Abstract:
A receiver for a digital broadcasting service have a compatibility between tuning to a desired channel at a short period of time and an improve ment of a bit error rate (BER) characteristics. In a case where a target wave is fluctuated, to prevent the receiver from having a significant declination of BER characteristics and to ensure the reproduction of high quality images, the receiver includes an automatic gain controller including a loop filter in which the frequency range of the loop filter can be switched according to an output of a synchronous state discriminator. The frequency band of the loop filter is selectively adjusted to one setting designed for minimizing the duration for tuning a desired channel before a synchronous state is established. After the synchronous state established, the frequency band of the loop filter is switched to another setting designed for enhancing BER characteristics.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a receiver for a broadcasting service and particularly to a digital broadcasting signal receiver. 
     BACKGROUND OF THE INVENTION 
     As a digital broadcasting service has recently been initiated with a ground wave, a digital modulation technique is required. Such a digital technique of modulating ground wave broadcasting signals commonly consists of superimposing multi-level data over a signal amplitude for increasing the transmission efficiency per unit frequency. In a digital broadcasting receiver, a received digital modified signal is digitally demodulated to a binary signal. It is, however, necessary for improving the quality of the binary signal defined by a bit error rate (BER) to precisely control the amplitude of the digital demodulated signal to a desired level, i.e., carry out automatic gain control. 
     FIG. 15 illustrates a conventional digital broadcasting signal receiver employing an automatic gain controller. As shown, an RF signal received at an antenna (not shown) is introduced into a mixer  1  where it is combined with an output of a local oscillator  2  to yield an IF signal. A desired frequency band is extracted from the IF signal by a Band Pass Filter (BPF)  3  and transferred to a gain-variable amplifier  4 . 
     The IF signal at mixer  1  is tuned by the frequency of the output of local oscillator  2  so that the IF signal can contain a particular channel signal which is selectively picked up. 
     The IF signal shaped by variable-gain amplifier  4  is further transferred to a digital signal demodulator  5  where the IF signal is digitally demodulated to generate a binary signal. 
     Variable-gain amplifier  4  and digital signal demodulator  5  form a feedback control loop together with an automatic gain controller  6  which consists mainly of an automatic gain control device  7 , a loop filter  8 , and a loop amplifier  9 . 
     Automatic gain control device  7  detects a difference in the amplitude between the IF signal and the desired channel signal from the output of digital signal demodulator  5 . The amplitude difference is smoothed by loop filter  8 , amplified by loop amplifier  9 , and fed back as a control signal to variable-gain amplifier  4  where it controls the signal amplifying rate, thus, to compensate for the amplitude difference in the IF signal. 
     Meanwhile, it is essential for any digital broadcasting signal receiver to establish the compatibility between quick tuning of a desired channel signal and use of higher BER. The reception of ground wave signals from ground broadcasting stations, unlike the direct transmission of satellite broadcasting signals from satellites, depends largely on the location of an antenna of the digital broadcasting receiver. As the transmission of ground wave signals is commonly disturbed by external obstacles, a received signal may contain static ghosts or Doppler ghosts, hence, declining BER. 
     In particular, any object located between the transmitter of a broadcasting signal and the antenna of the receiver (for example, the leaves or the branches of trees swayed by winds) may generate an external disturbance known as target wave fluctuation, where the amplitude (intensity of electric field) of a received signal is varied from time to time, hence, significantly declining BER and impairing the reproduction of images. The target wave fluctuation may cause a change of several decibels in the amplitude at intervals of several tens of hertz. 
     Since the band-width of the loop filter and the gain of the loop amplifier in the automatic gain controller of the conventional digital broadcasting signal receiver are fixed, it is required to increase the band-width of the loop filter and to broaden the control frequency range of the automatic gain controller for tuning a desired channel at a short period of time, thus enhancing the frequency response of the automatic gain controller. Alternatively, the control frequency range can be broadened by increasing the gain in the feedback control circuit. As the gain of the loop amplifier is increased, the control frequency range of the automatic gain controller expands to enhance the frequency response of the automatic gain controller. By enhancing the frequency response of the automatic gain controller, the amplitude of the IF signal introduced to the digital signal demodulator is quickly settled down to a desired range of a level, hence permitting the tuning to a desired broadcasting signal at a shorter period of time. 
     However, as the control system generally tends to be responsive to a too high frequency range including Doppler ghosts, the amplitude of an input signal to the digital signal demodulator may not stay in the desired range, thus declining the BER characteristics and deteriorating the performance of the digital broadcasting signal receiver. 
     On the contrary, when the band-width of the loop filer is narrowed to limit the control frequency of the automatic gain controller or the gain of the loop amplifier decreases to limit the control frequency range of the automatic gain controller, a control frequency range of the automatic gain controller becomes smaller than the frequency range of amplitude variation of the IF signal. As a result, the gain controller can hardly respond to any target wave fluctuation particularly with the antenna of the digital broadcasting signal receiver located in a region where the target wave fluctuation occurs frequently. This causes the amplitude of an input signal introduced to the digital signal demodulator not to stay in a desired level, hence declining the BER characteristics, seriously disturbing the production of images, and degrading the performance of the digital broadcasting signal receiver. 
     SUMMARY OF THE INVENTION 
     A digital broadcasting signal receiver has a compatibility between the tuning to a desired channel in a short period of time and an improvement of the BER characteristics, and in a case where a target wave fluctuates, prevent significant declination of the BER characteristics and ensure the reproduction of quality images. 
     The digital broadcasting signal receiver comprises a synchronous state discriminator or an electric field intensity detector for detecting a change in the intensity of an electric field of a received IF signal, so that the frequency range of a loop filter or the gain level of a loop amplifier in an automatic gain controller can be switched according to an output of the synchronous state discriminator or the electric field intensity detector. 
     The receiver consequently permits the frequency band of the loop filter or the gain level of the loop amplifier in the automatic gain controller to be selectively adjusted to one setting designed for minimizing the duration for tuning a desired channel before the synchronous state is established. Then, after the synchronous state is established, the frequency band of the loop filter or the gain level of the loop amplifier in the automatic gain controller is switched to another setting designed for enhancing the BER characteristics. As a result, the minimum duration for tuning to the desired channel and the higher BER rate will be compatible with each other. 
     Also, the digital broadcasting signal receiver allows the electric field intensity detector to monitor the amplitude of the intensity of the electric field of the received IF signal. When an amplitude change in the intensity of the electric field and the varying frequency exceed predetermined levels, it is judged that a target wave fluctuation occurs, and the frequency band of the loop filter or the gain level of the loop amplifier in the automatic gain controller is selectively switched. This limits the amplitude of the signal introduced to the digital signal demodulator to a desired range regardless of the occurrence of a target wave fluctuation in the antenna region of the digital broadcasting signal receiver, hence preventing significant declination of the BER characteristics and ensuring the reproduction of high quality images. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 1 of the present invention; 
     FIG. 2 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 2 of the present invention; 
     FIG. 3 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 3 of the present invention; 
     FIG. 4 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 4 of the present invention; 
     FIG. 5 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 5 of the present invention; 
     FIG. 6 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 6 of the present invention; 
     FIG. 7 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 7 of the present invention; 
     FIG. 8 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 8 of the present invention; 
     FIG. 9 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 9 of the present invention; 
     FIG. 10 is a diagram of a transmission format of the North American ATSC system; 
     FIG. 11 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 10 of the present invention; 
     FIG. 12 is a diagram of a transmission format of the European DVB-T system; 
     FIG. 13 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 11 of the present invention; 
     FIG. 14 is a diagram of a transmission format of the Japanese ISDB-T system; and 
     FIG. 15 a block diagram of a circuit of a conventional digital broadcasting signal receiver. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Some embodiments of the present invention will be described referring to the relevant drawings. 
     Embodiment 1 
     FIG. 1 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 1 of the present invention. As the digital broadcasting signal receiver of this embodiment is fundamentally identical in the arrangement to the conventional digital broadcasting signal receiver shown in FIG. 15, like components are denoted by like numerals and will not be explained in detail. 
     As shown, automatic gain controller  6  which comprises automatic gain control device  7 , a loop filter  11 , and loop amplifier  9  constitutes a feedback control loop together with variable-gain amplifier  4  and digital signal demodulator  5 . 
     A synchronous state discriminator  10  is provided for extracting a predetermined pattern of data from a digital signal released from digital signal demodulator  5 . When the data pattern fails to be extracted, the signal is found to be in a non-synchronous state. When the data pattern is successfully extracted, the signal is in a synchronous state. 
     The output of synchronous state discriminator  10  is thus a signal indicative of either the synchronous state or the non-synchronous state which is then transferred to loop filter  11 . Loop filter  11  has two different ranges of frequencies and in response to the output of synchronous state discriminator  10 , switches from one frequency range to the other. In a case of the non-synchronous state, one frequency range designed for minimizing the duration of tuning to a desired broadcasting signal is selected. When the synchronous state is acknowledged, the other frequency range designed for improving the BER characteristics of a received signal is selectively used. 
     For tuning to a desired channel, the frequency of an output of local oscillator  2  is determined so that an IF signal released from mixer  1  contains the desired channel. Similarly, fortuning to another channel after the synchronous state is acknowledged, the frequency of an output of local oscillator  2  is set so that the IF signal released from mixer  1  contains the another channel. At this time, as the synchronous state is eliminated, the output of synchronous state discriminator  10  indicates the non-synchronous state. 
     Embodiment 2 
     FIG. 2 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 2 of the present invention. As the digital broadcasting signal receiver of this embodiment is fundamentally identical in that of embodiment  1  shown in FIG. 1, like components are denoted by like numerals and will not be explained in detail. 
     As shown, automatic gain controller  6  which comprises automatic gain control device  7 , a loop filter  12 , and loop amplifier  9  constitutes a feedback control loop together with variable-gain amplifier  4  and digital signal demodulator  5 . 
     Loop filter  12  has two, wide and narrow, ranges of frequencies and switches from one to the other in response to the output of synchronous state discriminator  10 . In a case of the non-synchronous state, the wide range of frequencies is selectively used to enhance the frequency response of the control loop. This allows the amplitude of an IF signal introduced to digital signal demodulator  5  to be quickly settled down in a desired range, thus minimizing the duration required for tuning the desired channel. When the synchronous state is acknowledged, the filter frequency is switched to the narrow range, hence making the controlling action over the IF signal amplitude of automatic gain controller  6  less susceptible to Doppler ghosts and improving the BER characteristics. 
     Embodiment 3 
     FIG. 3 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 3 of the present invention. 
     As shown, automatic gain controller  6  which comprises automatic gain control device  7 , loop filter  8 , and a loop amplifier  13  constitute a feedback control loop together with variable-gain amplifier  4  and digital signal demodulator  5 . 
     Loop amplifier  13  has two different gain levels which are switched from one to the other in response to the output of synchronous state discriminator  10 . When the non-synchronous state is acknowledged, one gain level designed for minimizing the duration required for tuning to the desired channel is selected. In a case of the synchronous state, the other gain level designed for improving the BER characteristics of the received signal is selectively used. 
     Embodiment 4 
     FIG. 4 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 4 of the present invention. 
     As shown, automatic gain controller  6  which comprises automatic gain control device  7 , loop filter  8 , and a loop amplifier  14  constitute a feedback control loop together with variable-gain amplifier  4  and digital signal demodulator  5 . 
     Loop amplifier  14  has two, large and small, gain levels which are switched from one to the other in response to the output of synchronous state discriminator  10 . When the non-synchronous state is acknowledged, the large gain level is selected. The larger the gain, the wider the control frequency range of the feedback control system is broadened. As the gain of loop amplifier  14  is increased to broaden the control frequency of automatic gain controller  6  and to enhance the frequency response of the control loop of the automatic gain controller  6 , the amplitude of an IF signal introduced to digital signal demodulator  5  can quickly be settled down in a desired range, hence minimizing the duration required for tuning to the desired channel. In a case of the synchronous state, loop amplifier  14  is switched to the small gain level. The smaller the gain, the narrower the control frequency range becomes in the feedback control system. As loop amplifier  14  is switched to the small gain to limit the control frequency range of automatic gain controller  6 , the controlling action over the IF signal amplitude of automatic gain controller  6  can be less susceptible to Doppler ghosts, hence improving the BER characteristics. 
     Embodiment 5 
     FIG. 5 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 5 of the present invention. 
     As shown, automatic gain controller  6  which comprises automatic gain control device  7 , a loop filter  16 , and loop amplifier  9  constitute a feedback control loop together with variable-gain amplifier  4  and digital signal demodulator  5 . 
     After the desired channel is tuned, an electric field intensity detector  15  monitors a change with time in the intensity of an electric field and examines whether or not the varying frequency and the amplitude change in the electric field intensity exceed predetermined levels to find the occurrence of target wave fluctuation. The output of electric field intensity detector  15  is a signal indicating whether or not a target wave fluctuation occurs. In response to the output of electric field intensity detector  15 , loop filter  16  having two different frequency ranges can switch the frequency band from one to the other. When it is discriminated by electric field intensity detector  15  that a target wave fluctuation presumably occurs, loop filter  16  selects one frequency range designed for accepting the varying frequency due to the target wave fluctuation to set the amplitude of the input signal of digital signal demodulator  5  to a desired range, hence avoiding declination of the BER characteristics. 
     Embodiment 6 
     FIG. 6 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 6 of the present invention. 
     As shown, automatic gain controller  6  which comprises automatic gain control device  7 , a loop filter  17 , and loop amplifier  9  constitute a feedback control loop together with variable-gain amplifier  4  and digital signal demodulator  5 . 
     Loop filter  17  has two, wide and narrow, frequency ranges which are switched from one to the other in response to the output of electric field intensity detector  15 . When the desired channel has been tuned and electric field intensity detector  15  discriminated that a target wave fluctuation presumably occurs, loop filter  17  selects the wide frequency range to enhance the frequency response of the control loop. This allows automatic gain controller  6  to accept the varying frequency due to the target wave fluctuation and set the amplitude of the input signal of digital signal demodulator  5  to a desired range, hence avoiding declination of the BER characteristics. 
     Embodiment 7 
     FIG. 7 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 7 of the present invention. 
     As shown, automatic gain controller  6  which comprises automatic gain control device  7 , loop filter  8 , and a loop amplifier  18  constitute a feedback control loop together with variable-gain amplifier  4  and digital signal demodulator  5 . 
     Loop amplifier  18  has two gain levels which are switched from one to the other in response to the output of electric field intensity detector  15 . When the desired channel has been tuned and electric field intensity detector  15  discriminated that a target wave fluctuation presumably occurs, loop amplifier  18  selects the gain level designed for accepting the varying frequency due to the target wave fluctuation. This allows automatic gain controller  6  to accept the varying frequency due to the target wave fluctuation and set the amplitude of the input signal of digital signal demodulator  5  to a desired range, hence avoiding declination of the BER characteristics. 
     Embodiment 8 
     FIG. 8 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 8 of the present invention. 
     As shown, automatic gain controller  6  which comprises automatic gain control device  7 , loop filter  8 , and a loop amplifier  19  constitute a feedback control loop together with variable-gain amplifier  4  and digital signal demodulator  5 . 
     Loop amplifier  19  has two, large and small, gain levels which are switched from one to the other in response to the output of electric field intensity detector  15 . When the desired channel has been tuned and electric field intensity detector  15  discriminated that a target wave fluctuation presumably occurs, loop amplifier  19  selects the large gain level. The larger the gain, the wider the control frequency of the feedback control system is given. As the gain of loop amplifier  19  is increased, the control frequency of automatic gain controller  6  is broadened. Accordingly, the frequency response of the control loop of automatic gain controller  6  is enhanced. This allows automatic gain controller  6  to accept the varying frequency due to the target wave fluctuation and set the amplitude of the input signal of digital signal demodulator  5  to a desired range, hence avoiding declination of the BER characteristics. 
     Embodiment 9 
     FIG. 9 is a block. diagram of a circuit of a digital broadcasting signal receiver showing embodiment 9 of the present invention. The digital broadcasting signal receiver of this embodiment is fundamentally identical in arrangement to those shown in FIG. 1,  2 ,  3 , or  4 . 
     FIG. 10 illustrates a transmission format of the North American ATSC system, where each segment has a period of 77.3 microseconds and 313 of the segments form one field. The segment comprises 832 symbols and the first four symbols constitute a segment SYNC which indicates the front end of the segment. 
     The first segment of each field is a pseudo random signal (PN511, PN63), a field SYNC, for indicating the front end of the field. Synchronous state discriminator  20  is provided for, when detecting the segment SYNC or both the segment SYNC and the field SYNC from the digital signal output of digital signal demodulator  5 , discriminating that the signal is at the synchronous state and switching the control loop property of automatic gain controller.  6 . 
     Embodiment 10 
     FIG. 11 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 10 of the present invention. The digital broadcasting signal receiver of this embodiment is fundamentally identical in arrangement to those shown in FIG. 1,  2 ,  3 , or  4 . 
     FIG. 12 illustrates a transmission format of the European DVB-T system having a packet structure where “payload” of a transport packet stream specified by MPEG2-SYSTEMS (ISO/IEC13818-1) is joined with a 16-byte parity used for error correction, totaling 204 bytes. One byte at the front end of the packet is a header acting as a frame SYNC used for detection of the packet. Asynchronous state discriminator  21  is provided for, when detecting the frame SYNC from the digital signal output of digital signal demodulator  5 , discriminating that the signal is in the synchronous state and switching the control loop property of automatic gain controller  6 . 
     Embodiment 11 
     FIG. 13 is a block diagram of a circuit of a digital broadcasting signal receiver showing embodiment 11 of the present invention. The digital broadcasting signal receiver of this embodiment is fundamentally identical in arrangement to that shown in FIG.  11 . 
     FIG. 14 illustrates a transmission format of the Japanese ISDB-T system having a packet structure where “payload” of a transport packet stream specified by MPEG2-SYSTEMS (ISO/IEC13818-1) is joined with a 16-byte parity used for error correction, totaling 204 bytes. One byte at the front end of the packet is a header acting as frame SYNC used for detection of the packet. Asynchronous state discriminator  22  is provided for, when detecting the frame SYNC from the digital signal output of digital signal demodulator  5 , discriminating that the signal is in the synchronous state and switching the control loop property of automatic gain controller  6 . 
     As set forth above, the present invention includes a synchronous state discriminator or an electric field intensity detector for detection of a change in the intensity of the electric field of the received IF signal, allowing the frequency band of a loop filter or the gain level of a loop amplifier in the automatic gain controller to be switched in response to the output of either the synchronous state discriminator or the electric field intensity detector. 
     Consequently, while the frequency band of the loop filter or the gain level of the loop amplifier in the automatic gain controller is selectively adjusted to one setting designed for minimizing the duration required for tuning a desired channel before the synchronous state is established, it can be switched, after the synchronous state established, to another setting designed for enhancing the BER characteristics. As a result, the minimum duration for tuning the desired channel and higher BER will be compatible with each other. 
     Also, when the amplitude of the intensity of an electric field of the received IF signal is being monitored with time by the electric field intensity detector and a change in the amplitude and the varying frequency exceed predetermined levels, it is discriminated that a target wave fluctuation occurs and the frequency band of the loop filter or the gain level of the loop amplifier in the automatic gain controller is selectively switched from one setting to the other. This prevents a decline of the BER characteristics, even if the target wave fluctuation occurs, hence ensuring the reproduction of high quality images.