JPS63215176A - Sound demodulating device for television signal - Google Patents

Abstract

PURPOSE:To obtain a sound signal free from buzz disturbance by reproducing a buzz disturbance component essentially included in the demodulated output of a sound subcarrier, and subtracting the reproduced component from the demodulated output. CONSTITUTION:The output from a sound detecting circuit 35 is inputted to a subtractor 12 and the output from a voltage control oscillator(VOC) 44 is led into the other input of the subtractor 12 through an FM demodulator 11. The output signal from a mixer 42 includes an FM component and the FM component is generated in the output signal from the VOC 44, so that its own buzz disturbance can be extracted by demodulating the FM component by the FM demodulator 11. On the other hand, a mixer 41 executes synchronizing detection on the basis of the output of the VCO 44 including the FM component, its output includes an FM variance and a buzz disturbance component is included also in the output of a sound detecting circuit 35. When the output of the FM demodulator 11 is subtracted from the sound demodulation output, a sound signal from which the buzz disturbance component is removed is outputted from the subtractor 12.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to an audio demodulation device for television signals that employs an intercarrier system, and in particular,
This invention relates to a television signal audio salt m apparatus which performs synchronous detection of an IF multiplied signal using a reproduced video 1111F carrier reproduced from an F signal, obtains an audio subcarrier component from the signal obtained by this synchronous detection, and performs audio demodulation.

(Prior Art) In general, a transmitted television signal is transmitted using a vestigial sideband method, for example, from a video carrier to a 0. 75 [MHz
], both sidebands are transmitted as they are, and only the lower sideband, which is 1.25 [MHz] or more away from the video carrier, is completely suppressed. The video signal is transmitted by AM modulating the video carrier, and the audio signal is transmitted by FM modulating an independent audio carrier that is separated by exactly 4.5 MHz from the video carrier.

As a method for demodulating an audio signal from such a residual sideband signal, the most commonly used intercarrier method will be described below.

FIG. 2 shows an example of the configuration of an audio demodulation circuit based on the above method, in which 31 is a tee filter, 32 is an envelope detector, and 3
3 is a band pass filter, 34 is a limiter, and 35 is an audio detector. The Nyquist filter 31 is supplied with an IF signal made up of two IF carrier components, video and audio, amplified by an intermediate frequency (IF) amplification circuit (not shown). FIG. 3 shows the frequency characteristics of the Nyquist filter 31, and its characteristic characteristic is that, at the position of the IF image Gisselia frequency ωp, there is a frequency below (±) 0.75 [MH7] around the same frequency ωp. It has a characteristic of first-order slope in the frequency range ωd. In the input video IF signal, a lower sideband component in a residual sideband (hereinafter referred to as a VSB modulation signal) is cut due to the characteristics of the slope portion. As a result, the video low-frequency components (0 to 750 [
KH2])'s] is suppressed.

Note that in FIG. 3, ωa is the audio IF carrier frequency.

The envelope detector 32 detects the video AM component from the signal from the Nyquist filter 31. Among the outputs of the envelope detector 32, one output is guided to the video detection circuit, and the other is a band The signal is supplied to a pass filter 33.

The bandpass filter 33 extracts the high-frequency component of the envelope detection output, that is, the audio subcarrier signal of ω, a=ωp to ωa. After passing through the limiter 34, the audio subcarrier signal is subjected to FM detection by an audio detector 35 and becomes an audio output signal.

In the above, the signal passed through the Nyquist filter is
Generally, it can be expressed as the following orthogonal AM modulation signal.

e (t) −1(t) cos ωp t +Q(t)
sin ωp t...(1) Here, C is a DC term, 5lkcos (ωkt+θ
k) is the baseband signal corresponding to the modulated signal component at a frequency ω located below ωp by S, ksi
n (ωk t+θk) represents the baseband signal corresponding to the modulation signal component at the frequency ω located above (7Jp by ω). Hereinafter, I (t
) is called an in-phase component, and Q (t) is called a quadrature component, and will be further explained.

In the case of a television receiver, the orthogonal AM modulated signal as described above has a relationship of +Suk to 81 because either the upper side wave or the lower side wave is selected by a Nyquist filter. In particular, for the component in the non-pass band on one side outside the slope part of the Nyquist filter (ωk>ωd), either Slk or S becomes 0.

Furthermore, the audio subcarrier signal ωSa exists in the high range of ωk>ωd and is completely transmitted in a single sideband.

The only ideal detector that demodulates such a signal without distortion and obtains only the I ([) component is a fully synchronous detector, but realizing fully synchronous detection requires a complicated circuit configuration, and it is difficult to use a device like a television receiver. This is uneconomical for consumer electronics. Therefore, envelope detectors are often used. The detected output from the envelope detector generally produces distortion due to the orthogonal component Q(t). However, due to the nature of the video signal, that is, the high-frequency video component is small, the presence of a DC term C, and the characteristics of the Nyquist filter near the video carrier, the low-frequency range S
Due to the small difference between Ik and 5IJk, I(t)
)Q(t) is satisfied, and if the signal of equation (1) is envelope-detected, I(t) can be approximately obtained.

In other words, Than, The approximate output is the envelope detection output.

However, such an approximate output is 4 to 5 times the level of the audio subcarrier signal ωSa, which is always the maximum in Q(t), even in the case of a video signal where the level of ([) is the maximum. Therefore, in addition to I(t), it includes many beat frequency components involving ωSa. The main components of these components are ω8. There are beat frequency components between the subcarrier and the color subcarrier, and a buzz interference component as an interference signal to the audio FM signal.

In order to explain the generation process of the above-mentioned buzz disturbance, I of equation (1)
(t) and Q(t), the term of the audio subcarrier signal component is separated and shown as follows.

e (t) = I(t) cos ωD t 1Q(t
) sin ωp t+Acos (ωa t+fa(
t) dt) ・(4) (A is the audio carrier level, a
(t) is the audio signal) Now, when envelope detection is performed on equation (4), the audio beat frequency ωSa (ωP ~ ωa
) Listing the main components that appear in the vicinity, there are the following three types in descending order of magnitude.

■Video low-frequency components I (t) , Q, , ( around ωρ
t) [Beat between F and audio carrier ωa nearby component, ■Second harmonic of the beat between the video low frequency component around ωp and the video midrange component near the top of the car → who's land, ■Audio carrier near ωa and f
The second harmonic of the beat with the nearby video mid-range component is the audio subcarrier signal Aco obtained by linear synchronous detection of multiplying the signal shown in equation (4) by COSωpt.
s (ωsat + f a (t)) The d-th signal is the one that has received phase modulation and amplitude modulation interference. Amplitude modulation interference can be removed by a limiter, but the phase modulation component cannot be removed. The FM detection component of such a component is, and the second term represents the buzz interference output.

According to equation (6), Q of the video low-frequency components 5. (t
) component is 0, it can be seen that no buzz interference occurs.

This interference component QLF(t) becomes larger as the slope of the 00 section of the Nyquist filter shown in FIG.

In addition to this, disturbances (2) and (2) involving video mid-range components are added, so conventional demodulators that obtain audio subcarrier signals from video envelope detection outputs have a limit in obtaining high-quality audio. However, it has been difficult to adapt to today's high-definition television receivers.

A PLL synchronous detection method is a method suitable for audio demodulation of high-definition television receivers. This audio demodulation device using PLL has the advantage that it does not require AFC because the audio subcarrier frequency is stabilized on the transmitting side, and is an inexpensive alternative to envelope detection.

FIG. 4 shows an audio demodulation device for a television receiver using the above PLL synchronous detection method. This method will be explained. Note that parts common to those in FIG. 2 are denoted by the same reference numerals. In the figure, a portion surrounded by a dashed line indicates a PLL synchronous detection section. The F" L L synchronous detection section regenerates the carrier of the in-phase component 1 (t) (regenerated video IF carrier) from the IF signal from the Nyquist filter 31, and performs synchronous detection using the input IF multiplied signal regenerated IF carrier in the mixer 41. Therefore, video detection and audio demodulation are attempted using a signal containing only the in-phase component I(t) as an AM modulation component.The reproduced video IF carrier is used by the mixer 42, loop filter 43, and voltage control. Oscillator (hereinafter referred to as ■CO)
It is obtained by using the output of VC044 in the PLL loop consisting of 44. That is, the mixer 42, which performs multiplicative detection equivalent to the mixer 41, can be considered as a phase detector in the PLL loop, and calculates the phase of the output signal of the VCO 44 with respect to the COSωp key 17 rear in the input IF signal. 90°
When this relationship deviates, a phase error signal (actually a frequency fluctuation component) is output. As a result, an IF carrier signal having the same phase as sinωpt is output from the V CO 44, and the 90” phase shifter 45
The reproduced video IF rear obtained through the CO3ωp
This becomes a carrier signal that follows the carrier.

According to the above configuration, since the output signal phase of the 90° phase shifter 45 matches the carrier cos(1)p of the I(t) component, ideally only the I(t) component is used as the video detection component. Accordingly, the audio demodulation system can output an audio demodulated signal that is not affected by orthogonal components.

However, in reality, the control voltage supplied to the VCO 44 is
Since it essentially follows the low-frequency FM component caused by the 0(t) component that cannot be removed by the loop filter 43, an AC signal due to this appears in the output of the loop filter 43,
The output of VCO44 is low jii! It will be a modified version of FM. If you think of PLL synchronous detection as a circuit that converts the frequency of the input [F signal with the reproduced video IF key 1 aria,
The FM modulation component of this oscillation output directly generates the interfering FM component of the audio subcarrier in the frequency conversion process.

As described above, even when an audio signal is demodulated using the PLL synchronous detection method, it is not possible to completely eliminate audio buzz interference caused by low-frequency video components.

As a completely buzz-free receiver, there is a separate carrier method in which the audio carrier input to the receiver is detected from the beginning through a completely separate frequency conversion circuit, or the audio IF carrier is frequency-converted and detected separately. There are problems with linking with channel reception, and the need to provide a unique AFC for the audio system in the frequency conversion circuit, etc.
There are various restrictions.

(Problems to be Solved by the Invention) A conventional television receiver that uses PLL synchronous detection to reproduce video [F carriers and performs intercarrier audio demodulation]
Although C is not required, there is a problem in that the reproduced video IF carrier is FM modulated by the video modulation component that cannot be removed by the loop filter, and it is not possible to completely avoid the influence of buzz interference caused by the video signal.

The object of the present invention is to solve the above-mentioned problems and provide an intercarrier audio demodulator using PLL synchronous detection, which performs audio demodulation without buzz interference.

[Structure of the Invention (Means for Solving Problems)] This invention provides a carrier reproducing means for reproducing an IFF carrier of a video signal from an IF multiplied signal of an intermediate frequency obtained by heterodyne detection of a TV signal, and a carrier reproducing means for reproducing an IFF carrier of a video signal. synchronous detection means for synchronously detecting the IF multiplied signal on the reproduced video IF carrier; audio detection means for extracting an audio subcarrier component from the synchronous detection output from the synchronous detection means and performing audio detection;
It demodulates the audio signal in the television signal by comprising FM demodulation means for demodulating the reproduced video FF carrier FM, and subtraction means for subtracting the output of the FM demodulation means from the output of the audio detection means.

(Function) When reproducing a video IF-t signal using the IF multiplied signal from the Nyquist filter and a PLL circuit, the oscillation output from the voltage controlled oscillator of the PLL is based on a buzz component that cannot be removed by the loop filter. zu<FM component appears. Therefore, a buzz interference component essentially occurs in the output of the IF multiplied signal synchronously detected by the reproduced video IF carrier.
The audio output obtained by performing audio detection on this synchronous detection output using the intercarrier method inevitably contains buzz interference. In the present invention, the output of the voltage controlled oscillator is FM demodulated to generate the buzz component generated in the intercarrier demodulation process, and the buzz component is subtracted from the audio detection output, so that bus interference can be eliminated.

(Example) Hereinafter, the present invention will be explained with reference to the illustrated embodiments.

FIG. 1 is a block diagram showing an embodiment of a television signal audio demodulation device according to the present invention.

In FIG. 1, the same parts as in FIG. 4 are denoted by the same reference numerals, and an IF multiplied signal obtained by heterodyne detection of a television signal is input to the Nyquist filter 31. The Nyquist filter 31 attenuates the double-side wave transmission section from the transmission video signal transmitted in the vestigial sideband, and outputs an IF multiplied signal expressed by equation (1).

The IF multiplied signal from the Nyquist filter 31 is input to mixers 41 and 42 forming a PLL synchronous detection section. Synchronous detection is performed by one of the two mixers 41 and 42.

The reproduction video IF carrier, which becomes a reference phase signal for synchronous detection, is mixed by a mixer 42 and a loop filter 43 . It is obtained as the output of the VCO 44 in the PLL loop consisting of the VCO 44. However, the actual reproduced video IF carrier obtains the output of the V CO 44 through a 90" phase shifter 45, and the output of the phase shifter 45 is obtained from the mixer 4.
1 is supplied.

Specifically, the mixer 42 performs multiplicative detection of the output of the V CO44 and the input IF signal, and controls the oscillation frequency and phase of the C044 by passing the resulting multiplicative output through the loop filter 43. A control voltage is formed. The output of the VCO 44 generated by this control voltage is input to the mixer 42 and multiplied by the IF signal. In this way, the PLL loop is controlled by the VCO 44 from the loop filter 43.
A control voltage proportional to the phase difference between the output signal and the IF multiplied signal is output, and an oscillation signal with a frequency proportional to the control voltage is output from V(:, and the phase difference is generated to be the minimum.

On the other hand, a mixer 41 that performs synchronous detection (multiplicative detection) using the output of the phase shifter 45 supplies the output to a video detector (not shown) via a low-pass filter 46 and also extracts the audio subcarrier ωSa. is supplied to a bandpass filter 33 for use. Then, the limiter 34 removes the AM component from the audio subcarrier signal from the bandpass filter 33 and supplies it to the audio detection circuit 35 for audio detection. Thereby, the audio detection circuit 35 demodulates the FM audio signal superimposed on the audio subcarrier.

Here, in this embodiment, the output of the audio detection circuit 35 is not used as it is as an audio demodulated signal, but is supplied to the subtracter 12. The output from the VCO 44 in the PLL synchronous detection section is connected to the other side of the subtracter 12 by the FM demodulator 1.
1.

In such a configuration, the output signal of the mixed 2S42 is
The AC signal generated in the control voltage of the VCO44 cannot be removed even by the loop filter 43, and the output signal of the C044 contains an FM component resulting from the orthogonal component Q(t). <FM component will occur. This FM component corresponds to the buzz interference component in the IF multiplied signal shown in the second term of equation (6), and if the output of the VCO 44 is demodulated by the FM demodulator 11, the buzz interference itself can be extracted.

On the other hand, since the mixer 41 performs synchronous detection using the output of the VCO 44 that includes the FM component caused by the orthogonal component Q(1) as described above, the synchronous detection output includes the FM fluctuation due to the orthogonal component. This fluctuation affects the audio subcarrier, and even during the output of the audio detection circuit 35,
The buzz disturbance component shown by the second term of equation (6) is necessarily included. Therefore, the above FM return; Jl111
By subtracting the output from the audio demodulation output, the subtracter 12 can output an audio signal from which the buzz interference component has been removed.

To explain the above operation in terms of perspective, the output signal e1(t) of the VCO 44 is e, (t)=sin (ωp is 10(t))
- Expressed by (7), the output signal e2(1) of the 90° phase shifter 45 is expressed as C2(t)=cos(ωp by 10(t)
・Represented by (8). Therefore, the audio subcarrier in the output signal at the metal lid 41 is
), then from the product of equations (4) and (8), C3(1)
”C05((J5at 1J (1(t)'d, 19(t)11-(9) is obtained.

The audio detection circuit 35 reproduces the above e3(1) from FM:I!
11. This demodulated output E (t) is E (t) - a (1) (θ (1)) (10).

On the other hand, the output e4 (t) of the audio detector 11 is calculated as follows: KaiC4(t)=π(θ(1)) ・−・(1
1), by subtracting equation (11) from equation (10), it is possible to obtain an audio signal free of buzz interference.

In this way, this embodiment can perform audio demodulation without buzz interference caused by orthogonal components.

Note that this embodiment has the advantage that linearity of the FM modulator 11 is not required.

[Effects of the Invention] As described above, according to the present invention, the buzz interference component essentially included in the demodulated output of the audio 1 navigation carrier is reproduced and reduced from that in the demodulated output, thereby producing an audio signal free of buzz interference. This makes it suitable for audio demodulation circuits for high-definition television reception lines.

[Brief explanation of the drawing]

Figure 1 shows audio reproduction of a television signal according to the present invention: 1
FIG. 2 is a block diagram showing an example of a general audio demodulation device using the intercarrier method, and FIG. 3 shows the characteristics of the Nyquist filter used in the device shown in FIG. 2. FIG. 4 is a block diagram showing an example of an intergear type audio demodulation device using PLL synchronous detection. 11...FM demodulator, 12...subtractor, 31...
Nyquist filter, 41.42...mixer, 43f
...Loop filter, 44...Voltage control excavator, 45
... Phase shifter, 33... Bandpass filter, 35.
...Audio detection circuit.

Claims (1)

[Claims] In an audio demodulating device for a television signal that demodulates and outputs an audio signal using an intercarrier method, there is provided carrier reproducing means for reproducing an IF carrier of a video signal from an intermediate frequency IF signal obtained by heterodyne detection of the television signal. and synchronous detection means for synchronously detecting the IF signal with the reproduced video IF carrier from the carrier reproduction means; and audio detection means for extracting an audio subcarrier component from the synchronous detection output from this synchronous detection means and performing audio detection. , an audio demodulation device for a television signal, comprising: FM demodulation means for FM demodulating the reproduced video IF carrier; and subtraction means for subtracting the output of the FM demodulation means from the output of the audio detection means. .