KR800000204B1 - Frequency selective circuit for automatic frequency control and sound signals - Google Patents

Abstract

An intercarrier television receiver incorporates circuitry for providing automatic fine tuning signals and frequency translated sound carrier signals. a series shunt resonant circuit arrangement modifies the passband response of signals provided by an intercarrier If amplifier(16) and generates both a desired sound carrier and a desired automatic fine tuning response.

Description

Automatic frequency control and frequency selection circuit for voice signal

1 is a partial opening diagram, a partial schematic diagram of an intercarrier television receiver embodying the present invention;

2a-2d show bandpass characteristics of the FIG. 1 device.

The present invention relates to television circuits and, more particularly, to an apparatus for providing an automatic frequency control signal and a frequency converted speech signal for an intercarrier speech system.

In current intercarrier speech systems used in television receivers, an intermediate frequency (IF) amplifier amplifies both the modulated voice carrier and the modulated video carrier. The frequency separation of each of these carrier signals is adjusted by a particular broadcast standard (e.g., 4.5 MHz for US separation). The synthesized signal amplified by the IF amplifier is supplied to a regulating filter circuit, which is then coupled to appropriate detection stages for detecting the video and audio portions of the signal. The audio carrier is significantly attenuated compared to the video carrier before the video detection stage, but it is desirable to provide a good audio output while at the same time making the voice carrier supplied to the voice detector relatively less attenuated.

In addition, it is common for color television receivers (and deluxe monochrome receivers) to provide an automatic frequency control (or "auto-tuning") circuit to keep the on-board tuners at a suitable operating frequency. Such automatic frequency control circuits generally include a relatively narrowband tuning circuit to sense an intermediate frequency video carrier component (eg, normally 45.75 MHz under US broadcast standards) to control the tuner.

The present invention described herein is a relatively simple circuit that achieves both a satisfactory autotuning (AFT) response and a satisfactory voice carrier response. To provide such an audio and AFT response, an amplifier is coupled to an amplifying frequency amplifier of a television receiver for receiving both a modulated audio carrier signal and a modulated video carrier signal. A discriminator device comprising a first resonant circuit that resonates at a frequency substantially equal to the image carrier frequency well accepts the signals provided by the amplifier for providing an auto-tuned signal responsive to the image carrier frequency deviation from this resonant frequency. . The second resonant circuit is inserted in series between the amplifier and the discriminator and resonates at a frequency lower than the voice carrier frequency. The second resonant circuit serves as a load on the output of the amplifier that causes the amplifier to amplify a greater amount for a frequency closer to the resonance of the second resonant circuit than a frequency far from resonance.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Referring to FIG. 1, the signal received by the television receiver antenna is processed through amplifier 10 to combine the signal into mixer 12.

Mixer 12 receives the heterodyne signal from local oscillator 14 and provides the IF amplifier 16 with the converted signal at intermediate frequency. Amplifier 16 provides the signal to image detector 18 and a wideband amplifier 20 suitable for passing signals ranging from the chroma signal component to the audio signal component. The signal provided by detector 18 is a suitable circuit for coupling to kinescooff 22 (e.g. -68 type receiver).

The signal processed by the wideband amplifier 20 is coupled to an AFT amplifier detector 24 comprising a mid-tap secondary transformer winding 30 and a tuning capacitor 32 through a parallel resonant circuit comprising an inductor 26 and a capacitor 28. A shunt resonant circuit 52 comprising a transformer primary winding 34 and a capacitor 36 is also coupled to the transformer winding 30.

The bead frequency or mixer amplifier 44 is coupled to the output of amplifier 20 and feeds the frequency converted speech carrier signal to the speech detector or discriminator 46. The discriminator 46 modulates the voice carrier signal and provides a low frequency signal to the amplifier 48 and couples these signals to the loudspeaker 50.

The load resistor 38 is coupled to the operating voltage source (B ⁺ ) through the output terminal of amplifier 20 and resistor 40. Bypass capacitor 42 is connected to the contacts of resistors 38 and 40.

In operation of the circuit described above, IF amplifier 16 is arranged to provide sufficient bandwidth to pass both video and audio information. In the United States, the IF video carrier is normally located at 45.75MHz, and the IF audio carrier is normally located at 41.25MHz. The passband of IF amplifier 16 is determined by adding a tuning circuit (not shown) after the mixer circuit, to provide a relatively high attenuation ratio for signals less than 41.25 MHz or greater than about 48 MHz. Where the receiver is suitable for receiving color signals, the passband is adjusted to provide adequate amplification of the video, chromatic and audio carrier signals.

Under adverse reception conditions, it would be desirable to tune the receiver to receive a better video signal by shifting the video carrier away from the normal passband position of 45.75 MHz, which is somewhat below the peak amplitude response of the IF amplifier 16. For example, if the TV tuner is mis-tuned and the IF video carrier frequency and the IF audio carrier frequency are shifted to lower frequencies than normal, the video carrier information will be increased in amplitude, but the audio carrier information will be IF passband response. May be excessively damped by To correct this unique problem, the speech response, combined with the IF passband response, produces a modulated response with a low frequency region with a small amount of attenuation at frequencies where the speech carrier is reduced. The parallel combination circuit of inductor 26 and capacitor 28 is adjusted to resonate in parallel at, for example, about 37 MHz at frequencies below the normal IF negative subcarrier. Thus, resonant circuits 26 and 28 provide relatively high impedance at the output of amplifier 20 at the resonant frequency and relatively low impedance at other frequencies. After all, the IF signal through amplifier 20 tends to be relatively high impedance at frequencies near its resonance and relatively low impedance at frequencies far from resonance. Changing the output load of amplifier 20 as a function of signal frequency causes the signal gain provided by this amplifier to change equally. For example, it is relatively amplified for an input signal of 37 MHz rather than 41 MHz. A frequency response curve showing the relative output signal amplification (eg gain) of amplifier 20 is illustrated in FIG. 2b. When amplifier 20 is combined with IF amplifier 16, some synthesis response occurs. This synthesis response is expressed with the aid of the IF response curve shown in Figure 2a and the decibel curve of Figure 2b. The cooperation of these two curves is illustrated in Figure 2c. Referring to FIG. 2c, the voice carrier located at 41.25 MHz exhibits about 12 db attenuation, while the image carrier response (45.75 MHz) actually represents 0 db. This can compare the attenuation of the image carrier of 2.5db compared to the attenuation of the 18db audio carrier shown in the curve of FIG. 2a. When the untuning of the television receiver is manipulated to shift the voice carrier signal frequency lower, for example 41 MHz, the attenuation of the voice carrier, as measured by the IF amplifier, is about 22.5db, whereas in Figure 2c it is about 15db. At the same time, the image carrier is elevated in the response of Figure 2c with about 1.5db attenuation but lowered for 1db attenuation in Figure 2c. Therefore, the response derived from FIG. 2c shows a more gradual decrease compared to the attenuation of the voice carrier with the reduced frequency compared to that measured from the IF curve of FIG. 2a. It is also noted that the amplitude of the voice carrier measured at the output of amplifier 20 (FIG. 2C) is not significantly reduced from that provided by IF amplifier 16. If the untuning is very wrong, for example, when the voice carrier is shifted to a frequency lower than 1 MHz, for example 40 MHz, the attenuation of the voice carrier measured at the output of amplifier 20 is only 29.5 db, whereas the You can see that it is about 41.5db. This extreme misalignment can be exploited in suburban areas with very poor television reception sensitivity. Such exaggerated misalignment may be necessary to provide a viewable picture.

In another aspect of the apparatus of FIG. 1 an automatic untuned signal is generated at the output of amplifier 24. The AFT response is generated by providing a relatively high Q tuning circuit centered near the image carrier frequency that is coupled to the discriminator circuit having a response that changes as a function of the frequency deviation from the image carrier frequency. The tuning circuit comprising inductor 26 and capacitor 28 at the image carrier frequency (45.75 MHz) is relatively far from its resonance point. In this frequency domain, the signal provided from amplifier 20 is initially coupled to the AFT circuit through the low impedance of capacitor 28. The transformer with the primary inductor 34 and the secondary inductor 30 is arranged to have the first resonant circuit with capacitor 36 at the image carrier frequency, for example about 45.75 MHz. In this frequency domain, the resonant response generates a very high impedance for the signal provided directly to the center tap of the inductor 30 and also provides maximum energy transfer from the inductor 34 to the secondary inductor 30. As a result of the resonance response of the primary inductor 34, a narrow band high Q response appears at circuit point 52. A frequency response curve describing the signal level at point 52 as a reference for the input of amplifier 20 is illustrated in FIG. 2d.

Referring to Figure 2d, it can be seen that the peak response at about 45.5 MHz provides the desired response to the discriminator input (inductor 30) of the AFT amplifier 24. It can be seen that the response of the AFT circuit is relatively unobstructed by the circuit equipped to generate the above-mentioned voice response.

Therefore, it can be seen that the circuit for providing both the desired speech response and the desired AFT signal can be realized by combining the two parallel resonant circuits advantageously in the manner shown.

Claims (1)

Intercarrier intermediate with a bandwidth suitable for passing the modulated voice and video carrier components for amplifying both voice and video signal components, including modulated voice and video carrier components as described herein and shown in the figures. 1. A television receiver that accepts a composite video signal having a frequency amplifier, wherein the device providing a desired response to the audio and video carrier components and providing an automatic untuned voltage comprises an intermediate frequency comprising modulated low frequency and video carrier components. An auto-tuning device comprising an amplifier 20 for receiving a signal and first resonant circuits 34 and 36 which resonate at a frequency substantially equal to the image carrier frequency and responsive to an image carrier frequency deviation from the resonant frequency. A signal provided by the amplification device for providing a signal It is arranged in series between the receiving discriminator device 24 and the amplifying device and the discriminating device and provides a higher impedance load for the amplifier device at a frequency farther from the resonance than near the resonance, thereby providing a lower gain. And a second resonant circuit (26, 28) resonating at a lower frequency than said voice carrier frequency for providing.

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