Apparatus and method for masking audio signals in a signal distribution system
AssigneeScientific-Atlanta, Inc.
InventorsRaheel Khan, William P. Lafay
Life of this patent
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Abstract
An apparatus and method is provided for transmitting “masked” stereo audio signals in a signal distribution network such as a cable television system within the bandwidth constraints of conventional frequency allocation schemes. In various embodiments, left and right stereo audio signal components (L+R and L−R) are used to amplitude modulate a carrier using independent sideband (ISB) modulation without the transmission of a pilot signal. A jamming signal or low bandwidth data signal may be transmitted in place of the normal pilot tone. Various aspects of the invention contemplate the use of Hilbert transforms to perform the ISB modulation and demodulation.
Claims15
A method of transmitting stereo audio information across a distribution medium, the stereo audio information comprising a first audio component corresponding to a sum (L+R) of a left channel audio and a right channel audio signal, and a second audio component corresponding to a difference (L−R) between the left channel audio signal and the right channel audio signal, the method comprising the steps of:
- (1) subtracting the first audio component from the second audio component to produce a difference signal and passing the difference signal through a Hilbert transformer to produce a transformed difference signal;
- (2) using the transformed difference signal and the first audio component summed with the second audio component to modulate a carrier signal to produce an independent sideband (ISB) modulated signal comprising upper and lower sidebands one corresponding to the first audio component and the other corresponding to the second audio component;
- and introducing the independent sideband modulated signal into a distribution medium.
Apparatus for generating an amplitude-modulated independent sideband signal from a first signal comprising a sum (L+R) of a left and right stereo audio signal and a second signal comprising a difference (L−R) of the left and right stereo audio signals, the apparatus comprising:
- an in-phase signal generation circuit which generates an in-phase signal component from a sum of the first signal and the second signal;
- a quadrature signal generation circuit which generates a quadrature signal component by performing a Hilbert transformation, the quadrature signal generation circuit comprising a first summer that subtracts the first signal from the second signal to produce a difference signal, and a Hilbert transformer that receives the difference signal, and performs a Hilbert transformation;
- and
- an output summing circuit, coupled to the in-phase signal generation circuit and the quadrature signal generation circuit, which combines the in-phase and quadrature signal components to produce the amplitude-modulated independent sideband signal comprising a first independent sideband containing information from the first signal and a second independent sideband containing information from the second signal.
A system for transmitting masked stereo audio signals, comprising:
- a headend comprising an audio masking circuit which generates a modulated signal comprising a first independent sideband containing information from a L+R stereo signal and a second independent sideband containing information from a L−R stereo signal, the audio masking circuit comprising
- an in-phase signal generation circuit which generates an in-phase signal component from a sum of the first signal and the second signal;
- a quadrature signal generation circuit which generates a quadrature signal component by performing a Hilbert transformation, the quadrature signal generation circuit comprising a first summer that subtracts the first signal from the second signal to produce a difference signal, and a Hilbert transformer that receives the difference signal, and performs a Hilbert transformation;
- and
- an output summing circuit, coupled to the in-phase signal generation circuit and the quadrature signal generation circuit, which combines the in-phase and quadrature signal components to produce the modulated signal;
- a signal distribution network coupled to the headend;
- and
- at least one settop terminal, coupled to the signal distribution system, comprising an audio demasking circuit which demodulates the first and second independent sidebands from the modulated signal and provides as an output a stereo signal compatible with the Broadcast Television Standards Committee signal format, the audio demasking circuit comprising
- a quadrature settop component circuit which generates a quadrature settop component from the modulated signal, the quadrature settop component circuit comprising a Hilbert transformer;
- an in-phase settop component circuit which generates an in-phase settop component from the modulated signal and delays the in-phase settop component by an amount equivalent to the delay imposed by the Hilbert transformer;
- and
- a settop summing circuit, coupled to the quadrature settop component circuit and the in-phase settop component circuit, which sums the quadrature component and the delayed in-phase settop component.
Dependencies are inferred from the claim text.