JPS6130797B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention provides a separate carrier method (split carrier method) for transmitting audio signals of television broadcasting.
Regarding the configuration of a television audio receiving device that receives double super at
The object of the present invention is to provide a device that can reproduce high-quality audio by eliminating the generation of buzz caused by mixed modulation of video and audio signals during strong electric fields.

Conventional television receivers generally receive audio signals using an intercarrier system. The intercarrier method has advantages in terms of stability of audio reception, but in terms of sound quality, since the carrier consists of the beat components of the audio and video signals, it cannot avoid the influence of the video signal and may cause disturbances called buzz sounds. occurs. There is a separate carrier system as a reception system that eliminates this buzz component. This method extracts only the audio signal from the television signal, which consists of a video signal and an audio signal, so theoretically it does not generate buzz. However, during reception in a strong electric field, cross-modulation may occur in the tuner's high-frequency amplification stage, mixing circuit, etc., and video components may be mixed into the audio signal, causing buzz.

In addition, the audio intermediate frequency of general TV tuners is as high as 54.25MHz, making signal processing difficult.
A method is often used in which the signal is further converted to a second intermediate frequency of 10.7MHz, which is used in FM stereo receivers, using a double super. However, by setting the intermediate frequency to 10.7MHz, intermediate frequency amplification and FM
It has the great advantage of being able to share detection and other functions with an FM stereo receiver, but has the disadvantage that even the mixing circuit that converts 54.25MHz to 10.7MHz generates buzz during reception due to strong electric fields.

Therefore, the present invention aims to solve the problem of slow motion that occurs when receiving a strong electric field in the double super type receiving device as described above.
By applying AGC, the buzz sound caused by cross modulation is also eliminated, and a device is provided that realizes the reproduction of sound without buzz sound, which is an advantage of the separate carrier method.

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing its configuration. 1st
In the figure, 1 is an antenna, 2 is a tuner, and includes a high frequency amplification section 3, a first mixing circuit 4, and a first local oscillation circuit 5. The audio signal received by the tuner 2 is converted to a first intermediate frequency of 54.25MHz and output.

This output is selected and amplified by the first intermediate frequency amplification circuit 6, and then mixed with the output of the second local oscillation circuit (64.95MHz oscillation) 8 by the second mixing circuit 7.
Convert to an intermediate frequency of 10.7MHz. The second intermediate frequency signal converted to 10.7MHz is selected and amplified by the second intermediate frequency amplification circuit 9, and then passed through the limiter 10.
It is detected by the FM detection circuit 11. The detected signal is de-emphasized by a de-emphasis circuit 12,
After being low-frequency amplified by the low-frequency amplification circuit 13, the signal is output to the output terminal 14.

15 and 16 are circuits for applying slow-acting AGC, 15 is a level detection circuit that detects the level of the second intermediate frequency signal, and 16 is an AGC voltage increaser that generates and amplifies a slow-acting AGC voltage from the detected level. It is a width circuit.

By the way, in a television receiver, the video signal is
Since it is AM modulation, the detection output changes depending on the strength of the electric field. Therefore, AGC is applied to the IF signal using a low electric field to control the intermediate frequency output to be constant. In addition, as a countermeasure against cross-modulation when the tuner has a strong electric field, an AGC that starts operating when the input electric field becomes strong is applied to the high-frequency stage of the tuner.

On the other hand, with this receiver, the audio signal is FM
Since it is a signal, there is no need for an AGC that operates from a low electric field as described above, and an AGC that is applied to an intermediate frequency stage like the RF-AGC shown in Figure 2 is required.
It is also a slow-acting type that starts operating only after the electric field becomes strong.
AGC is good. However, the RF as shown in c in Figure 2
- If the IF level cannot be suppressed sufficiently by AGC alone and the electric field becomes strong, the input to the second mixing circuit 7 becomes large, and cross modulation is likely to occur here. Therefore, in this device, by supplying an AGC voltage having a rise similar to that of RF-AGC as shown in FIG. The IF level can be effectively suppressed even in a strong electric field, and cross-modulation in the second mixing circuit 7 can be eliminated. Note that FIG. 3 is a diagram for forward AGC, and it goes without saying that the polarity of the AGC voltage is reversed in reverse AGC. In addition, although the rising edges of a and b are made the same in Figure 2, it is not necessary to make them the same, and depending on the situation where cross modulation occurs, the RF
The rising edge may be different for AGC and IF-AGC.

FIG. 3 shows a specific circuit example of the level detection section 15 and the AGC voltage generation section 16. This circuit is applied to the high frequency amplification section 3 and the first intermediate frequency amplification section 6.
AGC is shown as forward AGC. Second
The output of the intermediate frequency amplifier circuit 9 is peak-detected by a diode 17, a capacitor 18, and a resistor 19.
Convert to DC voltage. As this output increases as the input increases, the DC voltage also increases, so the transistor 2
Invert and amplify at 0. The variable resistor 21 is for setting the rising point of the AGC voltage, and when the input electric field is weak, the transistor 22 is turned on and a voltage obtained by dividing the +B voltage by the resistors 23, 24, and 25 is supplied. In this case, the high frequency amplification section 3 and the second intermediate frequency amplification section 6 are
Set the AGC characteristics of. When the input electric field increases, the voltages at points a and b rise as shown in FIG. .

As described above, according to the present invention, when the audio signal of television broadcasting is received by double super in a separate carrier system, the difference between the video signal and the audio signal that occurs in the high frequency amplification stage and the second mixing circuit of the tuner during a strong electric field. Both cross-modulation can be effectively suppressed. In particular, in the second mixing circuit, the input level is amplified by the tuner and is likely to cause cross modulation, but by applying the AGC voltage to the intermediate frequency amplifier circuit in the previous stage of the second mixing circuit, The input level to the second mixing circuit can be prevented from increasing even in the case of a strong electric field, and an excellent television sound receiving device can be realized in which less buzz noise is generated due to cross modulation.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a television audio receiving device according to an embodiment of the present invention, FIG. 2 is a characteristic diagram explaining the operation of the device, and FIG. 3 is a specific circuit diagram of the main parts of the device. . 2... tuner, 3... high frequency amplification section, 4...
First mixing circuit, 5...First local oscillation circuit, 6
...First intermediate frequency amplification circuit, 7...Second mixing circuit, 8...Second local oscillation circuit, 9...Second intermediate frequency amplification circuit, 15...Level detection circuit, 16
...AGC voltage amplification circuit.

Claims (1)

[Claims] 1 a tuner that receives a television broadcast signal; a first tuner that mixes the output of this tuner with a first local oscillation signal to obtain a first audio intermediate frequency signal;
a mixing circuit and a second audio intermediate frequency signal;
a second mixing circuit which obtains a second audio intermediate frequency signal by mixing with the local oscillation signal of the audio intermediate frequency signal; and a demodulation circuit which processes the second audio intermediate frequency signal by at least an FM detection circuit and a low frequency amplification circuit. and a means for detecting the level of the second audio intermediate frequency signal, and a means for detecting the level of the second audio intermediate frequency signal;
means for generating an AGC voltage, a first audio intermediate frequency amplification stage provided before the second mixing circuit, one of the two slow-acting AGC voltages to the first high frequency amplification stage of the tuner; a television audio receiving apparatus, comprising: means for adding the first audio intermediate frequency amplification stage to the first audio intermediate frequency amplification stage.