JPH0114750B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an audio receiving circuit for a television receiver.

The intercarrier system is generally used for audio reception in current television receivers. This method is adopted because the configuration of the audio receiving circuit is simple and can be realized at low cost, and the audio signal can be sufficiently demodulated even if the local oscillation frequency of the tuner deviates from the normal value. However, on the other hand, this type of audio receiving circuit has a drawback in that it is susceptible to the influence of video signals and tends to generate so-called buzz.

There are various causes of buzz, but they can be broadly classified into the following four types. That is, the intercarrier signal undergoes phase modulation called differential phase (DP) by the video signal, and the intercarrier signal undergoes amplitude modulation called differential gain (DG) by the video signal. This is due to the harmonics of the video signal being mixed into the intercarrier signal band, so-called cross sound, and the video detection output being mixed into the demodulated audio signal. Among these, the intercarrier signal is directly affected by the video signal, and this is called an intercarrier buzz.
Since DG of this intercarrier buzz is the AM modulation component of the intercarrier signal, it can be almost completely eliminated by setting the amplitude modulation suppression ratio of the limiter of the audio demodulation circuit to a sufficiently large value. but,
Since the DP of is a PM modulation component of the intercarrier signal, and the cross sound component of is a signal of the intercarrier band, it is difficult to remove these.

For this reason, when high-quality audio signals are required, such as in TV audio multiplex receivers, it is better to use the separate carrier method audio receiving circuit, which is less likely to cause intercarrier buzz, than to use the intercarrier method described above. This means that it is more suitable than a circuit.

Therefore, the present invention proposes a new audio receiving circuit that can fully enjoy the advantages of the separate carrier system and eliminates the drawbacks of this system. The details will be explained below with reference to the drawings.

FIG. 1 shows the schematic configuration of the main parts of a television receiver that employs the audio receiving circuit of the present invention.
2 is a video signal system, and 3 is an audio signal system.

The tuner 1 is composed of a high frequency amplifier circuit 4, a mixer circuit 5, a first local oscillation circuit 6, and an AFT control circuit 7 for controlling the oscillation frequency, similar to a conventional receiver. There are no characteristics.

The video signal system 2 includes a video IF (intermediate frequency) filter 8, a video IF amplification circuit 9, a video detection circuit 10, and an intercarrier trap 11.
Although the configuration includes an AGC circuit 12 and an AGC delay circuit 13, there is no difference from a conventional receiver except that the band selection characteristics of the video IF filter 8 are selected as shown in FIG. That is, the audio IF component of the output of the mixer circuit 5 in the tuner is sufficiently attenuated by the filter 8 before being introduced into the video IF amplifier circuit 9.

Note that the first AGC circuit 12 is the same as the video detection circuit 1.
Obtain an output of 0 and create the AGC control voltage, and use this
The gain of the video IF amplifier circuit 9 is controlled by the AGC voltage,
Further, the AGC delay circuit 13 delays the AGC voltage until the start of gain control of the high frequency amplifier circuit 4 in the tuner, but this is the same as before.

On the other hand, the audio signal system 3 basically consists of an audio IF filter 1 having band selection characteristics as shown in FIG.
4 extracts an audio IF signal from the output of the mixer circuit 5, converts the frequency of this signal, and performs FM detection to obtain an audio signal.
The following points are featured here:

First, the AFT detection circuit 16 detects the phase of the audio carrier wave (54.25MHz) being output from the first audio IF amplification circuit 15 that amplifies the audio IF signal, and converts the obtained DC voltage to the AFT in the tuner. The reason for this is that the oscillation frequency of the first local oscillator 6 is controlled in accordance with the frequency shift of the audio carrier wave by applying it to the control circuit 7, and the reason is as follows.

In other words, the phase of the video carrier wave is detected as in the conventional method,
In the method of AFT controlling the tuner's local oscillator using this detection output, the filter for extracting the video carrier wave cannot be made narrow enough, so in addition to the audio carrier wave, the filter output contains extremely high frequencies such as the vertical frequency and its harmonics. Contains sideband components due to low frequency video signals. Therefore, a detection output based on the video side band component appears in the phase detection circuit, which causes the local oscillation frequency of the tuner to fluctuate. This means that the audio IF signal is phase modulated by the video signal component.
This is because a buzz will therefore occur.

Note that the pull-in range of AFT in FIG. 1 may be selected to be approximately (54.25±10) MHz.

The second option is to enable synchronous detection circuits, etc.
An AM detection circuit 17 is provided, and the output of the first audio IF amplifier circuit 15 and the output (64.95MHz) of the second local oscillation circuit 18 are supplied to this detection circuit, and thereby the detection circuit 17 generates an AGC voltage. The main difference is that it can be used as a wave detector for audio IF signals and as a mixer for frequency conversion of audio IF signals. That is, since the audio signal is an FM modulated wave, if it is directly detected by the AM detection circuit 17, a DC voltage proportional to the received electric field strength of the audio signal can be obtained as the detection output. Furthermore, since the detection circuit 17 operates as a mixer as described above, the first audio IF signal (center frequency: 54.25MHz) is output from the detection circuit 17.
This includes a second audio IF signal component of the beat frequency (center frequency: 10.7MHz) of the second local oscillation signal (64.95MHz) and the second local oscillation signal (64.95MHz).

Therefore, the DC component in the output of the detection circuit 17 is extracted by an appropriate low-pass filter in the second AGC circuit 19, thereby controlling the gain of the first audio IF amplifier circuit 15. At the same time, the second audio IF signal component in the detected output is extracted by a second audio IF filter 20 made of a ceramic filter, etc., and passed through a second IF amplifier circuit 21 and a limiter circuit 22, and then passed through an audio demodulation circuit. 23
FM detection is performed to obtain the audio signal.

Here, the AGC control of the first audio IF amplifier circuit 15 is to obtain the second audio IF signal with a substantially constant amplitude regardless of the received electric field strength and the video carrier level to audio carrier level ratio (P/S). It's for a reason. Furthermore, the reason why the output of the first audio IF amplification circuit 15 is not directly FM-detected, but is converted into a second audio IF signal and then FM-detected is because of the frequency shift of the IF signal with respect to the carrier wave. This is to increase the detection sensitivity during FM detection by increasing the degree.

Furthermore, thirdly, the output of the audio demodulation circuit 23, that is, the average DC level of the audio signal, is detected by an appropriate integrating circuit in the AFC circuit 24, thereby controlling the oscillation frequency of the second local oscillation circuit 18. This is what I decided to do. That is, since the average DC level of the demodulation circuit 23 changes when the center frequency of the second audio IF signal deviates from the normal value of 10.7MHz, the second local oscillation frequency is adjusted accordingly.
This is because AFC control allows the frequency deviation of the audio IF signal to be corrected, and therefore FM detection can be performed reliably. In this case, the AFC pull-in range may be selected to be approximately (64.95±0.3) MHz.

As explained above, the audio receiving circuit of the present invention has the following features: (a) Directly extracts the audio IF signal from the tuner output using a narrow band filter, converts this signal into a low frequency band, and demodulates the audio signal from the output. (b) The first and second local oscillators are operated independently, so the audio IF signal system itself is not directly affected by the video signal.
Because of AFC control, the center frequency of the second audio IF signal can always be made to match that of the audio demodulation circuit, without any influence of fluctuation components such as temperature drift of the second local oscillator on the first local oscillator.

(c) Output of the audio IF amplification circuit, that is, sufficient P/
Since the local oscillation frequency of the tuner is controlled according to the phase detection output of the audio IF signal having the S ratio, the oscillation frequency does not drift due to the video signal component.

Therefore, these synergistic effects can significantly reduce the buzz caused by the various causes mentioned above.

In addition, a single unit is used for the aforementioned low frequency conversion and AGC control.
When the AM detection circuit is also used, the circuit configuration becomes simple, and it is suitable as an audio receiving circuit for a TV audio multiplex receiver or the like.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a schematic configuration of a television receiver employing the present invention, and FIGS. 2 and 3 are diagrams showing the characteristics of a video IF filter and an audio IF filter used therein, respectively. 1 : Tuner, 2 : Video signal system, 3 : Audio signal system.

Claims (1)

[Scope of Claims] 1. An audio IF amplification circuit is independently provided in which the tuner output is introduced through a narrowband filter, and a second local oscillator is provided separately from the first local oscillator in the tuner. , a television receiver further comprising means for converting the output of the audio IF amplifier circuit to a low frequency range using the output of the second local oscillator, and means for performing FM detection on the low frequency converted output, the timbre IF amplifier circuit means for controlling the oscillation frequency of the first local oscillator according to the phase detection output of the audio carrier extracted from the FM detection; and means for controlling the oscillation frequency of the second local oscillator according to the DC level of the output after the FM detection. An audio receiving circuit for a television receiver comprising: