Abstract In a vestigial sideband modulator, an i.f. signal is modulated with a video input and the extreme part of one of the sidebands is removed to provide the right frequency response shape for transmission. The signal for transmission is usually produced by mixing the i.f. signal with an oscillator signal known as the heterodyne signal which differs from the desired carrier frequency by the i.f. frequency, and filtering the signal with a subsequent channel filter to remove any unwanted heterodyne oscillator components and the unwanted sum or difference product. To avoid the need for such a channel filter, the filtered video-modulated i.f. signal is demodulated to quadrature baseband components using the i.f. oscillator, and quadrature components at carrier frequency are modulated with these baseband components to produce on combination a v.s.b. modulated video output at carrier frequency.
A v.s.b modulator comprising first modulating means for modulating a signal derived from an i.f. oscillator with a baseband input signal, vestigial sideband filtering means for filtering the modulated i.f. signal, means for demodulating the filtered modulated i.f. signal to produce baseband components substantially in quadrature using signals derived from the i.f. oscillator, second modulating means for modulating signals which are substantially in quadrature and are derived from a carrier oscillator with the baseband quadrature components, and means for combining the modulated carrier signals to produce a vestigial sideband modulated carrier signal.
Claim 2
Dependent Claim
92 words
A v.s.b modulator comprising first modulating means for modulating a signal derived from an i.f. oscillator with a baseband input signal, vestigial sideband filtering means for filtering the modulated i.f. signal, means for demodulating the filtered modulated i.f. signal to produce baseband components substantially in quadrature using signals derived from the i.f. oscillator, second modulating means for modulating signals which are substantially in quadrature and are derived from a carrier oscillator with the baseband quadrature components, and means for combining the modulated carrier signals to produce a vestigial sideband modulated carrier signal.
Claim 3
Dependent Claim
65 words
A v.s.b. modulator as claimed in claim 1, in which the demodulating means includes means for splitting the i.f. signal into substantially in-phase and quadrature components, and phase shifting means to adjust the phase of the i.f. signal fed to the splitting means to maintain one of the components in quadrature with respect to the phase of the i.f. component of the modulated i.f. signal.
Claim 4
Dependent Claim
46 words
A v.s.b. modulator as claimed in claim 2, in which the phase shifting means is arranged to adjust the phase so as to maintain zero output of low frequencies in the baseband component that is in quadrature with the i.f. component of the modulated i.f. signal.
Claim 5
Dependent Claim
45 words
A v.s.b. modulator as claimed in claim 1, including means for combining substantially quadrature components of a subcarrier frequency modulated by a subsidiary input with the baseband quadrature components for modulation of the quadrature signals derived from the carrier oscillator in the second modulating means.