Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-07-22
2001-11-27
Pham, Chi (Department: 2731)
Pulse or digital communications
Receivers
Interference or noise reduction
C348S425300
Reexamination Certificate
active
06324233
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to reception of a modulated carrier having asymmetrical sidebands. An example of such a modulated carrier is a television (TV) signal for conventional terrestrial broadcasts.
2. Description of the Related Art
French Patent FR-A 2,656,187 mentions that, in a TV receiver, a specific type of filtering, called Nyquist filtering has to be carried out at an intermediate frequency in order to restore the integrity of the video spectrum after demodulation. This filtering ensures that the sidebands, which are asymmetrical, will always add up to “1”.
FR-A 2,656,187 also describes a prior-art TV receiver. In the prior-art TV receiver, a desired TV signal is converted in frequency to an intermediate-frequency signal. The intermediate-frequency signal passes a Nyquist filter, after which it is demodulated by means of a carrier which is extracted from the intermediate-frequency signal prior to any processing thereof, in particular prior to any filtering.
SUMMARY OF THE INVENTION
The invention seeks, inter alia, to provide reception of a modulated carrier having asymmetrical sidebands which, with respect to the background art, allows a better quality of reception.
This object is achieved in a receiver for receiving a modulated carrier having asymmetrical sidebands, characterized in that the receiver comprises a synchronous demodulator for deriving a vectorial baseband signal from the modulated carrier; and a filter for filtering the vectorial baseband signal so as to compensate for the sideband asymmetry.
This object is also achieved in a method of receiving, and in an integrated circuit forming the above receiver.
The invention takes the following aspects into consideration. A full compensation of a sideband asymmetry can be achieved only if the frequency characteristic of the filter, which has to compensate for the sideband asymmetry, is precisely positioned with respect to the modulated carrier. For example, in the case of TV reception, the modulated carrier should be located exactly halfway the relevant slope of the Nyquist filter. If the sideband asymmetry is compensated at an intermediate frequency, as in the background art, this will not be possible because the relevant filter, as well as a tuner which carries out the frequency conversion, will suffer front tolerances, temperature dependency and ageing.
In accordance with the invention, a synchronous demodulator derives a vectorial baseband signal from the modulated carrier, and a filter filters the vectorial baseband signal so as to compensate for the sideband asymmetry. The synchronous demodulator effectively converts the modulated carrier in frequency to exactly “zero” frequency, independent of the frequency of the modulated carrier. The vectorial baseband signal, which is derived from the modulated carrier, allows a filtering which is asymmetrical with respect to zero frequency and, therefore, allows compensation of the sideband asymmetry. The filtering can be very precise near zero frequency, even if filter components suffer from tolerances, temperature dependency, ageing and the like. As a result, the sideband asymmetry can be compensated with greater precision than in the background art. Consequently, the invention allows a better quality of reception.
The invention may be used in, for example, a TV receiver. In that case, a Nyquist-filter compensation of the sideband asymmetry is carried out at baseband frequencies after the modulated carrier has been synchronously demodulated, rather than at an intermediate frequency. Consequently, the modulated carrier itself does not need to be subjected to any type of Nyquist filtering which may otherwise cause unwanted phase modulation. As the modulated carrier is substantially free of unwanted phase modulation, any carrier recovery as part of the synchronous demodulation may be of a wide-band nature. A wide-band carrier recovery allows the synchronous demodulator to respond relatively quickly to any changes in the input signal it receives. Such changes may occur, for example, during search tuning in which the synchronous demodulator may receive various signals in succession. The quicker the synchronous demodulator responds, the quicker this succession of various signals may be and, consequently, the greater the speed at which the search tuning may be carried out. Thus, the invention allows a relatively fast search tuning.
The invention may also be used in, for example, a multi-standard TV receiver. In some TV standards, the upper sideband of the modulated carrier is complete and the lower sideband is truncated, whereas the opposite applies to other TV standards. Thus, in a multi-standard receiver, two different Nyquist slopes should be available: one which is negative and another which is positive. In the invention, the two different Nyquist slopes may be obtained by using only one filter by changing the sign of one of the orthogonal components of the vectorial signal, as a result of which, positive frequencies become negative frequencies, and vice versa. In contradistinction, in the background art, a multi-standard application would require two different filters, one providing a negative Nyquist slope and another providing a positive slope. Thus, the invention allows a relatively hardware-efficient, and thus cost-efficient, implementation of a multi-standard TV receiver.
The invention may be wholly or partially implemented as an integrated circuit. Since, in the invention, the filter which compensates for the sideband asymmetry operates at baseband frequencies, rather than at an intermediate frequency as in the background art, it will require relatively few external components or even none at all. As has been mentioned hereinbefore, any carrier recovery as part of the synchronous demodulation may be of a wide-band nature, so that it will also require relatively few external components or even none at all. For example, the carrier recovery may be carried out by a phase-locked loop having a relatively large loop bandwidth, such that any noise of the controllable oscillator will be effectively suppressed. This allows the use of, for example, an RC-type oscillator which is relatively noisy but can be realised without any external components. Thus, the invention allows an integrated-circuit implementation which has relatively few external components and is therefore uncomplicated and cost-efficient.
The invention and additional features, which may be optionally used to implement the invention to advantage, are apparent from and will be elucidated with reference to the drawings described hereinafter.
REFERENCES:
patent: 4263611 (1981-04-01), Gibson et al.
patent: 4716464 (1987-12-01), Parker
patent: 5103310 (1992-04-01), Gibson et al.
patent: 5298982 (1994-03-01), Lagoni
patent: 0609829A2 (1994-08-01), None
Bruekers Alphons A. M. L.
Gielis Gerardus C. M.
Sempel Adrianus
Stikvoort Eduard F.
van de Plassche Rudy J.
Pham Chi
Tran Khai
U.S. Philips Corporation
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