The present invention relates to a high-definition television signal receiving apparatus and an automatic gain controlling apparatus thereof for use in a circuit that extracts for processing only a desired signal from a sum signal of two or more signals having different frequency-division-multiplexed levels. More particularly, the present invention relates to a high-definition television signal receiving apparatus having a capability of suppressing interference between channels caused when broadcasts having different signal levels such as a high-definition television signal and a standard television signal are transmitted simultaneously.
Recently, multi-channel broadcasting has increased along with an increase of television broadcasting and CATV broadcasting. For the purpose of supporting multi-channel broadcasting, receiving apparatuses having a double conversion tuner have been used for reducing a deviation in in-band flatness (tilt), image suppression, and interference caused by leakage of locally oscillated signals also at the time of multi-channel reception.
FIG. 4 shows a conventional television receiver adopting a double conversion tuner. As shown in FIG. 4, a signal supplied from a television signal input terminal 10 is processed by an input filter 20 such as a band pass filter (BPF), a first automatic gain controlling amplifier 30, a first mixer 40, a first IF (intermediate frequency) filter 50 such as a band pass filter, a first IF (intermediate frequency) amplifier 60, a second mixer 70, a second IF amplifier 80, a second IF filter 90 such as a surface acoustic wave (SAW), and a second automatic gain controlling amplifier 100 in this order to be outputted from an output terminal 110. The first mixer 40 is connected with a first local oscillator 120. The first local oscillator 120 is connected with a reference oscillator 140 via a PLL (Phase Locked Loop) circuit 130. The second mixer 70 is connected with a second local oscillator 150. The output signal of the second automatic gain controlling amplifier 100 is branched to be supplied to an AGC detector 200. The AGC detector 200 is connected with an AGC signal switching circuit 190. The AGC signal switching circuit 190 is also connected with a reference voltage circuit 210. An output of the AGC signal switching circuit 190 is supplied to the first automatic gain controlling amplifier 30 and the second automatic gain controlling amplifier 100. An input signal from a tuning signal input terminal 230 is supplied to a controller 220. An output of the controller 220 is supplied to the PLL circuit 130 and the input filter 20.
In operation, an RF (Radio Frequency) signal AM-modulated with the standard television signal is supplied to the input terminal 10 and a band of the supplied signal is divided by the input filter 20. Only the channel containing a desired channel is selectively supplied to the first automatic gain control amplifier 30. Meanwhile, a tuning signal is supplied to the tuning signal input terminal 230. The input filter 20 is controlled by the controller 220 such that an appropriate pass band is selected according to a channel to be tuned in. The first automatic gain control amplifier 30 amplifies or attenuates the signal band-limited by the input filter 20 to an appropriate receive level and supplies an output of a resultant signal to the first mixer 40.
The first local oscillator 120 is controlled by the controller 220 and the PLL circuit 130 so that oscillation is performed at a frequency corresponding to the desired channel by the tuning signal supplied from the tuning signal input terminal 230. The PLL circuit 130 compares a signal obtained by dividing an oscillation frequency having a stable frequency from the reference oscillator 140 with a signal obtained by dividing an oscillation frequency from the first local oscillator 120 to control the oscillation frequency of the first local oscillator so that an error becomes zero. Appropriately varying these dividing ratios by the controller 220 allows the first local oscillator 120 to oscillate at a frequency corresponding to the desired channel.
The first mixer 40 mixes a signal from the first automatic gain control amplifier 30 and a locally oscillated signal from the first local oscillator 120 to output the first IF (intermediate frequency) signal 41. The first IF filter 50 selectively passes only the desired channel of the first IF signal 41. The first IF amplifier 60 amplifies the passed signal and supplies the amplified signal to the second mixer 70. The second mixer mixes the signal from the first IF amplifier 60 and the locally oscillated signal from the second local oscillator 150 to output the second IF signal 71. The second IF amplifier 80 amplifies this mixed signal and supplies the amplified signal to the second IF filter 90. The second IF filter 90 selectively passes only the desired channel of the second IF signal 71. The second IF signal 71 that passed the second IF filter is amplified by the second automatic gain controlling amplifier 100 to be outputted from the output terminal 110.
A gain control signal for controlling gains of the first automatic gain control amplifier 30 and the second automatic gain controlling amplifier 100 is generated by detecting through the AGC detector 200 a signal level obtained by branching the output signal of the second automatic gain controlling amplifier 100. The AGC signal switching or selector circuit 190 applies the generated gain controlling signal to either the first automatic gain controlling amplifier 30 or the second automatic gain controlling amplifier 100.
Namely, the output signal of the second automatic gain controlling amplifier is detected by the AGC detector 200 to provide the gain control signal. The AGC signal switching circuit 190, if the output signal became larger than the previous signal, first applies this signal to the second automatic gain controlling amplifier 100. Also after the gain attenuation of the second automatic gain controlling amplifier 100 reached the maximum level, the AGC signal switching circuit 190 applies the gain control signal to the first automatic gain control amplifier 30. Whether the gain attenuation of the second automatic gain controlling amplifier 100 has reached the maximum level or not is detected by comparing the output of the AGC detector 200 with the output of the reference voltage 210.
When the level of the output signal of the second automatic gain controlling amplifier 100 goes down, the AGC signal switching circuit 190 applies the gain control signal to the first automatic gain control amplifier 30 to increase the gain of the first automatic gain control amplifier 30. If the signal level is small after the gain of the first automatic gain control amplifier 30 has reached the maximum level, the AGC signal switching circuit 190 applies the gain control signal to the second automatic gain controlling amplifier 100 to increase the gain of the second automatic gain controlling amplifier 100. Whether the gain of the first automatic gain control amplifier 30 has reached the maximum level or not is detected by comparing the output of the AGC detector 200 with the output of the reference voltage 210.
Thus, if it is necessary to decrease the gain of the first automatic gain control amplifier 30 or the gain of the second automatic gain control 100 in order to keep constant the signal level outputted from the output terminal 110, the gain of the second automatic gain controlling amplifier 100 in the rear stage of the receiving apparatus is initially decreased as far as possible. If this is not sufficient, the gain of the first automatic gain control amplifier 30 in the front stage is decreased to keep the gain of the first automatic gain control amplifier 30 in the front stage as large as possible. Consequently, the degradation of the noise factor of the entire receiving apparatus can be suppressed.
Generally, in order to maintain a value of the noise figure of the system small, it is necessary to keep the gain of a first automatic gain controlling amplifier in front stage larger than a second automatic gain controlling amplifier in the rear stage. In the case of reducing of the gain of the first automatic gain controlling amplifier in the front stage and the second automatic gain controlling amplifier in the rear stage, first the gain of the second automatic gain controlling amplifier is reduced, and when the gain attenuation of the second automatic gain controlling amplifier reaches a maximum, the gain of the first automatic gain controlling amplifier is reduced. In the case of increasing the gain of the first and second automatic gain controlling amplifier, first the gain of the first automatic gain controlling amplifier is increased, and the gain of the second automatic gain controlling amplifier is increased when the gain of the first automatic gain controlling amplifier reaches the maximum value.
The gain control signal to the first automatic gain controlling amplifier 30 is set to have a comparatively long time constant and the gain control signal to the second automatic gain controlling amplifier 100 is set to have a comparatively short time constant. In the case where the signal level changes in a short period such as the case of flutter caused by aircraft or the like, the change is followed by the second automatic gain controlling amplifier 100.
The above-mentioned conventional receiving apparatus properly operates when receiving a television signal for which the received power between channels is approximately uniform. However, when broadcasting different transmission levels such as broadcasting by a standard television signal such as NTSC (National Television System Committee) broadcasting and a digitally transmitted high-definition television signal which are simultaneously frequency-division-multiplexed, such as the ATV (Advanced TV system in U.S.) and Perfect TV in Japan, it is difficult to remove a signal of an adjacent channel through the filter 20 and the filter 50, so that the signal of the adjacent channel is captured in the receiving apparatus together with the desired signal to be received. The desired signal captured together with the signal of the adjacent channel is then selected by the filter 90. This operation results because the frequencies of the signals when passing the filter 20 and the filter 50 are high and, currently, it is difficult to fabricate a filter having a sufficiently narrow pass band width for passing only the desired signal. Consequently, the desired signal to be received and the signal of the adjacent channel are processed together by each of the circuits before passing the filter 90 which may be SAW filter. Further, a problem occurs in that the NTSC signal of the adjacent channel is higher than the digital desired channel in level and therefore exceeds the dynamic range of each of the circuits in front of the filter 90, thereby possibly causing interference. Additionally, interference caused by the difference of received power between channels, as mentioned above, may also be caused when off-air TV signals are transmitted over a specific terrain or during a specific weather change.