JPS62136979A - Television tuner selecting sound demodulation system freely - Google Patents

Abstract

PURPOSE:To reproduce a voice signal with high S/N by detecting the phase modulation to a reception broadcast signal in detecting phase deviation of a video IF signal of a prescribed frequency, demodulating a sound IF signal by the intercarrier system and demodulating other signals by the separator carrier system. CONSTITUTION:The 3rd sound IF signal demodulated by the intercarrier system is fed to a normally ON contact of a relay switch 26 and the 2nd sound IF signal demodulated by the separate carrier system is fed to a normally OFF contact. When a reception broadcast signal is subject to phase modulation, since the sound IF signal is subject to phase modulation, it is detected by a phase difference detector 25 to operate the relay switch 26, the 3rd sound IF signal demodulated by the intercarrier system is FM-detected to obtain a sound signal. When the reception broadcast signal is not phase-modulated, the relay switch 26 is not activated, the 2nd sound IF signal demodulated by the separate carrier system is FM-detected to obtain a sound signal. Thus, a sound IF signal with better S/N in both the sound IF signal is always selected automatically.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a television tuner equipped with an audio demodulation circuit using different demodulation methods.

111'' A conventional example of an audio demodulation circuit for a television tuner that can perform two different demodulation methods will be described with reference to FIG. First, a broadcast signal desired to be received is selectively amplified by the front end 1 from a high frequency signal induced in an antenna (not shown), and converted to an intermediate frequency (hereinafter referred to as IF) to obtain an IF multiplied signal. This IF multiplied signal video signal and a video signal that is a video carrier signal that includes color video information such as a color signal and a first audio IF signal that is an audio carrier signal that includes audio information such as an audio multiplex signal and It is supplied to audio filter 2 and audio filter 3.

The video and audio filter 2 uses surface acoustic waves (hereinafter referred to as SAW).
Formed by filters etc., the above image! Only the 1llF signal and the first audio IF signal are passed.

Each signal that has passed through the video and audio filter 2 is amplified by an intermediate frequency amplification circuit 4 and supplied to a video detector 5, where a composite video signal is demodulated. This composite video signal includes the video IF signal (for example, 58.75MH2) and the first audio IF signal (
For example, the frequency difference with 54.25M1-(Z) is 4.5MH
A third audio IF signal equivalent to 2 is included, and is supplied to one input terminal A of the audio blocking filter 6 and the selection switch 7. The audio rejection filter 6 is a 4.5 MH2 band rejection filter for removing the third audio IF signal component from the composite video signal, and obtains only the composite video signal.

On the other hand, the first audio IF signal is supplied to one input end of the frequency mixing circuit 8 through the audio filter 3 that allows only the first audio IF signal to pass. A local oscillator 9 supplies a local oscillator signal with a fixed frequency 4.5 MH2 higher than the first audio IF signal to the other input terminal of the mixing circuit 8 . The mixing circuit 8 obtains a second audio IF signal of 4°5 MH2 and supplies it to the other input terminal B of the selection switch 7. The selection switch 7 is a manual switch and relays either the second or third audio IF signal to the audio filter 10. The audio filter 10 is a 4.5 MH2 band-pass filter, which blocks the passage of the composite video signal supplied together with the third audio IF signal, and allows only both 4.5 MH2 audio IF signals to pass. This audio IF signal is transmitted to intermediate frequency amplifier circuit 1.
1, the signal is amplified and amplitude limited, and then supplied to an audio detector 12 where FM detection is performed and the audio signal is demodulated.

In this configuration, the audio IF signal supplied to the input end A side of the selection switch 7 is a demodulated signal using the so-called intercarrier method, and the signal supplied to the input end B side of the selection switch 7 is demodulated using the so-called separate carrier method. It's a signal.

According to the intercarrier method, even if the received signal undergoes phase modulation due to interference waves, the video IF signal and the first I
Since the F-fold signal is modulated while maintaining a frequency difference of 4.5 MH2, this phase modulation has little influence on the second audio IF signal of this difference component. However, there is a problem that a so-called bass sound occurs in the audio signal due to the asymmetry of both sidebands of the video signal, and good sound quality cannot be obtained. On the other hand, in the separate carrier system, FM
Since it is configured in the same way as a broadcast receiver, it does not have such problems and can provide good sound quality, but if the received broadcast signal is phase modulated or has phase distortion, the sound quality deteriorates.

Therefore, we made it possible for viewers to switch the selection switch according to the situation and select the one with less phase distortion and higher signal-to-noise ratio. It is difficult to understand the selection switch, and it is troublesome to operate such a switch one by one, so it is not possible to utilize the selection switch and always enjoy high quality audio.

1g Therefore, an object of the present invention is to provide a television tuner that can automatically select an audio demodulation method according to the condition of a received broadcast signal and reproduce the audio signal at a higher SN ratio. It is.

In order to achieve the above object, the television tuner according to the present invention detects that the received broadcast signal has undergone phase modulation by detecting the phase deviation of the video IF signal of a constant frequency, and in this case, the audio IF signal is The configuration is such that demodulation is performed using a carrier method, and when the carrier method is not used, demodulation is performed using a separate carrier method.

Friend-1-1 Hereinafter, one embodiment of the present invention will be described with reference to FIG. In FIG. 1, parts corresponding to those shown in FIG. 2 are denoted by the same reference numerals, and a description of those parts will be omitted.

In FIG. 1, the video IF signal and the first audio IF signal output from the intermediate frequency amplification circuit 4 are supplied to a video synchronous detector 20 of synchronous detection type. Further, the video IF signal is supplied to one input terminal of the phase comparator 21. The other input terminal of the phase comparator 21 receives a signal from the VCO 23 at a frequency equal to the carrier signal of the video IF signal (for example, 58.
75MH2) output signal is supplied. Phase comparator 21
obtains a PWM signal with a pulse width corresponding to the phase difference between both signals, and supplies it to the low-pass filter 22 and the phase difference detector 25. This PWM signal is smoothed by the low-pass filter 22 and becomes a control signal for frequency control of vC023! Il
Supplied to the l end. The VCO 23 changes its phase in accordance with the control signal and oscillates in synchronization with the carrier signal, and uses the output signal as a switching signal for the video synchronous detector 20.
and the other input terminal of the comparator 21.

Phase comparator 21, low pass filter 22 and VC023
forms a PLL circuit. It is well known that the video synchronous detector 20 uses a multiplier configured as a double-balanced differential amplifier. When the PWM signal is generated, or when the pulse width of the PWM signal exceeds a predetermined value, the phase difference detector 25 issues a switching command to the relay switch 26, which is a signal selection circuit, and the video synchronous detector 20 If not, the output signal of the mixer 8 is relayed to the audio filter 10. Here, P.L.
The L circuit 24 and the phase difference detector 25 constitute phase deviation detection means. Other configurations are similar to the conventional example.

Next, the operation of the circuit will be explained. First, the normally closed contact of the relay switch 26 is supplied with the third audio IF signal demodulated using the intercarrier method, and the normally closed contact is supplied with the second audio IF signal demodulated using the separate carrier method.
signal is supplied. When the received broadcast signal is phase modulated, the audio IF signal is also phase modulated, so the phase difference detector 25 detects this and operates the relay switch 26 to output the third demodulated signal using the intercarrier method. The audio IF signal is subjected to FM detection to obtain an audio signal. When the received broadcast signal is not subjected to phase modulation, the relay switch 26 is not activated and the second audio IF signal demodulated by the separate carrier method is subjected to FM detection to obtain an audio signal. Therefore, of the two audio IF signals, the one with a better SN ratio is always automatically selected.

In the embodiment, a PLL circuit is used to detect the phase difference and obtain a switching signal for the video synchronous detector, but the phase deviation detection means is not limited to this, and may be a so-called Hege 0 dyne method. The phase difference may also be detected using
When a phase fluctuation of the F signal carrier is detected, the audio IF signal is automatically demodulated using the intercarrier method, so noise due to phase distortion is suppressed and reproduced audio with a low SN ratio is always obtained. It is preferable.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram showing a conventional example. Explanation of symbols of main parts 4...Intermediate frequency amplifier 21...Phase comparator 22...Low pass filter 23...vCO 25... phase difference detector

Claims (1)

[Claims] A filter that separates only the first audio carrier signal from an intermediate frequency signal consisting of a video carrier signal containing color video information and a first audio carrier signal containing audio information; a mixing circuit that mixes the signals to obtain a second audio carrier signal, an intermediate frequency amplification circuit that amplifies the intermediate frequency signal, and obtains a third audio carrier signal and a video signal from the output signal of the intermediate frequency amplification circuit. A television tuner comprising a video detection circuit and a signal selection circuit that selects one of the second and third audio carrier signals and supplies the selected one to the audio detection circuit, the television tuner comprising: a phase deviation of the video carrier signal; A television tuner comprising phase deviation detection means for causing the signal selection circuit to select a third audio carrier signal when detecting the third audio carrier signal.