1. Field of the Invention
The present invention relates to a intermediate-frequency signal processing device, which can be incorporated into television receivers, VTRs, or the like, for receiving FM broadcasts as well as television broadcasts.
2. Description of the Prior Art
Recent years have seen an increase in television receivers capable of receiving general FM broadcasts, such as portable color televisions, and liquid crystal televisions. Referring now to FIG. 2, there is illustrated a block diagram showing the structure of an example of a prior art video intermediate-frequency signal processing device which can be incorporated into such television receivers. In the figure, reference numeral 1 denotes an antenna for receiving broadcast waves, and 2 denotes a tuner which can select a desired broadcast wave from among the plurality of broadcast waves received via the antenna 1, and which, when receiving television broadcast waves, can convert a selected television broadcast signal into an intermediate-frequency signal including a video modulated component and a sound modulated component, and, when receiving FM broadcast waves, can convert a selected FM broadcast signal into an intermediate-frequency signal having the same frequency as the sound intermediate-frequency component generated when receiving television broadcast waves. Furthermore, reference numeral 3 denotes a video intermediate-frequency surface acoustic filter, which is hereafter abbreviated as a VIF SAW filter, for extracting a video intermediate-frequency component of a certain frequency, for example 58.75 MHz, in the case where the television receiver is destined for Japan, from the output of the tuner 2, 6 denotes a sound intermediate-frequency SAW filter, which is hereafter abbreviated as an SIF SAW filter, for extracting a sound intermediate-frequency component of a certain frequency, for example 54.25 MHz, in the case where the television receiver is destined for Japan, from the output of the tuner 2, and 4 denotes a video intermediate-frequency signal processing IC for detecting or demodulating the output of the VIF SAW filter 3. In addition, reference numeral 12 denotes a VIF amplifier for amplifying the output of the VIF SAW filter 3, 13 denotes a video detector for demodulating the output of the VIF amplifier 12, 14 denotes an intermediate-frequency automatic gain control circuit, which is hereafter abbreviated as an IF AGC circuit, for controlling the gain of the VIF amplifier 12 according to the output of the video detector 13, 15 denotes an IF AGC filter terminal for connecting an IF AGC filter 16, comprised of a capacitor, to the IF AGC circuit 14, 31 denotes a switch which can be switched to a TV terminal so as to open both ends of IF AGC filter 16 when receiving television broadcast waves, and which can be switched to an FM terminal so as to close both ends of the IF AGC filter 16 when receiving FM broadcast waves, and 17 denotes an automatic phase control detector, which is hereafter abbreviated as an APC detector, for comparing the phase of an output signal from the VIF amplifier 12 and that of an output signal from a voltage-controlled oscillator or VCO 18 so as to furnish a signal indicating the phase difference between them to the VCO 18 and perform an automatic phase control operation on the VCO 18. The VCO 18 furnishes a control voltage of a certain frequency, dependent on the output of the APC detector 17, to both the video detector 13 and the APC detector 17. In addition, reference numeral 19 denotes a VCO coil for setting the free-running frequency of the VCO 18 to 58.75 MHz. The VCO coil 19 includes an inductance component and a capacitance component. Reference numeral 20 denotes an APC filter terminal for connecting an APC filter 21 to the APC detector 17. The APC filter 21 is comprised of a capacitor and a resistor. After the APC filter 21 filters the output of the APC detector 17, the output of the APC detector 17 is applied as a control input to the VCO 18.
Reference numeral 23 denotes a sound intermediate frequency detector for receiving both the output of the SIF SAW filter 6 and the output of the VCO 18, and for an SIF detection, 24 denotes an FM detector for performing an FM detection on the output of the SIF detector 23, and 25 denotes an FM detection coil terminal for connecting an FM detection coil 26 to the FM detector 24. The FM detection coil 26 is connected between the FM detection coil terminal 25 and a ground potential, and includes an inductance component and a capacitance component. Furthermore, reference numeral 27 denotes an output terminal through which a detected sound (or voice) output from the FM detector 24 is furnished, 28 denotes a low-pass filter comprised of a resistor and a capacitor, for extracting low-frequency components from the detected sound output from the FM detector 24, and 30 denotes a switch which can be switched to a TV terminal so as to disconnect the output of the low-pass filter 28 from the APC filter terminal 20 when receiving television broadcast waves, and which can be switched to an FM terminal so as to connect the output of the low-pass filter 28 to the APC filter terminal 20 when receiving FM broadcast waves.
Next, a description will be made as to the operation of the prior art video intermediate-frequency signal processing device. When receiving a television broadcast, the video detector 13 demodulates the video intermediate-frequency signal which has been amplified by the VIF amplifier 12 so that the video intermediate-frequency signal has a certain amplitude. The video detector 13 then furnishes the demodulated video signal by way of the output terminal 5. Since the switch 31 does not short circuit the IF AGC filter 16, the IF AGC circuit 14 produces an AGC voltage from the demodulated video output of the video detector 13, and the IF AGC filter 16 then filters the AGC voltage from the IF AGC circuit 14. The filtered AGC voltage is applied to the VIF amplifier 12. Thus, the IF AGC loop is formed when receiving a television broadcast. The phase-locked loop or PLL comprised of the APC detector 17, the APC filter 21, and the VCO 18 generates a reference carrier wave used for demodulating the video intermediate-frequency signal from the VIF SAW filter. The reference carrier wave furnished by the VCO 18 has the same frequency as the VIF carrier wave, and is in phase with the VIF carrier wave.
For a sound signal in the received television broadcast wave, the SIF detector 23 performs a detection by multiplying the reference carrier wave from the VCO 18 by a sound intermediate-frequency signal extracted by the SIF SAW filter 6 (e.g. a sound signal at a frequency of 54.25 MHz in the case where the video intermediate-frequency signal processing device is destined for Japan), so as to convert the sound intermediate-frequency signal into an intermediate-frequency signal at a frequency of, for example, 4.5 MHz. After that, the FM detector 24 demodulates the intermediate-frequency signal to produce a sound signal and then furnishes it by way of the output terminal 27. At that time, since the switch 30 is switched to the TV terminal, the demodulated sound signal filtered by the low-pass filter 28 does not reach the APC filter 21.
On the other hand, when receiving an FM broadcast, the switch 31 is switched to the FM terminal and shorts circuits the IF AGC filter 16. As a result, the gain of the VIF amplifier 12 is reduced to a minimum. Since the APC detector 17 does not output anything when the gain of the VIF amplifier 12 is set to its minimum, the VCO 18 oscillates freely while it is unaffected by the APC detector 17. The other switch 30 is then switched to the FM terminal and the DC voltage that appears at the output terminal 27 therefore enters the low-pass filter 28. The low-pass filter 28 then rejects a sound signal from the DC voltage and then furnishes the filtered DC voltage to the APC filter terminal 20 by way of the switch 30. If automatic fine tuning or AFT is carried out so that the polarity of the FM detection characteristics matches the polarity of the AFT, the VCO 18 can be stably controlled and therefore the FM detection can be performed. Thus, by using the same circuit as that used when demodulating a modulated sound signal included in a television broadcast wave, the prior art video intermediate-frequency signal processing device can convert an FM broadcast signal into an intermediate-frequency signal at a frequency of 54.25 MHz by means of the tuner 2, further convert the intermediate-frequency signal into anther intermediate-frequency signal whose carrier frequency is 4.5 MHz by means of the SIF detector 23, and then produce an FM broadcast sound signal.
A problem with the prior art intermediate-frequency signal processing device capable of receiving FM broadcasts is that the cost of manufacturing is increased because it needs the FM detection coil 26 and the FM detection coil terminal 25 for connecting the coil to the FM detector 24 to control the VCO 18 stably when receiving an FM broadcast, and the FM detection coil 26 needs to be adjusted so that the frequency characteristic of the output voltage of the APC detector 17 that appears at the APC filter terminal 20 is symmetric with respect the intermediate frequency of 4.5 MHz.