JP2719467B2 - Audio signal modulation circuit - Google Patents

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 a television signal in an RF band.
In particular, the present invention relates to an audio signal modulation circuit suitable for use in a case where audio FM signals of a plurality of frequencies exist, such as the PAL system.

[0002]

2. Description of the Related Art Sound F of television broadcasting of the PAL system
The M signal frequency varies from region to region. For example, the frequencies include 4.5 MHz, 5.5 MHz, 6.0 MHz, and 6.5 MHz. When there are a plurality of frequencies as described above, a plurality of VCO resonators constituting the FM modulator are required. FIG. 2 shows an RF modulator provided with such an FM modulator. A video signal is generated from a first signal source (1), and an audio signal is generated from a 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 of the plurality of resonators is selected by the switch (8) in accordance with a desired FM frequency. The audio FM signal converted to the desired FM signal frequency is converted to a high frequency by the up-converter (9), and is added to the adder (1).
0) is added to the video AM signal.

Accordingly, a television signal converted into an RF band can be obtained at the output terminal (11). still,
A 500 KHz reference signal is applied from a fixed oscillator (13) to a TSG (12) that generates a test video signal. In an area where multiple PAL broadcasts coexist, the frequency of the empty channel differs for each area, so when using one VTR in multiple areas, it is necessary to check whether the image is correctly displayed on the selected empty channel There is a TS
G (12) is required.

[0004]

However, since the VCO (7) in FIG. 2 is of a self-oscillation type, the oscillation frequency is unstable and the frequency must be adjusted. To stabilize the oscillation frequency, V
Although it is conceivable to dispose the CO (7) in the loop of the PLL, the PLL circuit requires a reference oscillator and a resonator external to the oscillator (external to the IC). An increase in the number of attached parts became a problem.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and is an audio signal modulation circuit capable of converting one audio signal into a plurality of audio FM signals having different frequencies. VCO whose center frequency is switched according to the frequency of the audio FM signal, a fixed oscillator used for a test signal generator for generating a video signal as a test signal, and a signal obtained by dividing 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 phase of the frequency-divided output signal of the programmable divider with the output signal of the oscillator; A smoothing circuit for smoothing the output signal of the filter, an input terminal to which an audio signal to be subjected to 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 M signals are obtained.

[0006]

According to the present invention, when a PLL circuit is used in the circuit of FIG. 2, a fixed oscillator for a test signal built in a PAL RF modulator 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 system, a VCO with high selectivity can be used, and a plurality of resonators are not required, which is particularly effective when implementing an IC.

[0007]

FIG. 1 shows an embodiment of the present invention.
(14) a BPF (16) whose center frequency is switched to near a desired audio signal frequency in accordance with a control signal from a control circuit (15) for audio FM signal selection, and an amplifier (17) and a capacitor (18). VCO (19) is a VCO
(14) A programmable divider whose frequency division ratio is switched according to the control signal so that the oscillation output signal frequency becomes 500 KHz, (20) is a phase comparator, (21) is a smoothing circuit, and (22) is an input terminal. A resistor for superposing the audio signal (AC) from (23) on the DC voltage from the smoothing circuit (21), and (24) is an output terminal 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 have a center frequency of the VCO (14). Locks to the frequency of the oscillator (13) and is modulated by an audio signal. The oscillator (13) is of a fixed type using a ceramic vibrator, and the oscillation is very stable. Therefore, VCO
In (14), stable oscillation can be achieved 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 both are switched in 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 via the phase comparator (20) and the smoothing circuit (21), and a 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 to a desired FM signal, and can be output to an output terminal (24). FIG.
Shows a specific circuit of the VCO (14) in FIG. 1, and its 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 that generates the M signal is arranged in the PLL loop, it is easy to change the oscillation frequency of the VCO, and it is not necessary to use a resonator. In particular, according to the present invention, since an oscillator for TSG is used as a 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 the drawings]

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

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

FIG. 3 is a table provided for description of FIG. 1;

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

[Explanation of symbols]

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

Claims (2)

(57) [Claims] An audio signal modulation circuit capable of converting one audio signal into a plurality of audio FM signals having different frequencies, a VCO whose center frequency is switched according to a desired audio FM signal frequency, and a test signal oscillator and the desired programmable divider that frequency dividing ratio is switched in accordance with the audio FM signal frequency, the programmable divider of the divided output signal of the test signal generator oscillation frequency fixed type used to generate a video signal A phase comparator for comparing the phase of the output signal of the oscillator with the output signal of the oscillator; a smoothing circuit for smoothing the output signal of the phase comparator; an input terminal to which an audio signal to be FM-modulated is applied; and an audio signal from the input terminal And an adding means for adding the output DC voltage of the smoothing circuit and applying the control signal to the VCO as a control signal, and obtaining a voice FM signal from the VCO. Audio signal modulation circuit, characterized in that the the like. Wherein said VCO includes a BPF which the center frequency is changed, an amplifier for amplifying the output signal of the BPF, a feedback means for the output signal of the amplifier fed back to the BPF, to form an oscillation loop, the audio signal modulation circuit according to claim 1, characterized in that it comprises.