GB2328822A

GB2328822A - NTSC co-channel interference rejection filter

Info

Publication number: GB2328822A
Application number: GB9726243A
Authority: GB
Inventors: Hwak-Dong Park
Original assignee: Daewoo Electronics Co Ltd
Current assignee: WiniaDaewoo Co Ltd
Priority date: 1997-08-30
Filing date: 1997-12-11
Publication date: 1999-03-03
Anticipated expiration: 2017-12-11
Also published as: CN1210421A; GB9726243D0; GB2328822B; JPH1175133A; KR100256449B1; KR19990021153A

Abstract

A technique for rejecting NTSC co-channel interference uses 3-stage notch filters. In received I-channel data symbols 101 and data field sync reference patterns 102, frequency components causing NTSC co-channel interference, which are an NTSC visual carrier, an NTSC chrominance subcarrier and an NTSC aural carrier, are 3-stage notch filtered 120,130. A first energy of digital broadcasting signals 321, before being 3-stage notch filtered, is produced from a first error signal between received I-channel data symbols and data field sync reference patterns. A second energy of digital broadcasting signals 331, after being 3-stage notch filtered, is produced from a second error signal between received I-channel data symbols and data field sync reference patterns which are 3-stage notch filtered. After comparing the first energy with the second energy 400 for detecting NTSC co-channel interference, an output path 500 is controlled so that a signal having a lower NTSC co-channel interference energy out of the I-channel data symbols and the 3-stage notch filtered I-channel data symbols is selectively outputted. Consequently, a received digital TV signal which does not contain frequency components of the three carriers causing NTSC co-channel interference is outputted 530.

Description

METHOD FOR REJECTING NTSC INTERFERENCE AND NTSC INTERFERENCE REJECTION FILTER CIRCUIT USING 3-STAGE NOTCH FILTER The present invention relates generally to advanced television (ATV) services, and more particularly, to improved techniques for rejecting national television system committee (NTSC) co-channel interference in a terrestrial digital television signal receiver.

ATV is a planned U.S. terrestrial broadcast television service for digitally transmitting one high definition television (HDTV) signal or a number of lower resolution television signals over a standard 6 MHz television channel. The video and audio components of the television signal are compressed using, for example, MPEG-2 and Dolby AC-3 compression methods, respectively, and are multiplexed into a single transport bit stream for transmission as a plurality of N-level data symbols using a vestigial sideband (VSB) form of modulation.

ATV will broadcast over channels which were previously unused in a given area, i.e. so called taboo channels, and must co-exist with NTSC broadcast services for some period of time: In order to provide ATV service while utilizing the large installed base of NTSC receivers, simultaneous transmission of identical program material encoded in two different formats over respective 6 NffIz television channels is performed. For this, within a given NTSC service area, an additional 6 NtHz television channel is considered for transmission of the ATV encoded signal. While such additional channels are generally available for this purpose, at least some of the same channels are also quite likely to be allocated for NTSC transmissions in nearby television service areas. This raises the problem of co-channel interference where ATV and NTSC transmissions over the same channel in nearby television service areas interference with one another. Excessive NTSC co-channel interference into a received ATV signal may degrade the ability of signal reproduction in an ATV receiver. Accordingly, a treatment for minimizing effects of NTSC co-channel interference into received ATV signals is necessary.

For satisfying the above requirement, as prior arts for rejecting the NTSC cochannel interference in an ATV receiver, there are two prior inventions. The first is U.S.

Pat. No. 5,087,975 entitled 'VSB HDTV TRANSMISSION SYSTEM WITH REDUCED NTSC CO-CHANNEL INTERFERENCE', and the second is U.S. Pat. No. 5,602,583 entitled 'NTSC REJECTION FILTER WITH SWITCHED TOMLINSON PRECODER FOR REDUCING NTSC CO-CHANNEL INTERFERENCE IN ATV RECEIVERS', both issued to Richard W. Citta. In addition, there is also another related technique, which is U.S. Pat. No. 5,606,602 entitled 'METHOD AND APPARATUS FOR COMBATING CO-CHANNEL NTSC INTERFERENCE FOR DIGITAL TV TRANSMISSION HAVING A SIMPLIFIED REJECTION FILTER', suggested by Samir N. Hulyalkar.

On the other hand, Advanced Television Systems Committee (ATSC), in a specification entitled 'GUIDE TO THE USE OF THE ATSC DIGITAL TELEVISION STANDARD' and issued on October 4, 1995, suggests a technique for rejecting NTSC co-channel interference using comb filters. The interference rejection properties of a VSB transmission system using the comb filter are based on the frequency location of the principal components of the NTSC co-channel interfering signal within the 6 MHz television channel and the periodic nulls of a VSB receiver baseband comb filter.

FIG. 2A shows the location and approximate magnitude of the three principal NTSC components: (1) a visual carrier (V) located 1.25 lHz from the lower band edge.

(2) a chrominance subcarrier (C) located 3.58 MHz higher than the visual carrier frequency, and (3) an aural carrier (A) located 4.5 MHz higher than the visual carrier frequency.

In the specification, ATSC suggests a filter circuit shown in FIG. 1 for rejecting NTSC interference. The NTSC interference rejection filter is a comb filter, which is a one tap feed-forward filter. NTSC interference can be detected by the circuit shown in FIG. 1, where the signal-to-interference plus noise ratio of the binary Data Field Sync is measured at an input and an output of the comb filters 10 and 20, and compared to each other. This is accomplished by creating two error signals. The first is created by comparing the received signal with a stored reference of the field sync. The second is created by comparing the rejection filter output with a combed version of the internally stored reference field sync. The errors are squared and integrated. After a predetermined level of confidence is achieved, the path with the largest signal-to-noise ratio (lowest interference energy) is switched in and out of the system automatically.

The frequency response characteristic is shown in FIG. 2B and provides periodic spectral nulls spaced 57*fH (10.762 MHz I 12 or 896.85 KHz) apart, where fah is an NTSC horizontal line velocity. There are 7 nulls within the 6 MHz channel. The NTSC visual carrier frequency falls close to the second null from the lower band edge. The 6th null from the lower band edge is correctly placed for the NTSC chrominance subcarrier, and the 7th null from the lower band edge is near the NTSC aural carrier.

However, as part of the frequency response characteristic of the comb filter, while the prior NTSC interference rejection filter circuit shown in FIG. 1 rejects the visual carrier, the chrominance subcarrier. and the aural carrier which cause the NTSC cochannel interference, the filter circuit also rejects the digital components of 1sot, 3rd. 4th and 5th nulls, which should not be rejected. Consequently, the filter circuit creates a problem where the reproduction characteristic of the received signal becomes worse due to an unnecessary loss of the received I-channel data.

The present invention is intended to overcome the above described disadvantages.

Therefore, it is an object of the present invention to provide a method and an apparatus for rejecting NTSC co-channel interference where the frequency components causing the NTSC co-channel interference, that is, a visual carrier, a chrominance subcarrier and an aural carrier, are selectively rejected, without causing an unnecessary digital signal loss from the desired received signal.

In order to achieve the above object of the present invention, there is provided an NTSC interference rejection filter using a 3-stage notch filter comprising: (i) NTSC rejecting means for 3-stage notch filtering, after receiving an I-channel data symbol and a data field sync reference pattern, frequency components of an NTSC visual carrier, an NTSC chrominance subcarrier and an NTSC aural carrier causing NTSC co-channel interference, to produce an I-channel data symbol and a data field sync reference pattern in which the frequency components causing the NTSC co-channel interference are not included; (ii) first energy creating means for producing a first energy of a received digital signal before being 3-stage notch filtered, from a first error signal between the received Ichannel data symbol and the data field sync reference pattern; (iii) second energy creating means for producing a second energy of a received digital signal after being 3-stage notch filtered, from a second error signal between the received I-channel data symbol and the data field sync reference pattern which were 3-stage notch filtered by said NTSC rejecting means; (iv) minimum energy detecting means for detecting NTSC co-channel interference by comparing the first energy with the second energy. and for producing a selection signal for controlling an output path so that a signal having a lower NTSC co-channel interference energy out of the I-channel data symbol and the 3-stage notch filtered Ichannel data symbol can be selectively outputted; and (v) selectively outputting means for controlling the output path so that a signal out of the I-channel data symbol and the 3stage notch filtered I-channel data symbol can be selectively outputted under a control of the selection signal.

According to the present invention employing the NTSC interference rejection filter circuit using a 3-stage notch filter, a quality of digital broadcast reception is improved by eliminating the phenomenon that an ATV digital broadcasting signal is also rejected as well as NTSC interference signals when using the comb filter.

The above object, characteristics and other advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: FIG. 1 is a prior NTSC interference rejection filter circuit using a conventional comb filter; FIG. 2A is a spectral view showing a frequency spectrum within one channel and the locations of NTSC carriers; FIG. 2B is a spectral view showing a frequency response characteristic of the prior comb filter; FIG. 2C is a spectral view showing a frequency response characteristic of a 3stage notch filter employed by the present invention; FIG. 3 is an NTSC interference rejection filter circuit using a 3-stage notch filter according to an embodiment of the present invention; FIG. 4 is a flowchart explaining an operation principle of an NTSC interference rejection filter circuit using the 3-stage notch filter shown in FIG. 3; FIG. 5A is an 8-levels symbol mapping table with respect to received I-channel data symbols; FIG. 5B is an example wave form of the received I-channel data symbols mapped by the mapping table shown in FIG. 5A; and FIG. 5C is a wave form showing a data field sync reference pattern.

Hereinafter, the present invention according to a preferred embodiment of the present invention will be explained in more detail with reference to the accompanying drawings.

FIG. 3 is an NTSC interference rejection filter circuit using a 3-stage notch filter according to a first embodiment of the present invention.

As shown in FIG. 3, an NTSC interference rejection filter circuit using a 3-stage notch filter includes an NTSC rejection section 100, a first energy creating section 200, a second energy creating section 300, a minimum energy detector 400 and an output section 500.

The NTSC rejection section 100 has two 3-stage notch filters 120 and 130. Input of the first 3-stage notch filter 120 is connected to a source 101 of a received I-channel data symbol 103, and input of the second notch filter 130 is connected to a source 102 of a data field sync reference pattern 104.

The first energy creating section 200 includes a first adder 210, a first squaring device 220 and a first integrator 230. Two inputs of the first adder 210 are connected to the source 101 of the received I-channel data symbol 103 and the source 102 of the data field sync reference pattern 104, respectively, and to the output of the first adder 210, the first squaring device 220 and the first integrator 230 are successively connected.

The second energy creating section 300 includes a second adder 310. a second squaring device 320 and a second integrator 330. Two inputs of the second adder 310 are connected to outputs of the first 3-stage notch filter and the second 3-stage notch filter, and the first squaring device 220 and the first integrator 230 are successively connected to the output of the second adder 310.

The minimum energy detector 400 is connected to the first integrator 230 and to the second integrator 330 and compares the two energy values from the integrators 230 and 330 to provide the output section 500 with a predetermined selection control signal 301.

A multiplexer constitutes the output section 500. The multiplexer 500 takes two signals, that is, the received I-channel data symbol 103 and the output from the first 3stage notch filter 120, as inputs, receives the output from the minimum energy detector 400 as the selection control signal thereof, and multiplexes the two input signal 103 and 121 based on the selection control signal 301.

Next, an operation of the embodiment according to the present invention will be explained with reference to the accompanying drawings.

In the received I-channel data symbol 103, one symbol, which is illustrated as an example, consists of 3 digital data as shown in FIG. 5B, and each symbol is 8-level mapped by a predetermined mapping table shown in FIG. 5A. Additionally, in the data field sync reference pattern 104, one digital data corresponds to one symbol as shown in FIG. 5C, where a binary value '1' is mapped to '5' and a binary value '0' is mapped to '-5'. As known from the FIGs. 5B and 5C, the received I-channel data symbol 103 and the data field sync reference pattern 104 are applied into the sources 101 and 102 in the form of a bit stream.

With reference to the flowchart of FIG. 4. in the operation of the NTSC interference rejection filter using the 3-stage notch filter shown in FIG. 3, first the Ichannel data symbol 103 and the data field sync reference pattern 104 are received through the input source terminals 101 and 102 (ST 2). Next, the received I-channel data symbol 103 and the data field sync reference pattern 104 are 3-stage notch filtered by the first 3-stage notch filter 120 and the second 3-stage notch filter 130, respectively (ST 6).

Generally, the notch filter is, as is well known, a filter for absorbing and attenuating a given frequency. The present invention adapts 3-stage notch filters in place of the comb filters employed by the prior filter circuit. The frequency response characteristic of the 3-stage notch filter employed by the present invention is shown in FIG. 2C. As known in FIG. 2C, the 3-stage notch filters 120 and 130 selectively filter only three particular frequencies, that is, a visual carrier, a chrominance subcarrier and an aural carrier which create NTSC co-channel interference. When an undesired cochannel interference by NTSC carriers into the received I-channel data symbol 103 and the data field sync reference pattern 104 is created, the co-channel interference is rejected by the 3-stage notch filters 120 and 130. However, when such co-channel interference does not occur, the desired digital broadcasting signals are rather lost.

On the other hand, the first energy creating section 200 produces a first energy 231 of the digital broadcasting signal before 3-stage notch filtering (ST 4). To reject the sync signal within the digital broadcasting signal, the first adder 210 gives a voltage dimensional error signal 211 between the received I-channel data symbol 103 and the data field sync reference pattern 1W. Next, the error signal 211, which has a predetermined magnitude and phase, is squared by the first squaring device 220 and continuously integrated by the first integrator 230. As a result, rhe first integrator 230 produces the first energy of the I-channel digital signal without the sync signal before 3-stage notch filtering.

Additionally, the second energy creating section 300 also produces a second energy 331 of the digital broadcasting signal which is 3-stage notch filtered (ST 8). To reject the sync signal within the digital broadcasting signal, after receiving the 3-stage notch filtered I-channel data symbol 121 and the 3-stage notch filtered data field sync reference pattern 131, the second adder 310 produces a voltage dimensional error signal 311 by subtracting the 3-stage notch filtered data field sync reference pattern 131 from the 3-stage notch filtered I-channel data symbol 121. Next, the error signal 311, which has a predetermined magnitude and phase, is squared by the second squaring device 320 and continuously integrated by the second integrator 330. Consequently, the second integrator 330 produces the second energy of the I-channel digital signal without the sync signal after 3-stage notch filtering.

Receiving the first energy 231 and the second energy 331, the minimum energy detector 400 compares the first energy 231 with the second energy 331 and provides the multiplexer 500 with a predetermined selection control signal 301 based on the comparison (ST 10). Here, a case where the first energy 231 is larger than the second energy 331 is interpreted as a state where NTSC co-channel interference into the Ichannel data symbol 103 does occur Conversely, a case where the first energy 231 is smaller than the second energy 331 is interpreted as a state where NTSC co-channel interference into the I-channel data symbol 103 does not occur. Accordingly, to produce a data output 530, the multiplexer 500 selects the output 121 from the first 3-stage notch filter 120 in the former case, while the multiplexer j00 selects the received I-channel dara symbol 103 which is not 3-stage notch filtered in the later case (ST 12). The selection control signal 301 required for this multiplexing is provided by the minimum energy detector 400.

Consequently, the multiplexer 510 can supply to an output port 530 a VSB(Vestigial Side-Band) digital broadcasting signal 513 having removed analogue NTSC components comprising a visual(V) carrier, a chrominance(C) subcarrier and an aural(A) carrier.

As described above, in a case of receiving digital ATV signals for terrestrial broadcasting, the present invention, by using the NTSC interference rejection filter circuit using the 3-stage notch filter, can overcome the disadvantage of the prior art using comb filters, which is that the loss of digital TV broadcasting signals inevitably accompanies rejecting of analog NTSC co-channel interfering components, that is, the visual carrier, the chrominance subcarrier and the aural carrier. Consequently, the present invention can give an improved quality of digital broadcasting signals.

In addition, the NTSC interference rejection filter circuit using 3-stage notch filter can use an 8-VSB demodulation chip in receiving digital TV for terrestrial broadcasting.

Although the preferred embodiment of the invention has been described, it is understood that the present invention should not be limited to this preferred embodiment, but various changes and modifications can be made by one skilled in the art within the scope of the invention as hereinafter claimed.

Claims

1. A method for rejecting NTSC co-channel interference using 3-stage notch filtering, characterized by comprising the steps of: (i) 3-stage notch filtering, after receiving an I-channel data symbol and a data field sync reference pattern, frequency components of an NTSC visual carrier, an NTSC chrominance subcarrier and an NTSC aural carrier causing NTSC co-channel interference, to produce an I-channel data symbol and a data field sync reference pattern in which the frequency components causing the NTSC co-channel interference are rejected; (ii) producing a first energy of a received digital signal, which is a signal before 3stage notch filtering, from a first error signal between the received I-channel data symbol and the data field sync reference pattern; (iii) producing a second energy of a received digital signal, which is a signal after 3-stage notch filtering, from a second error signal between the received I-channel data symbol and the data field sync reference pattern which were 3-stage notch filtered at said step (i); (iv) detecting NTSC co-channel interference by comparing the first energy with the second energy, and producing a selection signal for controlling an output path so that a signal having a lower NTSC co-channel interference energy out of the I-channel data symbol and the 3-stage notch filtered I-channel data symbol can be selectively outputted; and (v) controlling the output path so that a signal out of the I-channel data symbol and the 3-stage notch filtered I-channel data symbol can be selectively outputted under a control of the selection signal, wherein the selectively outputted signal is the received r- channel data symbol, which is a signal before 3-stage notch filtering, in a case where the NTSC co-channel interference does not occur, and is the received I-channel data symbol, which is a signal after 3-stage notch filtering, in a case where the NTSC co-channel interference does occur.

2. The method as claimed in claim 1, characterized in that said step (ii) comprises the steps of: producing the second error signal, which is a voltage dimensional signal, by subtracting the data field sync reference pattern from the received I-channel data symbol; producing a first power signal by squaring the first error signal; and producing the first energy by integrating the first power signal.

3. The method of claim 1 or 2 wherein said step (iii) comprises: producing the first error signal, which is voltage dimensional signal, by subtracting the data field sync reference pattern which is 3-stage notch filtered from the received I-channel data symbol which is 3-stage notch filtered; producing a second power signal by squaring the second error signal; and producing the second energy by integrating the second power signal.

4. The method of any preceding claim wherein detection of the case where the NTSC co-channel interference does not occur is accomplished at a state where the first energy is smaller than the second energy, while detection of a case where the NTSC co-channel interference does occur is accomplished at a state where the first energy is larger than the second energy.

5. An NTSC interference rejection filter circuit using a 3-stage notch filter, characterized by comprising: (i) NTSC rejecting means for 3-stage notch filtering, after receiving I-channel data symbol and a data field sync reference pattern, frequency components of an NTSC visual carrier, an NTSC chrominance subcarrier and an NTSC aural carrier causing NTSC cochannel interference, to produce an I-channel data symbol and a data field sync reference pattern in which the frequency components causing the NTSC co-channel interference are not included; (ii) first energy creating means for producing a first energy of a received digital signal, which is a signal before 3-stage notch filtering, from a first error signal between the received I-channel data symbol and the data field sync reference pattern; (iii) second energy creating means for producing a second energy of a received digital signal, which is a signal after 3-stage notch filtering, from a second error signal between the received I-channel data symbol and the data field sync reference pattern which were 3-stage notch filtered by said NTSC rejecting means; (iv) minimum energy detecting means for detecting NTSC co-channel interference by comparing the first energy with the second energy, and for producing a selection signal for controlling an output path so that a signal having a lower NTSC co-channel interference energy out of the I-channel data symbol and the 3-stage notch filtered Ichannel data symbol can be selectively outputted; and (v) selectively outputting means for controlling the output path so that a signal out of the I-channel data symbol and rhe 3-stage notch filtered I-channel data symbol can be selectively outputted under a control of the selection signal.

6. The NTSC interference rejection filter circuit as claimed in claim 5, characterized in that said NTSC rejection means comprises: a first 3-stage notch filter, for 3-stage notch filtering the frequency components causing the NTSC co-channel interference which are an NTSC visual carrier, an NTSC chrominance subcarrier and an NTSC aural carrier to produce the I-channel data symbol and a data field sync reference pattern in which the frequency components causing the NTSC co-channel interference are not included, after being provided with the received Ichannel data symbol; and a second 3-stage notch filter, for 3-stage notch filtering the frequency components causing the NTSC co-channel interference which are an NTSC visual carrier, an NTSC chrominance subcarrier and an NTSC aural carrier to produce the I-channel data symbol and a data field sync reference pattern in which the frequency components causing the NTSC co-channel interference are not included, after being provided with the received Ichannel data symbol.

7. The NTSC interference rejection filter circuit of claim 5 or 6 characterized in that said first energy creating means comprises: means for producing the first error signal, which is a voltage dimensional signal, by subtracting the data field sync reference pattern from the received I-channel data symbol; means for producing a first power signal by squaring the first error signal: and means for producing the first energy by integrating the first power signal.

8. The NTSC interference rejection filter circuit of any one of claims 5-7 wherein said second energy creating means comprises: means for producing the second error signal, which is a voltage dimensional signal, by subtracting the data field sync reference pattern which is 3-stage notch filtered" from the received I-channel data symbol which is 3-stage notch filtered; means for producing a second power signal by squaring the second error signal; and means for producing the second energy by integrating the second power signal.

9. The srsc interference rejection filter circuit of any of claims 5-8 wherein the output signal from said selective outputting means is the received I-channel data symbol, which is a signal before 3-stage notch filtering, in a case where the NTSC co-channel interference does not occur, and is the received I-channel data symbol, which is a signal after 3-stage notch filtering, in a case where the NTSC co channel interference does occur.

10. The NTSC interference rejection filter circuit of any of claims 5-9 wherein said minimum energy detecting means detects a case where the NTSC co-channel interference does not occur when the first energy is smaller than the second energy, and detects a case where the NTSC co-channel interference does occur when the first energy is larger than the second energy.

11. A method of rejecting NTSC co-channel interference substantially as herein described with reference to Figures 2C, 3, 4 and 5A-5C.

12. An NTSC filter constructed and arranged substantially as herein described with reference to or as shown in Figures 3-5 of the accompanying drawings.

Amendments to the claims have been filed as follows

1. A method for rejecting NTSC co-channel interference using 3-stage notch filtering, characterized by comprising the steps of: (i) 3-stage notch filtering, after receiving an I-channel data symbol and a data field sync reference pattern, frequency components of an NTSC visual carrier, an NTSC chrominance subcarrier and an NTSC aural carrier causing NTSC co-channel interference, to produce an I-channel data symbol and a data field sync reference pattern in which the frequency components causing the NTSC co-channel interference are rejected; (ii) producing a first energy of a received digital signal, which is a signal before 3stage notch filtering, from a first error signal between the received I-channel data symbol and the data field sync reference pattern; (iii) producing a second energy of a received digital signal, which is a signal after 3-stage notch filtering, from a second error signal between the received I-channel data symbol and the data field sync reference pattern which were 3-stage notch filtered at said step (i); (iv) detecting NTSC co-channel interference by comparing the first energy with the second energy, and producing a selection signal for controlling an output path so that a signal having a lower NTSC co-channel interference energy out of the I-channel data symbol and the 3-stage notch filtered I-channel data symbol can be selectively outputted; and (v) controlling the output path so that a signal out of the I-channel data symbol and the 3-stage notch filtered I-channel data symbol can be selectively outputted under a control of the selection signal, wherein the selectively outputted signal is the received I channel data symbol, which is a signal before 3-stage notch filtering, in a case where the NTSC co-channel interference does not occur, and is the received I-channel data symbol, which is a signal after 3-stage notch filtering, in a case where the NTSC co-channel interference does occur.

2. The method as claimed in claim 1, characterized in that said step (ii) comprises the steps of: producing the first error signal, which is a voltage dimensional signal, by subtracting the data field sync reference pattern from the received I-channel data symbol; producing a first power signal by squaring the first error signal; and producing the first energy by integrating the first power signal.

3. The method of claim 1 or 2 wherein said step (iii) comprises: producing the second error signal, which is voltage dimensional signal, by subtracting the data field sync reference pattern which is 3-stage notch filtered from the received I-channel data symbol which is 3-stage notch filtered; producing a second power signal by squaring the second error signal; and producing the second energy by integrating the second power signal.

4. The method of any preceding claim wherein detection of the case where the NTSC co-channel interference does not occur is accomplished ar a state where the first energy is smaller than the second energy. while detection of a case where the NTSC co-channel interference does occur is accomplished at a state where the first energy is larger than the second energy.

5. An NTSC interference rejection filter circuit using a 3-stage notch filter, characterized by comprising: (i) NTSC rejecting means for 3-stage notch filtering, after receiving I-channel data symbol and a data field sync reference pattern, frequency components of an NTSC visual carrier, an NTSC chrominance subcarrier and an NTSC aural carrier causing NTSC cochannel interference, to produce an I-channel data symbol and a data field sync reference pattern in which the frequency components causing the NTSC co-channel interference are not included; (ii) first energy creating means for producing a first energy of a received digital signal, which is a signal before 3-stage notch filtering, from a first error signal between the received I-channel data symbol and the data field sync reference pattern; (iii) second energy creating means for producing a second energy of a received digital signal, which is a signal after 3-stage notch filtering, from a second error signal between the received I-channel data symbol and the data field sync reference pattern which were 3-stage notch filtered by said NTSC rejecting means; (iv) minimum energy detecting means for detecting NTSC co-channel interference by comparing the first energy with the second energy, and for producing a selection signal for controlling an output path so that a signal having a lower NTSC co-channel interference energy out of the I-channel data symbol and the 3-stage notch filtered Ichannel data symbol can be selectively outputted; and (v) selectively outputting means for controlling the output path so that a signal out of the I-channel data symbol and the 3-stage notch filtered I-channel data symbol can be selectively outputted under a control of the selection signal.

6. The NTSC interference rejection filter circuit as claimed in claim 5 characterized in that said NTSC rejection means comprises: a first 3-stage notch filter, for 3-stage notch filtering the frequency components causing the NTSC co-channel interference which are an NTSC visual carrier, an NTSC chrominance subcarrier and an NTSC aural carrier to produce the I-channel data symbol in which the frequency components causing the NTSC co-channel interference are not included, after being provided with the received Ichannel data symbol; and a second 3-stage notch filter, for 3-stage notch filtering the frequency components causing the NTSC co-channel interference which are an NTSC visual carrier, an NTSC chrominance subcarrier and an NTSC aural carrier to produce the data field sync reference pattern in which the frequency components causing the NTSC co-channel interference are not included, after being provided with the received data field sync reference pattern.

7. The NTSC interference rejection filter circuit of claim 5 or 6 characterized in that said first energy creating means cornprises: means for producing the first error signal, which is a voltage dimensional signal.

by subtracting the data field sync reference pattern from the received I-channel data symbol; means for producing a first power signal by squaring the first error signal: and means for producing the first energy bv integrating the first power signal.

8. The NTSC interference rejection filter circuit of any one of claims 5-7 wherein said second energy creating means comprises: means for producing the second error signal, which is a voltage dimensional signal, by subtracting the data field sync reference pattern which is 3-stage notch filtered' from the received I-channel data symbol which is 3-stage notch filtered; means for producing a second power signal by squaring the second error signal; and means for producing the second energy by integrating the second power signal.

9 The NTSC interference rejection filter circuit of any of claims 5-8 wherein the output signal from said selective outputting means is the received I-channel data symbol, which is a signal before 3-stage notch filtering, in a case where the NTSC co-channel interference does not occur, and is the received I-channel data symbol, which is a signal after 3-stage notch filtering, in a case where the NTSC co channel interference does occur.