Tone signal detection circuit for detecting tone signals

Tone signal detection circuit for a receiving circuit for detecting at least one tone signal of predetermined tone signal frequency (fE) which is contained in a received analog input signal, comprising a reference signal generator (41) for generating an analog converter reference signal which consists of a reference DC (VrefDC) and a periodic reference AC (VrefAC) having a variable fundamental frequency (fG), which is superimposed on the reference DC, an analog/digital converter (11) for converting the analog input signal into a digital data stream in dependence on the analog converter reference signal (Vref); and comprising a digital control circuit (20) which adjusts the variable fundamental frequency (fG) of the reference signal (Vref) generated by the reference signal generator (42) in accordance with the predetermined tone signal frequencies (fG) of the tone signals to be detected and evaluates the digital data stream output by the digital analog/digital converter (11) for detecting a data pattern corresponding to the tone signal.

TECHNICAL FIELD

The invention relates to a tone signal detection circuit for detecting at least one tone signal having a predetermined tone signal frequency, the tone signal being contained in a received analog input signal.

RELATED ART

Tone signals are added to an analog data signal as control signals by transmitting devices so that the associated receiving circuit or the receiver can be driven in dependence on the received tone signals. For example, modem circuits contain signal receivers which extract tone signals for controlling the modem from the received analog data signal.

FIG. 1shows a tone signal detection circuit according to the prior art. An analog input signal which, in addition to the analog data signal, also contains tone signals having certain tone signal frequencies passes via a signal input E to an automatic gain control (AGC) circuit which outputs this amplified analog input signal to a following anti-aliasing filter. From the AAf, the filtered analog input signal passes to a signal input of an analog/digital converter (ADC) which, in dependence on a reference DC level VrefDCconverts the analog input signal present at the signal input into a digital data stream. The analog reference voltage level VrefDCis generated by a reference voltage source UQ. The digital data stream is supplied to an interface circuit IF and passes from there into a data processing unit DVE in which the digital data stream is decoded and evaluated for further data processing.

The analog input signal present at the signal input E contains tone signals having discrete frequencies which are supplied to a tone signal amplifier TSV. The amplified tone signals, which are used for controlling the receiver, pass from the tone signal amplifier TSV to adjustable band-pass filters BP, the frequency of which can be adjusted in dependence on the discrete frequencies of the tone signals to be detected. As an alternative, a separate band-pass filter BP is provided for every known tone signal frequency. The band-pass filters BP are in each case followed by a comparator circuit K having an adjustable threshold value. If the threshold value level of the comparator K is exceeded, the associated tone signal detection circuit TD connected via the signal output of the comparator K detects that the associated tone signal is contained in the analog input signal and outputs a corresponding detection signal to a central control circuit of the receiver.

The tone signal detection circuit according to the prior art, shown inFIG. 1, has the disadvantage that either a separate band-pass filter BP is necessary for each tone signal or an analog band-pass filter BP having an elaborate circuit must be provided which can be adjusted to all tone signal frequencies occurring. The circuit expenditure for detecting the tone signals is, therefore, very high in the conventional tone signal detection circuit shown inFIG. 1.

A further disadvantage of the tone signal detection circuit according to the prior art, shown inFIG. 1, consists in that the tone signal detection circuit is inflexible with respect to changes of the tone signal frequency standard, i.e. cannot be reprogrammed when the tone signal frequencies are changed.

SUMMARY OF THE INVENTION

It is, therefore, the object of the present invention to create a tone signal detection circuit which can be programmed for detecting various tone signals and which can be implemented with little circuit expenditure.

According to the invention, this object is achieved by a tone signal detection circuit for detecting at least one tone signal having a predetermined tone signal frequency, having the features specified in patent claim1.

The invention creates a tone signal detection circuit for detecting at least one tone signal having a predetermined tone signal frequency, the tone signal being contained in a received analog input signal, comprising

a reference signal generator for generating an analog converter reference signal which consists of a reference DC and a periodic reference AC signal, which is superimposed on this reference DC and having a variable fundamental frequency,

an analog/digital converter for converting the analog input signal into a digital data stream in dependence on the analog converter reference signal,

and comprising a digital tone signal detection circuit which adjusts the variable fundamental frequency of the reference signal generated by the reference signal generator in accordance with the predetermined tone signal frequencies of the tone signals to be detected and evaluates the digital data stream output by the analog/digital converter for detecting a data pattern corresponding to the tone signal.

The basic concept of the tone signal detection circuit according to the invention consists in also using the analog/digital converter, which is already contained in the receiver for converting the useful analog input signal, for detecting tone signals which are contained in the analog input signal.

For this purpose, the analog/digital converter is additionally used as mixing stage for mixing the analog input signal with the analog converter reference signal generated by the reference signal generator.

The reference signal generator of the tone signal detection circuit according to the invention preferably consists of a reference voltage source for generating a reference DC,

a controllable signal generator for generating a periodic reference AC signal in dependence on a fundamental-frequency adjusting signal, received by the digital tone signal detection circuit, for adjusting the fundamental frequency of the periodic reference AC signal, and

an adder which adds the difference DC to the periodic reference AC for forming the analog converter reference signal.

In a particularly preferred embodiment of the tone signal detection circuit according to the invention, the digital control circuit contains a zero transition counting device which counts the number of zero transitions of the digital data stream output by the analog/digital converter, the control circuit detecting a tone signal when the number of zero transitions per time essentially corresponds to a predetermined nominal zero transition rate.

The various nominal zero transition rates for the various tone signals to be detected can be preferably adjusted in the digital control circuit.

The digital control circuit of the tone signal detection circuit according to the invention preferably contains a comparator circuit with adjustable signal threshold values.

The digital control circuit preferably also exhibits digital band-pass filters for band-pass filtering the digital data stream.

In the digital control circuit, the detected tone signals are preferably temporarily stored in a memory.

The digital control circuit preferably outputs a corresponding interrupt control signal to a central controller of the receiver circuit with a predetermined temporarily stored tone signal combination which consists of at least one tone signal.

The analog/digital converter preferably contains a digital filter and a decimation filter.

The received analog input signal is preferably an xDSL signal.

The receiver circuit in which the tone signal detection circuit according to the invention is provided is preferably a modem receiver circuit.

The data modem in which the tone signal detection circuit according to the invention is contained, preferably switches from a standby mode to a data reception mode on reception of an interrupt signal from the control circuit by the central controller of the receiver circuit.

In a preferred embodiment of the tone signal detection circuit according to the invention, the analog/digital converter is preceded by an anti-aliasing filter (AAF).

The anti-aliasing filter is also preferably preceded by an automatic gain control circuit.

The digital data stream output by the analog/digital converter is preferably evaluated by a following data evaluation circuit of the receiver.

According to the invention, a conventional analog/digital converter is used as mixing stage for mixing an analog input signal with an analog reference signal.

In this arrangement, the analog/digital converter converts the analog input signal supplied to it into a digital data stream which is evaluated for detecting a tone signal.

DETAILED DESCRIPTION OF THE INVENTION

As can be seen fromFIG. 2, the tone signal detection circuit1according to the invention exhibits a signal input2for applying a received analog input signal. The tone signal detection circuit1shown inFIG. 2forms, for example, a part of a receiver circuit of a data modem. The analog input signal present at signal input2consists of a useful analog signal and various tone signals having predetermined tone signal frequencies which are used for controlling the receiver circuit. The analog input signal present at the signal input2is supplied via a signal line3to the input4of a gain control circuit5which supplies the amplified analog input signal via an output6and a signal line7to a following anti-aliasing filter8. The output of the anti-aliasing filter8is connected via a signal line9to an analog signal input10of an analog/digital converter11. The analog/digital converter11converts the analog input signal present at the analog signal input10into a digital data stream in dependence on a converter reference signal present at a reference signal connection12and outputs the digital data to digital signal lines14via digital data outputs13. The digital data generated in accordance with the analog input signal pass via the digital data lines14to an interface circuit15which forwards the digital data via digital data lines16to a data processing circuit17for data decoding and data evaluation. The analog/digital converter11preferably exhibits a digital filter and a decimation filter. In an operating mode for detecting tone signals, the A/D converter11is clocked at a lower rate than in a normal operating mode for converting useful signals into useful data. In the tone signal detection mode, the data processing circuit17is also clocked at a low clock rate so that the power dissipation Pvof the tone signal circuit drops overall.

The converted digital data present on the data lines14are also supplied to a digital data input19of a digital control circuit20via data lines18. The digital data input connection19of the digital control circuit20is connected to a digital band-pass filter22via a line21. The digital band-pass filter22is preferably a third-order digital band-pass filter. The output of the band-pass filter22is connected via lines23to an adjustable comparator circuit24, the adjustable signal threshold values of which have a hysteresis. The programmable signal threshold values can be adjusted or, respectively, programmed via adjusting lines25and an adjusting connection26of the digital control circuit20. The output of the comparator circuit24is connected via lines27to a zero transition counting device28which counts the number of signal transitions of the digital data stream output by the analog/digital converter11and outputs the count value determined to a control logic30via a line29.

The control logic30is connected via lines31to a memory32in which various nominal zero transition rates for the various tone signals to be detected are stored. The control logic30compares the number of zero transitions per time determined by the zero transition counting device28with the nominal zero transition rates stored in the memory32. If the number of zero transitions per time determined essentially corresponds to a stored predetermined nominal zero transition rate, the control logic30of the digital control circuit20detects that the analog input signal present at the signal input2contains a tone signal. When a tone signal is detected, the tone signal is temporarily stored in the memory32by the control logic30until a predetermined tone signal combination consisting of at least one tone signal is present. As soon as a particular tone signal combination is detected by the control logic30, the control logic30outputs, via a line33and an interrupt control connection34of the digital control circuit20, an interrupt signal via an interrupt line35to a central controller36of the receiver. The central controller36detects the interrupt signal and changes the operating mode of the receiver in accordance with the interrupt signal received via the interrupt signal line35.

The control logic30is connected to an adjusting connection38of the digital control circuit20via a line37. The adjusting connection38is connected to a control connection40of a reference signal generator41via an adjusting line39. The reference signal generator41contains a controllable signal generator42which is controlled by an adjusting signal present on the line44via a control connection43. The controllable signal generator42generates a periodic reference AC signal VrefACin dependence on an adjusting control signal which is output by the control logic30. The adjusting control signal adjusts the fundamental frequency of the periodic reference AC signal output by the controllable signal generator42. The reference AC signal VrefACgenerated is supplied to a first input47of an adder48by the controllable signal generator42via a signal output45and a signal line46. The adder48exhibits a further signal input49which, via a line50, is present at a reference voltage source51for generating a reference DC VrefDC. The adder48adds the reference DC VrefDCgenerated in the reference voltage source51and the periodic reference AC VrefACgenerated by the controllable signal generator42and outputs the composite signal Vrefto a signal output54of the reference signal generator41via a signal output52and a line53. The signal output54of the reference signal generator41is connected to the reference signal connection12of the analog/digital converter11via a line55.

The controllable reference signal generator42for generating a periodic reference AC signal having a variable fundamental frequency fGpreferably generates a periodic squarewave signal, the fundamental frequency fGof which is varied in accordance with the control signal output by the control logic30. The periodic squarewave signal is superimposed on the reference DC VrefDCin the adder48. The reference DC with superimposed squarewave signal is supplied to the analog/digital converter via the line55.

The following holds true for the digital output value present at the digital signal output13of the analog/digital converter11:digital⁢⁢value=VinVref(1)
where Vinis the input voltage of the analog input signal at the sampling time and Vrefis the analog converter reference signal generated by the reference signal generator41.

The analog converter reference signal Vrefis composed of the reference DC VrefDCand a periodic reference AC signal VrefAChaving a variable fundamental frequency fG:
Vref=VrefDC+VrefAC(2)

The reference DC VrefDCis generated by the reference DC source51and the periodic reference AC signal VrefACis generated by the reference signal generator42. The fundamental frequency fGof the periodic reference AC signal VrefACis varied by the control logic30.
VrefAC=A·sin(2πfG·t)  (3)
where A is the amplitude of the periodic reference AC signal and fGis the adjustable variable fundamental frequency of the reference AC signal.

The analog/digital converter11multiplies the inverse value of the analog reference signal Vrefgenerated by the analog input signal Vinpresent at the analog input connection10, in accordance with equation (1) and thus acts like a mixing stage. The adjustable variable fundamental frequency fGof the periodic reference AC signal forms the mixing frequency of this mixing stage so that high-frequency tone signals having a certain tone signal frequency are down-converted to a certain intermediate frequency fZby the analog/digital converter11. The mixing frequencies or, respectively, the fundamental frequencies fGof the periodic reference AC signal can be programmed for all tone signals to be detected and can be stored in the memory32of the digital control circuit20. The control logic30reads the necessary fundamental frequencies fG, for detecting an associated tone signal, out of the memory32and drives the signal generator42for generating corresponding periodic reference AC signals. In this method, all fundamental frequencies fGconsidered for the various tone frequencies are preferably progressively switched through so that the reference signal generator42successively outputs various periodic reference AC signals having different fundamental frequencies fGwhich are superimposed on the reference DC VrefDCin the adder48. The switching-through of the various fundamental frequencies fGis preferably cyclic until a tone signal is detected by the digital control circuit20.

FIG. 3shows a simplified flowchart of various method steps for detecting tone signals within the digital controller20.

In a step S1, the variable fundamental frequencies necessary for the various tone signals are adjusted or, respectively, programmed for the tone signal detection.

In step S2, the digital controller20switches over to an operating mode for detecting tone signals.

In step S3, a periodic reference AC signal VrefAChaving the variable fundamental frequency fGfor a tone signal to be detected is generated and applied to the analog/digital converter1.

After a certain period of time has elapsed, the number of zero transitions of the digital data signal is measured by the zero transition counting device28of the digital control circuit20in step S4and output to the control logic30.

In step S5, the control logic30calculates the number of signal zero transitions per time and compares them with the stored nominal zero transition rates for the various tone signals. If the number of calculated zero transitions per time corresponds to a predetermined nominal zero transition rate, a tone signal is detected. If necessary, the detected tone signal is temporarily stored in the memory32for detecting a particular tone signal combination, the tone signal combination consisting of at least one predetermined tone signal.

If all tone signals of a tone signal combination have been detected, the control logic30outputs a corresponding interrupt signal to the central controller36in a step S6.

In a step S7, the variable fundamental frequency fGis set for the next tone signal to be detected. The various fundamental frequencies fGfor the various tone signals to be detected are cyclically set by the control logic30at the signal generator42.

The input tone signal having a tone signal frequency fE, which is present at the signal input10, is multiplied or, respectively, mixed with the inverse converter reference signal Vrefin the analog/digital converter11, the reference signal Vrefhaving a discrete spectral frequency line at the fundamental frequency fG. Multiplication in the time domain results in a convolution in the frequency domain so that the tone signal present is down-converted. The input frequency fEis down-converted to the intermediate frequency fZ. The frequency interval Δf between the fundamental frequency and the input frequency fEof the tone signal to be expected is adjusted in such a manner that the various tone signals are down-converted to the same intermediate frequency fZ.

The number of various tone signals to be detected, and their signal frequencies with associated variable fundamental frequencies fGto be adjusted can be programmed in a simple manner in the signal detection circuit according to the invention. The tone signal detection circuit1according to the invention as shown inFIG. 2can, therefore, be flexibly used for the most varied applications without selective band-pass filters with elaborate circuits becoming necessary.

The digital tone signal detection circuit1according to the invention additionally utilizes the existing analog/digital converter11as a mixing stage, the mixed signal obtained already being present in digital form and thus also being capable of being evaluated digitally. Since the analog input signals are down-converted to the same intermediate frequency fZ, the digital control circuit20only needs a single fixed digital band-pass filter22in order to select this intermediate frequency fZ.