Abstract:
A method and circuit for distinguishing between a first voltage signal having a high amplitude and a second voltage signal having a low amplitude, the first and second voltage signals comprising respective AC components superimposed on a DC level. The DC level of the first and second voltage signals is blocked and the respective AC components of the first and second signals are attenuated by an attenuation factor which is voltage dependent and substantially frequency independent, so that the respective AC components of the first and second voltage signals after attenuation have similar amplitudes. The respective attenuated AC components of the first and second voltage signals are then processed so as to distinguish between the first and second voltage signals according to their respective frequencies.

Description:
FIELD OF THE INVENTION 
     This invention relates to telephone ring and Caller ID detector circuits. 
     BACKGROUND OF THE INVENTION 
     So-called “smart telephones” are often provided with circuitry which decodes a Caller-ID signal sent by a central office between the first and second ring signals, so as to identify the calling party prior to answering the call. Typically, frequency-shift keying (FSK) encoded caller ID data is transmitted between first and second ring signals. The called party&#39;s telephone extension unit includes a detector, which demodulates the FSK signal and allows display of the Caller-ID before the called party answers the call. 
     U.S. Pat. No. 5,796,815 (Guercio et al.) discloses a communications device that is adapted to detect a ring and a Caller ID signal with the same detection circuitry. The communications device includes an off-hook detector coupled between a telephone line and a communications circuit. When the telephone is on-hook, an electrical resistance of at least 25 MΩ exists between two switch terminals of the off-hook detector, thereby effectively d.c. decoupling the telephone. A capacitor connected between the two switch terminals of the off-hook detector couples a.c. signals from the telephone line to the communications circuit when the telephone is on-hook. The ring signal has an amplitude of 100 volt peak to peak and a frequency in the range of 20 Hz. The Caller-ID signal has an amplitude of approximately 1 volt peak to peak and employs FSK using frequencies in the range of 1,200 Hz and 2,200 Hz to encode logic “1” and logic “0” respectively. 
     The value of the coupling capacitor is selected so that the amplitude of the ring signal is attenuated to approximately 1 volt peak to peak, this being the maximum allowable input voltage range of commonly available A/D converters. On the other hand, the amplitudes of the Caller-ID FSK frequencies are changed by no more than 5%. The signal amplitudes of both signals are thus brought into an overlapping range, allowing both signals to be detected with a single detector according to their respective frequencies. 
     There are several drawbacks with such an arrangement. First, the input impedance to the communications circuits series is substantially equivalent to a series combination of an inductance and a resistance. Together with the coupling capacitor, this forms an R-L-C high pass filter whose output is thus frequency dependent. The frequency of the ring signal varies from one country to another, typically lying within a range of 15 to 80 Hz. Thus, at the higher frequency range of the ring signal, the lower frequency of the FSK encoded Caller-ID signal is only fifteen times larger than the ring signal frequency. It is not possible, to attenuate an 80 Hz signal sufficiently using the coupling capacitor proposed by Guercio et al. to the same extent as may be done for a 20 HZ signal. Thus, effective translation of the ring and Caller-ID signals at all frequencies into the permitted range is impossible. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a detector for detecting high voltage ring signal and the low voltage Caller-ID signal, wherein the drawbacks associated with hitherto-proposed circuits are eliminated. 
     According to a first aspect the invention, there is provided a method for distinguishing between a first voltage signal having a high amplitude and a second voltage signal having a low amplitude, said first and second voltage signals comprising respective AC components superimposed on a DC level, the method comprising the steps of: 
     (a) blocking the DC level of the first and second voltage signals,. 
     (b) attenuating the respective AC components of the first and second signals by an attenuation factor which is voltage dependent and substantially frequency independent, so that the respective AC components of the first and second voltage signals after attenuation have similar amplitudes, and 
     (c) processing the attenuated respective AC components of the first and second voltage signals so as to distinguish between the first and second voltage signals according to their respective frequencies. 
     According to a second aspect of the invention, there is provided a circuit for distinguishing between a first voltage signal having a high amplitude and a second voltage signal having a low amplitude, said first and second voltage signals comprising respective AC components superimposed on a DC level, the circuit comprising: 
     a capacitor for blocking the DC level of the first and second voltage signals, 
     an attenuator for attenuating the respective AC components of the first and second signals by an attenuation factor which is voltage dependent and substantially frequency independent, so that the respective AC components of the first and second voltage signals after attenuation have similar amplitudes, and 
     a processor for processing the attenuated respective AC components of the first and second voltage signals so as to distinguish between the first and second voltage signals according to their respective frequencies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic circuit diagram showing a detector circuit according to a first embodiment of the invention; 
     FIGS. 2 a  and  2   b  show graphically transient characteristics of the detector circuits according to the invention for small and large signals, respectively; and 
     FIG. 3 is a schematic circuit diagram showing a detector circuit according to a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic circuit diagram showing a detector circuit  10  employing a non-linear attenuator  11  to distinguish between ring and Caller-ID signals appearing across a telephone subscriber line shown generally as  12 . The non-linear attenuator  11  comprises an operational amplifier  13  having an output terminal  14 , an inverting input terminal  15  and a non-inverting input terminal  16 . A first linear gain setting resistor  17  couples a TIP terminal  18  of the telephone subscriber line having a RING terminal  19  to the inverting input terminal  15  of the operational amplifier  13 . A first non-linear gain setting resistor  20  is coupled between the inverting input terminal  15  and the output terminal  14  of the operational amplifier  13 . The first non-linear gain setting resistor  20  shows a low resistance to a ring signal having high amplitude (constituting a first voltage signal) and a high resistance to a low amplitude Caller-ID signal (constituting a second voltage signal). The TIP terminal  18  constitutes a first voltage rail of a voltage source feeding the ring and Caller-ID signals. The non-linear attenuator  11  is AC coupled to telephone line Tip and Ring terminals  18  and  19 , respectively, via respective DC blocking capacitors  21  and  21 ′. 
     In practice, the ring and Caller-ID signals are applied differentially by the central office between the TIP and RING terminals  18  and  19 . Therefore, a symmetrical arrangement is provided wherein a second linear gain resistor  23  couples the RING terminal  19  to the non-inverting terminal  16  of the operational amplifier  13 . Likewise, a second non-linear gain resistor  24  is coupled between the non-inverting input terminal  16  and a virtual ground connection  25  of the operational amplifier  13 . The second non-linear gain setting resistor  24  shows a low resistance to the ring signal and a high resistance to the Caller-ID signal. A capacitor  26  is connected in parallel with the first non-linear gain setting resistor  20 . The capacitor  26  in combination with the non-linear resistor  20  operates as a low-pass filter for diminishing spurious high frequency out-of-band signals. In order to ensure proper symmetry, the values of equivalent components associated with the TIP and RING terminals should be identical in value. 
     The first non-linear gain setting resistor  20  comprises a linear resistor  27  connected in parallel with a series connection of a pair of back-to-back rectifier diodes  28  and  29  and a linear resistor  30 . Current flows through the linear resistor  30  when the voltage of the signal across the first non-linear gain setting resistor  20  exceeds the V BE  breakdown voltage of the diode (equal to 0.7 volts for a silicon device). The two back-to-back rectifier diodes  28  and  29  ensure that under these conditions, current flows regardless of the polarity of the signal. In like manner, the second non-linear gain setting resistor  24  comprises a linear resistor  32  connected in parallel with a series connection of a pair of back-to-back rectifier diodes  33  and  34  and a linear resistor  35 . 
     The output terminal  14  of the operational amplifier  13  is connected to an A/D converter  37  which converts the analog output signal to a digital equivalent, which is processed by a processor  38  connected to the A/D converter  37 . The processor  38  is preferably a Digital Signal Processor (DSP). 
     The circuit operates as follows. The TIP and RING terminals  17  and  18 , respectively, are connected to the telephone exchange, which feeds the ring and Caller-ID signals to the detector  10 . The amplitude of the Caller-ID signal is less than the V BE  threshold of the diodes  28 ,  29 ,  33  and  34 , which therefore remain in their non-conductive state regardless of the polarity of the Caller-ID signal. Therefore, the first and second non-linear resistor presented to the Caller-ID signal is constituted by the linear resistor  27  and  32 , respectively. On the other hand, the ring signal greatly exceeds the V BE  threshold of the diodes  28 ,  29 ,  33  and  34 , one of each pair of which therefore conducts depending on the polarity of the Caller-ID signal. Therefore, the first non-linear resistor presented to the ring signal is constituted by the parallel combination of the linear resistors  27  and  30 . Likewise, the second non-linear resistor presented to the ring signal is constituted by the parallel combination of the linear resistors  32  and  35 . 
     FIGS. 2 a  and  2   b  show graphically transient characteristics of the detector circuits according to the invention for low and high amplitude signals, respectively. It is seen that the attenuation (or amplification) factor is non-linear, comprising three linear sections: signal between Vbe and −Vbe, signal below −Vbe and above Vbe. In FIG. 2 a , the low amplitude Caller-ID signals are attenuated by a low attenuation factor chosen to be around unity i.e. 0 dB. In FIG. 2 b , the high amplitude ring signals are attenuated by a high attenuation factor chosen to be around −30 dB. 
     FIG. 3 is a schematic circuit diagram showing a different highly symmetrical implementation of the invention, especially suitable for integrated circuit utilization. A detector circuit  40  employs a non-linear attenuator  41 , comprising first and second operational amplifiers  42  and  43 . The first operational amplifier  42  has respective output, inverting input and non-inverting input terminals  44 ,  45  and  46 . The second operational amplifier  43  has respective output, inverting input and non-inverting input terminals  47 ,  48  and  49 . 
     The first and second operational amplifiers  42  and  43  are coupled to respective TIP and RING terminals  50  and  51  of a telephone exchange via respective first and second DC blocking capacitors  52  and  52 ′ in series with first and second linear gain setting resistors  53  and  54 , respectively. The TIP and RING terminals  50  and  51  constitute respective first and second voltage rails of a voltage source feeding the ring and Caller-ID signals to the respective inverting input terminals of the first and second operational amplifiers, via the first and second linear gain setting resistors  53  and  54 , respectively. 
     A first non-linear gain setting resistor  55  is coupled between the inverting input terminal  45  and the output terminal  44  of the first operational amplifier  42  and has a low value to the high amplitude ring signal and a high value to the low amplitude Caller-ID signal. Likewise, a second non-linear gain setting resistor  56  is coupled between the inverting input terminal  48  and the output terminal  47  of the second operational amplifier  43  and has a low value to the high amplitude ring signal and a high value to the low amplitude Caller-ID signal. 
     The first non-linear gain setting resistor  55  comprises a linear resistor  60  connected in parallel with a series connection of a pair of back-to-back rectifier diodes  61  and  62  and a linear resistor  63 . A capacitor  64  is connected in parallel with the first non-linear gain setting resistor  55 . Current flows through the linear resistor  55  when the voltage of the signal across the first non-linear gain setting resistor  55  exceeds the V BE  breakdown voltage of the diode (equal to 0.7 volts for a silicon device). The two back-to-back rectifier diodes  61  and  62  ensure that under these conditions, current flows regardless of the polarity of the signal. In like manner, the second non-linear gain setting resistor  56  comprises a linear resistor  65  connected in parallel with a series connection of a pair of back-to-back rectifier diodes  66  and  67  and a linear resistor  68 . A capacitor  69  is connected in parallel with the first non-linear gain setting resistor  56 . The capacitors  64  and  69  in combination with the respective non-linear resistors  55  and  56  operate as a low-pass filters for diminishing spurious high frequency out-of-band signals. 
     The respective output terminals  44  and  47  of the first and second operational amplifiers  42  and  43  are connected to an A/D converter  70  which converts the analog output signal to a digital equivalent, which is processed by a processor  71  connected to the A/D converter  70 . The processor  71  is preferably a Digital Signal Processor (DSP). 
     In such an arrangement, the non-inverting terminals  46  and  49  of the first and second operational amplifiers are floating and take no part in the behavior of the attenuator  41 . Each of the first and second operational amplifiers  42  and  43  has a non-linear attenuation depending on the amplitude of the respective signal fed thereto, according to which part of the transient characteristic shown in FIGS. 2 a  and  2   b  is operative. 
     Although the invention has been described with particular regard to distinguishing the Caller-ID signal from the ring signal, it can equally well be used as a mechanism to monitor a data signal on the line without going “off hook”. The data signal may be any low amplitude signal representative of voice, music, facsimile or other analog data, which may be passed unattenuated for processing by the DSP. 
     It will be understood that the invention differs over hitherto-proposed circuits in that the attenuation factor is dependent on the voltage level of the incoming signals. Thus, providing there exists sufficient difference between the two signals, the larger of the two can be attenuated by a much greater factor than the other allowing both signals to pass to the A/D converter using a single detector only. Actual discrimination between the signals is then a function of some other characteristic, such as frequency, cadence and amplitude.