telegraph) signal is comparatively low, such a system is often quite satisfactory. However, in modern telecommunications systems it may be desired to use a digitized speech signal, or a method of transmission involving a digitization of the combined signals, for instance delta-modulation. In these circumstances, quantizing noise generated as a result of the digital nature of the transmitted signals can be troublesome. Quantizing noise generated from the transmission of the digital data signals spreads into the frequency band allocated to the speech signal and tends to degrade the quality of the speech signal, while quantizing noise generated from the transmission of the digitized speech signals spreads into the frequency band allocated to the data signals and tends to degrade their signal to noise ratio. Normal speech signals vary over a very wide dynamic range; when the speech is loud, the noise derived from it may seriously interfere with the reception and decoding of the data signals, yet when the speech is soft, the noise derived from the data signals may seriously degrade the quality and intelligibility of the speech signal.

It is an object of the present invention to provide telecommunications apparatus arranged to mitigate these effects.

According to the present invention, there is provided telecommunications transmitting apparatus for transmitting a speech signal and at least one digital data signal in different frequency ranges over a common telecommunications channel, wherein it is arranged that the data signal (or data signals) shall be amplitude modulated by a modulator responsive to the amplitude of the speech signal. The modulator may be responsive to the amplitude of the speech signal only, or alternatively it may be responsive to the amplitude of the combined signal of which the speech signal forms the major part. Preferably the arrangements for producing the amplitude modulation of the data signals should be arranged to operate with a time constant in the range from 100 milliseconds to 300 milliseconds.

By this arrangement, the amplitude of the data signal or data signals is made to follow the mean level of the speech signal. The amplitudes of the quantizing noise signals generated naturally follow the mean levels of the signals causing them, and so the signal-to-noise ratios for both the speech signals and the data signals are stabilized in spite of the wide dynamic range of the speech signal. If the time constant of the amplitude modulator is suitably chosen within the preferred range specified hereabove, the data signals can be received on existing apparatus since such apparatus is conventionally arranged to operate satisfactorily over a considerable range of signal amplitudes, to allow for "fading" effects on radio communication channels.

To improve the quality of the transmissions, or to maintain it in difficult circumstances, the speech signal may be passed through an amplitude limiter circuit. However, any amplitude limitation of the speech signal must also be regarded as a distortion which degrades it.

In some embodiments of the invention, the apparatus includes a voltage-controllable amplitude limiter circuit connected in series with the speech signal input channel and control means for sampling the speech signal and deriving from it a signal to control the amplitude limiter. The control means may include a band-stop filter connected to receive the speech signal or the output of the amplitude limiter, a line simulator circuit connected to the output of the band-stop filter, for simulating the distorting and cross-modulating effects of a transmission system on the speech signal, a band-pass filter connected to the output of the line simulator circuit and a control circuit for rectifying the output of the band-pass filter and deriving therefrom a voltage to control the amplitude limiter. The control circuit may be arranged to have a threshold effect.

The action of the control means is preferably arranged to have a decay time constant of approximately 100 milliseconds but a comparatively fast onset.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which:

FIG. 1 is a schematic circuit diagram of transmitting apparatus for a voice-frequency telegraphy and telephony system using delta-modulation,

FIG. 1a is a schematic circuit diagram of a modified form of the apparatus of FIG. 1,

FIG. 2 is a schematic circuit diagram of a modified form of the apparatus of FIG. 1, and

FIG. 3 is a circuit diagram of some circuits used in the apparatus of FIG. 2.

FIG. 1 shows a microphone 1 connected to apply a speech signal through an amplifier 2 to a band-stop filter 3. The drawing also shows a teleprinter 4 controlling a tone modulator 5. The tone modulator 5 is arranged to gate a 1,680 cycles per second tone signal generated by a tone source 6. The output of the tone modulator 5 is passed through an amplitude modulator 7 to a band-pass filter 8. The output of the amplifier 2 is also applied to a detector 9, whose output controls the amplitude modulator 7.

The outputs of the band-stop filter 3 and the band-pass filter 8 are combined and connected to the input of a delta modulator 10. The output of the delta modulator 10 is connected to a transmitter 11 and the output of the transmitter 11 is connected to an aerial 12. The stop band of the band-stop filter 3 and the pass band of the band-pass filter 8 are similar, having a bandwidth of about 400 cycles/second centered on 1,680 cycles per second.

In the operation of this apparatus, the output of the detector 9 follows the main amplitude of the speech signal at the output of the amplifier 2, and controls the amplitude of the tone pulses which form the data signals so that their amplitude is proportional to the mean level of the speech signals. These tone pulses pass through the band-pass filter 8. A corresponding part of the spectrum of the speech signal is obstructed by the band-stop filter 3, which permits the remainder of the speech signal to pass to the delta-modulator 10. The tone pulse data signals from the band-pass filter 8 are also passed to the input of the delta-modulator 10. The mean amplitude of the speech signal varies considerably, but the amplitude of the data signals is varied in unison with it. The amplitude modulator 7 is adjusted so that the quantizing noise resulting from the delta-modulation of the data signals does not become excessively large relative to the speech signal, even in the softest passages of the speech signal, whereas the quantizing noise resulting from the delta-modulation of the speech signals does not become excessively large relative to the data signals, even in the loudest passages of the speech signal.

Various modifications may be made to the above-described embodiment. For instance, two or more data signals may be multiplexed and then modulated as described hereinabove. The invention is not restricted to systems using conventional delta modulators. Various known types of modulator and transmitters may be employed, for instance a pulse code modulator may be used. The invention may even be found useful in some cases where the speech signal is an analogue signal and the transmission does not involve a digitization of the combined signal.

In most cases the speech signal forms the major part of the combination of signals at the input of the modulator, so the amplitude modulator in the data signal path may be made responsive to the amplitude of the combination of signals rather than to the amplitude of the speech signal alone. For instance in the embodiment described hereinabove the input of the detector 9 may be connected to the input of the delta-modulator 10 instead of to the output of the amplifier 2; this arrangement is illustrated in FIG. 1a.

In cases where the level of the speech signal may be subject to extreme variations, there may be occasions when the combined signals overload the transmitting or receiving apparatus, causing undesirable distortion and unusually strong mutual interference. Am amplitude limiter circuit may be connected in series with the speech signal channel, to avoid this. It also limits the maximum amplitude of the data signals, thereby making the requirements for the band-pass filters less stringent so that simpler filters may be used than might otherwise be considered advisable. However, in practice the apparatus may be required to operate in conjunction with various communication channels, which may have uncertain or unknown overload characteristics. To ensure that overloading effects are avoided on the worst channel the amplitude limiter would have to be set to operate at a speech signal level which would cause it to produce an unnecessary degradation of the speech quality when the better channels are in use. FIG. 2 shows a modified form of the apparatus, including an amplitude limiter circuit with a variable control arrangement, so that the extent of the amplitude-limiting action is no greater than is required to prevent intermodulation noise and quantizing noise from exceeding a predetermined level in a telegraph receiver circuit.

FIG. 2 shows parts 1 to 11 inclusive arranged as in FIG. 1. However, an amplitude-limiter circuit 13 is connected in series with the speech signal path from the output of the amplifier 2 to the detector 9 and the filter 3. The output of the transmitter 11 is shown connected through a communications channel 16 to a receiver 17. The receiver 17 includes complementary filters (not shown) for separating the speech signal and the data signal, and has a speech signal output connected to a loudspeaker 18 and a data signal output connected to a teleprinter 19.

The output of the limiter 13 is also connected through a band-stop filter 20, a line simulator circuit 21 and a band-pass filter 22 to a control circuit 23. The output of the control circuit 23 is connected to control the amplitude limiter 13.

The filters 20 and 22 have characteristics similar to those of the filters 3 and 8 respectively, but they do not have to carry any digital signals and can be of much simpler construction than the filters 3 and 8. The signal passing through the filter 20 corresponds to, and may be regarded as a sample of, the speech signal component of the composite signal transmitted by the delta-modulator 10, the transmitter 11, and the communications channel 16. The line simulator circuit 21 is a network arranged to produce distortion and intermodulation effects similar to those of the actual modulator 10, transmitter 11 and communications channel 16, so that its output can be regarded as a sample of that part of the signal at the receiver 17 which is due to the speech signal transmission. Since the filter 22 simulates the effect of the band-pass filter used in the receiver 17 to pass the data signals to the teleprinter 19, it passes a signal which can be regarded as a sample of the noise which reaches the teleprinter 19 as a result of the transmission of the speech signal through the system. The control circuit 23 rectifies this signal to provide a direct-voltage control signal for controlling the amplitude limiter circuit 13; it is arranged to have a threshold action, so that the amplitude-limiting action is only provided when the noise reaching the teleprinter 19 approaches an amplitude which is liable to cause the teleprinter 19 to print incorrect characters.

FIG. 3 shows the amplitude limiter circuit 13 and the control circuit 23 in detail. The speech signal input connections 30 of the limiter circuit are connected through resistance R1 and R2 to the primary of a transformer 33. The transformer 33 has four secondary windings, of which three are connected to feed the filter 20, the filter 3 and the detector 9 respectively and the fourth secondary winding is connected to the AC inputs of a bridge rectifier circuit comprising diodes D1, D2, D3 and D4. The DC output connections of the bridge rectifier circuit are connected to the emitters of a complementary pair of transistors Q1 and Q2.

A connection from the output of the filter 22 (FIG. 2) is capacitively coupled to two half-wave rectifier circuits 34 and 35. The output of the circuit 34 is connected between a power supply positive voltage connection 36 and the base of the PNP transistor Q1. The output of the circuit 35 is connected between a power supply negative voltage connection 37 and the base of the NPN transistor Q2. Four resistors R8, R9, R10 and R11 are connected in series between the connections 36 and 37. The junction of the resistors R9 and R10 is connected to earth and to the collectors of the transistors Q1 and Q2. The junction of the resistances R8 and R9 is connected to the emitter of the transistor Q1, while the junction of the resistances R10 and R11 is connected to the emitter of the transistor Q2. The voltages developed at these junctions are equal and opposite and shall be referred to hereinafter as +Vb and -Vb respectively.

Normally, the diodes D1 to D4 of the bridge rectifier circuit are reverse-biased by the voltage +Vb and -Vb, so that the rectifier circuit has a comparatively high impedance. However, when the amplitude of the speech signal exceeds the voltage Vb, the diodes D1 and D4 conduct and present, in effect, a comparatively low impedance across the primary of the transformer 33, causing the peaks of the speech signal to be flattened. The threshold amplitude at which the flattening effect commences is set by the voltage +Vb and -Vb.

When the amplitude of the signal from the filter 22 is comparatively small, so that the output voltage of the rectifier circuit 34 is less than the voltage drop across the resistor R8, and the output voltage of the rectifier circuit 35 is less than the voltage drop across the resistor R11, then the transistors Q1 and Q2 are nonconductive and the voltage +Vb and -Vb remain at preset values determined by the resistances R8 to R11 and the voltages on the connections 36 and 37.

When the amplitude of the signal from the filter 22 becomes larger, the outputs of the rectifier circuits 34 and 35 exceed the voltage drops across the resistors R8 and R11 respectively and cause the transistors Q1 and Q2 to conduct. This reduces the voltages +Vb and -Vb, so that the onset of the limiting action of the bridge rectifier circuit occurs at lesser amplitudes of the speech signal.

In many applications, the impedance of the speech signal source (not shown) connected to the input connections 30 (for instance the output impedance of the amplifier 2 in FIG. 2) may be sufficient in conjunction with the loading effect of the bridge circuit to achieve an adequate control and the resistances R1 and R2 may be omitted.

The action of the control circuit is initiated with a time constant determined mainly by the values of the capacitors in the rectifier circuits and the series resistance of the line feeding them, and decays with a longer time constant determined by the product of the values of the capacitors and the resistances connected in parallel with them. The initiation time constant may be of the order of 1 millisecond or less; the decay time constant is preferably about 100 milliseconds.

Various other modifications of the invention will be apparent to persons skilled in the art. For instance, other forms of amplitude limiter circuits and other suitable control circuits may be used in place of the circuits of FIG. 3. The line simulator circuit 21 may be simplified or even omitted, a direct connection being made from the output of filter 20 to the input of filter 22. The arrangement of the amplitude limiter circuit and its control loop comprising the units 20 to 23 inclusive could be applied to a system in which the data signals are of constant amplitude; for example in FIG. 2 the units 7 and 9 could be omitted, and the output of the tone modulator 5 directly connected to the input of the filter 8.

Claims

What I claim is:

1. Telecommunications transmitting apparatus for transmitting a speech signal and at least one digital data signal in different frequency ranges over a common telecommunications channel, said apparatus comprising:

a speech signal path,

a data signal path,

transmitting means responsive to signals from said speech signal path and to signals from said data signal path for transmitting said signals over said common telecommunications channel,

an electrically controllable amplitude limiter circuit connected in said speech signal path,

amplitude adjusting means series connected in one of said signal paths,

first control means connected both to said speech signal path and to said amplitude adjusting means for controlling the amplitude adjusting means to cause data signal amplitude applied to said transmitting means to increase when mean speech signal amplitude concurrently applied to said transmitting means increases, and

second control means connected to said transmitting means and to said amplitude limiter circuit for deriving a signal representative of interference and noise effects liable to effect reproduction of said data signals due to transmission of the speech signal in a common telecommunications channel and for controlling said amplitude limiter circuit to intensify the amplitude limiting action thereof, whenever said interference and noise effects exceed a predetermined amplitude.

2. Apparatus as in claim 1 wherein said transmitting means comprises:

a band stop filter connected to said speech signal path,

a band pass filter connected to said data signal path,

a transmitter connected to outputs of said band stop filter and said band pass filter; and where said second control means comprises:

a second band stop filter connected to said speech signal path,

line simulator means connected to an output of said second band stop filter for simulating probable intermodulation, distortion and noise-producing effects of said common telecommunications channel,

a second band-pass filter connected to an output of said line simulator means, and

a control circuit connected to an output of said second band pass filter and responsive to signal amplitudes obtained therefrom.

3. Apparatus as in claim 2 wherein said amplitude limiter circuit comprises:

an impedance in series with said speech signal path, and

a variable load in parallel with said speech signal path.

4. Apparatus as in claim 3 wherein said variable load comprises:

a rectifier circuit, and

a bias voltage source circuit connected thereto for applying a reverse bias voltage to said rectifier circuit.

5. Apparatus as in claim 4 wherein said control circuit comprises:

rectifying means having input connections to an output of said second band pass filter, and

said rectifying means also having output connections to said bias voltage source circuit.

6. Apparatus as in claim 5 wherein said bias voltage source circuit comprises:

a potentiometer chain, and

at least one transistor connected in parallel with part of said potentiometer chain and connected to an output of said rectifying means.

7. Telecommunications transmitting apparatus for transmitting a speech signal and at least one digital data signal in different frequency ranges over a common telecommunications channel, said apparatus comprising:

a speech signal path,

a data signal path,

transmitting means having a common input connection responsive to signals from said speech signal path and to signals from said data signal path for transmitting said signals over said common telecommunications channel,

amplitude adjusting means, series-connected in said data signal path, and having a control input, for continuously adjusting the amplitude of the signals in the said path in response to variations of a signal applied to said control input, while preserving the waveform of said signals substantially undistorted, and

control means connected to said speech signal path and to said control input of said amplitude adjusting means for controlling said amplitude-adjusting means to cause the amplitude of data signals applied to said transmitting means to follow variations in the mean amplitude of speech signals concurrently applied to said transmitting means so that the data signal amplitude will increase whenever the speech-signal mean amplitude increases.

8. Apparatus as in claim 7 wherein an amplitude limiter circuit is provided in said speech signal path.

9. Apparatus as in claim 7 wherein said transmitting means includes equipment of the class comprising pulse code modulators and delta modulators.

10. Telecommunications transmitting apparatus for transmitting speech signals and at least one digital date signal in different frequency ranges over a common telecommunications channel, said apparatus comprising:

a speech signal path for carrying said speech signals,

a data signal path for carrying said data signals,

transmitting means for transmitting both said speech and said data signals over said common telecommunications channel, and

means for maintaining a substantially constant ratio between the transmitted speech signal amplitudes and the transmitted data signal amplitudes, said means comprising:

amplitude adjusting means, series connected in one of said signal paths, and having a control input for continuously adjusting the amplitude of signals in the said one of said signal paths in response to variations of a signal applied to said control input, while preserving the waveform of said signals substantially undistorted, and

control means connected both to said transmitting means and to said control input of said amplitude adjusting means, for varying signal amplitude in at least one of said signal paths so as to maintain said ratio substantially constant despite natural variations in the amplitude of said speech signals.

11. Apparatus as in claim 10 wherein said amplitude means comprises a variable gain amplifier series connected in said data signal path.

12. Apparatus as in claim 11 wherein said first control means is connected to said speech signal path for adjusting the gain of said variable gain amplifier in accordance with the amplitude of said speech signal.