JPH06113233A - Reduction method for intercarrier buzz - Google Patents

Abstract

PURPOSE:To prevent intercarrier buzz (buzz noise) from being caused in an audio signal obtained by applying FM detection to an intermediate frequency video signal and an intermediate frequency audio signal even when the intermediate frequency video signal outputted from a TV modulator has a residual phase modulation component. CONSTITUTION:An intermediate frequency audio signal obtained by applying FM modulation 23 to an audio signal at a TV modulator 1 is mixed 29 to an intermediate frequency video signal obtained by AM modulation 12 to a video signal at the same TV modulator 1, and a beat signal generated by detecting both signals simultaneously is FM-detected to obtain the audio signal. A residual phase modulation component generated at the intermediate frequency video signal is added in advance at AM modulation of the video signal before the mixing.

Description

Detailed Description of the Invention

[0001]

INDUSTRIAL APPLICABILITY The intercarrier buzz reduction method of the present invention is used, for example, for voice demodulation in a head-end TV modulator in a CATV system or a TV modulator in a TV receiver of a subscriber's house. is there.

[0002]

2. Description of the Related Art In current television broadcasting, an intercarrier system is adopted for receiving audio signals. This is because the transmitting side sets the frequency interval between the signals of the video carrier and the audio carrier to be exactly 4.5 MHz and transmits, and the receiving side transmits the video carrier and the audio carrier to the video intermediate frequency signal of 58.75 MHz and the audio of 54.25 MHz. The intermediate frequency signal is converted into an intermediate frequency signal, and the amplitude of the video intermediate frequency signal is increased by 20 dB or more than that of the audio intermediate frequency signal to detect both intermediate frequency signals at the same time. A beat frequency of 4.5 MHz is generated, and a beat signal of 4.5 MHz including this audio FM signal is F
That is, M detection is performed to obtain an audio signal.

A TV receiver for receiving such an inter-carrier type signal has a TV modulator A as shown in FIG.
Is used. This is to convert high-frequency video signals and audio signals output from a high-frequency amplifier circuit (not shown) into intermediate frequency signals, and to mix and output both intermediate frequency signals. Specifically, the VIDEO of FIG.
The high-frequency video signal input to the INPUT is AM-modulated by the AM modulator B and converted into a 58.75 MHz video carrier, and the high-frequency audio signal input to the AUDIO INPUT is FM-modulated by the FM modulator C. F
The M-modulated voice signal is frequency-converted by the converter D using the same local oscillator signal as that used in the AM modulator B and converted into a 54.25 MHz voice carrier, and both carriers are mixed. Mixed in E and IF OUTP
It is designed to be output from the UT.

[0004]

However, the AM modulator B of the TV modulator A, when AM-modulating a video signal,
The residual phase modulation component is generated in the video carrier from the physical characteristics of the balanced modulator (double balanced mixer) (not shown) of the AM modulator B. When a video carrier in which such a residual phase modulation component is generated is mixed with an audio carrier in which no phase modulation is generated and output, and the reception side detects the same to obtain a 4.5 MHz beat signal, There has been a problem that a residual phase modulation component of the video carrier is generated in the beat signal, and an intercarrier buzz (buzz sound) is generated in an audio signal obtained by FM detection of the beat signal.

According to the intercarrier buzz reduction method of the present invention, even if a residual phase modulation component occurs in the intermediate frequency video signal output from the TV modulator, the intermediate frequency video signal and the intermediate frequency audio signal are FM-detected. The purpose is to prevent the intercarrier buzz (buzz sound) from occurring in the audio signal obtained in this way.

[0006]

As shown in FIG. 1, a method for reducing inter-carrier buzz according to the present invention uses a video signal T
The intermediate frequency video signal AM-modulated by the V modulator 1 is mixed with the intermediate frequency audio signal FM-modulated by the TV modulator 1 and the beat signal generated by simultaneously detecting both signals is FM-detected. In an audio demodulation method by an inter-carrier method for obtaining an audio signal by means of an intermediate frequency audio signal to be mixed with the intermediate frequency video signal, a residual generated in the intermediate frequency video signal in advance during the AM modulation of the video signal before the mixing. A phase modulation component is added.

[0007]

According to the intercarrier buzz reduction method of the present invention, as shown in FIGS. 1 and 2, a 58.75 MHz intermediate frequency image obtained by AM-modulating a high frequency image signal (F _v ) by the TV modulator 1. Before mixing the signal (IF _V ) with the intermediate frequency audio signal (IF _A ) of 54.25 MHz obtained by FM-modulating the audio signal (F _A ) by the TV modulator 1, the intermediate frequency audio signal is converted into the intermediate frequency audio signal. Since the residual phase modulation component (A) generated when the video signal is AM-modulated is added, the frequency of the beat signal output as the difference between the mixed intermediate frequency signals is (58.75 MHz + A)-(54.25 MHz +
A) = 4.5 MHz and the residual phase modulation component (A) is canceled to form a beat signal, and the intercarrier buzz does not occur in the audio signal obtained by FM detection of this beat signal.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram of a TV modulator manufactured to carry out the intercarrier buzz reduction method of the present invention. The TV modulator 1 has VIDEO INPUT and A
A high-frequency video signal and an audio signal output from a high-frequency amplifier (not shown) are input to the UDIO INPUT.

The video signal input to VIDEO INPUT is VIDEO MODURATION 10, CL
AM MODRATION through the AMP circuit 11
(AM modulator) 12 is input. A local oscillating circuit 13 using a crystal is attached to the AM modulator 12, where the high frequency video signal is AM-modulated and
It is converted to an intermediate frequency video signal (video carrier) of 8.75 MHz. At this time, the AM modulator 12 is used for the video carrier.
Due to the physical characteristics of the balanced modulation section (double balanced mixer), the residual phase modulation component is present. This video carrier is amplified by IF _V AMP14, VSBF
An unnecessary frequency signal is cut by the BPF 15 and further amplified by the IF _V AMP 14 to output IF OUTPUT.
It is output to T.

The audio signal input to the AUDIO INPUT is AUDIO MODURATION 20, PR
E. AF through EMP (pre-emphasis circuit) 21
FM MODUR with C (automatic frequency control circuit) 22
It is input to the ATION (FM modulator) 23. Where F
The M-modulated audio signal is input to the CONV (converter) 24. This converter 24 converts the audio signal into 54.2.
A carrier reproducing circuit 25 is attached to the converter 24 for converting to a 5 MHz audio carrier, and the residual phase modulation component generated in the video carrier at the time of AM modulation is detected by the carrier reproducing circuit 25 and the sound is reproduced. The carrier undergoes the same phase modulation as the video carrier.

The voice carrier is a BPF 26, F _a L
MIX2 through EVEL27, IF _A AMP28
IF OUTPUT mixed with the video carrier at 9
Is output to.

[0012]

According to the intercarrier buzz reduction method of the present invention, a TV modulator in a CATV system,
It is possible to easily reduce the inter-carrier buzz generated in the TV modulator of the TV receiver at the receiver's house.

[Brief description of drawings]

FIG. 1 is a block diagram of a TV modulator showing an embodiment of a method for reducing inter-carrier buzz according to the present invention.

FIG. 2 is an explanatory diagram showing a method of demodulating an audio signal by an intercarrier method.

FIG. 3 is a block diagram showing an example of a conventional TV modulator.

[Explanation of symbols]

 1 TV modulator

Claims (1)

[Claims] 1. An intermediate frequency video signal obtained by AM-modulating a video signal by a TV modulator 1 and an FM signal by an audio signal by the TV modulator 1.
In an audio demodulation method by an inter-carrier method, in which a modulated intermediate frequency audio signal is mixed and a beat signal generated by simultaneously detecting both signals is FM-detected to obtain an audio signal, an intermediate frequency mixed with the intermediate frequency video signal A method for reducing inter-carrier buzz, characterized in that a residual phase modulation component generated in an intermediate frequency video signal at the time of AM modulation of the video signal is added to the audio signal in advance before mixing.