JPS6365195B2 - - Google Patents

Description

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

In current television receivers, the AFT voltage for controlling the local oscillation frequency of the tuner is created from the VIF (video intermediate frequency) carrier signal, and the demodulation of the audio signal is performed using the SIF (audio intermediate frequency) signal and the video carrier signal. Generally, this is done by obtaining an intercarrier signal based on the beat of .
However, this method requires a frequency discrimination circuit for creating the AFT voltage and an FM detection circuit for audio demodulation, and also requires the generation of a "bus", which is a drawback of the intercarrier method.

Therefore, by taking these points into consideration, the present applicant has proposed the following method in the patent application filed in 1983-
Proposed in issue 34470. That is, a separate carrier type SIF signal derived from a tuner is guided to a single frequency discrimination (FM detection) circuit to perform frequency discrimination, and an audio signal and an AFT voltage are obtained from the discrimination output.

FIG. 1 shows a schematic configuration of the main parts of a television receiver adopting such a system, and the circuits indicated by blocks in the figure perform the operations described in each block. The following points are of interest to me. That is, in the frequency discrimination circuit 3 shown in the figure, the linear detection area W (see FIG. 3) is
SIF derived from tuner 1 (center frequency: 54,
25MHz) signal frequency deviation range (±25KHz), it is selected to be sufficiently wide (approximately ±500 to 700KHz).

If the linear detection area of the frequency discrimination circuit 3 is selected in this way, if the center frequency of the SIF signal deviates from the normal value mentioned above, such as when the tuner is detuned (immediately after turning on the power or immediately after changing the channel), , the operating point of the discriminator circuit moves from point A to point B or C in FIG. 3, but does not leave the linear detection region. This means that the discrimination operation is performed well even in the above-mentioned case, and therefore, especially with regard to audio demodulation, as with the intercarrier method described above, there will be no loss of audio or distortion when the tuner is detuned. This means that it does not occur.

However, on the other hand, when the operating point of the discrimination circuit 3 moves from point A to point B or C in FIG. 3, the following problem occurs. That is, generally at the midpoint of the frequency discrimination circuit's operation (point A in Figure 3),
It is thought that there is some AM suppressing effect, so
If the center frequency of the SIF signal exactly matches this midpoint of operation, a discrimination output (that is, an audio demodulation output) from which noise has been removed will be obtained.
Further, even if the noise is not completely removed, in this case, the noise N appearing in the discrimination output is added vertically symmetrically as shown in FIG. 5a, so that it is difficult to perceive as noise. but,
If the operating point shifts to point B or point C in FIG. 3, the balance of the AM suppression effect of the frequency discrimination circuit 3 will be lost. This also applies to peak differential detectors that have a particularly high degree of AM suppression.
Therefore, in this case, a large amount of noise N appears in the discrimination output.
As can be seen from the characteristics in the figure, when the deviation is to point B, it appears largely on the negative side of the discrimination output as shown in Figure 5b,
If it deviates to point C, it will appear largely on the positive side of the discrimination output as shown in c in the same figure, so these will be perceived as large noise.

However, the present invention has been made to eliminate such drawbacks, and an embodiment of the present invention shown in FIG. 2 will be described below.

The circuit in Figure 2 consists of the part surrounded by the broken line in Figure 1.
This shows an example of an IC implementation, and parts corresponding to the blocks in FIG. 1 are given the same figure numbers. In the circuit shown in Figure 2, the SIF signal (center frequency 54, 25MHz) is introduced into the terminal _P1 of the IC,
This SIF signal is guided to a frequency discrimination (FM detection) circuit 3 .

The frequency discrimination circuit 3 includes transistors T ¹ and T ₂
A buffer amplification stage 3a consisting of a transistor, etc.
It is configured as a peak differential type with a differential discrimination stage 3b consisting of T ₃ to T ₅ , etc., and a 180° phase shifter consisting of an external coil L ₁ and capacitors C ₁ and C ₂ . The output is T ₃ of the differential discrimination stage 3b,
Each collector of T ₄ is led to the next stage AFT voltage extraction circuit 4 via common base type transistors T ₆ and T ₁₁ , respectively. and,
The values of the capacitors C ₁ , C _{2 ,} etc. are selected so that the width of the linear detection area (W in FIG. 3) of the discrimination circuit 3 is, for example, about ±700 KHz as described above.

Note that an external capacitor _C9 is connected to the collector (terminal _P2 ) of T3 of the differential discrimination stage _3b , and this capacitor and the resistor _R9 on the emitter side of _T6 provide the following It suppresses the carrier component in the discrimination output, and also performs a high frequency de-emphasis effect on the audio signal component of the discrimination output. Further, a capacitor _C4 is connected to the collector of the other _T4 of the differential discrimination stage 3b, and the carrier component in the discrimination output is suppressed by this capacitor and resistor _R16 . that time,
Since C ₄ and R ₁₆ do not perform a de-emphasis effect on the audio signal, C ₄ has a small capacity formed by the internal capacity of the IC, unlike C ₉ described above. Further, the capacitor _C3 is also used for suppressing carrier components.

The AFT voltage extraction circuit 4 includes a first, a second, a third
It consists of current mirror circuits 4a, 4b, 4c, resistors Ra, Rb and capacitor _C5 connected to terminal _P6 . Then, the input side transistors T ₈ and T ₁₂ of the first and second current mirror circuits 4a and 4b
is connected to the differential discrimination stage 3 via the aforementioned T ₆ and T ₁₁ , respectively.
connected to each collector of T ₃ and T ₄ of b, and
The output transistor T ₁₀ of the second current mirror circuit 4b is connected to the input transistor T ₁₅ of the third current mirror circuit 4c. Therefore, if the values of the resistors R ₁₃ , R ₁₄ and R ₁₇ to _{R 21} of each current mirror circuit are all equal, then the second current mirror circuit 4b
A current equal to the collector current of one T4 of the differential discrimination stage _3b flows through the output side transistor _T13 of the third current mirror circuit 4c, and the output side transistor _T17 of the third current mirror circuit 4c flows.
A current equal to the collector current of the other _T3 of the differential discrimination stage flows through. As a result, its T ₁₃ , T ₁₇
A DC voltage corresponding to the current difference will be generated at the terminal _P6 , and this voltage will be taken out as the AFT voltage.

Here, when creating the AFT voltage, the AC component in the discrimination output from the differential discrimination stage 3b, that is, the audio signal component, is smoothed by the capacitor _C5 , so that the AFT voltage obtained at the terminal P6 is smoothed by the capacitor _C5 . The voltage changes depending on the DC component of the discrimination output, that is, the carrier frequency of the SIF signal. Here, the width W' of the linear variation region of the AFT voltage characteristic shown in Fig. 4 is narrower than that in Fig. 3 (for example, ±
(approximately 200 KHz), and the slope of the linear variation region is selected to be steep. This is to prevent the AFT sensitivity from decreasing due to the wide selection of the linear detection area W (Fig. 3) of the frequency discrimination circuit 3 , and such characteristic switching
This can be easily achieved by setting the values of the external resistors Ra and Rb connected to _P6 , but since such a point is outside the gist of the present invention, detailed explanation will be omitted.

Next, the audio amplification circuit 5 includes an emitter follower stage 5a consisting of a transistor T18 whose base receives the collector output of the other output side transistor _T7 of the _first current mirror circuit 4a, and a transistor _T18 ' which operates as its load. , a differential amplification stage 5b consisting of transistors T ₁₉ , T ₂₀ and their constant current transistor T ₂₁ , whose emitter follower outputs are applied to each base via resistors R ₂₄ , R ₂₅ having the same resistance value, and the differential amplifier stage 5b. One side of dynamic pair transistor T ₂₀
It consists of an external capacitor _C6 connected to the base of the audio frequency component for attenuation of audio frequency components. Therefore, the discrimination output taken out by _T7 of the first current mirror circuit 4a is transmitted to the emitter follower stage 5.
When guided to the differential amplification stage 5b through a,
The DC component of the discrimination output is applied to the bases of T ₁₉ and T ₂₀ by the resistors R ₂₄ and R ₂₅ , but the AC component, that is, the audio signal component, is applied to the base of T ₁₉ due to the capacitor C ₆ . only is applied. the result,
The differential amplification stage 5b performs amplification only on the alternating current component of the discrimination output, so even if the operating point of the frequency discrimination circuit 3 moves as shown in FIG. An audio signal output whose DC level is maintained at a constant potential is obtained from each collector of T ₁₉ and T ₂₀ of the dynamic amplification stage 5b. Then, the audio signal obtained from T ₁₉ is guided to the noise detection circuit 7 , and the audio signal obtained from T ₂₀ is guided to the gain control circuit 6 .

The gain control circuit 6 controls the differential amplification stage 5b.
A first differential pair of transistors T ₂₂ and T ₂₃ forming a shunt path for the collector current of T ₂₀ and a transistor T ₂₆
A second differential pair transistor T ₂₄ with T 24 as a constant current source,
T ₂₅ are cross-connected and their respective bases (S point and Q
The configuration is such that a DC control voltage obtained from a volume control drive circuit 8 , which will be described later, is applied to each of the points). Therefore, if the control voltage at point S is higher than that at point Q, the alternating current component of the current flowing through the collector load resistor _R31 will increase, the audio signal obtained from the collector of _T25 will increase, and the Q If the control voltage at the point is higher, on the contrary, the audio signal will decrease. The audio signal whose volume has been controlled in this manner is then guided to the next audio drive circuit 9 .

On the other hand, in the noise detection circuit 7 , the bases of the transistors T ₃₇ and T ₃₉ of the first and second comparison stages 7a and 7b are connected to the collector of the transistor T ₁₉ in the audio amplifier circuit 5 , and the transistors T ₃₆ and Bias voltages E ₂ and E ₁ determined by voltage dividing resistors R ₄₂ to R ₄₄ are applied to the base of T ₄₀ , respectively. The respective collector output voltages of T ₃₇ and T ₄₀ of the first and second comparison stages 7a and 7b are led to inverting amplification transistors T ₄₂ and T ₄₃ ,
After the output voltages of T ₄₂ and T ₄₃ are subjected to the integral action of capacitor C ₈ , switching transistor T ₄₄
is applied to the base of the Therefore, if it is assumed that impulse noise N as shown in FIG. 5 is superimposed on the audio signal taken out from the collector of T ₁₉ of the audio amplification circuit 5 , negative polarity (E ₁ or less) of the noise is superposed. (Figure b)
is detected by the first comparison stage 7a, positive polarity (E ₂ or more)
(C) is detected by the second comparison stage 7b. As a result, the first and second comparison stages
Negative noise pulses taken out from the respective collectors of T ₃₇ and T ₄₀ are inverted at T ₄₂ and T ₄₃ , converted to a slightly wider width at R ₅₁ and C ₈ , and applied to T ₄₄ . Therefore, this _T44 is on during the period of the noise, and the transistor of the volume control drive circuit 8 is turned on.
Forcibly lower the emitter potential of T ₄₆ .

The volume control drive circuit 8 is a transistor.
The first differential stage 8a consisting of T ₄₅ to T ₄₇ , etc.
The first differential stage 8a consists of transistors _T50 to _T54 , etc. _, which DC amplify the collector voltages of T45 and _T46 .
T ₄₅ has an external variable resistor connected to terminal P ₉
A variable DC voltage is applied by VR, and a fixed bias voltage is applied to T ₄₆ by resistors R ₅₈ , R ₅₉ and the like. That is, the second differential stage 8b
The above variable resistor VR is connected to each collector of T ₅₁ and T ₅₂ .
Two control voltages whose magnitude relationship changes are generated by the adjustment, and these voltages are applied to the S point and Q point of the gain control circuit 6 described above. Therefore, when _T44 of the noise detection circuit 7 is turned on during the noise period and lowers the emitter potential of _T46 of the first differential stage 8a, the collector potential of _T46 becomes low, so that the second difference The collector potential of _T51 of the dynamic stage 8b, ie, the control voltage at point S, is lower than that at point Q. As a result, the gain control circuit 6 reduces the gain (volume) of the audio signal during the noise period, making the noise less noticeable.

The audio signal subjected to volume control and noise removal in this manner is introduced from the collector of T ₂₅ of the gain control circuit 6 to the audio drive circuit 9 . This audio drive circuit 9 , like the audio amplifier circuit 5 described above, includes an emitter follower input stage 9a consisting of transistors T ₂₉ to _{T 32} and a diode D ₃ , etc., and a differential output stage 9b consisting of transistors T ₃₃ to T ₃₅ , etc. It consists of However, here, the emitter of T ₃₂ is connected to the bases of T ₃₃ and T 34 of the differential output stage 9b through resistors R ₃₅ and R ₃₇ of the same resistance value, and the terminal P ₉ is connected to the base _of T ₃₄ . The reason why the external capacitor _C7 for attenuating the audio signal is connected through the capacitor C7 is not for the purpose of preventing fluctuations in the DC level as described above, but for the following reason. That is, the differential output stage 9
The audio output from b is taken out from terminal P ₈ through transistor T ₃₅ and guided to an audio output circuit (not shown).
There are cases where it is desired to extract an audio signal with a lower gain than the audio signal obtained from the terminal _P8 . Therefore, in such a case, the external capacitor _C9 may be removed and the audio output may be taken out from the terminal _P9 . In this case, the differential output stage 9b does not function as an amplifier circuit at all, so the terminal P ₉
This means that an audio signal with sufficiently low gain can be obtained.

In the audio drive circuit 9 , the transistors T ₂₇ , T ₂₈ and the resistor R ₃₃ constitute a current mirror circuit with the transistors in each of the circuits whose common base is connected to T ₂₈ . A predetermined bias current is applied to the

As detailed above, the audio signal processing circuit of the present invention detects noise in the audio demodulation output in a television receiver that performs audio demodulation by directly detecting a separate carrier type SIF signal. Since the gain of the audio demodulation output is reduced over the period during which noise is detected, impulse noise caused by external factors, which is particularly noticeable when the tuner is detuned in this type of television receiver, can be removed. There are advantages.

In addition, noise appearing on the positive side and negative side of the audio demodulation output waveform is detected respectively.
Even if the frequency of the SIF signal deviates in either the high or low direction, the above-mentioned noise removal operation can be achieved.

Furthermore, the circuit that obtains the noise detection output and controls the gain of the audio demodulation output uses a circuit that adjusts the volume using an external DC voltage.
The circuit configuration becomes simpler.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a schematic configuration of the main parts of a television receiver using the audio signal processing circuit of the present invention, FIG. 2 is a circuit diagram showing an embodiment of the main parts, and FIGS. The figure is a characteristic diagram for explaining the operation, and FIG. 5 is a signal waveform diagram for explaining the operation.

Claims (1)

[Claims] 1. In a television receiver that performs audio demodulation by directly detecting a separate carrier type audio intermediate frequency signal derived from a tuner using a frequency discrimination circuit, the audio demodulation output and the first reference voltage _E2 are a first comparison stage that compares and detects impulse noise of negative polarity; and a second comparison stage that compares the audio demodulated output with a second reference voltage _E1 and detects impulse noise of positive polarity. and a volume control drive circuit that outputs a control voltage over the noise detection period based on the output of the noise detection circuit, and a gain control circuit that controls the gain of the audio demodulation output using the control voltage,
An audio signal processing circuit for a television receiver, wherein a gain of the audio demodulation output is reduced during the noise detection period.