Abstract: A telephone subscriber system comprising a speech circuit and a ring circuit adapted to be integrated monolithically to a semiconductor material substrate and being coupled to a telephone subscriber line, respectively through first and second circuit switch of the diode bridge type, further comprises a first circuit changeover switch, under control by the user, which drives at least second and third circuit switching means connected between the first diode bridge circuit and the speech circuit. The third circuit changeover switch is connected to the speech and ring circuits at a circuit communication node which forms the system point of reference, physically coincident with the substrate. The system includes a circuit interface means for controlling the second and third circuit changeover switch by external electrical signals which are related to the circuit communication node.

Abstract: A ringing generator includes a binary signal source generating a waveform with eight degrees of freedom controlled to suppress or cancel certain harmonics to produce a binary low frequency signal substantially free of low frequency components other than the desired ringing frequency. The binary signal is amplified and filtered to remove unwanted frequency components above a certain threshold to produce a substantially sinusoidal ringing signal.

Abstract: In accordance with an embodiment of the invention, a method of establishing call progress tones for a switching system having a generic signal generating apparatus, comprising the steps of displaying at least one form on a display and inserting into fields of the form definitions of the cadencing and particular generic signals associated with each call progress tone for a tone plan, temporarily storing the definitions in a memory, compiling the definitions as control signals into object code, storing the object code in a random access memory, and using the object code in the processing of a call by a switching system processor to control provision of particular signals from the generic signal generating apparatus with a particular cadence and signal level to a subscriber or operator.

Abstract: In a ringing signal control circuit for controlling supply of a ringing signal from a ringing signal generator to a subscriber's line in response to a call signal destined thereto, a switching element is repeatedly switched to connect the ringing signal generator to the subscriber's line under control of a switching signal from a switch controller. In order to suppress a surge voltage due to connection and disconnection of the subscriber's line for the ringing signal generator, a memory stores a time data signal representative of a transition time from a reference voltage level to an optimum voltage level for the subscriber's line. The optimum voltage level is the level at which the connection and disconnection can be carried out without generation of the surge voltage. A voltage comparator compares the ringing signal with the reference voltage to produce a coincident signal.

Abstract: A telephone ringing arrangement where a non-sinusoidal ringing voltage waveform is used that has significantly longer intervals during which the waveform is near zero as compared to a sinusoidal waveform. A connect interval is defined as the time during which the magnitude of the non-sinusoidal ringing voltage waveform remains less than a predefined threshold. The connect interval includes a zero-crossing of the ringing voltage waveform. For an illustrative ringing voltage waveform, the zero-crossing has zero slope. A ringing relay is controlled to connect the non-sinusoidal waveform to a telephone line such that the connection time, including relay bounce time, occurs only during one of the connect intervals. Similar principles may be used based on a disconnect interval of the ringing current waveform, to reduce transients on disconnection.

Abstract: An aural annunciator circuit for a radio receiver and a radio transceiver incorporating such. The annunciator circuit is operative to generate an aural signal when a signal is transmitted to the receiver in a manner analogous to telephonic ringer circuitry of conventional telephonic apparatus. The aural signal is of a signal level of a relatively high magnitude when the receiver in which the aural annunciator circuit is embodied is configured in a closed configuration and the aural signal is of a relatively low magnitude when the receiver is configured in an open configuration.

Abstract: A call signal generating circuit includes a step-up transformer having a primary winding and secondary windings and a first switching circuit coupled between the primary winding of the step-up transformer and a D.C. power source for turning ON/OFF at a frequency higher than a frequency of a call signal. A rectifying circuit is coupled to the secondary windings of the step-up transformer for rectifying voltages induced at the secondary windings into a positive polarity voltage and a negative polarity voltage. A second switching circuit is coupled to the rectifying circuit for alternatively outputting the positive polarity voltage and the negative polarity voltage with a quiescent time in which both the positive and negative polarity voltages are not output. A capacitor is coupled to the second switching circuit for receiving the positive and negative polarity voltages output from the second switching circuit.

Abstract: An ADPCM transcoder (60) includes an integral tone generator (65) which inserts a linear tone signal, such as a conventional DTMF tone signal, into either the transmit or receive data stream, or both. A digital PCM input signal is first converted to a first linear signal. If tone generation is enabled for transmission, then the linear tone signal is substituted for or added to the first linear signal and provided to an ADPCM encoder (63), which provides an ADPCM output signal in response. An ADPCM decoder (66) receives an ADPCM input signal and provides a second linear input signal in response. If tone generation is enabled for reception, then the linear tone signal is substituted for or added to the second linear input signal, and converted to a digital PCM output signal. The ADPCM transcoder (60) may also be integrated with other components of a signal processing system.

Abstract: Sine waves are generated by a port circuit processor of a telephone switching office by accessing a look-up table stored in a memory associated with the port circuit processor. Entries in the look-up table are accessed by using table steps or .DELTA.'s which correspond to the frequencies of the sine waves to be generated. The table steps or .DELTA.'s are changed for different sine wave frequencies and may be changed each bit period. Whether changed or not, a first fractional portion of the table step or .DELTA. for the previous frequency is combined with a second fractional portion of the table step or .DELTA. for the current frequency with the first and second fractional portions adding up to one. In this way, an X sample per second data stream, X equaling the base sample rate of the look-up table, that precisely matches a Y bits per second rate, Y equaling the transmission rate of the data stream, is generated.

Abstract: An inband tone signaling method results in improvements in talk-off and talk-down performance without sacrificing important human factors considerations. The inband tone signal transmitter has a monitor (60) for monitoring the power level in one or more guard bands (1 and 2) of a voiceband signal. A tone generator (64) which is responsive to the monitor (60) generates the inband tone signal consisting of one or more signaling frequencies each at a level such that a desired signal-to-guard ratio is achieved at a detector (50).

Abstract: A wireless telephone has a function of monitor-outputting an operation sound as an operation key is pressed. The wireless telephone can be operated in one of three modes, namely, none push-tone output mode, normal push-tone output mode and prolonged push-tone output mode. The operation sound is generated as an user selects a certain output mode by pressing the operation key. Monitor outputs from a speaker of tile wireless telephone vary with tile output mode. Specifically, a single sound is output from the speaker if the none push-tone output mode is selected. The push-tone is output from the speaker if the normal or prolonged push-tone output mode is selected. In the normal push-tone output mode, tile push-tone is output for a constant predetermined period of time (for example, 100 msec). In the prolonged push-tone output mode, the push-tone is output for a period of time as the operation key is kept pressed.

Abstract: A portable telephone set having a ringing circuit for producing different ringing tones depending upon the use mode of the portable telephone set, a portable mode or a mobile mode. The ringing circuit comprises a mode detecting circuit for detecting the use mode, an internal loudspeaker housed in a housing of the portable telephone set for producing a first ringing signal when the portable telephone set is in the portable mode, an external loudspeaker connected externally of the housing for producing a second ringing signal when it is in the mobile mode and a ringing tone generator for generating a first and a second ringing tones for driving the respective loudspeakers producing first and second ringings. The first and second ringing tones have frequency spectra compatible with frequency characteristics of these loudspeakers to produce the respective ringings efficiently.

Abstract: A method and apparatus, of producing an alarm or ringing signal by providing a first pulse signal having a predetermined pulse width to define an alarm or ringing signal amplitude, and modulating the pulse signal with a second pulse signal having a pulse rate at least an order lower than the frequency of the first pulse signal to define the alarm or ringing signal frequency, averaging the modulated first pulse signal in a loudspeaker voice coil and reproducing the result in the loudspeaker to provide the alarm or ringing signal.

Abstract: The ringer comprises a loudspeaker receiving signals from an amplifier, the amplifier having both a power supply input connected to a ringer line providing a ringer signal via a chopper power supply means, and a melody input which is connected to the ringer line via a regulation unit that provides an image signal representative of the power of the ringer signal, and via a transformation unit that transforms the image signal by means of a melody signal applied to a terminal.

Abstract: A DC to DC switch mode power supply for a service subscriber interface unit of a fiber optic network includes a source of DC primary power, a main DC to DC switched mode power supply connected to the source of DC primary power and including switching circuitry and rectifier circuitry for supplying DC operating power to the service subscriber interface unit including central office battery power to at least one subscriber loop connected thereto, and a ringing voltage DC to AC switched mode generator connected to the source of DC primary power and including switching circuitry and synchronous rectification circuitry for generating ringing ringing voltage approximating a sine wave supplied as a reference to the synchronous rectification circuitry. A Johnson counter and resistor capacitor network is preferably employed to convert a reference ringing square wave into a sine wave.

Abstract: A low frequency high voltage AC power for a telephone system is provided by a ring signal generator, which produces a ring signal from a DC power supply using a pair of switches controlled by pulse width modulation (PWM) circuit. The ring signal generator includes a PWM circuit driving a pair of MOSFET transistors via a respective pair of opto-isolators. The output of the MOSFET transistor switches is commonly connected to a low pass filter network. The ring signal generator of the present invention advantageously produces a 65 VRMS ring signal at very high efficiency.

Abstract: A portable telephone set having a ringing circuit for producing different ringing tones depending upon the use mode of the portable telephone set, a portable mode or a mobile mode. The ringing circuit comprises a mode detecting circuit for detecting the use mode, an internal loudspeaker housed in a housing of the portable telephone set for producing a first ringing signal when the portable telephone set is in the portable mode, an external loudspeaker connected externally of the housing for producing a second ringing signal when it is in the mobile mode and a ringing tone generator for generating a first and a second ringing tones for driving the respective loudspeakers producing first and second ringings. The first and second ringing tones have frequency spectra compatible with frequency characteristics of these loudspeakers to produce the respective ringings efficiently.

Abstract: A ringing signal generation circuit contains a direct current power source, a transformer, a control transistor, a rectifying and smoothing circuit, an invertor circuit, and an invertor controlling circuit. The direct current power source generates a direct current voltage. The transformer contains a primary winding and a base-driving winding in a primary side, and a secondary winding in a secondary side. The control transistor is connected in series to the primary winding and the direct current power source, and controls a current flowing in the primary winding. The rectifying and smoothing circuit rectifies and smooths a current generated in the secondary winding. The invertor circuit for converting a direct current output of the rectifying and smoothing circuit into an alternating current. The invertor controlling circuit controls the invertor circuit.

Abstract: An electronic ringing generator circuit with provision for power efficient current limiting and preservation of sinusoidal or near sinusoidal output waveform under overload conditions. A pulsewidth modulated DC-DC power convertor produces supply voltages for a linear class B power amplifier which has a low frequency low level sinusoidal signal applied to its input through a first controllable attenuator. The DC-DC convertor's reference input is also driven through a second controllable attenuator which is coordinated with the first controllable attenuator. The attenuators are operated in such a way that if the amplifier's output current exceeds a predetermined value, both the input signal and amplifier supply voltages are reduced in a coordinated manner so that simultaneously the output AC waveshape is preserved and the amplifier continues to operate at or very near optimum efficiency. The amplifier may be operated to produce a sinusoidal output (crest factor =1.

Abstract: A subscriber line control circuit for controlling a reversing switch to couple a subscriber station either to an AC signalling voltage generator or to a DC line voltage supply, depending on whether the subscriber station is on-hook or off-hook. The control circuit includes a capacitor connected between the subscriber line and the reversing switch and which by means of a rectifier is charged to a DC charge voltage by alternate half cycles of the AC signalling voltage. During the other half cycles the DC charge voltage superimposes on the AC signalling voltage and so is available for actuating an automatic switching arrangement by which the subscriber station is selected for coupling to the reversing switch. When the subscriber station goes off-hook the capacitor discharges over the subscriber line, resulting in a change in voltage or current which is detected by a detection circuit.

Abstract: A sound generation circuit for producing a dual tone which includes memory circuitry for storing data for controlling production of the dual tone, selection circuitry for selecting data stored in the memory circuitry and frequency dividers for producing an output signal having a frequency which varies based on the data selected by the selection circuitry. The production of the dual tone is based on the output signal of the frequency dividing circuitry. The sound generation circuit also includes logic circuitry to inhibit selection by the selection circuitry of data for controlling production by the sound generation circuit of a musical note when the sound generation circuit is currently producing a dual tone. The melody and dial sections of the sound generation circuit employ common elements.

Abstract: The device includes an acoustic transmitter (10), a tone generator (12), a memory (14) containing an identification code (N) and a service key (C), and an electronic circuit (16). A digital message transmitted depends on N and C and changes with each use. The time limitation moment of the device is determined. The device appears in the form of a token.

Abstract: A telephone circuit arrangement allows the use of a (high voltage) dialler chip (3) without a mode select output in a low voltage telephone. A filter (R1,C1) derives control signals from the DTMF output of the dialler chip. The control signals switch a control switch (TR1) connected to the Mode Select input (MS) of a transmission circuit (2) and the DTMF signals are applied to an input (T1) of transmission circuit (2). This enables the use of a dialler chip without a dedicated mode select output in a circuit which normally requires a dialler chip with a mode select output.

Abstract: An added main line system is disclosed with particular utility in connection with equipment for providing the services of two telephone lines on a single conductor pair. The ring generator of the added main line system has two transistor bridges, one for the ringing signal on each of two subscriber lines. In each bridge, diagonally opposed pairs of transistors are turned on in alternation, providing alternating current for the ringing signal. The two bridges, each of which has four transistors, share two transistors. Comparators provide off-hook detection during the ringing cycle. Off-hook detection circuits are provided in series with selected driver transistors of the bridges, so that the condition of customer equipment going off-hook may be detected even though limited power is available for testing for the off-hook condition.

Abstract: A dialing confirmation tone (DCT) output apparatus including a variable attenuator connected between a DCT generator and an adder adding a received voice signal and a dialing confirmation tone. The attenuator attenuates the confirmation tone based on a control signal from a control circuit. The control circuit produces the control signal using a conversation condition signal and a receiving situation signal. The conversation condition signal is provided from a conversation condition detector and indicates whether conversation is taking place or not. The receiving situation signal is provided from an earphone jack circuit and indicates whether or not an earphone is used for outputting the added signal. The attenuated confirmation tone can be heard while conversation takes place or the earphone is used.

Abstract: A tone and announcement PCM code generator (10) outputs to selected multiple channels of a TDM telephonic switching system (12) selected tones and voice announcements stored in a message memory (16) according to command signals from a telephonic switching system central controller (12A) through a share memory (20) to a state machine, or sequencer, (22). The sequencer controls the message memory (16) to provide selected tones and voice announcements through a serial interface (24) on the selected TDM channels. The entire repertoire of tones and announcements can be down loaded from a central memory (12B) of the telephonic switching system (12), and customer designed voice messages from any channel of the telephonic switching system can be stored in the message memory (16) for later local playback or for up loading to the central memory (12B). All tones and announcements are processed uniformly and any tone or announcement can be preselected to any or all channels simultaneously for maximum system flexibility.

Abstract: In a telephone system, several telephone terminals are connected to a single telephone line including several telephone interface circuits (6, 7, 8), each interface circuit being energized by ringing signals appearing on the line and being arranged between the telephone line (3) and one of the telephone terminals (13, 14, 15, 16), the ring of which is disconnected from the line. Each interface circuit comprises a circuit (26) for counting ringing signals and picking up the line, this first circuit receiving at the input the ringing signals appearing on the line (L1-L2) during a telephone call, and counting a predetermined number of these ringing signals, then picking up the line by a relay (27) at the moment the last ringing signal counted stops. The system is powered only by the signals on the telephone line.

Abstract: A telephone system with a clock includes a circuit which produces a first tone indicating a connection of a transmission line is on hold or that a preset time has arrived, and a different time announcing tone for announcing the arrival of the preset time while the connection of the transmission line is on hold. When a calling tone is to be produced upon reception of the call while the time announcing tone is emitted, the telephone system with the clock produces the calling tone in a manner different from the time announcing tone. Thus, it is possible to be alerted to both the time announcing tone and the holding tone and both the time announcing tone and the calling tone when either combination is initiated at the same time.

Abstract: A method of automatically and equally distributing the ring load for an exchange system. The subscribers are grouped so that at any one time the subscribers of one group only is supplied with the ring current for ringing the telephone. If the number of the subscribers exceeds the allowable limit within the specified on time, the on time is automatically reset by use of a subtracting time to limit the ring load within a specified value.

Abstract: A method and apparatus for generating dual tone multiple frequency signals employing a low frequency quartz oscillator, wherein a phase accumulation dual tone multiple frequency generator is comprised of a low frequency quartz oscillator. Two phase selectors are connected to produce digital phase signals in response to a signal input. Two phase accumulators, each composed of an adder and a register, are connected to accumulate the phase signals. Two ROM based sine wave tables are addressed by the outputs of the accumulators. An adder adds the outputs of the ROMs, and a D/A converter produces an analog output from the output of the adder.

Abstract: A dialer circuit comprising a microcomputer for controlling the dialing of a telephone. This dialer circuit is capable of storing a telephone number and transmitting a signal concerning this telephone number to a telephone circuit according to a given command. The dialer circuit is equipped with a means for detecting the ON state or the OFF state of a hook switch; upon detection of the state of the hook switch, the dialer circuit performs a resetting of the entire system except a memory in which the telephone number is stored. The result is that when operating the hook switch, it is possible to automatically reset only the system, thus effectively preventing any reckless driving of the system.

Abstract: A call management system connected locally between a central office telephone company switching station (CO) and a conventional single telephone line having multiple extensions provides call paging and intercom capabilities between the multiple telephone extensions. An integrated voltage generator supplies a high voltage, local ringing voltage for ringing the local extensions independent from the CO and a low voltage talk or "battery" feed to the local telephone when they are operated independently from the CO. Line splitting apparatus temporarily separates the CO from the local extensions when the local ringing voltage is generated by the integrated voltage generator and injected into the local telephone extensions. A hold device mimics an active local telephone off the hook, thereby keeping a call from the CO holding when the local telephones are being used in the paging or intercom modes.

Abstract: A ringing signal generator includes a low power signal generator for generating a relatively low power signal representing a ringing signal, a bipolar pulse generator for generating a series of bipolar pulses having widths representing the amplitude of the low power signal, wherein adjoining pulses have opposite polarities, and a high power ringing signal generator for producing a high power ringing signal in response to the series of bipolar pulses. An analog sine wave signal is superimposed upon a DC bias signal to produce the low power signal.

Abstract: A card-like device having a microprocessor and a speaker to communicate with telephone line, the device also including a single switch which when depressed by a user first activates the device and then causes a DTMF signal to be generated.

Abstract: Within a digital PBX system or the like, an appartaus permits provision of digital composite tone alerting particularly suitable for piezoelectric transducers with one arbitrarily selectable tone, by means of logic circuits for latching the sign-bit in a received PCM signal that exceeds a validity threshold, and by combining the toggling sign-bit with another tone to yield the composite tone.

Abstract: A system transmits information over telephone lines using ring signals. The ring signals have bursts of selectable duration separated by selectable time intervals between bursts. A ring signal translator receives the ring signals from telephone lines whereupon a computer activates selected indicators in response to the particular ring signal.

Abstract: A power supply (ringer power supply) for a telephone exchange comprising a first switching circuit for receiving and subjecting direct current to high frequency switching by means of a switching element, a converter transformer connected to the switching circuit, a smoothing circuit connected to the secondary side of the converter transformer, a second switching circuit connected to the output stage of the smoothing circuit and used for switching the input current by means of a transistor bridge of a plurality of transistors for being alternately turned on, a direct-current bypassing diode connected between the input and output terminals of each transistor, a driving circuit for driving each of the transistors in accordance with a predetermined driving signal, and a driving signal generator circuit for generating the driving signal for setting timing to prevent the period during which a transistor is turned on from overlapping the one during which another transistor is subsequently turned on, permitting a red

Abstract: A series ringing signal generator is disclosed which comprises a source of a user control signal. A programmable ringing signal generator is provided for generating an analog sine wave signal having a selectable amplitude and frequency in response to a selection signal. Finally, sequencing means, coupled between the source of the user control signal and the programmable ringing signal generator, is provided for generating a sequence of selection signals in response to said user control signal.

Abstract: A call signal generating circuit includes a DC/DC converter, an ON/OFF control unit, including a control circuit and a phototransistor, for ON/OFF-controlling an output voltage from the DC/DC converter, and a polarity inverter cooperating with the control circuit, for switching polarities of an ON/OFF-controlled voltage. The ON/OFF timings of the ON/OFF control unit and a polarity switching timing of the polarity inverter are adjusted to generate a call signal having an arbitrary frequency.

Abstract: A central office line card with distributed signaling voltages comprised of a plurality of subscriber line interface circuits connected to a common signal bus. Standard ringing signal is received from a remote generator within the central office and applied to the common signal bus via a first relay. A receptacle is provided on the line card for receiving a removable module containing circuitry for generating further signaling voltages such as party line ringing and coin telephone test and control voltages. Signals from the module are applied to the common signal bus via a further plurality of relays. Each of these subscriber line interface circuits is connected to the common signal bus via additional respective relays.

Abstract: A programmable ring signal generator includes a source of a user control signal. A controlled clock signal generator produces one of a set of respective clock signals responsive to the user control signal. A switched capacitor filter is coupled to the clock signal generator for converting the clock signals to an analog sine-wave. Finally, a power amplifier, coupled to the sine-wave converter, produces the ring signal.

Abstract: The present invention provides a power drive circuit which is transformer isolated from the circuit developing the drive signals, but which does not lose its DC reference voltage. This technique results in the desired isolation without the undesirable possibility of operating the output transistors in an unsafe DC region, which is a particular problem when the output transistors are FET devices. In accordance with the present invention, pulse transformers are used to drive the power FET devices.

Abstract: A switching power amplifier circuit has the functions of a power inverter and a switching amplifier embodied into one integral circuit. Two high frequency switching inverter circuits are connected back to back sharing the same inverter power transformer. One switching inverter circuit processes power from an input rail to an output capacitor while the other switching inverter circuit processes power from the output capacitor to the input rail. The output capacitor voltage is regulated with respect to a signal input in order to obtain the desired output signal waveform. A blocking capacitor in the output absorbs the DC component of the output capacitor voltage and allows a pure AC ringing signal to be applied to the load. In an application as a ringing generator circuit the signal input is supplied by a ringing signal oscillator circuit.

Abstract: A wristwatch equipped with a dialing tone generator employs a low-frequency oscillator as a reference signal oscillator. The reference signal is frequency-divided into first and second frequency-divided signals, the first frequency-divided signal being used to produce a low-frequency tone signal, the second frequency-divided signal being used to produce a high-frequency tone signal. Both these tone signals are rectangular waveforms which are filtered in corresponding low-pass filters, to output first and second sine-wave signals, which are then synthesized with each other to produce a desired dialing tone.

Abstract: A constant current circuit, including a constant current source which draws a current and a constant current source which sends a current, is connected to a two-wire subscriber line. A ringing signal made up of constant stream of positive and negative current signals is transmitted to the telephone line. The OFF-hook condition of a telephone set is detected by comparing the output voltage of a constant current circuit with a reference voltage for ring trip.

Abstract: A key-in sound-generating circuit comprises a keyboard including dial number keys and function keys, a DTMF signal generator coupled, at the output terminal, with a telephone network, a sound generator for converting, into a sound, the output signal from the DTMF signal generator, and a data processor circuit for executing the key data processing according to the keying operation performed using the keyboard, and supplying a control signal to the DTMF signal generator, the data processor being responsive to the operation of the dial number key, to supply a DTMF signal to the keyboard and the sound generator, and responsive to the operation of the function key, to supply a single-frequency signal to the keyboard and the sound generator.

Abstract: A dialing tone generator employs a low-frequency oscillator as a reference signal oscillator. The reference signal is frequency-divided into first and second frequency-divided signals. The first frequency-divided signal is used to produce a low-frequency tone signal, whereas the second frequency-divided signal is used to produce a high-frequency tone signal. These tone signals are rectangular waveforms. The rectangular waveforms are filtered in corresponding low-pass filters to output first and second sine-wave signals. Then, these sine-wave signals are synthesized with each other to produce a desired dialing tone.

Abstract: The invention concerns an electronic ringing circuit for telephone systems. A capacitor is connected to the center tapping of a first coil inductively coupled to a transistor-controlled supply line. The other side of the capacitor is connected to the junction between two switches each switch being connected via an induction coil to one end of the first coil so that when the switches are switched in opposite senses load current is drawn in alternate directions from the capacitor.

Abstract: A circuit for sending out a ringing signal of a high voltage and a low frequency to a telephone set through a telephone line repeatedly with a predetermined time interval. In order to omit use of a high power oscillator and relays, a D.C. power supply of a high voltage is operatively connected to the telephone line through phototransistors of photocouplers being driven by a pulse signal having a low voltage and the low frequency under control of pulses occurring at the predetermined time intervals so that the ringing signal is sent out to the telephone set repeatedly with the predetermined time interval.

Abstract: A stepwise approximation of a sinewave is generated by inputting different values representing different points on the waveform from a ROM (6) to a step duration counter (5). At the end of each step, a step counter (7) is incremented causing the ROM to input the next value. An up/down counter (3) is also incremented at the end of each step and caused to count in the opposite direction after a preset number of steps. The counter (3) produces an output representing the sinewave where the amplitude of all of the steps are equal but where their duration varies to produce the sinewave shape and this output is then passed to a D/A converter (9). An oscillator (1) is used to provide clock pulses to the step duration counter (5) and also to the up/down counter (3). Two such sinewave generators can be used in a dualtone multifrequency dialling circuit.