Ghost canceling circuit

Abstract: A ghost canceling circuit comprising a Fourier transforming circuit (14) for Fourier transforming a reference waveform contained in the received television signal while converting (11) the analogue signal into digital form, means (15) for holding and outputting a reference waveform Fourier coefficient obtained by Fourier transforming the reference waveform contained in the television signal at the transmission side, a signal processing portion (16) in which the outputted reference waveform Fourier coefficient is divided by the result of the A/D conversion, the divided Fourier coefficient is reverse Fourier transformed (18), and the reverse Fourier transformed Fourier coefficient is supplied to a transversal filter (1) recognizing it as a tap gain.

Description: The present invention relates to a ghost canceling circuit which is installed in a television receiver. Generally, a television receiver receives through an antenna a television signal composed by multiplying a directly received signal and delay signals which reach the antenna through several reflecting routes in which the signals are reflected by neighboring geographical features, neighboring buildings, moving vehicles or the like. Due to such delay signals, generally, multiple images may be come out in some extent on the received picture. Such delay signals causing the multiple images are called "a ghost" and a phenomenon that large multiple images deteriorate a quality of the picture is called "a ghost obstacle". There has been known a conventional ghost canceling circuit having a transversal filter for generating a dummy ghost, a tap gain control circuit for controlling a tap gain of the transversal filter and a signal composing circuit. The transversal filter of the conventional circuit is provided with a coefficient circuit group, an adding circuit and a delay circuit group which connects in series a delay, an attenuation and an adding which would cause a ghost based on multiple reflections, so that the dummy ghost is generated. The dummy ghost thus generated reverse in polarity is added to the original received television signal by the signal composing circuit, thereby cancelling the ghost component contained in the original signal. Generally, an influence of a ghost varies moment by moment in accordance with a change in frequency of a received radio wave raised by changing receiving channels, with a change in position of a moving vehicle, a ship or an air plane passing near the television receiver, or the like. Therefore, the tap gain of the transversal filter should be controlled, moment by moment, to be an optimum value. In order to obtain the tap gain control, a reference waveform for detecting an amount of the ghost is included in a television signal by a transmitter side. Further, the tap gain control circuit installed in the television receiver generates an optimum tap gain in accordance with a result of analysis of the deformation of the reference waveform extracted from the received television signal and with a result of the ghost cancellation fed back from the following detection. The optimum tap gain is then supplied to the transversal filter. The conventional ghost cancelling circuit described above is of a feed-back type in which the result of the ghost cancellation is detected by a following detection circuit and fed back to the forwarding tap gain control circuit. Therefore, such a circuit would not be sufficient in response speed and in ghost cancelling accuracy. The Document IEEE transactions on broadcasting, Vol. BC-29, No. 2, pages 60-71 (June 1983) discusses various methods for eliminating ghost images with a transversal filter for generating a dummy ghost and a tap gain control circuit for controlling the tap gain of the transversal filter. In view of the above-noted defects of the conventional device, an object of the present invention is to provide a ghost cancelling circuit of a feed-forward type which is improved in response speed and in ghost cancelling accuracy. This object is solved by the ghost cancelling circuit of claim 1. Further advantageous features of the invention are evident from the dependent claims. The invention provides a ghost cancelling circuit comprising a Fourier transforming circuit for Fourier transforming a reference waveform which is extracted from an original reference waveform that is inserted into a received television signal before transmission thereof; means for holding and outputting a reference waveform Fourier coefficient group obtained by Fourier transforming the original reference waveform contained in the television signal at the transmission side; a signal processing portion in which the extracted reference waveform Fourier coefficient group is divided by the reference waveform coefficient group obtained by transforming the original reference waveform; wherein the divided Fourier coefficient group is reverse Fourier transformed, and the reverse Fourier transformed Fourier coefficient group is supplied to a transversal filter as a tap gain. Fig. 1 is a circuit diagram showing a ghost canceling circuit according to an embodiment of the present invention. Fig. 2 is a circuit diagram showing a transversal filter shown in Fig. 1. Fig. 3 shows a reference waveform contained in a transmitted television signal and a Fourier coefficient thereof. Fig. 4 shows a reference waveform extracted from a received television signal and a Fourier coefficient thereof. The preferred embodiments will now be described in detail with reference to accompanying drawings. Fig. 1 is a block diagram showing a ghost canceling filter circuit according to an embodiment of the present invention. The circuit shown in Fig. 1 is provided with a transversal filter 1 for generating a dummy ghost signal, a tap gain control circuit 2 and an adder 3. The tap gain control circuit 2 is provided with an A/D (analogue/digital) converter 11, a reference waveform memory 12, a reference waveform detecting circuit 13, a Fourier transform circuit 14, a reference waveform Fourier coefficient memory 15, a dividing circuit 16, a compensation circuit 17, a reverse Fourier transform circuit 18 and a tap gain holding register 19. As shown in Fig. 2, the transveral filter 1 consists of a plurality of delay circuits 22a, 22b, 22c,...22n connected in series, a plurality of multipliers 23a, 23b, 23c,...23n for multiplying a tan gain with each of outputs of the delay circuits, a transversal filter portion including an adder 24 which adds all of outputs of the multipliers and a D/A (digital/analogue) converter 25. The transversal thus constructed generates a dummy ghost signal in accordance with tap gains supplied by the tap gain control circuit 2. A reference waveform S₀(t) shown in Fig. 3A for detecting a ghost is added to a predetermined portion of the television signal on a vertical blanking period, for example, by a transmission side of the television signal. The band-width of the reference signal waveform S₀(t) is limited by passing through a low pass filter circuit, which has an amplitude-frequency characteristic as is shown in Fig. 3B. The reference waveform Fourier coefficient memory 15 preliminary stores therein a discrete Fourier coefficient group F₀(u). The discrete Fourier coefficient group F₀(u) is obtained by converting an analogue signal of the original reference waveform S₀ shown in Fig. 3A to a digital form and Fourier transforming the digital signal of the reference waveform. The amplitude-frequency characteristic of the Fourier coefficient F₀(u) is shown in Fig. 3B as a discrete sampling value group. An actual reference waveform S(t) contained in the television signal which is received by a television receiver is supplied through the input terminal IN and the A/D converter 11 to the reference waveform detecting circuit 13 where the reference waveform S(t) is extracted from the predetermined portion of the television signal in accordance with a timing control of the reference waveform detecting circuit 13. Then the reference waveform is stored in the reference waveform memory 12. The reference waveform S(t) thus received is deformed as shown in Fig. 4A relative to the original reference waveform S₀(t) included by the transmitter side by an influence of the transmitting characteristic of the signal containing therein the ghost. The received reference waveform S(t) is read from the reference waveform memory 12 and supplied to the Fourier transform circuit 14 where the reference waveform is Fourier transformed to the discrete Fourier coefficient group F(u). Fig. 4B shows an example of the discrete Fourier coefficient group F(u) where R(u) designates an amount of phase rotation. The dividing circuit 16 divides the reference Fourier coefficient F₀(u) read from the reference Fourier coefficient memory 15 by the Fourier coefficient F(u) having a corresponding frequency components received from the Fourier transforming circuit 14, thereby generating a tap gain coefficient 1/G(u) expressed by following equation. 1/G(u) = F₀(u) / F(u) Now assuming that the transmission characteristic containing therein the ghost is g(t), the reference waveform S(t) is expressed by following equation. S(t) = g(t) ⊗ S₀(t) The equation (2) is Fourier transformed thereby obtaining the Fourier transforming coefficient F(u) according to following equation. F (u) = G(u) F₀(u) The reference waveform S(t) deformed by an influence of the ghost can be compensated in order to attain the original reference waveform S₀(t) by following equation. S₀(t) = [ 1/g (t) ]⊗ S(t) The equation (4) is Fourier transformed to obtain the reference Fourier coefficient F₀(u) by the equation (5) below. F₀(u) = F(u) / G(u) The equation (5) is the same as the equation (1). Accordingly, a dummy ghost can be obtained by reverse Fourier transforming, by the reverse Fourier transforming circuit 18, the result of division of the dividing circuit 16, and then employing the reverse Fourier transformed signal as a tap gain for the transversal filter 1. The compensation circuit 17 enhances or mitigates a high frequency component of the output of the dividing circuit 16 or rises a rapid down part or lowers a rapid up part of the divided value at a particular frequency to about a degree of the circumferential values recognizing it as a beat disturbance, or the like. The dummy ghost generated by the transversal filter 1 is reverse in phase to that contained in the received television signal. The dummy ghost is added by the adder 3 to the ghost in the television signal so that the latter ghost is canceled by the dummy ghost. The television signal in which the ghost is canceled is supplied to the following signal processing part of the television receiver through the output OUT. The embodiment described above employs as the reference waveform the waveform the impulse of which is band filtered. However, the reference waveform may be a stepped waveform, for example, which is band filtered if applicable. In an embodiment in which a waveform impulse of which is band filtered is employed as a reference waveform, a Fourier coefficient F₀(u) is constant in the band. In this case, therefore, the reference waveform Fourier coefficient memory 15 and the dividing circuit 16 shown in the present Fig. 1 may be eliminated. Further, in case that the reference waveform is made by integrating the waveform mentioned above, that is, the stepped waveform, such a reference waveform is differentiated in the television receiver in order to obtain the above impulse waveform and, accordingly, the reference waveform Fourier coefficient memory 15 and the dividing circuit 16 may be eliminated. According to the present invention, as described above, the ghost cancelling circuit is of a feed-forward type comprising a Fourier transforming circuit for Fourier transforming a reference waveform contained in the received television signal while converting the analogue signal into digital form, means for holding and outputting a reference waveform Fourier coefficient obtained by Fourier transforming the reference waveform contained in the television signal at the transmission side, a signal processing portion in which the outputted reference waveform Fourier coefficient is divided by the result of the A/D conversion, the divided Fourier coefficient is reverse Fourier transformed, and the reverse Fourier transformed Fourier coefficient is supplied to a transversal filter recognizing it as a tap gain. Therefore, such a feed-forward system is largely advantageously improved in response speed and ghost cancelling accuracy relative to the conventional feedback system. Further, since the system of the present invention processes the signal on frequency axis, the tap gain can readily be compensated such as enhancing a high frequency component of the signal or eliminating a beat disturbance.

Claims: A circuit for cancelling a ghost portion of a received television signal, comprising: means (1) for generating a dummy ghost in accordance with a signal output by said circuit and in accordance with a tap gain, said tap gain being derived from said received television signal, said dummy ghost having a reverse phase to that of said ghost portion of said received television signal; means (2) for controlling said tap gain of said dummy ghost generating means (1), such that said dummy ghost has a reverse phase to that of said ghost portion of said received television signal, and such that said dummy ghost corresponds to said ghost portion of said received television signal; and means (3) for adding the dummy ghost output of said dummy ghost generating means (1) to said received television signal; characterized by said tap gain controlling means (2) comprising: means (13) for extracting a reference waveform S(t) from the received signal, originating from an original reference waveform So(t) which is inserted into said received television signal before transmission and reception of said television signal; means (15) for preliminary Fourier transforming said original reference waveform So(t); means (14) for Fourier transforming said reference waveform extracted from said received television signal, said Fourier transforming means outputting a Fourier coefficient group F(u); and means (15) for storing a Fourier coefficient group Fo(u) obtained from said original reference waveform preliminary Fourier transforming means; means (16) for dividing an output Fo(u) of said Fourier coefficient group storing means by said Fourier coefficient group F(u), said dividing means outputting a signal group G(µ) to a reverse Fourier transforming means (18); and said means (18) for reverse Fourier transforming said signal group G(µ) output by said dividing means, outputting a reverse Fourier transformed group g(t) to said dummy ghost generating means as said tap gain. The ghost cancelling circuit of Claim 1, wherein said reference waveform is made by band filtering an impulse waveform. The ghost cancelling circuit of Claim 1, wherein said reference waveform is made by band-filtering a stepped waveform. The ghost cancelling circuit of one of the preceding claims, wherein said reference waveform S(t) extracting means (13) further comprises means (11) for converting said reference waveform S(t) from an analogue form into a digital form; and means (12) for storing said digital form of said reference waveform S(t). The ghost cancelling circuit of one of the claims 2-4, further comprising compensating means (17) for enhancing or mitigating a high frequency component of said output of said dividing means (16). The ghost cancelling circuit of Claim 5, wherein said compensating means (17) enhances a high frequency components of said output of said dividing means. The ghost cancelling circuit of Claim 5, wherein said compensating means (17) mitigates a high frequency components of said output of said dividing means. The ghost cancelling circuit of Claim 5, wherein said compensating means (17) at any given frequency raises an output frequency of said dividing means when said output frequency is rapidly falling and lowers an output frequency of said dividing means when said output frequency is rapidly rising to substantially a degree of the circumferential frequency of the output of said dividing means to thereby provide beat cancellation. The ghost cancelling circuit of one of the preceding claims, wherein said dummy ghost generating means (1) comprises a transversal filter (1) coupled to receive said tap gain from said tap gain controlling means (19) and also coupled to provide said dummy ghost signal to said adding means in response to said tap gain. The ghost cancelling circuit of one of the preceding claims, said tap gain controlling means (2) further comprising a tap gain register (19) coupled to receive said signal output by said reverse Fourier transforming means and also coupled to said dummy ghost generating means to provide a signal for controlling the tap gain of said dummy ghost generating means. A circuit for cancelling a ghost portion of a received television signal comprising: means (1) for generating a dummy ghost in accordance with a signal output by said circuit and in accordance with a tap gain, said tap gain being derived from said received television signal, said dummy ghost having a reverse phase to that of said ghost portion of said received television signal; means (2) for controlling said tap gain of said dummy ghost generating means (1), such that said dummy ghost has a reverse phase to that of said ghost portion of said received television signal, and such that said dummy ghost corresponds to said ghost portion of said received television signal; and means (3) for adding the dummy ghost output of said dummy ghost generating means (1) to said received television signal; characterized by said tap gain controlling means (2) comprising: means (13) for extracting a reference waveform S(t) from the received signal, originating from an original reference waveform So(t) which is inserted into said received television signal before transmission and reception of said television signal, and which is made either by band filtering an impulse waveform or by band filtering a stepped waveform; means (14) for Fourier transforming said reference waveform extracted from said received television signal, said Fourier transforming means outputting a Fourier coefficient group F(u); and means (18) for reverse Fourier transforming said Fourier coefficient group F(u), said reverse Fourier transforming means outputting a reverse Fourier transformed coefficient group to said dummy ghost generating means as said tap gain.

Inventors: KOBAYASHI MICHIO C O NEC HOME, KOBAYASHI REIICHI C O NEC HOME, MIYAZAKI TETSURO C O NIPPON HO, SASAKI FUMIYOSHI C O NIPPON HO, SHIKI TATSUYA C O NEC HOME ELE, TANAKA TADAAKI, KOBAYASHI, MICHIO C/O NEC HOME ELECTRONICS LTD., KOBAYASHI, REIICHI C/O NEC HOME ELECTRONICS LTD., MIYAZAKI, TETSURO C/O NIPPON HOSO KYOKAI, SASAKI, FUMIYOSHI C/O NIPPON HOSO KYOKAI, SHIKI, TATSUYA C/O NEC HOME ELECTRONICS LTD., TANAKA, TADAAKI
Applicants: JAPAN BROADCASTING CORP, NIPPON DENKI HOME ELECTRONICS, NEC HOME ELECTRONICS, LTD., NIPPON HOSO KYOKAI
Application year: 1989
Main classification: H04N5
IPC codes: H04N5