Abstract:
In an apparatus for supplying a tip and ring telephone line with voice band and broadband signals, a first pair of drivers supply at least DC signals to the respective tip and ring line components, and a second pair of drivers supply broadband signals to the tip and ring line components. Feed components combine the outputs of said respective drivers for the respective tip and ring components of the telephone line.

Description:
[0001]    Method and apparatus for connecting broadband voice and data signals to telephone systems  
           [0002]    1. Field of the Invention  
           [0003]    This invention relates to the field of telephony, and in particular to a method and apparatus for connecting broadband voice and data signals to telephone systems. The invention is suitable for, but not limited to, interfacing a DSL analog front end circuit to a subscriber line in the presence of a voice band (DC-4000 Hz) signal. DSL (Digital Subscriber Line) is a service that permits broadband data signals to be superimposed on local subscriber loops.  
           [0004]    2. Background of the Invention  
           [0005]    With the popularity of the Internet, telephone companies are offering more and more broadband services to subscribers. One such service is DSL (Digital Subscriber Line) wherein the voice band Plain Old Telephone Service (POTS) and the higher frequency band DSL service are put onto the same pair of copper wires at the same time. However, this gives rise to several problems. Both the DC and AC requirements for the voice band POTS and the DSL service are different. POTS requires a DC voltage and current for line signaling and voice transmission while DSL service only operates in AC mode. The line impedance for POTS lines ranges from 600 to 1000 ohms either real or complex, while the DSL line impedance is around 100 ohms.  
           [0006]    When POTS is in the on-hook state, no voice (AC) signal is transmitted onto the tip/ring pair. There is about 40 V to 48 V DC voltage across tip and ring. Usually, the tip DC voltage is around 0 V to −5 V and the ring DC voltage is around −40 V to −56 V. A typical DSL signal has a peak voltage of about 18 V. When the DSL signal is transmitted during the POTS on-hook state, the typical signal at tip and ring is as shown in FIG. 2. There are currently two common approaches for providing both POTS and DSL service on the same tip/ring pair. One approach is to use a transformer and a splitter. The splitter consists of two capacitors to the DSL interface connection and a low pass filter (LPF) for the POTS connection as shown in FIG. 3.  
           [0007]    The two capacitors act as a high pass filter and isolate the DSL line impedance from the voice band POTS line. The capacitors also isolate the DC voltage for POTS line from the DSL line driver. The drawback of this method is the need for the relatively expensive and bulky transformer and splitter.  
           [0008]    The other approach is to use common solid state differential drivers to directly drive both POTS signals and DSL signal onto the common copper wire pair as illustrated in FIG. 4. The total feeding impedance,  2 Z f , is set to that of the DSL load impedance. As shown in FIG. 2, the DSL signal requires an AC swing of about 40 Vpp. POTS requires a DC voltage of about 40 V to 48 V between tip and ring when the line is in onhook (idle) state to meet Central Office (CO) requirements. In the case of DSL signal transmission during POTS on-hook state, the drivers need a DC supply voltage of about 80 V (V+minus V−equals to 80 V) to drive signals onto tip and ring as illustrated in FIG. 2.  
           [0009]    For a typical DSL load of 100 ohms and a signal of 3 Vrms, the drivers need to provide about 30 mA. Hence the power consumption is around 2.4 W for a 80 V power source. In this configuration, about 50% of the power is wasted in the line drivers due to the fact that they need to be biased to 80 V instead of 40 V to provide the required tip/ring DC voltage. The power dissipated through the DSL load and the feeding component is about 0.2 W, (2×(100 ohms×30 mA 2 )). Hence the power dissipation in the drivers is about 2.2 W. For normal integrated voice and data telephone line service, the majority of the time the POTS line is in on-hook state while DSL line is active. It makes the common solid state driver very power inefficient.  
           [0010]    An object of the invention is to alleviate this problem.  
         SUMMERY OF THE INVENTION  
         [0011]    According to the present invention there is provided an apparatus for supplying a tip and ring telephone line with voice band and broadband signals, comprising a first pair of drivers for supplying at least DC signals to said respective tip and ring line components, a second pair of drivers for supplying broadband signals to said tip and ring line components, and feed components for combining the outputs of said respective drivers for said respective tip and ring components of the telephone line.  
           [0012]    The broadband signals are typically DSL signals. The voice band signals can either be passed through the first pair of drivers with the DC signals or the second pair of drivers with the broadband signals.  
           [0013]    In one embodiment the feed components match the output impedance to the line impedance in the frequency band of the broadband signals, whereas the output impedance in the voice band is matched by feedback of the voice band transmit signal.  
           [0014]    The drivers are typically solid state integrated circuits devices.  
           [0015]    The invention also provides a method of improving power efficiency in the transmission of integrated voice and broadband service on a tip and ring telephone, comprising supplying at least DC signals through a first pair of drivers for to said respective tip and ring line components, supplying broadband signals to a second pair of drivers for said tip and ring line components, and combining the outputs of said respective drivers with feed components for said respective tip and ring components of the telephone line. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The invention will now be described in more detail by way of example only, with reference to the accompanying drawings, in which:  
         [0017]    [0017]FIG. 1 is a block diagram of a connection circuit in accordance with the principles of the invention;  
         [0018]    Figure shows the tip/ring voltage in a POTS on-hook state with DSL transmission;  
         [0019]    [0019]FIG. 3 shows a transformer and capacitor coupled DSL driver; and  
         [0020]    [0020]FIG. 4 shows a direct drive for POTS and DSL transmission. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    In FIG. 1, which shows a circuit in accordance with the principles of the invention, input signals  105 ,  110  are fed to respective differential amplifiers  104 ,  109 . Signal  105  can include DC signals only, or it can also include voice band, but not broadband signals. Signal  10  includes the broadband signals, typically DSL signals, and optionally the voice band signals if they are not included in signal  105 . The non-inverting outputs of differential amplifiers  104 ,  109  are fed respectively to the inputs of drivers  103  and  108 , connected through feed network  102 ,  106 ,  107  to tip component  101  of the telephone line. The inverting outputs of differential amplifiers  104 ,  109  are fed respectively to the inputs of drivers  116 ,  112  connected through feed network  111 ,  115 ,  113  to ring component  114  of the telephone line.  
         [0022]    Drivers  103  and  116  have a working bandwidth from DC to a minimum of 4 KHz. There are at least two ways to use the drivers  103  and  116 .  
         [0023]    Drivers  103  and  116  can function in a similar way to conventional POTS only SLIC (Single Line Interface Circuit) drivers. Driver  103  and driver  116  are used to provide the tip/ring DC line voltage and loop current, to transmit the voice, ringing, and other voice band signals onto tip  101  and ring  114  through feeding impedances  102  and  115  in both on-hook or off-hook state. The DC voltages on tip and ring can vary between around 0 V to around −60 V as defined in the specific CO application requirement. Depending on whether balance or unbalance ringing is supported or not, the DC supply to the drivers  103  and  116  can vary between around 50 V to 200 V. Any DC voltages from data drivers  108  and  112  are blocked by capacitors  106  and  113  from interfering with the DC voltages on tip  101  and ring  114  and vice versa.  
         [0024]    Drivers  103  and  116  can also only provide the conventional POTS DC line signaling voltages and loop current without transmission of any voice band AC signal. Depending on the overall system architecture of the SLIC, drivers  103  and  116  can only provide DC signals or DC and AC voice band signals simultaneously.  
         [0025]    AC Signal Drivers  
         [0026]    The differential drivers  108  and  112  are used to transmit AC signal onto tip  101  and ring  114 . The AC signal can be DSL type data band signal only or the DSL type data band signal and the POTS voice band signal simultaneously. Depending on the overall system architecture of the SLIC, the type of AC signal can be DSL type signal only or DSL type plus voice band signals.  
         [0027]    The differential drivers  108  and  112  transmit the AC signal onto tip  101  and ring  114  through feeding impedances  107  and  111  and capacitors  106  and  113 . As capacitors  106  and  113  block any DC voltages from tip  101  and ring  114  from the AC signal drivers  108  and  112 , the output of drivers  108  and  112  can be biased at any DC voltage level as is convenient for the driver design. The AC signal drivers  108  and  112  only need to meet the maximum AC signal amplitude requirement. The maximum amplitude is around 40 Vpp for DSL type signal and is much lower for POTS voice band signal. Assuming that the output impedance at tip  101  and ring  114  equals to the DSL load impedance at the DSL signal frequency band, the maximum swing of driver  108  or  112  equals to that of the maximum data band signal. As the voice band signal also has a much less maximum amplitude, therefore the supply voltage to drivers  108  and  112  equals to the maximum data band signal swing plus the driver overhead. For DSL type signal, the maximum signal swing is about 40 Vpp. Therefore the supply voltage needed for drivers  108  and  112  is around 45 V to 50 V. This supply voltage is totally independent of the POTS DC voltage requirement.  
         [0028]    The line impedance can be set by either the passive feeding components  102 ,  106 ,  107 ,  111 ,  113 , and  115  alone or the passive feeding components together with transmit signal feedback from tip  101  and ring  114  through a real time transfer function back onto tip  101  and ring  114  again.  
         [0029]    The line impedance value changes according to the signal frequency band. In the voice band (300-4000 Hz), the line impedance meets POTS line impedance requirement. The actual value depends on the CO application and ranges from 600 ohms to 1000 ohms, real or complex. For DSL transmission, the line impedance is around 100 ohms within the transmission bandwidth. The requirement for line impedance for POTS is much more stringent than that for the DSL type lines. One preferred way to achieve both the POTS line impedance requirement and DSL line impedance is by setting the passive feeding components  102 ,  106 ,  107 ,  111 ,  113 , and  115  to the DSL line impedance in the DSL frequency band and using voice band transmit signal feedback with the passive feeding components to provide the POTS line impedance within the voice frequency band. The total power consumption of the Universal SLIC tip/ring drivers is the sum of the power consumption of the drivers  103  and  116  and the drivers  108  and  112 . The power consumption of each set of drivers is independent of the operation of the each other. The biggest improvement in power consumption when compared to a single differential tip/ring drivers is when DSL line is operating during the POTS on-hook state.  
         [0030]    In POTS on-hook state, there is no DC current going through the tip/ring loop. The power consumption of the drivers  103  and  116  is only needed to set up the DC biasing of tip  101  and ring  114 . This power consumption is totally independent of the operation of the AC signal drivers  108  and  112  as there is no DC current flowing through the DSL load and drivers are all AC coupled. For high efficient driver design, drivers  103  and  116  can consume as low as 0.1 W.  
         [0031]    The power consumption for the AC drivers  108  and  112  equals to the sum of the currents through the DSL load and the POTS driver feeding components  102  and  115  multiplied by the driver supply voltage. For typical DSL operation, the total current is about 30 mA and the driver supply voltage is about 40 V. Therefore, the power consumption for the DSL line operation is about 1.2 W. Together with the 0. 1 W for the DC drivers, the total power consumption is about 1.3 W. This saves about 1.1 W when compared to 2.4 W consumed by a single differential tip/ring driver pair as illustrated in section  2 .  
         [0032]    With feeding impedance matching that of the DSL load at around 100 ohm, the power dissipated in the DSL load and feeding impedance is about 0.18 W (200 W×30 mA 2 ). The power dissipation in the drivers is about 1.12 W (1.3 W-0.18 W) compared to 2.22 W (2.4 W-0.18 W) for a common AC and DC driver design as illustrated in section  2 .  
         [0033]    When DSL line is in idle, the power consumption of the data band drivers  108  and  112  is about 0.1 W.  
         [0034]    In POTS ringing or off-hook states, the power consumption of the drivers  103  and  116  is of that of typical POTS only driver designs plus the product of the current through the AC driver feeding components  106 ,  107 ,  111 , and  113  and the POTS driver supply voltage, in the case that voice band signal is transmitted through drivers  101  and  106 . In the case that voice band signal is transmitted through the AC drivers  108  and  112 , the overall power consumption is less than that of using drivers  103  and  116  due to the fact that the supply voltage for the AC drivers  108  and  112  is less than that for the drivers  103  and  116 .  
         [0035]    The described apparatus avoids the use of a transformer and splitter in a system requiring the transmission of both voice and data over the same communications medium while the power efficiency of integrated voice and DSL service.