JPS6331154B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an audio discrimination output 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 is unavoidably subject to the drawbacks 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 that adopts this system, and the circuits indicated by blocks in the figure perform the operations described in each block. The following points need to be noted here. That is, in the case of the same figure, since the AFT voltage and the audio signal are obtained from the output of the frequency discrimination circuit 3, the output of the discrimination circuit 3 is sent to the AFT voltage extraction circuit 4 and the audio amplification circuit 5.
It is necessary to ensure that both circuits are not influenced by the other circuit when applying the power to the other circuit.

However, the present invention has been made to solve this problem, and one embodiment of the present invention shown in FIG. 2 will be described below.

FIG. 2 shows an example in which the portion including the processing circuit of the present invention surrounded by the broken line in FIG. 1 is integrated into an IC, and parts corresponding to the blocks in FIG. are doing. In this circuit shown in Figure 2,
The SIF signal (center frequency 54.25MHz) is introduced into terminal P1, and this SIF signal is used for frequency discrimination (FM detection).
It leads to circuit 3.

The frequency discrimination circuit 3 includes transistors T1, T
It is configured as a peak differential type including a buffer amplification stage 3a consisting of 2 etc., a differential discrimination stage 3b consisting of transistors T3 to T5 etc., and a 180° phase shifter consisting of an external coil L1 and capacitors C1 and C2. The discrimination output is the differential discrimination stage 3.
From the collectors of T3 and T4 of b to the next stage through common base type transistors T6 and T11
It is designed to be guided to the AFT voltage extraction circuit 4. The characteristics of this frequency discrimination circuit 4 are as shown in FIG.

The AFT voltage extraction circuit 4 includes first, second,
Third current mirror circuits 4a, 4b, 4c and terminal P
It consists of external resistors Ra and Rb connected to 6 and a capacitor C5. And the first and second
The input transistors T8 and T12 of the current mirror circuits 4a and 4b are respectively connected to the collectors of T3 and T4 of the differential discrimination stage 3b via the aforementioned T6 and T11, respectively, and the first current mirror circuit The output side transistor T10 of the third current mirror circuit 4a is connected to the input side transistor T15 of the third current mirror circuit 4c. Therefore, if the values of the resistors R13, R14 and R17 to R21 of each current mirror circuit are all equal, the output side transistor T13 of the second current mirror circuit 4b has one T of the differential discrimination stage 3b.
4, and a current equal to the collector current of the other transistor T3 of the differential discrimination stage flows through the output transistor T17 of the third current mirror circuit 4c. As a result, T13, T17
A DC voltage corresponding to the current difference is generated at the terminal P6, and this voltage is 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 the AFT voltage obtained at the terminal P6 is , will change depending on the DC component of the discrimination output, that is, the carrier frequency of the SIF signal.

Note that the slope of the linear variation region (W') of the above AFT voltage characteristics (Fig. 4) is much greater than that of the linear detection region (W) of the characteristics of the frequency discrimination circuit 3 (Fig. 3). It is chosen to be steep,
This is aimed at improving AFT sensitivity, 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 a transistor T18 whose base receives a current equal to the collector current of the other output side transistor T7 of the first current mirror circuit 4a, that is, the collector current of T3 of the differential discrimination stage 3b, and its load. An emitter follower stage 5a consisting of a transistor T19 operating as a transistor T19, and a resistor R24 whose emitter follower output has the same resistance value
A differential amplification stage 5b consisting of transistors T19, T20 and a constant current transistor T21 applied to each base via R25, and an audio frequency component attenuator connected to the base of one of the differential pair transistors T20. It consists of an external capacitor C6, etc. Therefore, when the discrimination output taken out by T7 of the first current mirror circuit 4a is led to the differential amplification stage 5b through the emitter follower stage 5a, the DC component of the discrimination output is transferred to the resistor R24. , T17, T2 by R25
However, the alternating current component, ie, the audio signal component, is applied only to the base of T19 due to the capacitor C6. As a result, the differential amplification stage 5b performs amplification only on the alternating current component of the discrimination output. Therefore, when the tuner is detuned, such as immediately after power-on or channel switching, the frequency discrimination circuit 5b amplifies only the AC component of the discrimination output. A in Figure 3, where the operating point of
Even if the DC level of the discrimination output changes by moving from point B to point C, T1 of the differential amplifier stage 5b
An audio signal whose DC level is kept at a constant potential is obtained from each of the collectors 9 and T20. This audio signal is then guided to the audio output circuit through a volume control circuit (not shown) or the like.

As explained above, the audio discrimination output processing circuit of the present invention is configured to take out the differential type discrimination output from the frequency discrimination circuit which inputs the separate carrier type SIF signal derived from the tuner, and Lead the discrimination output to the current mirror circuit,
This current mirror circuit separates the output for creating the AFT voltage and the output for taking out the audio signal, so the circuit for creating the AFT voltage and the circuit for taking out the audio signal are not influenced by each other. Therefore, the AFT voltage and audio signal can be obtained accurately.

In addition, the AFT voltage is created using both of the differential discrimination outputs mentioned above, and the audio signal is obtained from one of the differential discrimination outputs, so a large AFT
In addition to being able to obtain voltage (greater AFT control sensitivity), the parts for audio signal de-emphasis [the capacitor (C ₉ ) in Figure 2] are 1
It is only necessary to provide the circuit of the present invention.
If it is implemented as an IC, the number of external parts and external terminals will be reduced, which is preferable.

In the example shown in Fig. 2, a peak differential detector is used as the frequency discrimination circuit for extracting the differential type discrimination output, but this is done at the expense of reducing the degree of AM suppression in the frequency discrimination operation. If so, it may be possible to use a quadrature detector.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a schematic configuration of main parts of a television receiver to which the present invention is applied, and FIG. 2 is a circuit diagram showing an embodiment of the audio discrimination output processing circuit.
FIGS. 3 and 4 are characteristic diagrams for explaining the operation. 3: Frequency discrimination circuit, 4: AFT voltage extraction circuit, 5: Audio amplification circuit, 3a: Differential discrimination stage, 4
a, 4b, 4c: current mirror circuit.

Claims (1)

[Claims] 1. In a television receiver in which a separate carrier type audio intermediate frequency signal derived from a tuner is discriminated by a single frequency discrimination circuit, and an AFT voltage and an audio signal are obtained from the discrimination output, the above-mentioned A differential type discrimination output is taken out from the frequency discrimination circuit, and a current of the same magnitude according to one of the differential discrimination outputs flows through the two output side transistors of the first current mirror circuit, respectively. , an audio signal is obtained from one of the output side transistors, and a current corresponding to the other of the differential discrimination outputs is caused to flow to the output side transistor of the second current mirror circuit, and the output side of the second current mirror circuit is A smoothing capacitor connected to the connection midpoint of the output transistor of a third current mirror circuit connected in series so that a current equal to that of the other output transistor of the first current mirror circuit flows through the transistor. 1. A sound discrimination output processing circuit for a television receiver, characterized in that an AFT voltage is obtained from a television receiver.