1. Field of the Invention
The present invention relates to a ghost canceller for removing ghosts by using a GCR signal or a vertical synchronizing signal.
2. Description of the Prior Art
Recently there is an increasing demand for a larger screen and higher picture quality of television systems. To meet this need, the attention has been directed again to the ghost interference that has been hitherto known as the greatest factor of deteriorating the receiving picture quality of television systems. In the existing ghost cancelling technology, the leading edge of the vertical synchronizing signal of the television signal is used as the reference signal. On the other hand, from August 1989, EDTV (extended definition television) broadcasting was started. As a result it has been made possible to remove ghosts efficiently by making use of a special signal (ghosts cancelling reference signal) superposed in the vertical blanking period of the television signal for detecting the ghosts.
As a prior example, a ghost canceller was reported in the ITEJ Technical Report RE80-6, pp.9 to 14 (February 1980) (ITEJ: the Institute of Television Engineers of Japan). This is intended to cancel ghosts by using the vertical synchronizing signal of television signal as a reference signal. This ghost canceller comprises a serially connected FIR (finite impulse response) filter and an FIR/IIR (infinite impulse response) filter, a tap gain control circuit for obtaining the tap gain of these transversal filters, and a tap gain hold circuit for holding the tap coefficients. The FIR filter equalizes the waveform near the peak of the impulse response, using the impulse signal obtained by differentiating the leading edge of the vertical synchronizing signal as the reference signal. The FIR/IIR filter is changed over to the FIR mode to correct the tap coefficients by calculating the correlation of the input and output of the transversal filter, thereby cancelling the ghosts. Afterwards, without changing the tap coefficients, the FIR/IIR filter is changed over to the IIR mode.
An example of a ghost canceller using a GCR (a ghost cancel reference) signal was reported in the ITEJ Technical Report ROFT 89-6, pp.31 to 36 (June 1989). This ghost canceller comprises a transversal filter, a waveform memory for taking in the input and output of the transversal filter, and a CPU (central processing unit) for controlling them. The GC signal superposed in the vertical blanking interval of television signal is taken out and put into the CPU through the waveform memory. The CPU adds the GCR signals for improving the S/N (signal/noise) ratio in a sufficient number of fields (synchronous addition). Since the GCR signals are sent out in a sequence of a cycle of 8 fields (field sequence), the CPU also executes interfield processing (field sequence processing) on the basis of the field sequence. After synchronous addition and field sequence processing, the tap coefficients of the transversal filter are corrected on the basis of such algorithms as a MSE (mean square error) method and a ZF (zero forcing) method.
In these ghost cancellers, the arithmetic operation for tap coefficient correction of the transversal filter is effected in the time domain. The calculation for determining the optimum tap coefficients is repeated to correct sequentially. Accordingly, as the number of synchronous additions is increased, the operation time becomes longer, and it takes a longer time to cancel ghosts. Besides, since the GCR signal is taken in on every correction of tap coefficients, it is likely to be effected by disturbances such as flutter and impulse noise, and it is highly possible that ghost cancelling action will be unstable.