1. Field of the Invention
The present invention relates to a bilingual television signal processor, and more particularly, to an improved bilingual television signal processor which is capable of automatically controlling a pilot signal included in a bilingual signal received from a television having a bilingual function without an external adjustment.
2. Description of the Prior Art
Conventionally, as shown in FIG. 1, a bilingual television signal processor includes a main audio signal processor 130 for amplifying and FM(frequency demodulation)-detecting a main audio signal (MAS) carried on a carrier wave of 4.5 MHz to detect only a main audio signal, a sub audio signal processor 100 for detecting a sub audio signal (SAS) having a pilot signal included therein and carried on a carrier wave of 4.72 MHz through the same course as the main audio signal processor 130, a pilot signal detector 110 for filtering and detecting the sub audio signal outputted from the sub audio signal processor 100 to detect whether a present TV program is in a stereo or dual state or in neither state, a phase locked loop (hereinafter, called PLL) detector 120 for locking the pilot signal detected from the pilot signal detector 110 and outputting the locked information, a microcomputer 160 for outputting an on-screen display (OSD) message notifying a bilingual broadcasting on the television screen when the signal from the PLL detector 120 is outputted, a matrix unit 140 for selecting and outputting a mono type or a stereo type, a main audio unit or a sub audio unit in accordance with control signals (ctrl1,ctrl2) applied from a user by using the main audio signal and the sub audio signal detected from the main audio signal processor 130 and the sub audio signal processor 100, and a tone controller 150 for receiving an output from the matrix unit 140 to determine an output tone and output the same through a speaker.
The main audio signal processor 130 includes an amplifier 131 for amplifying the inputted main audio signal carried on the carrier wave of 4.5 MHz, and an FM-detector 132 for FM-detecting the amplified signal to detect a main audio signal. The sub audio signal processor 100 includes an amplifier 101 and an FM-detector 102 having the same functions as those of the main audio signal processor 130.
The pilot signal detector 110 includes a bandpass filter 11 for filtering the signals around the sub audio signal of 55 kHz outputted from the sub audio signal processor 100, and an AM(amplitude modulation)-detector 112 for detecting the signals outputted from the band filter 11 to detect a pilot signal. The PLL detector 120 includes a stereo PLL detector 121 and a dual PLL detector 122 for locking the pilot signal detected from the AM-detector 112.
The operation of the conventional bilingual television signal processor having the above-described construction will now be described.
The main audio signal (MAS) carried on the carrier wave of 4.5 MHz is inputted to the amplifier 131 of the main audio signal processor 130, amplified by a predetermined gain and inputted to the FM-detector 132. As a result, only the main audio signal is detected through the FM detection.
The sub audio signal (SAS) having the pilot signal included therein is carried on the carrier wave of 4.724 MHz to be inputted to the sub audio signal processor 100. The sub audio signal is detected through the same course as that of the main audio signal processor 130. The sub audio signal is inputted to the bandpass filter 111 of 55.069 kHz of the pilot signal detector 110 to filter the signals around 55 kHz and then a carrier wave of 55.069 kHz is detected by the AM-detector 112 to detect a pilot signal.
The pilot signal detected from the AM-detector 112 is inputted to the stereo PLL detector 121 and the dual PLL detector 122 of the PLL detector 120 and locked therein. The pilot signal locked in the PLL detector 120 and then outputted therefrom is inputted to the microcomputer 160, which outputs an on-screen display (OSD) message notifying a bilingual broadcast or a stereo broadcast on the television screen.
The main audio signal and the sub audio signal detected from the main audio signal processor 130 and the sub audio signal processor 100 are inputted to the matrix unit 140, which selects and outputs a mono type or a stereo type, a main audio signal or a sub audio signal as a desired output, in accordance with the control signals (ctrl1,ctrl2) applied by the user. The selected output signal is inputted to the tone controller 150 to determine an output tone and to be outputted to the outside through a speaker.
The bandpass filter 111 of the pilot signal detector 110 receives the sub audio signal outputted from the sub audio signal processor 100 and filters the frequencies around 55.069 kHZ in accordance with a central frequency adjusted by a DC voltage externally applied.
That is, the user must adjust the center frequency to be exactly 55.069 kHz, and for the general TV products, the center frequency is set using a variable resistance.
The bandpass filter of 55.069 kHz has a significantly high Q value to exactly detect the pilot signal carried on the carrier wave of 55.069 kHz and reject other frequencies to a maximum degree. In accordance with such high Q value, a center frequency of the band filter must be exactly adjusted externally.
However, in the conventional bilingual television signal processor, when the adjusted center frequency is out of the bandwidth of 55.069 kHz by around 3 dB, the detectability becomes lower, and a shift in the center frequency is generated by a change in the external DC voltage, resulting in lowering the detectability of the pilot signal and causing erroneous operation.