JPH0117634B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an intermediate frequency processing device for, for example, a television receiver. FIG. 1 shows the configuration of an intermediate frequency processing device for a television receiver that is commonly used. In Figure 1, 1 is an intermediate frequency signal (hereinafter referred to as IF
A video carrier wave (58.75MHz in Japan) that is amplitude-modulated with the video signal and an audio carrier wave (58.75MHz in Japan) that is frequency-modulated with the audio signal are input at the input terminal (referred to as signal).
54.25MHz) is input. 2 is IF
IF amplifier that amplifies the signal to the required level, 3
4 is an audio carrier remover (hereinafter referred to as an audio trap) that removes only the audio carrier from the IF signal; 4 is a video detector; its detection output is supplied to the video signal output terminal 5; 6 is an audio first detector which converts the audio carrier wave from the video carrier wave and the audio carrier wave of the output signal of the above-mentioned IF amplifier 2 to the difference frequency (4.5MHz in Japan), and the output terminal 7 is a 4.5MHz audio intercarrier. A signal is output. As mentioned above, the reason for separating the video detection and the first audio detection is as follows. In other words, the video detector 4 is the main source of the striped pattern that appears on the screen, the so-called 920KHz beat, which is the beat between the audio carrier and the color subcarrier (55.17MHz in Japan) in the IF signal. Audio trap 3 is installed to remove unnecessary audio carrier waves to prevent the occurrence of 920KHz beats.
The signal before the audio trap is applied is input to the audio first detector 6. Next, a conventional example of the audio trap 3 will be specifically explained. An example of this is audio traps used in integrated circuits. In FIG. 2, 8 and 9 are IF signal input terminals to which the output of the IF amplifier 2 is supplied. 10,
Reference numeral 11 denotes an NPN transistor, the base of which is connected to the input terminals 8 and 9, the collector connected to the power supply terminal 12, and constant current circuits 13 and 14 constituting DC bias means between the emitter and ground.
are connected to each other and operate as an emitter follower. 15 and 16 are both transistors 10 and 1
The output terminals are respectively connected to the emitters 1 and 1, and are connected to the first audio detector 6 with reduced impedance. 22 and 23 are transistors 1
It is an output terminal connected to emitters 0 and 11 via resistors 17 and 18, respectively, and is connected to the video detector 4.
connected to. 19 and 21 are output terminals 22,
A coil and a capacitor 23 are connected in series, and 20 is a capacitor connected in parallel to the coil 19, forming a series-parallel resonant circuit that constitutes a reactance network 26 together with the coil 19 and the capacitor 21. The inductance of the coil 19 is L, and the capacitance of the capacitors 20 and 21 is C ₁ , C ₂
Then

[Formula] causes series resonance and the impedance decreases,

Impedance increases due to parallel resonance in [Equation]. f ₁ to 54.25MHz and f ₂ to
When approaching 54.25MHz as much as possible, the frequency characteristics at output terminals 22 and 23 are as shown in Figure 3.
It is possible to realize a voice trap that rapidly decays at 54.25MHz. However, as audio multiplexing has become more widespread, market demands for audio have become more stringent, and with the above conventional configuration, noise due to the effects of video signals and synchronization signal components, so-called buzz, audio S/N, etc. This was not sufficient, and there were drawbacks such as the need for many external parts in order to improve the above-mentioned characteristics, and the need for pins when integrated. The present invention has been made in order to eliminate the above-mentioned drawbacks of the conventional technology.The present invention utilizes a reactance network of a voice trap, operates as a bandpass filter at the input of a voice detector, and increases the level of the voice carrier wave. It is an object of the present invention to provide an intermediate frequency processing device that improves the buzz and audio S/N. An embodiment of the present invention will be described below with reference to the drawings. This invention is characterized in that the configuration of the audio trap 3 is changed. The differences in Figure 4 from Figure 2 are as follows:
Load resistors 24 and 25 constituting a load circuit are inserted between the collectors of the NPN transistors 10 and 11 and the power supply terminal 12, respectively, and the output terminals 15 and 16 leading to the first audio detector 6 are connected to the NPN transistors 10 and 11, respectively. This is the point taken from the collector. Next, the operation will be explained. The voice trap operates exactly the same as in FIG. 2, so its explanation will be omitted. In the conventional example, the input signal to the first audio detector 6 is
Although the current was taken out from the emitter side of the NPN transistors 10 and 11, in the present invention, it is taken out from the collector side of the transistors 10 and 11. In this way, a differential amplifier is formed by the transistors 10 and 11, the series-parallel resonant circuit consisting of the resistors 17 and 18, the coil 19, and the capacitors 20 and 21, and the load resistors 24 and 25.
Since the impedance of the series-parallel resonant circuit enters the emitter resistance, it becomes an amplifier with characteristics exactly opposite to those of an audio trap. Figure 5 shows its frequency characteristics. As shown in the figure, the impedance decreases at the series resonance point of 54.25MHz, increasing the gain, and the impedance reaches its maximum and the gain reaches its minimum at the parallel resonance point. In other words, the reactance network 2 of the audio trap
6 operates as a bandpass filter such that the gain at the audio carrier frequency of 54.25 MHz is higher than the gain at the video carrier frequency, and the gain is lower at the unwanted video carrier sidebands. When input to the first audio detector 6 through amplification with such characteristics, the level of the output 4.5MHz signal increases,
At the same time, the influence of video sidebands is reduced, so when demodulating into an audio signal, the S/N ratio is improved, especially in a weak electric field, and buzz noise is also reduced. In the above embodiment, a differential amplifier configuration that is often used when integrated is shown, but a common emitter type amplifier using a single transistor can also produce similar effects. Further, in the above embodiment, a constant current source was used between the emitter and the ground, but it goes without saying that the same effect can be obtained even if a resistor is used instead. As explained above, according to the present invention, the emitter output of the transistor whose base receives the video intermediate frequency signal and the audio intermediate frequency signal is input to the video detector through the audio trap, and the collector output is input to the audio detector. However, by using the reactance network of the audio trap as a bandpass filter at the input of the audio detector, it is possible to improve the audio S/N ratio, especially in weak electric fields, without the need for external components, and to reduce buzz noise. This has the effect of reducing the amount of

[Brief explanation of drawings]

Fig. 1 is a block diagram of a commonly used intermediate frequency processing device, Fig. 2 is a circuit diagram showing a specific example of a conventional audio trap, Fig. 3 is a frequency characteristic diagram of the audio trap, and Fig. 4 is a feature of the present invention. FIG. 5 is a circuit diagram showing an example of a voice trap, and FIG. 5 is a diagram showing the frequency characteristics of the circuit of FIG. 4 as an amplifier. In the figure, 6 is an audio detector, 10 and 11 are transistors, 12 is a power supply terminal, 13 and 14 are DC bias means, 24 and 25 are load circuits, and 26 is a reactance circuit network. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1 A transistor into which a video intermediate frequency signal and an audio intermediate frequency signal are input to the base electrode, a DC bias means provided between the emitter electrode of this transistor and ground, and a DC bias means provided between the collector electrode of the transistor and a power source. a video detector that receives the emitter output of the transistor as an input; and a video detector that receives the collector output of the transistor as an input, and converts the audio intermediate frequency signal into a frequency that is the difference between the video intermediate frequency signal and the audio intermediate frequency signal. an audio detector, comprising a reactance network connected between the emitter electrode of the transistor and the video detector, forming a remover for the audio intermediate frequency signal and forming a bandpass filter for the collector output of the transistor; Intermediate frequency processing equipment.