JPH0591426A - Sound signal modulation circuit - Google Patents

Abstract

PURPOSE:To stabilize the frequency of a VCO with use of a small number of elements and to convert a sound signal into plural sound FM signals of different frequency by using a fixed oscillator used for a test signal generator which produces the video signals to a part of a PLL circuit. CONSTITUTION:A VCO consists of a BPF 16, an amplifier 17, and a capacitor 18. A PLL circuit consists of a VCO 14, a programmable divider 19, an oscillator 13, a phase comparator 20, and a smoothing circuit 21. The center frequency of the VCO 14 is locked to the frequency of the oscillator 13 and then modulated by a sound signal. The oscillator 15 can use a fixed oscillator which is built in an RF modulator for PAL end used for 8 test signal. In such a constitution, the VCO is controlled via the divider 19 end a stable PLL operation is always secured. Therefore the sound signal applied to an input terminal 23 is modulated into a desired FM signal and outputted through an output terminal 24.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an audio signal modulation circuit used in an RF modulator for converting a baseband video signal into an RF band television signal.
In particular, the present invention relates to an audio signal modulation circuit suitable for use in an audio FM signal having a plurality of frequencies such as the PAL system.

[0002]

2. Description of the Related Art Audio F of PAL television broadcasting
The M signal frequency differs depending on the region. For example, the frequencies include 4.5 MHz, 5.5 MHz, 6.0 MHz and 6.5 MHz. When a plurality of frequencies exist in this way, a plurality of VCO resonators constituting the FM modulator are also required. FIG. 2 shows an RF modulator including such an FM modulator, in which a video signal is generated from the first signal source (1) and an audio signal is generated from the second signal source (2). The video signal is clamped by a clamp circuit (3), clipped by a white clip circuit (5) via a switch (4), and AM-modulated by an AM modulator (6). The audio signal is FM-modulated by the VCO (7). At this time, one resonator is selected by the switch (8) from the plurality of resonators according to the desired FM frequency. The audio FM signal converted into the desired FM signal frequency is converted into a high frequency by the up converter (9) and added by the adder (1
In 0), the video AM signal is added.

Therefore, a television signal converted into the RF band can be obtained at the output terminal (11). still,
A fixed-type oscillator (13) applies a reference signal of 500 KHz to the TSG (12) that generates a test video signal. In an area where multiple PAL broadcasts coexist, the frequency of the empty channel is different for each area, so when using one VTR in the multiple areas, it is necessary to check whether or not the image can be captured correctly on the selected empty channel. There is TS
G (12) is required.

[0004]

However, since the VCO (7) of FIG. 2 is of a self-oscillation type, there is a problem that the oscillation frequency is unstable and the frequency needs to be adjusted. To stabilize the oscillation frequency, V in Fig. 2
It is conceivable to arrange the CO (7) in the loop of the PLL, but the PLL circuit requires a reference oscillator and a resonator external to the oscillator (external to the IC), which increases the number of elements and the external IC. The increase in attached parts became a problem.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and is a voice signal modulation circuit capable of converting one voice signal into a plurality of voice FM signals having different frequencies. Of a VCO whose center frequency is switched according to the audio FM signal frequency, a fixed oscillator used in a test signal generator for generating a video signal as a test signal, and a signal obtained by modulating the oscillation output signal of the VCO. A programmable divider whose frequency division ratio is switched so that the frequency becomes equal to the output signal frequency of the oscillator, a phase comparator for comparing the frequency division output signal of the programmable divider and the output signal of the oscillator, and the phase comparison. Circuit for smoothing the output signal of the output device, an input terminal to which an audio signal for FM modulation is applied, an audio signal from the input terminal and an output DC voltage of the smoothing circuit Adding, voice F from the VCO and an adder means for applying a control signal to the VCO
It is characterized in that the M signal is obtained.

[0006]

According to the present invention, when the PLL circuit is used in the circuit of FIG. 2, the fixed oscillator for the test signal built in the RF modulator for PAL is used as a part of the PLL circuit. Therefore, the frequency can be stabilized with a relatively small number of elements. By adopting the PLL method, a VCO with high selectivity can be used, and a plurality of resonators are not required, which is particularly effective when integrated into an IC.

[0007]

FIG. 1 shows an embodiment of the present invention.
(14) is a BPF (16) whose center frequency is switched to the vicinity of a desired audio signal frequency in accordance with a control signal from a control circuit (15) for audio FM signal selection, an amplifier (17) and a capacitor (18). VCO, (19) is VCO
A programmable divider whose frequency division ratio is switched according to the control signal so that the oscillation output signal frequency of (14) becomes 500 KHz, (20) a phase comparator, (21) a smoothing circuit, and (22) an input terminal. A resistor for superimposing the audio signal (AC) from (23) on the DC voltage from the smoothing circuit (21), and (24) are output terminals from which an audio FM signal is obtained.

In FIG. 1, a VCO (14), a programmable divider (19), an oscillator (13), a phase comparator (20) and a smoothing circuit (21) constitute a PLL circuit, and a center frequency of the VCO (14). Is locked to the frequency of the oscillator (13) and is modulated by the audio signal. The oscillator (13) is a fixed type that uses a ceramic oscillator, and the oscillation is very stable. Therefore, VCO
In (14), stable oscillation is possible without using a resonator. The control circuit (15) switches the center frequency f ₀ of the VCO (14) and the frequency division ratio 1 / N of the programmable divider (19) according to the frequency of the desired audio FM signal. When the two are switched according to the relationship as shown in FIG. 3, the output signal frequency of the programmable divider (19) is always about 500 KHz. Therefore, the VCO (14) is controlled through the phase comparator (20) and the smoothing circuit (21), and stable PLL operation can always be continued.

Therefore, according to the circuit of FIG. 1, the input terminal (2
The audio signal added to 3) can be FM-modulated into a desired FM signal and output to the output terminal (24). Figure 4
Shows a concrete circuit of the VCO (14) in FIG. 1, whose center frequency can be adjusted by changing the gains of the operational amplifiers (25) and (26).

[0010]

As described above, according to the present invention, the voice F
Since the VCO for generating the M signal is arranged in the PLL loop, it becomes easy to change the oscillation frequency of the VCO and it becomes unnecessary to use the resonator. Particularly, according to the present invention, since the oscillator for TSG is used as the reference oscillator of the PLL loop, there is an advantage that an increase in the number of elements can be suppressed even if a PLL circuit is used.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an audio signal modulation circuit of the present invention.

FIG. 2 is a circuit diagram showing an RF converter including a conventional FM modulator.

FIG. 3 is a table used to explain FIG.

4 is a circuit diagram showing a specific circuit example of a VCO (14) shown in FIG.

[Explanation of symbols]

 (12) TSG (13) Oscillator (14) VCO (19) Programmable divider (20) Phase comparator (21) Smoothing circuit (22) Resistor

Claims (2)

[Claims] 1. A voice signal modulation circuit capable of converting one voice signal into a plurality of voice FM signals having different frequencies, wherein a VCO whose center frequency is switched according to a desired voice FM signal frequency, and a test signal Fixed oscillator used for the test signal generator for generating the video signal of the above, and the frequency division ratio is switched so that the frequency of the signal obtained by dividing the oscillation output signal of the VCO becomes equal to the output signal frequency of the oscillator. A programmable divider, a phase comparator for phase-comparing the divided output signal of the programmable divider and the output signal of the oscillator, a smoothing circuit for smoothing the output signal of the phase comparator, and an audio signal for FM modulation are applied. And an audio signal from the input terminal and an output DC voltage of the smoothing circuit are added and applied as a control signal to the VCO. Comprising a stage, a sound signal modulation circuit, characterized in that to obtain a sound FM signal from the VCO. 2. The VCO comprises: a BPF whose center frequency is switched; an amplifier for amplifying an output signal of the BPF; and feedback means for feeding back the output signal of the amplifier to the BPF to form an oscillation loop. The audio signal modulation circuit according to claim 1, wherein the oscillation frequency is easily changed.