Abstract:
Method and apparatus for use with digital television receivers are included among the embodiments. In exemplary systems, the presence of an interfering analog television signal (e.g., an NTSC signal) in the same channel as a DTV signal is verified by detecting the presence of analog sync signals in the received signal band. When sync signals of the proper frequency are detected, an analog TV signal rejection filter is inserted in the DTV received signal processing path. Other embodiments are described and claimed.

Description:
RELATED APPLICATIONS  
         [0001]    This application claims the benefit of Korean Patent Application No. 10-2003-0039898, filed Jun. 19, 2003, the disclosure of which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to co-channel interference filtering for digital television signals, and more particularly to methods and apparatus for detecting the presence of co-channel interference in order to selectively apply a co-channel interference filter.  
           [0004]    2. Description of the Related Art  
           [0005]    Conventional broadcast television signals are strictly analog in nature. These conventional signals generally conform to one of three broadcast formats in wide adoption: the NTSC (National Television Standards Committee) format adopted in the United States and a few other countries, and the PAL (Phase Alternation by Line) and SECAM (Systeme Electronique Couleur Avec Memoire) formats adopted in most other countries.  
           [0006]    High-Definition Television (HDTV), or more generally Digital Television (DTV), formats abandon the conventional analog television signal format in favor of a digitally coded signal. Due to the high redundancy found in most video signals, it is possible to digitally compress a video sequence in a manner that will be visually imperceptible (or mostly so) once uncompressed. Such DTV signals can therefore transmit much more detail than is possible with an equivalent analog signal of the same bandwidth. With the current HDTV format being implemented in the United States, HDTV bandwidth has been set to occupy roughly the same bandwidth as an analog NTSC broadcast, with channels assigned from the same channel space as NTSC channels.  
           [0007]    Although the long-term plan is to phase out NTSC channels, the vast majority of television users do not yet own HDTV receivers and a complete switchover does not appear imminent. In the interim, television stations that broadcast an HDTV signal may have viewers that receive both the desired HDTV signal and a relatively strong but undesired NTSC signal on the same channel. In this circumstance, the NTSC and HDTV signals interfere with each other, producing what is known as “co-channel” interference.  
           [0008]    Referring to FIG. 1, the frequency spectrum  100  of an NTSC signal is depicted. Envelope  110  represents the DTV information transmitted within the NTSC signal channel space when a DTV signal occupies the same channel space as an NTSC signal. Video carrier V, located 1.25 MHz from the lower edge of the allotted frequency spectrum, is used to demodulate the luminance component of the original NTSC signal. Color subcarrier C, located 3.58 MHz above video carrier V, is used to demodulate the NTSC quadrature chrominance signals in a color television receiver. Audio carrier A, located 4.5 MHz above the video carrier, is used to demodulate the NTSC frequency-modulated (FM) audio signal transmitted in a relatively small frequency band centered about carrier A.  
           [0009]    When an HDTV signal occupies the same channel space as an NTSC signal, the NTSC signal can produce strong interference. It is therefore desirable to pre-filter the received HDTV signal with an NTSC rejection filter that can remove predictable components of the NTSC signal, i.e, the video, color, and audio carriers. Typically, a comb filter is used as the NTSC rejection filter. As shown in FIG. 1, the comb filter  120  has nulls spaced 57 f H  Hz apart, where f H  is the horizontal scan frequency of the analog video signal (15.734 kHz for NTSC video). One comb filter null aligns approximately with the video carrier V, another comb filter null aligns approximately with the color subcarrier C, and a third comb filter null aligns approximately with the audio carrier A.  
           [0010]    From FIG. 1, it can be appreciated that the comb filter contains other nulls within the HDTV channel space that in all likelihood will not improve co-channel interference. In fact, the NTSC rejection filter degrades the signal-to-noise ratio (SNR) of the HDTV signal by approximately 3 dB when no NTSC signal is present. Therefore, when the NTSC co-channel interference is less than 3 dB, it is preferable to not use the filter at all. Accordingly, it is recommended that the NTSC rejection filter be switched out of the HDTV signal path when no NTSC signal is present.  
           [0011]    Several methods have been used to determine whether an NTSC signal is present in an HDTV channel. U.S. Pat. No. 6,201,576 teaches calculating two noise power estimates on the received signal, one with the NTSC rejection filter in place and one without. An NTSC signal is detected when the difference between the two noise power estimates exceeds a threshold. When the NTSC signal is detected, the NTSC rejection filter is switched into the HDTV processing stream.  
           [0012]    U.S. Pat. No. 6,421,077 teaches a slightly different method for determining whether NTSC co-channel interference is present. The error/detecting correcting circuitry in the HDTV receiver outputs an error rate for the received digital signal. This error rate is compared for two conditions—one with an NTSC rejection filter in place, and one without. The configuration that results in the lowest error rate is chosen as the HDTV signal processing path. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 illustrates an interfering frequency spectrum comprising an NTSC television signal and a DTV channel occupying the same band, and how that spectrum aligns with a comb filter used for NTSC signal rejection;  
         [0014]    [0014]FIG. 2 depicts a DTV receiver according to some embodiments of the present invention;  
         [0015]    [0015]FIG. 3 contains a flow chart showing a filter/no filter decision process useful in some embodiments of the present invention;  
         [0016]    [0016]FIG. 4 shows a television receiver having both DTV and analog television receiver components, wherein the analog television receiver cooperates with the DTV receiver to make filter/no filter decisions; and  
         [0017]    [0017]FIGS. 5 and 6 show two DTV receiver embodiments, respectively specific to NTSC and PAL co-channel interference rejection. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0018]    The present invention includes embodiments that determine whether an interfering NTSC (or other analog) television signal occupies the same channel as a DTV signal by detecting characteristic features of the analog television signal. For instance, NTSC signals contain vertical synchronization pulses that repeat at a 59.94 Hz field rate, and horizontal synchronization pulses that repeat at a 15.734 kHz line rate. PAL signals contain vertical synchronization pulses that repeat at a 50 Hz field rate and horizontal synchronization pulses that repeat at a 15.625 kHz line rate. These sync signal features are used in some embodiments to determine whether a given analog-television-formatted signal is present in the currently tuned HDTV frequency space.  
         [0019]    Referring to FIG. 2, a first television receiver embodiment  200  is depicted. The DTV processing path  210  contains a tuner  220 , a demodulator  230 , a multiplexer (MUX)  260 , and other DTV processing functions (not shown) that are well-known and not critical to the present invention. In addition, an analog television signal rejection filter  240  can be placed in DTV processing path  210  when an analog television signal matched to filter  240  is detected.  
         [0020]    Tuner  220  receives an input signal IN, from which tuner  220  selects a channel. The selected channel is supplied to demodulator  230 , where the signal timing and synchronization are recovered and other pre-filtering operations are performed. The demodulator output is supplied directly to a first input of MUX  260  and to the input of analog TV signal rejection filter  240 . The output of analog TV signal rejection filter  240  is supplied to a second input of MUX  260 . A SELECT signal is used to switch either the first or second input of MUX  260  to the output of MUX  260  for further DTV processing.  
         [0021]    Outside DTV processing path  210 , a sync signal detector  250  receives the output from tuner  220  and attempts to detect one or more analog TV sync signals in the tuned channel. If the sync signal(s) of interest is/are present, sync signal detector  250  asserts the SELECT switching input on MUX  260  to place analog TV signal rejection filter  240  in the DTV processing path; otherwise, sync signal detector  250  deasserts the SELECT switching input such that MUX  260  passes the output of demodulator  230  to downstream DTV processing. Optionally, an ENABLE output of sync signal detector  250  can turn the analog TV signal rejection filter off when it is not needed.  
         [0022]    [0022]FIG. 3 shows one general method  300  for processing a DTV signal, e.g., with receiver  200  of FIG. 2. In method  300 , detection of one or more analog TV sync signals is attempted. If a sync signal is not detected, the DTV signal is not filtered. Otherwise, the sync pulse repetition rate (PRR) is measured and compared to the expected PRR. If the PRR is of the wrong rate, an analog TV signal may exist but it is not of the type that can be filtered by rejection filter  240 . If the PRR has an erratic rate, a weak analog TV signal may exist, in which case the decision can be made to not filter the DTV signal. Finally, if the measured PRR agrees with the expected PRR, the DTV signal is filtered using rejection filter  240 .  
         [0023]    [0023]FIG. 4 illustrates a second television receiver  400  according to other embodiments of the present invention. Receiver  400  contains a tuner  420  with an output shared by a DTV receiver  410  and an analog TV receiver  470 , both of which can supply video output signals. The DTV receiver contains a demodulator  430 , analog TV signal rejection filter  440 , and MUX  460 , e.g., connected and functioning similar to the same components in receiver  200  of FIG. 2. In addition, a signal controller  450  supplies the SELECT signal to MUX  460 .  
         [0024]    Analog TV receiver  470 , as part of its normal processing, contains a sync separator that detects vertical and horizontal synchronization pulses in an analog TV signal when an analog TV signal is present. In FIG. 4, at least the sync separator portion of analog TV receiver  470  is enabled when the DTV receiver video output is selected for output from the receiver. Accordingly, the sync pulses detected by analog TV receiver  470  are supplied to signal controller  450  within DTV receiver  410 . Signal controller  450  uses the sync pulse signals from analog TV receiver  470  to determine whether or not to assert its SELECT output. In some implementations, the analog and digital TV receivers can reside on a common integrated circuit. In some other implementations, a commercially available analog TV receiver circuit can be used.  
         [0025]    [0025]FIG. 5 illustrates a DTV receiver  500  according to some embodiments that are specific to NTSC co-channel interference. DTV receiver  500  contains a tuner  520 , demodulator  530 , NTSC rejection filter  540 , and MUX  560 , e.g., connected and functioning similar to the same components in receiver  200  of FIG. 2. Several other typical DTV processing components (an equalizer  570  and forward error correction circuitry  580 ) are also shown downstream of MUX  560 .  
         [0026]    Demodulator  530  contains additional sub-block detail. The output of tuner  520  is connected to an analog-to-digital converter  532 , which feeds digital samples to a clock recovery and synchronization timing recovery block  534 . A matched filter  536  then operates on the samples to detect a properly timed binary stream. This binary stream is supplied to MUX  560  and to NTSC rejection filter  540 .  
         [0027]    The output of tuner  520  is also supplied to a sync separator  552 . Sync separator  552  attempts to detect NTSC vertical and/or horizontal synchronization pulses in the tuned signal. When synchronization pulses are detected, the pulses are transmitted to a signal controller  554 . Signal controller  554  decides, based on whether sync pulses are detected and their repetition rates, whether or not to select the output of NTSC rejection filter  540  as the output of MUX  560 .  
         [0028]    [0028]FIG. 6 illustrates a DTV receiver  600  according to some embodiments that are specific to PAL co-channel interference. DTV receiver  600  contains a tuner  620 , demodulator  630 , PAL rejection filter  640 , and MUX  660 , e.g., connected and functioning similar to the same components in receiver  200  of FIG. 2. Forward error correction circuitry  680  is also shown downstream of MUX  660 .  
         [0029]    Demodulator  630  contains additional sub-block detail. The output of tuner  620  is connected to an analog-to-digital converter  632 , which feeds digital samples to a clock recovery and synchronization timing recovery block  634 . A Fast Fourier Transform  636  and an equalizer/mapper  638  provide a frequency-domain representation of the binary input stream. This frequency-domain signal is supplied to MUX  660  and to PAL rejection filter  640 .  
         [0030]    The output of tuner  620  is also supplied to a sync separator  652 . Sync separator  652  attempts to detect NTSC vertical and/or horizontal synchronization pulses in the tuned signal. When synchronization pulses are detected, the pulses are transmitted to a signal controller  654 . Signal controller  654  decides, based on whether sync pulses are detected and their repetition rates, whether or not to select the output of PAL rejection filter  640  as the output of MUX  660 .  
         [0031]    In the above embodiments, a variety of different synchronization signal analysis techniques can be used to determine whether, e.g., an NTSC or PAL rejection filter should be used. In perhaps the simplest case, the existence of a horizontal sync pulse train with a repetition rate near the expected repetition rate can be used to make the filter/no filter decision. The existence of a vertical sync pulse train can be used instead, or the signal controller can require both horizontal and vertical sync pulses to be evident. The signal controller can, in some embodiments, also count the horizontal pulses detected between successive vertical sync pulses.  
         [0032]    More sophisticated methods can also be used. For instance, an AGC (automatic gain control) function in the sync separator can indicate the relative level of the sync pulses with respect to the output of the tuner. The AGC output level can be supplied to the signal controller as an additional metric. When the AGC output level indicates a relatively weak NTSC or PAL signal, signal controller  654  may choose to deselect the rejection filter even though weak or intermittent sync pulses are detectable.  
         [0033]    The sync separator can also operate on the digital samples produced within the HDTV demodulator. For instance, the sync separator can low-pass filter the ADC output and then analyze the low-pass filtered version (e.g., by FFT) for frequency components corresponding to the horizontal line rate and vertical field rate of an analog TV signal. Relative strengths of such signals can be analyzed by the signal controller to determine whether insertion of a rejection filter is appropriate.  
         [0034]    With any of these techniques, it is possible to construct a system that can be programmed to detect co-channel interference from a selectable one of several analog TV formats (e.g., NTSC, NTSC variants, PAL, SECAM, etc.). If a system is capable of inserting multiple types of rejection filters, the system can also be designed to detect interference from multiple analog TV formats and insert an appropriate rejection filter.  
         [0035]    Although comb filters have been described, other filtering techniques can be used to remove one or more spectral components of an interfering analog television signal.  
         [0036]    Those skilled in the art will recognize that many other device configuration permutations can be envisioned and many design parameters have not been discussed. Likewise, functionality shown embodied in a single functional block may be implemented using multiple cooperating circuits or blocks, or vice versa. Such minor modifications and implementation details are encompassed within the embodiments of the invention, and are intended to fall within the scope of the claims.  
         [0037]    The preceding embodiments are exemplary. Although the specification may refer to “an”, “one”, “another”, or “some” embodiment(s) in several locations, this does not necessarily mean that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment.