Abstract: A method and apparatus are provided for allowing two different television signals to be combined and sent to a common propagation device, such as a tower antenna. The apparatus utilizes a number of appropriately loaded hybrids, hybrid waveguide junctions, or other appropriate equivalent devices to combine the signals in such manner that there is a minimum of intersignal interference or distortion. The apparatus utilizes bandpass/reject filters and notch filters in conjunction with the hybrids to achieve the desired results. A television broadcast system using the hybrid combining apparatus is also provided.

Abstract: An exciter for advanced television signals such as a digital television signal. The exciter corrects for instabilities in the television signal provided to it so as to produce a signal with tightly controlled frequency and reduced co-channel interference. The exciter inserts a pilot frequency into the digital television signal after the signal has been shaped, thereby reducing the difficulties of shaping. Also disclosed is a rate converter for use in an exciter so that a digital signal can be received at a first frequency and can be output at a second frequency in a manner in which the output of a signal is assured even in the absence of an input signal.

Abstract: An N-level digitally encoded signal at a symbol rate fs substantially equal to three times the NTSC color subcarrier frequency, N being a plural integer, modulates a carrier signal of a frequency within a few hundred kilocycles of the NTSC audio carrier, for transmission through a channel subject at times to the presence of a co-channel NTSC analog television signal. The modulation is suppressed-carrier amplitude-modulation generating first and second amplitude-modulation sidebands. Any portion of the amplitude-modulation sidebands extending outside the channel is suppressed in the transmitted signal, thereby making the first amplitude-modulation sideband a vestigial sideband providing an image for only a portion of the second amplitude-modulation sideband closer in frequency to the carrier signal.

Abstract: A method and apparatus are provided for enabling multiple users to concurrently access a PC-based server in a home local area network using conventional TVs as display devices. A client system includes a TV, conventional input devices, such as a keyboard and a mouse, and a set top box for interfacing the TV to the network. The server maintains a system work area and multiple processes corresponding to user applications. The system work area is partitioned in the server into multiple independent, simultaneously active desktops, one desktop for each of the client systems. Individual processes are mapped to the appropriate desktop. Multiple frame buffers are maintained in the server, such that a different frame buffer is assigned to each client system. Each desktop is rendered within the server and stored in the corresponding frame buffer. The contents of each frame buffer are transmitted over a transmission medium to the set top box of the corresponding client system.

Abstract: Apparatus and method are provided by which television signals may be transmitted from adjacent NTSC and DTV channels. The NTSC television signals for a first channel are selected from a frequency band including a plurality of channels and wherein this first channel includes NTSC aural and NTSC visual signals. A source of DTV television signals are selected for a second channel in the same frequency band and wherein the second channel is adjacent the first channel. The NTSC visual signal and the DTV signal are combined to provide an intermediate combined signal. The intermediate combined signal and the NTSC aural signal are then combined to provide a combined output signal for application to antenna means for transmitting television signals.

Abstract: A multichannel television access control system is disclosed wherein processing of the broadband signal is performed by gated coherent RF injection on a plurality of channels, thereby effecting selective descrambling of an arbitrary subset of channels in a channel group. The system provides for a closed loop injection signal phasor control so as to maintain substantially constant relationship between the television signals and the injected signals. The combined signals are supplied to subscribers enabling the use of their cable ready equipment.

Abstract: A multi-standard receiver is which is particularly suited for reception of B/G-PAL and D/K-PAL TV signals. In the receiver, a tuner (TUN) selects a desired TV signal which is passed to a multi-standard demodulator (DEM) via an intermediate-frequency filter circuit (IFF). To obtain good performance, particularly in view of EN 55020 requirements, at moderate costs, the intermediate-frequency filter circuit (IFF) includes a main filter (FIL1) and an auxiliary filter (FIL2). The main filter (FIL1) may have a relatively narrow band, for example, just capable of passing a full B/G-PAL TV IF signal to the demodulator circuit. For D/K-PAL reception, the auxiliary filter (FIL2) is effectively switched (SW) in parallel with the main filter (FIL1). The auxiliary filter (FIL2) transfers the D/K-PAL sound carrier to the demodulator circuit (DEM), which transfer is not possible via the relatively narrow-band main filter (FIL1).

Abstract: A method for compatibly tuning a video signal receiver to an image processing device involves generating a prompting message for display to prompt a user to initiate a command response. First and second signals are generated. Each signal incorporates the prompting message but only one of the signals is compatibly tuned to the display. The first and second signals are alternately provided to the display and the prompting message is legibly displayed when the compatibly tuned signal is provided. The signal that is compatibly tuned to the display is selected in response to the command.

Abstract: A method and apparatus for transmitting video over telephone lines is provided. FM data is produced representative of a video picture to be transmitted and synchronization data concerning the transmitted picture is also produced. The FM data and the synchronization data are transmitted via telephone lines to a receiver. A pair of memories is provided in the receiver. The receiver uses the synchronization data to coordinate the storage and display of the received pictures. One memory displays the picture while the other memory is being loaded. The memories are toggled so that the memory from which the previously stored data is displayed becomes the memory being loaded and the memory being loaded becomes the memory from which previously stored data is displayed. Alternatively, one memory is a load memory while the other memory is a display memory. After the incoming picture is stored in the load memory, it is transferred to the display memory.

Abstract: The present invention provides a device (200, 300), system (400, 500) and method for real-time streaming of a multimedia file stored in a remote server over a packet network to a multimedia client connected to the same network via a low-speed access link such as an analog telephone line. The invention operates to provide significantly improved QoS with respect to both quality and delay.

Abstract: A cable modem provides upstream data signals in a cable system on a return channel. The upstream data signals are provided in a 50-550 MHz frequency range and yet do not affect the picture quality associated with conventional cable television signals. The data is provided on vestigial sidebands associated with the cable television signals or during black periods associated with the cable television signals. The data can be modulated in accordance with quadrature amplitude modulation (QAM) techniques.

Abstract: In a single-chip multistandard video modulator arrangement in which audio signals for transmission with video signals are first modulated onto a subcarrier and then combined with the video signals for modulation onto a VHF or UHF main carrier, the modulated subcarrier signals are passed through a high-pass filter before being combined with the video signals in order to reduce the amplitude of any audio frequency components, and the resulting variations in main carrier amplitude, which tend to produce "sound in vision" effects.

Abstract: An electrical signal transmission system is disclosed and includes a transmission unit and a receiver unit. The transmission unit converts the relative amplitudes of an input amplitude signal to a time domain representative signal of varying pulse widths, and the receiver unit receives the time domain representative signal and generates an output signal representative of the input amplitude signal.

Abstract: An automatic gain controlling circuit and a high-definition television signal receiving apparatus includes an input terminal for receiving a signal including first and second signal components creating interference, a first automatic gain controlling amplifier for amplifying the received signal and providing an amplified output signal, a filter for selectively passing a signal portion in a predetermined band of the amplified output signal from the first automatic gain controlling amplifier, and a second automatic gain controlling amplifier for amplifying the signal portion passed by said filter and providing amplified output signal portion.

Abstract: A telecommunication system for wideband analog input video signals, data signals, facsimile signals, and the like which comprises a synchronous pixel analog modulator circuit for converting the wideband analog input signals into a multi-bit multi-level discrete code signal, and a transmission gate modulator circuit for modulating the multi-level discrete code signal on a carrier at high speed and within a limited bandwidth.

Abstract: A method and apparatus for reducing video crosstalk in a data carrier located within a lower vestigial sideband (VSB) of a standard television signal. Specifically, an apparatus according to the invention comprises a single bandpass filter for conditioning a VSB television signal, and a frequency interleaver for spectrally adding the data carrier to the lower VSB of the standard VSB video signal. The filter removes removing video components from the lower VSB of a standard VSB video signal, including frequencies greater than the highest frequency nominally associated with the lower VSB of the standard television signal, illustratively 250 Khz above the nominal VSB bandedge. The filter also boosts frequencies in a spectral boost region within a passband for boosting region. The boosted frequencies comprise image frequencies associated with the non-VSB spectrum attenuated by the filter.

Abstract: A wireless video and audio transmission device is provided that comprises a transmitter, a receiver and a TV. The transmitter has a 2.4 GHz transmission module that modulates the video signals, audio signals from a microphone that have been amplified by the amplifying circuit, and signals from a detector The transmitter transmits those signals from a 2.4 GHz transmission antenna, and after reception by the receiver, the signals will be demodulated and processed into regular A/V signals. The A/V signals are coupled to the TV, and the detected signals are coupled to a buzzer to sound an alarm. A UHF transmitter of the receiver modulates the audio signals from a microphone that have been amplified by an amplifying circuit, and transmits them from a UHF transmission antenna. After the signals have been received by a UHF reception antenna, they are transmitted to a UHF reception circuit to be demodulated, amplified and processed into regular audio signals The audio signals are output to a speaker.

Abstract: A wideband phase and frequency modulator using a direct digital synthesis alias frequency band is described. The modulator is spurious free by the utilization of a fixed frequency at the output of the direct digital synthesizer wherein only certain submultiples of the clock frequency are used. Known frequency ratios relative to the clock frequency of the direct digital synthesizer (DDS) cause the DDS to function as a simple divider resulting in extremely low spurious levels when additional conditions are met. An additional frequency synthesizer may be utilized to achieve a tuning range since the DDS output is preferably held at a fixed frequency. At least one additional mixing operation is utilized for upconversion to a microwave frequency. Since fixed frequency ratios are used, implementation simpler than a full DDS may also be constructed consisting of digital modulators, multiplexers, digital to analog convertors, and filters.

Abstract: In optically transmitting a video signal comprised of luminance and chrominance signals which are frequency modulated to provide an FM luminance signal and an FM chrominance signal in respective adjacent frequency bands, such as, 6 to 20 MHz and 20 to 30 MHz, respectively, whereupon, infrared signals corresponding to the FM luminance and chrominance signals are output by respective emitting diodes, a carrier frequency for the FM chrominance signal is set, for example, between 25.93 and 26.08 MHz, so that the lower sideband wave of the FM chrominance signal is spaced by a substantial frequency from a frequency band in which a secondary distortion of the FM luminance signal is substantially generated. Further, the upper sideband wave of the FM chrominance signal is removed, as in a trap circuit, for ensuring that the transmitted FM chrominance signal will be accommodated in the respective frequency band therefor when its carrier frequency is set, as above.

Abstract: A system of sending digital high definition television pictures and sound in a limited bandwidth using the principle of analyzing and sending changes in pictures and coded instructions rather than entire pictures. Delaying the presentation of the pictures slightly permits major picture changes and sound to be sent at a lower bit rate over a longer period of time. Non-changing areas of the pictures are sent to the receiver and remain on the screen until changes occur or refresh is desired. When TV cameras shift orientation, backgrounds of pictures or objects which do not change are shifted on the receiver with appropriate codes rather than repeating the entire picture. Fast changing portions of pictures are determined and sent more frequently than slower or non-changing ones. Sound information is acquired and compared to libraries of sound fragments.

Abstract: At the transmitter side, carrier waves are modulated according to an input signal for producing relevant signal points in a signal space diagram. The input signal is divided into, two, first and second, data streams. The signal points are divided into signal point groups to which data of the first data stream are assigned. Also, data of the second data stream are assigned to the signal points of each signal point group. A difference in the transmission error rate between first and second data streams is developed by shifting the signal points to other positions in the space diagram. At the receiver side, the first and/or second data streams can be reconstructed from a received signal. In TV broadcast service, a TV signal is divided by a transmitter into, low and high, frequency band components which are designated as a first and a second data stream respectively.

Abstract: A television (TV) transmission system is adapted to receive a plurality of TV signals and to transmit these TV signals at a plurality of channel frequencies using multi-bay antenna. The system includes a plurality of exciters, each exciter receiving a one of the TV signals and operative to combine the TV signal with a carrier signal at a respective one of the channel frequencies to produce a transmittable signal. Each exciter is coupled to a signal splitter which produces a plurality of split signals from the transmittable signal. A separate power amplifier is coupled to receive each split signal and passes an amplified signal to a power combiner which produces a single combined output signal. A plurality of such power combiners pass their respective output signals to a plurality of antenna elements arranged to radiate as a group.

Abstract: A UHF transmitter for digital TV signals includes, for example, twelve silicon carbide transistor RF output power amplifier panels driven in parallel by two silicon carbide amplifier driver panels and where each panel is comprised of two modules each further including a 180.degree. signal splitter, a pair of quadraphase signal splitters and combiners, a 180.degree. signal combiner, and five RF amplifier pallets which themselves are comprised of two silicon carbide transistors located on a common flange and operated in push-pull relationship so as to suppress even order distortion products and reduce sensitivity to ground currents. A monitoring system, including a touch screen control display and control panel senses the status of all assemblies so as to allow an operator to control the transmitter and inquire about the status of the various components and power supplies, and to replace one or more units as required without the need for turning the transmitter off.

Abstract: At the transmitter side, carrier waves are modulated according to an input signal for producing relevant signal points in a signal space diagram. The input signal is divided into, two, first and second, data streams. The signal points are divided into signal point groups to which data of the first data stream are assigned. Also, data of the second data stream are assigned to the signal points of each signal point group. A difference in the transmission error rate between first and second data streams is developed by shifting the signal points to other positions in the space diagram expressed at least in the polar coordinate system. At the receiver side, the first and/or second data streams can be reconstructed from a received signal. In TV broadcast service, a TV signal is divided by a transmitter into, low and high, frequency band components which are designated as a first and a second data streams respectively.

Abstract: The BASEBAND VIDEO/AUDIO/DATA TRANSCEIVER invention simultaneously transmits and receives baseband video signals, audio signals, and associated digital data and control signals within a single continuous signal over a single physical transmission line. The device is a single cohesive module, best embodied as an integrated circuit. The audio signals are sampled, and subsequently reconstructed. Audio samples and digital signals are multiplexed, then fed to a multirate elastic buffer. The multiplexed signals are extracted from the elastic buffer coincidentally with the video signal horizontal synchronization pulses and applied to these same pulses, then placed on a channel having a similar transceiver at the other end. The transceiver invention separates the received signals from the transmission line by canceling out its own transmitted signal. The received audio and digital data bursts are culled and reshaped.

Abstract: An HDTV broadcast transmission system and an analog NTSC broadcast transmission system are carried on the same tower and broadcast to the same area by adaptation of both systems: the NTSC system including: (a) a source of analog NTSC video signals, (b) an NTSC analog video signal exciter producing NTSC IF modulated analog video signals, (c) an NTSC frequency up converter and power amplifier producing NTSC RF modulated analog video signals (NTSC RF signals), (d) an NTSC broadcast antenna and (e) an all band VHF and UHF signal transmission line having an input end and an output end; the HDTV system including: (1) a source of digital video signals, (2) a digital video signal encoder producing IF modulated digital video signals and (3) a frequency up converter and power amplifier producing RF modulated digital video signals (HDTV RF signals) and an HDTV broadcast antenna; by combining the HDTV RF signals and NTSC RF signals at the input end of the transmission line and separating them at the output end and feedin

Abstract: A low power, multi-function cellular television system for transmitting signals at super high frequencies. Each cell transmitter radiates signals with one polarization over one sector, and signals with a different polarization over the balance of 360.degree.. Sectors of adjoining cells are aligned to minimize interference from the adjoining cell. Adjoining cells may transmit different pluralities of carrier frequencies which are interleaved to minimize interference further, with each cell having one set of transmitted frequencies radiated for both polarizations, occupying at least 90% of a band used in common for the system.

Abstract: An electronic camera system includes a programmable transmission capability for selectively transmitting electronic image data to a plurality of remote base units. In one embodiment, a camera module is detachably coupled to a portable computer including a display screen and a data entry device. The electronic image data generated by the camera module is supplied to the portable computer for display on the display screen. The data entry device is used by an operator to select which of the plurality of base units are to receive the digital image data. The digital image data is supplied by the portable computer to a radio-frequency transmitter module for transmission to the selected base units. The radio-frequency transmitter module is formed either integral with the portable computer or, like the camera module, is detachably coupled to the portable computer.

Abstract: A video camera/camcorder mounted transmitter for transmitting audio, video, time code and tally status signals through an omnidirectional antenna to a receiver. The receiver demodulates the signal into its original components, and uses the tally status signal to start and stop a recorder associated with a monitor receiving the transmitted image. The monitor/recorder can, at the option of the user, display and record the time code in a superimposed window on the video screen, thereby producing an accurate window dub of the video being recorded at the camera.

Abstract: Television signal transmission and reception system includes a broadcasting center, a transmission network and many reception terminals. In the broadcasting center, plural television signals corresponding to individual channels are modulated into transmittable form, and a synthesis circuit synthesizes the television signals to produce a synthesis image signal for a multi-screen display comprising plural small screens. Both the ordinary television signal and the synthesis image signal is transmitted with control information to the reception terminals through the transmission network. In the reception terminal, for selecting a desired channel out of the individual channels, the synthesis image signal is used for displaying a multi-screen, a moveable cursor image is superimposed on the multi-screen for designating one small screen corresponding to the desired channel.

Abstract: In signal processing circuitry using active devices, non-linearity of the device causes distortion of pure tones and the generation of intermodulation products for more complex signals. The transfer characteristics of such active devices may be considered as the accumulated characteristics of input and output filter stages separated by a non-linear region. To compensate for non-linearities and reduce intermodulation products, a drive circuit comprises pre- and post-correction filters separated by a compensating amplifier. The frequency response of the pre-correction filter corresponds to that of the input filter stage but is reversed in the frequency spectrum; with conventional I.F. the transfer characteristic of the amplifier and frequency response characteristic of the post-correction filter are the respective complements, of those of the non-linear region and pre-correction filter.

Abstract: A paging system (200)is provided having at least one television subscriber unit (222) with a television identification number and at least one paging subscriber unit (236) with a pager identification number. The paging system (200)includes a paging control station (240) for processing a page. This page includes the pager identification number that is intended for the at least one paging subscriber unit (236). The paging control station (240) includes a circuit (226) for determining when the at least one paging subscriber unit (236) is not in use. The paging system (200) also includes a television transmitter (216) which is coupled to the paging control station (240) for selectively transmitting the page to the television subscriber unit (222) in response to the circuit (226) for determining when the at least one paging subscriber unit (236) is not in use.

Abstract: A selective intermodulation correction system which can correct for in-band intermodulation products which are formed in a high end of a video sideband region generated in the common amplification of a television signal. The selective intermodulation correction system operates to generate an intermodulation product which is equal in amplitude but opposite in phase to in-band intermodulation products formed at the high end of the video sideband region. By combining such a signal to a conventionally corrected television signal, the in-band intermodulation products formed in the high end of the video sideband region can be cancelled.

Abstract: For transmitting digital HDTV signals, the HDTV signal, which is either fed with a data rate of .ltoreq.20.48 Mbit/s or which is data-reduced to such a data rate of .ltoreq.20.48 Mbit/s by means of a method utilizing the psycho-optical phenomena of the human eye, is split at the transmitter side of a broadband transmission link operating on the DSR method into two data streams of .ltoreq.10.24 Mbit/s each, and is fed to the I/Q interface of the 4PSK modulator of the DSR transmission link. These two data streams whch are transmitted to the I/Q output of the 4PSK demodulator of the DSR transmission link are then further processed thereat.

Abstract: Simulcasting from a television transmitter facility of both NTSC video signals, via a UHF/VHF portion of the broadcast spectrum, and digital HDTV signals, over a prescribed UHF portion of the broadcast spectrum, is accomplished by converting the NTSC video signals into digital format at the television studio and multiplexing the digitally formatted NTSC signals and digital HDTV signals into a combined NTSC/HDTV digital television signal. The combined NTSC/HDTV digital television signal is transmitted over a common microwave radio frequency communication channel linking the television studio with the television transmitter facility. At the television transmitter facility, the combined NTSC/HDTV digital television signal is demultiplexed into separate HDTV and NTSC video digital signals. The digital NTSC video signals are converted into analog NTSC format, and the NTSC video signals and the HDTV signals are simulcast from the television transmitter facility.

Abstract: A cochannel interference filter for a HDTV transmission system comprising a post-comb filter. The post-comb filter comprises first and second feedforward circuits. The first feedforward circuit includes a first adder and a first delay circuit. The first adder has a first input connected to an output of a data clock recovery circuit. The first delay circuit has an input connected to the output of the data clock recovery circuit and an output connected to a second input of the first adder. The second feedforward circuit includes a second adder and a second delay circuit. The second adder has a first input connected to an output of the first adder. The second delay circuit has an input connected to the output of the first adder and an output connected to a second input of the second adder. Also, the post-comb filter comprises third and fourth feedforward circuits. The third feedforward circuit includes a third adder and a third delay circuit.

Abstract: Disclosed is a method and apparatus for combining a plurality of television signal channels which are immediately adjacent in frequency to each other. A main transmission line transmits a plurality of television signals therethrough in a predetermined direction of transmission. The visual signal of a first of the television channels is inserted into the main transmission line in the predetermined direction. The visual signal of a second, immediately adjacent television channel is then inserted into the main transmission line in the predetermined direction, using a frequency band otherwise occupied by the aural signal of the first television channel as a guard band to separate the combined visual signals of the first and second television channels.

Abstract: A spread spectrum system provides bidirectional digital communication on a vacant television (TV) channel for simultaneous use by more than 75,000 subscribers using time and frequency division multiplex signals locked to horizontal and vertical sync pulses of an adjacent channel Host TV station. The system, whose operation is analogous to a radar system, comprises: (1) the Host TV station to send down-link sync and data pulses to subscribers during the horizontal blanking interval (HBI), (2) subscriber "transponders" which detect those signals and transmits up-link "echo" data pulses only during the HBI to eliminate interference to TV viewers, and (3) a central receiver which also uses the host TV sync pulses to trigger range gates to detect the up-link data pulses.

Abstract: A method and apparatus for modulating and demodulating an MCM television signal is described which is particularly useful in proposed "simulcast" HDTV terrestrial broadcasting. The MCM spectrum of the television signal is shaped using weighting coefficients which are derived from either an idealized NTSC signal or an actual NTSC co-channel signal. These coefficients are used to shape the respective MCM carriers to provide an energy distribution in the MCM signal which provides greater degree of robustness with a minimum degree of co-channel interference to and from an NTSC co-channel signal. The instant invention further takes advantage of the interference characteristics of composite television signals described above, to position the placement of the MCM carriers in frequency, to minimize interference with an NTSC co-channel signal.

Abstract: A system for minimizing co-channel interference into NTSC receivers from digitally transmitted ATV signals. The NTSC receiver subjective random noise sensitivity characteristic is utilized to shape the ATV transmitter power curve. A complementary filter incorporated in ATV receivers for compensating for the shaped ATV transmitter power curve. The shaped ATV power curve emphasizes signals at the frequencies where the NTSC subjective random noise sensitivity is low and deemphasizes signals at the frequencies where the NTSC subjective sensitivity is high.

Abstract: According to an embodiment of the present invention, a high power RF electric signal transmission system such as a TV broadcast system operating in the range of 100 kW at a UHF band includes one or more high power klystrons whose outputs are combined and fed to the first port of a high power Y-junction three port ferrite circulator, the second port of the circulator feeds the broadcast system radiating antenna system, the third port of the circulator feeds a non-reflecting RF load and means are provided for tuning the circulator to compensate for changes in the temperature of the circulator ferrite material, so that the klystron(s) are isolated from reflections from the antenna system.

Abstract: A noise-reduced method of transmitting a video signal including separating the video signal into a plurality of frequency subbands representing a corresponding plurality of subband images, each of which is made up of a corresponding array of blocks of samples; for a preselected one of the subband images, measuring the busyness of each of the blocks of that subband image; for the preselected subband image, assigning each of the blocks of that subband image to one of a plurality of groups, the plurality of groups including a group of more busy blocks and a group of less busy blocks; adaptively modulating the samples of the selected subband image by multiplying the selected subband image by a set of adaptive modulation factors, wherein a greater quantity of adaptive modulation factors is used for the blocks assigned to the group of more busy blocks as compared to the blocks assigned to the group of less busy blocks; and transmitting the adaptively modulated selected subband image along with the set of adaptive m

Abstract: Television signal transmission and reception system includes a broadcasting center, a transmission network and many reception terminals. In the broadcasting center, plural television signals corresponding to individual channels are modulated into transmittable form, and a synthesis circuit synthesizes the television signals to produce a synthesis image signal for a multi-screen display comprising plural small screens. Both the ordinary television signal and the synthesis image signal is transmitted with control information to the reception terminals through the transmission network. In the reception terminal, for selecting a desired channel out of the individual channels, the synthesis image signal is used for displaying a multi-screen, a moveable cursor image is superimposed on the multi-screen for designating one small screen corresponding to the desired channel.

Abstract: In signal processing circuitry using active devices, non-linearity of the device causes distortion of pure tones and the generation of intermodulation products for more complex signals. The transfer characteristics of such active devices may be considered as the accumulated characteristics of input and output filter stages separated by a non-linear region. To compensate for non-linearities and reduce intermodulation products, a drive circuit comprises pre- and post-correction filters separated by a compensating amplifier. The frequency response of the pre-correction filter corresponds to that of the input filter stage but is reversed in the frequency spectrum; with conventional I.F. the transfer characteristic of the amplifier and frequency response characteristic of the post-correction filter are the respective complements, of those of the non-linear region and pre-correction filter.