Television sub-carrier transmission

A receiver for receiving a television signal from an earth orbiting satellite, wherein the receiver comprises a first demodulator stage for extracting a video signal and at least one information-carrying sub-carrier signal from the received signal, at least one sub-carrier processing channel coupled to an output of the first demodulator stage and including a signal averaging circuit followed by a second demodulator stage for producing a respective demodulated information signal, and an impulse noise reduction circuit upstream of the signal averaging circuit, the impulse noise reduction circuit being contained in the first demodulator stage in the form of a mixer arranged to receive an intermediate signal which has been split into two components, each component being fed to a respective input of the mixer, wherein one of the said components is delayed with respect to the other by a time T, defined by (0.25/B).ltoreq.T<1/B, where B is the demodulator bandwidth.

A communicaton system for transmitting data by means of a television satellite link comprising:

• (a) a transmitter for transmitting a television signal including a plurality of sub-carriers, (b) an earth-orbiting satellite transponder, and (c) a receiver assembly comprising an antenna coupled to a receiver unit which has a first demodulator stage operable to demodulate the television signal, a video output stage coupled to an output of the first demodulator stage to provide a picture signal for driving a television display, and a plurality of sub-carrier channels also coupled to an output or outputs of the first demodulator stage to provide data output signals from the received sub-carrier signals, each sub-carrier channel comprising a signal averaging circuit followed by a further demodulator stage and an impulse noise reduction circuit upstream of the averaging circuits whereby said at least one sub-carrier signal can be satisfactorily demodulated when the received signal strength is low such that said video signal is substantially unrecoverable.

A method of receiving information transmitted as data by means of a sub-carrier forming part of a satellite television signal, the method inlcuding the steps of demodulating the television signal in a receiver demodulation stage to provide a video signal and at least one sub-carrier signal, reducing impulse noise in the subcarrier signal, averaging the sub-carrier signal and demodulating it to produce a data signal containing the said information by a demodulator having a mixer coupled to receive components of a received intermediate frequency signal delayed with respect to each other by a time T, where (0.25/B.ltoreq.T<1/B), B being the bandwidth of the signal to be demodulated.

A method of receiving information transmitted as data by means of a sub-carrier forming part of a satellite television signal, the method including the steps of demodulating the television signal in a receiver demodulation stage to provide a video signal and at least one sub-carrier signal, reducing impulse noise in the sub-carrier signal, averaging the sub-carrier signal and demodulating it to produce a data signal containing the said information.

A receiver for receiving a television signal from an earth orbiting satellite, wherein the receiver comprises,

• a first demodulator stage for extracting a video signal and at least one information-carrying sub-carrier signal from the received signal, each sub-carrier being received as a spread spectrum signal,
• at least one sub-carrier processing channel coupled to an output of the first demodulator stage and including a signal averaging circuit followed by a second demodulator stage for producing a respective demodulated information signal,
• means coupled between the first demodulator stage and the or each averaging circuit for converting the or each spread spectrum sub-carrier signal to a narrow band signal.

A communication system for transmitting data by means of a television satellite link comprising:

• (a) a transmitter for transmitting a television signal including a plurality of sub-carriers, said transmitter including a means of applying a direct sequence code to each sub-carrier to reduce its spectral power density, (b) an earth orbiting satellite transponder, and (c) a receiver assembly comprising an antenna coupled to a receiver unit which has a first demodulator stage operable to demodulate the television signal, a video output stage coupled to an output of the first demodulator stage to provide a picture signal for driving a television display, a plurality of sub-carrier channels also coupled to an output or outputs of the first demodulator stage to provide data output signals from the received sub-carrier signals, each sub-carrier channel comprising a signal averaging circuit followed by a further demodulator stage, and at least one code lock loop circuit coupled to receive spread spectrum sub-carrier signals from the first demodulator stage and to convert the said sub-carrier signals into signals having a relatively narrow bandwidth.

A method of receiving information transmitted as data by means of a sub-carrier forming part of a satellite television signal, the method including the steps of demodulating the television signal in a receiver demodulation stage to provide a video signal and at least one sub-carrier signal, averaging the sub-carrier signal and demodulating it to produce a data signal containing the said information wherein the sub-carriers are received as spread spectrum signals and wherein the demodulated sub-carrier signals are converted to narrow band signals.