Abstract:
A digital telephone includes a connector to a telephone line and a unit operative with current provided on the telephone line to minimally distort high amplitude voice signals during low current operation.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to telephones generally and to telephone operation in the presence of low line current in particular.  
       BACKGROUND OF THE INVENTION  
       [0002]     Telephones are well-known in the art. They are typically a combination of analog and digital elements with the trend, over the years, to reducing the number of analog elements. Digital elements require a steady source of power and thus, digital phones typically plug into a power socket near their location. This is especially true for wireless telephones.  
         [0003]     Unfortunately, power supplies occasionally are cut, at which point, most digital phones are not operative. There is a trend in digital phones to enable at least minima telephone operation during power cuts, using the 50V power that the central office of the telephone system supplies.  
         [0004]     The central office has a predefined amount of power which must suffice for the telephones connected to it. To ensure that the system works during power cuts, the telephone companies typically specify a desired DC impedance for an OFF-HOOK state (i.e. for when a user picks up the telephone headset. In North America, according to TIA-EIA-470B, the voltage drop across the telephone at 20 mA should be less than or equal to 6V. This voltage drop must be divided between the analog activity of the telephone and the digital activity performed by a digital signal processor (DSP) providing the digital operation of the digital telephone.  
         [0005]     DSPs take a fixed amount of power, which must be provided to them during a power cut, leaving the rest of the power to handle the voice signals. As a result, a digital telephone typically cannot easily handle high voltage audio signals (3 dBm is typically the maximum allowed amplitude), such as occur when the speaker shouts. The result is a distorted voice signal. This is shown in  FIG. 1 , to which reference is now made. As can be seen, the negative dips of the periodic signal are cut, giving the signal a flattened bottomlook. The person listening to this will hear a distorted voice signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:  
         [0007]      FIG. 1  is a graphical illustration of a distorted voice signal;  
         [0008]      FIG. 2  is a block diagram illustration of a power line unit, constructed and operative in accordance with the present invention;  
         [0009]      FIG. 3  is a graphical illustration of the voice signal produced by the unit of  FIG. 2 ;  
         [0010]      FIG. 4A  is a circuit diagram illustration of a distortion minimizer forming part of the unit of  FIG. 2 ;  
         [0011]      FIG. 4B  is a circuit diagram illustration of a voltage maintainer and of a hold and transmit amplifier forming part of the unit of  FIG. 2 ; and  
         [0012]      FIG. 5  is a circuit diagram illustration of an alternative embodiment of the unit of  FIG. 2 . 
     
    
       [0013]     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.  
         [0015]     Reference is now made to  FIG. 2 , which illustrates a basic power line circuit for a digital telephone  8  with two additional units, constructed and operative in accordance with the present invention. The basic power line circuit may operate when there is no main power and the only power available is from a telephone line  9 . To this end, it may comprise a diode bridge  10 , a hold and transmit amplifier  12 , a 2 wire to 4 wire (2W/4W) conversion unit  14 , a voltage regulator  16 , an output capacitor  18  and a digital telephone controller  20 . To this, the present invention may add a low current, voice signal maintainer comprising a distortion minimizer  24  and a voltage maintainer  26 .  
         [0016]     Briefly, diode bridge  10  may correct the arbitrary polarity of the signal on telephone line  9  to that which telephone controller  18  may require. Hold and transmit amplifier  12  may hold line  9  when the user takes telephone  8  off-hook and may amplify and transmit the user&#39;s voice signals. In telephony, “transmission” (Tx) indicates signals produced by the user (on a headset microphone or a box microphone) and “reception” (Rx) indicates signals received from telephone line  9  (and provided to a headset speaker or a box speaker).  
         [0017]     2W/4W conversion unit  14  may convert between the two wire format of telephone line  9  and the four wire format of telephone  8  and may provide voice signals for the user to hear, from both telephone line  9  and feedback of his own voice to be transmitted. Voltage regulator  16  may act as a power supply, controlling and regulating the voltage (a supply voltage Vdd) to telephone controller  20 . In the present invention, voltage regulator  16  may ensure a 3V supply voltage at 10 mA.  
         [0018]     Voltage regulator  16  may comprise a controlled isolation coil  32 , a power supply operational amplifier (op-amp)  34  and a voltage divider  36 . Isolation coil  32  may be a high impedance element connected before telephone controller  20 . Due to its high impedance to audio frequencies, isolation coil  32  may generally isolate the telephone line from noise that is generated in the digital side by telephone controller  20 . Isolation coil  32  may also convert the voltage Vline (comprised of DC (direct current) and audio signals) to a direct 3V DC signal.  
         [0019]     Op-amp  34  may act as a feedback controller to isolation coil  32 , measuring the level of supply Vdd and changing the impedance of isolation coil  32  to maintain the level of supply Vdd at the desired input level, such as 3V@10 mA. Voltage divider  36  may be connected between the output of isolation coil  32  and ground and may provide op-amp  34  with a voltage proportional to Vdd.  
         [0020]     Output capacitor  18  may be a power supply output capacitor that may filter the voltage and may hold the energy, for the digital circuit, during line disconnect (such as during—Hook flash and pulse dialing).  
         [0021]     As mentioned hereinabove, during power cuts, there may be distortions of the voice signal due to the lack of extra power. These distortions may be unpleasant for the user to hear. In accordance with a preferred embodiment of the present invention, units  24  and  26  may be added to the power line circuit to minimize the affect of the distortions.  
         [0022]     Applicant has realized that isolation coil  32  should not operate in the presence of high audio levels, as it causes the distortions. Applicant has further realized that, in this situation, output capacitor  18 , rather than isolation coil  32 , may supply the 3V power to telephone controller  20 .  
         [0023]     Distortion minimizer  24  may sense the presence of high amplitude signals, which may come when a user shouts and may reduce the current flowing through isolation coil  32 . This may increase the voltage drop on isolation coil  32 , which, in turn, may reduce the distortion.  FIG. 3 , to which reference is now made, shows the audio signal of  FIG. 1  but as output of the power line circuit of the present invention. As can be seen, the audio signal is a relatively clean sine wave. Only the first negative cycle, labeled  40 , may suffer from distortion.  
         [0024]     Unfortunately, when distortion minimizer  24  may operate for too long, the power supply voltage Vdd may start to drop, possibly to the point where telephone controller  20  may reset itself, which, in turn, may disconnect the telephone call, an undesirable result.  
         [0025]     Voltage maintainer  26 , which may monitor the 3V supply to telephone controller  20 , may begin operation when the power supply drops significantly and may raise the level of supply voltage Vdd. For example, voltage maintainer  26  may raise the voltage level of amplifier  12  such that the voltage drop on telephone line  9  may be higher than specified. For example, if the specified voltage drop is 6V, voltage maintainer  26  may raise the voltage drop by 1-1.5V. While this voltage drop may be above that which is specified, it is not expected to last for a long time, since it may be present only when a speaker is speaking loudly and such does not happen for long periods of time. Moreover, it occurs only during a power cut, a not very common occurrence.  
         [0026]     It will be appreciated that the present invention may increase the dynamic range of the audio signals while maintaining a stable power supply to telephone controller  20 .  
         [0027]     Reference is now made to  FIGS. 4A and 4B , which together illustrate one embodiment, in circuit format, the elements of the power line circuit of the present invention. Since many of the elements are known, they will not be described in detail, it being understood that persons skilled in the art know how to build such elements.  
         [0028]     In  FIG. 4A , isolation coil  32  may be connected between a point A (the line input) and a point B (the power supply output) and may comprise two transistors Q 10  and Q 11 , two resistors R 23  and R 24  and a capacitor C 21 . Transistors Q 10  and Q 11  may be connected as high current gain NPN transistors. The voltage drop on controlled isolation coil  32  (which is equivalent to the impedance of the coil) may be controlled by the current flows through resistor R 23 . The equivalent inductance of the circuit is: 
 
 L=R   23 * R   24 * C   21  
 
         [0029]     Voltage divider  36  may be connected between point B and ground and may comprise two resistors R 26  and R 27  in series, connected in parallel with the output capacitor, here labeled C 23 . In addition, voltage divider  36  may comprise a capacitor C 22  connected in parallel with resistor R 27 . The voltage on resistor R 27  and capacitor C 22 , labeled V 4 , may be the input to op-amp  34 .  
         [0030]     Op-amp  34  may comprise three transistors Q 12 , Q 13  and Q 14  and a resistor R 25 . Transistors Q 13  and Q 14  may be connected as an operational amplifier, as is known in the art, and transistor Q 12  may act as an output driver. The base of transistor Q 14  may receive V 4 , a sampled version of power supply Vdd, and the base of transistor Q 13  may receive a reference voltage Vref 1  The Op-amp of transistors Q 13  and Q 14  may compare the two inputs and, in response, may control the current through driver Q 12  which, in turn, may control the voltage drop across isolation coil  32 . For example, if power supply Vdd goes high, transistor Q 14  may conduct more, which may cause transistor Q 12  to conduct more, which may increase the voltage drop across isolation coil  32 , which may reduce power supply Vdd back towards the desired 3V value.  
         [0031]     In accordance with a preferred embodiment of the present invention, transistor Q 12  may also operate as an “OR” function and may have a second input, controlled by distortion minimizer  24 , which may be connected to its base,  
         [0032]     In this embodiment, distortion minimizer  24  may receive the Vline signal and may comprise a voltage divider  38  (a capacitor C 20  and a resistor R 20  in parallel, connected in series with a resistor R 21 ), a diode D 16  and a resistor R 22 .  
         [0033]     Capacitor C 20  in voltage divider  38  may emphasize the AC signals over the DC level. Thus, in the presence of high AC amplitude in the Vline signal, that may cause isolation coil  32  not to behave like a coil and to load the AC signal on telephone line  9 , diode D 15  may conduct. This, in turn, may increase the current through driver Q 12 . By increasing the current and by bypassing the op-amp of transistors Q 13  and Q 14 , transistors Q 10  and Q 11  of isolation coil  32  may conduct less, thereby not loading the AC signals. The lowered voltage across isolation coil  32  may temporarily not reduce power supply Vdd because output capacitor C 23  may store sufficient charge to temporarily sustain power supply Vdd.  
         [0034]     However, as the stored charge may be utilized, the voltage level of power supply Vdd may decrease. If it decreases below 2.5V, telephone controller  20  may reset itself, an undesired action. Voltage maintainer  26  ( FIG. 4B ) may temporarily raise the voltage of the connection to input telephone line  9  in order to raise power supply Vdd.  
         [0035]     Voltage maintainer  26  may receive power supply Vdd and a second reference voltage Vref 2  and may produce a signal V 6  to hold and transmit amplifier  12 . If desired, second reference voltage Vref 2  may be the same signal as the first reference voltage Vref 1 . Voltage maintainer  26  may comprise three transistors Q 3 , Q 4  and Q 5  and  7  resistors R 5 , R 6 , R 7 , R 8 , R 9 , R 10  and R 11 . Transistors Q 4  and Q 5  may be connected as an operational amplifier. Transistor Q 3  may form the basis of an output stage  40  that operates as a current sink.  
         [0036]     Resistors R 9  and R 11  may provide a sampled version of power supply Vdd to one input of the operational amplifier (e.g. to the base of transistor Q 5 ). The second input of the operational amplifier, the base of transistor Q 4 , may be connected to reference voltage Vref 2 .  
         [0037]     As power supply Vdd may start to drop, transistor Q 5  may conduct less and the voltage at the collector of transistor Q 5  may rise. The collector voltage of transistor Q 5  may be fed to output stage  40  through resistor R 6 . As the collector voltage of transistor Q 5  rises, transistor Q 3  of output stage  40  may start to sink more current. Resistor R 5  may set the lower limit of the current that transistor Q 3  may sink and resistors R 6  and R 7  may set the bias and gain of output stage  40 .  
         [0038]     Hold and transmit amplifier  12  may receive an output V 6  of current sink  40 . Amplifier  12  may be based on a “coil”  42  formed of two transistors Q 1  and Q 2  that may operate as an NPN transistor with high current gain. Amplifier  12  may additionally comprise four resistors R 1 , R 2 , R 3  and R 4  and a capacitor C 5 .  
         [0039]     The transmitted signal Tx may be injected directly to the base of transistor Q 2 . Resistor R 4  may route the audio signal via capacitor C 5 , thereby to reduce any shorting of the signal to ground. Since the value of resistor R 4  may be small compared to the values of resistors R 2  and R 3  and since the base current of transistor Q 2  may be small, due to the high gain configuration of transistors Q 1  and Q 2 , resistor R 4  generally does not effect the DC bias or the equivalent inductance of the circuit.  
         [0040]     Resistors R 2  and R 1  and capacitor C 5  may set the equivalent inductance of the circuit as: 
 
 L=R   2 * R   1 * C   5 . 
 
         [0041]     Resistors R 2  and R 3  may set a minimum DC voltage across coil  42  to be: 
 
 V=Vbe*[ 1+( R   2 )/( R   3 )]
 
         [0042]     If power supply Vdd starts to drop, then transistor Q 3  of current sink  40  may start to conduct, as described hereinabove, which, in turn, may change the value of the impedance that resistor R 2  may see. For example, when transistor Q 3  may be in a cutoff state, resistor R 2  may see the impedance of resistor R 3 , while, when transistor Q 3  may be in saturation (i.e. when power supply Vdd starts to drop and transistor Q 3  may be sinking current, the equivalent impedance may be the impedance of resistor R 3  in parallel with resistor R 5 . This temporary impedance may temporarily raise the voltage drop across amplifier  12  and may raise the level of incoming voltage Vline.  
         [0043]     Reference is now made to  FIG. 5 , which illustrates a second embodiment, constructed and operative in accordance with the present invention, in which distortion minimizer  24  and voltage maintainer  26  may be implemented using the capabilities of telephone controller  18 . In particular, the DX  36  family of DSP chips, commercially available from The DSP Group Ltd. of Israel, include in them both a digital signal processor  50  and peripheral analog elements, such as analog-to-digital (A/D) converters  52  and digital-to-analog (D/A) converters  54 , analog comparators  56  and other simple analog devices. Both types of elements may be utilized to implement distortion minimizer  24  and voltage maintainer  26 .  
         [0044]     In this embodiment, distortion minimizer  24  may comprise analog elements and digital elements. Its analog elements may comprise a control transistor Q 27 , a resistor R 29  and comparator  56  (within telephone controller  18 ). Its digital elements may comprise A/D converter  52  and D/A converter  54 A.  
         [0045]     For distortion minimizer  24 , the voltage Vline of telephone line  9  may be provided, through a resistor R 30 , to telephone controller  18 . The resultant signal, within the voltage range that telephone controller  18  may read, is labeled Vline-Mon. Power supply Vdd, output of isolation coil  32 , may also be provided to telephone controller  18 . Both signals may be provided to A/D converter  52  and the resultant digital signals may be provided to processor  50  for processing. Moreover, the Vline-Mon signal may be fed to comparator  56  for detecting the high audio amplitude, as described hereinbelow.  
         [0046]     Processor  50  may measure the strength of power supply Vdd and may determine an appropriate voltage level (exported through D/A  54 A as Vfdbk) to change the impedance of isolation coil  32  accordingly. Transistor Q 26  may receive the feedback voltage Vfdbk and may shift it from the level at which telephone controller  18  operates to an appropriate analog level to affect the flow of current through transistor Q 26 . In a normal mode, the changing current flow changes the impedance of isolation coil  32 .  
         [0047]     In addition, transistor Q 26  may operate with transistor Q 27  in a “wired or” function, as follows. In the normal mode, transistor Q 27  may not operate. However, comparator  56  may compare the voltage Vline of the telephone line (the DC and the AC voltages) with a reference voltage Vref 3  (which may be the same as Vref 1  or a different signal) and may issue a positive signal whenever there is a high negative peak in voltage Vline. The positive signal may saturate transistor Q 27 , which may enable it to dominate transistor Q 26 . Moreover, conduction of transistor Q 27  may discharge capacitor C 32  which in turn, may cause transistors Q 24  and Q 25  of isolation coil  32  not to conduct.  
         [0048]     In this embodiment, voltage maintainer  26  may comprise a holding coil voltage controller  58 , A/D converter  52  and D/A converter  54 B. For voltage maintainer  26 , processor  50  may monitor power supply Vdd using A/D  52 . When power supply Vdd may start to drop, processor  50  may increase the output of D/A  54 B to controller  58 .  
         [0049]     Holding coil voltage controller  58  may comprise a transistor Q 23 , two resistors R 36  and R 38  and a capacitor C 34 . The increased voltage from D/A  54 B may cause transistor Q 23  to conduct more which, in turn, may increase the voltage drop across holding coil and transmit amplifier  12  in a manner similar to that explained hereinabove. Resistor R 36  may form a base resistor and capacitor C 34  may filter the D/A output.  
         [0050]     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.