fig3 illustrates a power supply system for generating ringing voltage signals in a remote telecommunications subscriber interface unit ( siu ). the system comprises a flyback power converter 10 , for converting a dc voltage supply v siu into ac ringing voltage v ring . the converter 10 includes a primary side transistor switch 12 , such as a field - effect - transistor switch ( fet ) or the like , operated by a converter control circuit 14 , which periodically causes current to be transferred through a pair of secondary transformer windings , 16a and 16b , respectively , wherein winding 16a circulates current through a first , &# 34 ; positive &# 34 ; rectifying diode 18a , and winding 16b circulates current through a second , &# 34 ; negative &# 34 ; rectifying diode 18b , respectively . a pair of switches 20a and 20b build a ringing voltage signal 22 from positive and negative current supplied through rectifying diodes 18a and 18b , respectively . ringing voltage signal 22 is smoothed through an lc filter circuit 24 and provided as ringing voltage vrmng for on - demand use in subscriber line ( s ) served by the siu . in preferred embodiments , additional ringing voltage signals may be generated for use by substantially similar converter circuit operations ( not shown ) provided in parallel with converter 10 . a voltage detection circuit 26 monitors the siu operating voltage level v siu at the primary side input to power converter 10 . under &# 34 ; normal &# 34 ; siu operating voltage conditions , i . e ., where v siu is equal to or greater than a predetermined minimum required operating voltage level v min , a first , low voltage control signal 28 is output by the detection circuit 26 into a reference signal selection circuit 30 , wherein control signal 28 triggers the reference signal selection circuit 30 to output a low voltage ( ac ) reference sine wave 32 . sine wave 30 is preferably transmitted within a range of 16 to 67 hertz , with the actual frequency selected according to the desired ringing frequency for the siu subscriber lines . when an &# 34 ; undervoltage &# 34 ; condition is detected , e . g ., where v siu is less than v min , a second , low voltage control signal 34 is output by voltage detection circuit 26 into the reference signal selection circuit 30 , triggering circuit 30 to output a steady - state , low ( dc ) voltage signal 36 , in lieu of ( ac ) reference sine wave 32 . various known circuit design and switching techniques may be employed to achieve the aforementioned reference signal generation and selection of either an ( ac ) sine wave signal 32 or a steady - state low ( dc ) voltage signal 36 , depending on the voltage level v siu detected at the input to the ringing signal power converter 10 . by way of example ( s ), a low voltage ( ac ) reference sine wave may be generated by imputing a high frequency ac sine wave reference , ( e . g ., 200 hertz or greater ), into a step counter , such as a johnson step counter , and then converting the counter output signal into a smooth , low frequency sine wave via a filter circuit . a steady - state , low ( dc ) voltage signal may be provided by a reference voltage source , preferably no more than 2 . 5 volts and substantially accurate . an operational - amplifier type feedback hysteresis may be employed in conjunction with one or more transistor switches to select between the ( ac ) sine wave and ( dc ) steady - state voltage , based on a selected voltage level differential between control signals 28 and 34 . other known circuit switching techniques may also be employed to achieve the change in ringing reference signals based on the detected voltage level of the siu operating voltage . an electrical path 38 is provided between the output side of the power converter 10 and a summing junction 40 , wherein ringing voltage signal 22 is input into summing junction 40 via path 38 . reference signal 32 or 36 is also input into summing junction 40 , wherein summing junction 40 compares reference signal 32 or 36 with ringing signal 22 . a feedback signal 42 is output from summing junction 40 and input into positive and negative switch drivers 44 and 46 , respectively , which drive positive current switch 20a and negative current switch 20b , respectively , in order to shape ringing voltage signal 22 into the same form as the particular reference signal 32 or 36 . preferably , a resistor 48 is provided across electrical path 38 to reduce the voltage of signal 22 prior to when it is compared to reference signal 32 or 36 , in order to increase the accuracy of feedback signal 42 . as illustrated in fig4 during normal siu voltage conditions , i . e ., when v siu ≧ v min , ringing output voltage v ring is shaped by ( ac ) reference signal 32 in the form of an ac sine wave . in the embodiment shown , v ring is supplied at 56 volts - rms by the power converter operation , having a peak nominal voltage of around 82 volts . v ring is centered at a - 48 volts offset in order for the siu to detect an off - hook condition and is transmitted at a frequency of 20 hertz , i . e ., with a peak to peak period of 50 ms . as depicted in fig5 during an undervoltage condition , i . e ., when v siu & lt ; v min , illustrated as occurring at t = a , ringing output voltage v ring is shaped by ( dc ) reference signal 36 in the form of a steady - state , - 48 volts dc signal . as demonstrated by comparing shaded areas v ac and v dc of fig4 and 5 , respectively , when implemented as a dc offset signal , v ring consumes considerably less operating voltage during a 50 ms cycle than when implemented as a normal ac ringing signal . preferably , voltage detection circuit 26 is provided with delay means ( not shown ), such as an rc or lc delay circuit , to maintain the ( undervoltage ) control signal 34 &# 34 ; on &# 34 ; for a predetermined amount of time , t dc , when an undervoltage condition is first detected , regardless of whether the siu operating voltage v siu returns to a sufficient operating level prior to the duration of t dc . t dc is preferably selected to be at least 20 ms to allow for voltage recovery in an siu and is preferably greater , e . g ., 50 ms in the embodiment illustrated in fig5 . by implementing a &# 34 ; minimum &# 34 ; recovery time t dc , v siu is allowed to restore to a safe operating level and then be maintained at that level for a sufficient period of time to ensure stable operation . in this manner , rapid , alternating &# 34 ; on / off &# 34 ; selection between ac and dc ringing reference signals by selection circuit 30 is avoided . a maximum holding time , preferably no more than 100 ms , -- i . e ., no more than two ( ac ) sine wave cycles at 20 hertz , is also preferred in order to avoid unnecessary disruption of audible ringing signal transmission over the subscriber line . if , at the end of the predetermined time interval , an undervoltage condition in v siu is still detected by circuit 26 , then control signal 34 and corresponding dc reference signal 36 are maintained for an additional time interval t dc , with the process repeated until sufficient siu operating voltage is finally restored , or until ringing signal generation is no longer required in the siu . thus , a ringing voltage power supply system for a remote telecommunications subscriber interface unit has been disclosed , including siu voltage detection circuitry operating in conjunction with variable - state ringing reference signal generation . while embodiments and applications of the present invention have been illustrated and described , it would be apparent to those skilled in the art that many other modifications are possible without departing from the inventive concepts herein . the scope of the invention , therefore , is not to be restricted except in the spirit of the appended claims .