JPS63158983A - Voice demodulator for television signal - Google Patents

Abstract

PURPOSE:To eliminate buzz harm that may harm an output after FM modulation by generating a mixing signal for a video component in the vicinity of video carrier, FM detecting this mixing signal to generate a component coincident with a buzz noise. CONSTITUTION:In the output of a mixer 14, a voice subcarrier signal equivalent to the difference frequency between the video carrier and a voice carrier is obtained, which is subjected to a BPF 15, and a voice signal is demodulated in and FM detector 16. In the mean time, a filter 12 extracts a video carrier component with its characteristic that is sharper than that of a filter 11, and supplies the result to a mixer 18. From a signal including the difference signal obtained from the mixer 18, the difference frequency is separated through a BPF 19, and it is supplied to an FM detector 20. An FM demodulated output from the detector 20 is subtracted from a voice demodulated output from the detector 16. As a result, a buzz harm occurring in the output of the detector 20 can be eliminated.

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 provides a dedicated circuit for the audio system. The present invention relates to an audio demodulation device for television signals that removes buzz interference generated during the FM detection process from the output after demodulation.

(Prior Art) Generally, a transmitted television signal is transmitted using a vestigial sideband method, for example, at a frequency of 0.75 [MHz] from a video carrier.
Within the lower sideband, both sidebands are transmitted as they are, and only the lower sideband where the frequency is 1.25 [MHz] or more away from the video carrier is completely suppressed. Then, the video signal is transferred to the video carrier A.
The audio signal is FM-modulated and transported on an independent audio carrier that is exactly 4.5 [MH2] away from the video carrier.

An intercarrier type receiver that receives such residual sideband signals converts the two signals of video and audio through a frequency conversion circuit.
By directly detecting the two intermediate frequency signals in the same intermediate frequency processing circuit, an audio subcarrier signal corresponding to the difference frequency between both carriers is generated, and this is used to demodulate the audio signal.

FIG. 2 shows an example of the configuration of the audio 111 circuit based on the above method, where 31 is a Nyquist filter, 32 is an envelope detector, 33 is a bandpass filter, and 34 is an audio FM detector. , an IF low signal consisting of two IF low signals of video and audio amplified by an intermediate frequency (IF) amplification circuit (not shown) is supplied. FIG. 3 shows the frequency characteristics of the Nyquist filter 31, and its characteristic characteristics are 0, 7 before and after (±) around the same frequency ωp in the IF video image filter 9F.
It has a characteristic of first-order slope in a certain frequency ωd range less than 5 [MH2]. 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 transmitted in both sidebands (O~750 [KH2]
) can be suppressed from increasing.

In addition, in FIG. 3, ωa is the audio IF key 11 rear frequency.

The envelope detector 32 detects the video AM acid 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 bandpass detector. It is supplied to the filter 33. The bandpass filter 33 extracts the high-frequency component of the envelope detection output, that is, the audio subcarrier signal of ω −ωa p to ωa. The 1M detector 34 performs FM detection on the audio subcarrier signal ωSa and extracts an audio demodulated output.

The signal that has passed through the Nyquist filter as described above can generally be expressed as the following quadrature AM modulation signal.

e (t) − 1(t) cos ccB+ t + (
1(t) sin ωp t...(1) T(t)=C+Σ((8 1 +8, 1,) − c
os(ωk t+θk)) ...(1
-1 )Q(t)-Σ((SH S,k) ・sin
(ωk t+θk )) ...(1
-2) Here, C is the DC term. 5IICO3 (ωk
t+θk) is lower than ωp by ω, S, kSi
n ((i) k i +θk) represents a modulation signal component corresponding to a baseband signal at a frequency ω located above ωp by ω.

In the case of a television receiver, the quadrature AM modulated signal as described above has a relationship of SIk+suk 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 of the Nyquist filter, 51kh%Suk
One of the chairs 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 (t) component is a fully synchronous detector, but realizing fully synchronous detection requires a complicated circuit configuration, and it is difficult to use a device such as a television receiver. This is uneconomical for consumer electronics. Therefore, envelope detectors are often used. The detection output from an envelope detector generally produces distortion due to orthogonal components. 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 difference between Suk and Suk is smaller at lower frequencies. etc., 1t))Q (
It is assumed that if the condition t) is satisfied and the signal of equation (1) is envelope-detected, I (t) can be approximately obtained.

In other words, xcos [ωp t −jan −1(−′?−H+
−)] ...From (2), &! I (t)...
The approximate output (3) is used as the envelope detection output.

However, with this approximate output, Q (t) even when the video signal has the maximum level of
Since the level is only 4 to 5 times the level of the audio subcarrier signal ωSa, which is always the maximum among the
Contains many humming frequency components involving The main components of these components are the beat frequency component of ω, a and the color subcarrier, which acts as an interference signal to the video signal, and the buzz interference component, which acts 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) −1(t) cos ωp t +Q(t)
sin ωp t+Acos (ωa t+f a(
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) in the output is
Listing the main components that appear in the vicinity, there are the following three scales in descending order of magnitude.

■Video low frequency component 111(t) -Q,F(t
) and the audio carrier ωa nearby component, es (t
) −A (1(t) )r; −7Qπ tv[[ xcos (ωsa 10(t)) −(5)ω
p+ωa ■The second harmonic of the beat of the video low-frequency component around ωp and the video mid-range component around -2-·, ■The audio carrier around ωa and the video mid-range component around (I)p+″a The second harmonic of the beat, ■, is the signal shown in equation (4),
Audio subcarrier signal Acos (ωsat +
f a (t) dt) is affected by phase modulation and amplitude modulation interference, and although the amplitude modulation interference can be removed by a limiter, the phase modulation component cannot. The FM detection component of such a component is 111! Electricity = (θ (t)) - a (on -... (6
), and the second term represents the buzz interference output.

According to equation (6), QLl(t) of the video low-frequency components
It can be seen that if the component is 0, 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, the disturbances (2) and (2) involving video mid-range components are added, so conventional demodulators that obtain audio subchirelia signals from video envelope detection outputs have a limit in obtaining high-quality audio. be.

Figure 4 shows selected carriers obtained by extracting video IF components near ωρ with steep characteristics without using a Nyquist filter.
An example of a method for generating audio subcarriers is shown. The circuit shown in FIG. 4 does not include a Nyquist filter, but uses a filter 41) to extract only the components around ωρ of the video IF carrier and around ωa of the audio IF carrier, and a limiter 43 which outputs the output of a filter 42 that sharply extracts the area around ωρ. The signal is input to the mixer 44 via the filter 41 and multiplied by the output from the filter 41 to obtain an audio subcarrier signal. Note that 45 is a filter that functions equivalently to the bandpass filter shown in FIG. 2, and 46 is an FM detector.

This type of circuit extracts the video IF carrier ωp that does not pass through the Nyquist filter, so ideally ωρ
There are no modulation components due to Q and F(t), and (6
) Buzz interference like the second term of the equation should not appear. In fact, in this circuit, the interference caused by the components (2) and (2) among the components (1) to (2) above is prevented. However, in reality, due to the asymmetry of the filters 41 and 42 for extracting ωp and the presence of slight orthogonal modulation components Q and F(t) caused by the VSB filter on the transmitting side, the auditory sense may vary depending on the content of the video signal. Buzz noise may occur.

Furthermore, in the circuit shown in FIG. 4, when the video carrier is overmodulated, the inevitable video carrier phase inversion causes this inversion key 1? Since the phase of the audio subcarrier signal is also reversed when converted by the rear, a spike-like buzz is generated.

In this case, the IF low signal with the characteristics regulated by the intermediate frequency amplification stage is supplied to the video detection circuit. Then, image detection is performed through a Nyquist filter as shown in FIG.

On the other hand, there are television receivers that are designed to prevent buzz interference in principle, but they are not widely used because they have complicated configurations and are uneconomical. For example, in a buzz interference-free television receiver, the audio carrier is received by a completely different frequency conversion circuit in the tuner, or the audio IF
There are other methods that perform frequency conversion at the signal stage, but with the former, it is difficult to accurately link video channel switching and audio channel switching, and a common problem between the former and the latter is that It is necessary to apply AFC for audio detection to the frequency conversion circuit.

(Problems to be Solved by the Invention) Even if a television receiver that adopts the conventional intercarrier system uses a filter that sharply extracts only the components near the video carrier, due to the asymmetry of its characteristics, or the transmission Due to the side VSB filter, orthogonal components Q and Fm of the video low frequency components exist, causing a problem of buzz interference.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and provide a television signal audio demodulation device that eliminates buzz interference from the output after FM demodulation and achieves a reliable prevention effect.

[Structure of the Invention] (Means for Solving Problems) The present invention provides video carrier component output means for extracting only signal components near the video carrier from an intermediate frequency signal including video and audio signals, and and a subtraction means that subtracts the noise component from the buzz component generation means from the audio demodulation output from the FM detection means for audio demodulation. It is characterized in that the generated audio buzz interference is removed from the i-key audio output.

(Function) The intercarrier type audio v11 circuit converts the beat signal, which is the result of multiplication of the video carrier and the audio carrier, into FM
Detect and demodulate the audio signal. In one example of the present invention, as a second beat signal equivalent to the audio beat signal, a mixed signal with an arbitrary frequency is created for video components near the video carrier, and this mixed signal is subjected to FM detection.
Generates a component that matches the buzz noise generated in the iron platform demodulation system. Then, the generated buzz noise is transmitted to the audio FM11'l! i
Buzz noise is removed by subtracting it from the audio signal from the device.

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

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

In FIG. 1, reference numeral 11 denotes a first filter that receives an IF low signal input from a video intermediate frequency amplification circuit (not shown) and extracts a video carrier component and an audio carrier component. The output of this first filter 11 is further supplied to a limiter 13 via a second filter 12 that extracts a component centered on the video carrier. The output of the limiter 13 is input to a mixer 14 and multiplied by the signal from the first filter 11. As a result, an audio subcarrier signal corresponding to the difference frequency between the video carrier and the audio carrier is obtained as the output of the mixer 14. This signal is supplied to a first FM detector 16 via a bandpass filter 15 that extracts the audio subcarrier signal, and the audio signal is demodulated.

On the other hand, the second filter 12 inputs the video carrier component extracted with a steeper characteristic than the first filter 11 to a mixer 18 to which a carrier signal having an arbitrary angular frequency ωe is supplied at one end.

The signal ωe is supplied from an oscillator 17. The signal 9 obtained from the mixer 18 is ωe
The signal includes a difference frequency signal between ωp and ωp. Mixer 18
The signal is input to the next stage bandpass filter 19 to separate the difference frequency components ωe to ωp, and the second FM detector 20 is supplied with the same difference frequency signal. The second FM detector 20 FM demodulates the difference frequency signal, and its output is supplied to the other end of a subtraction circuit 21 into which the audio detection output from the first FM detector 16 is input. In the present invention, the output of the fraud reduction circuit 21 is supplied to the audio value system as an audio demodulation output.

This embodiment is configured as described above.

In such a configuration, the output e1 of the second filter 12
(t) is x C03(ωpt-θ(t))...
(7) (θ(t) −tan −1(fence)) From this, the output of the limiter 13 is C03(
ωpt−θ(t)) ・−(8)
becomes.

Also, the audio carrier ω at the output of the first filter 11
If the signal of a encounter is C2(t), then e (t)−
coS(ω t+fa(t)dt) ・(9
)a is represented by 6.

The signal e3(t) (audio subcarrier signal) near ω, a−ωp to ωa in the output of the mixer 14 is expressed as (8), (9
), so e 3(t) −CO3(ω, t + f a (t)
dt-θ(t))... (10) It becomes.

Here, the buzz disturbance component in the above equation (10) is ΔL a(t)-(θ([))...(11)
dt.

On the other hand, the signal @ e 1(t) is frequency-converted by the mixer 18, but the output of the second FM detector 20 that performs FM detection on this output is equal to the FM detection result of el(t), θ(t)) ・・−(12)
dt, which is the phase modulation component of the video carrier subjected to FM detection.

The component of equation (12) is the buzz component itself that occurs in the output of the first FM detector 20, and therefore, (11)
By subtracting equation (12) from equation (12), the first FM detector 20
It is possible to remove the buzz interference generated in the output of the .

In this way, the present embodiment includes a mixer 189, an oscillator 17. Bandpass filter 19 and F
In addition to the configuration consisting of the M detector 20, the audio FM detector 16
Since the buzz interference component generated by the additional configuration described above is subtracted from the audio demodulated output from the audio demodulation output, more reliable buzz removal performance is achieved than in the past. In other words, the present invention takes measures against buzz interference for the II copper tone post-output.

In another embodiment, since the second FM detector 20 is not required to have good detection performance (as long as the detection range is wide, linearity is not a problem), the components near the video carrier from the second filter 12 are The buzz component may be extracted by direct FM detection.

[Effects of the Invention] As explained above, according to the present invention, since measures against buzz interference are taken for the output after audio FM demodulation, the effect is that the performance for removing buzz components generated during the demodulated output is high. be. According to experiments, it has become possible to remove approximately 20 [dB] of audible buzz interference.

[Brief explanation of the drawing]

FIG. 1 is a circuit block diagram showing an embodiment of a television signal audio demodulation device according to the present invention, FIG. 2 is a circuit block diagram showing an example of a conventional audio 1a modulation device, and FIG. 3 is a circuit block diagram showing an example of a conventional audio demodulation device. A characteristic diagram showing the frequency characteristics of the Nyquist filter used, and FIG. 4 is a circuit block diagram showing another conventional example. 12...Second filter, 14...Mixer, 15
... Band pass filter, 16... First FM detector, 17... Oscillator, 18... Mixer,
19...Band pass filter, 20...Second F
M detector, 21... subtraction circuit.

Claims (3)

[Claims] (1) In an audio demodulation device for television signals that demodulates and outputs audio signals using the intercarrier method, an audio subcarrier signal obtained by multiplying both carrier components in an intermediate frequency signal containing video and audio signals is F.
FM detection means for performing M detection and demodulating the audio signal; video carrier component extraction means for inputting the intermediate frequency signal and deriving a signal component near the video carrier as an output; and a subtraction circuit means for subtracting the signal component from the buzz component generation means from the audio signal from the FM detection means. Device. (2) The buzz component generation means includes means for generating a mixed signal of the video carrier of the signal component from the video carrier component extraction means and a carrier of an arbitrary frequency, and detecting the FM component from the mixed signal generation means. The audio demodulation device for a television signal according to claim 1, characterized in that it comprises FM detection means. (3) The audio demodulation device for a television signal according to claim 1, wherein the buzz component generation means uses means for directly FM detecting the signal component from the video carrier extraction means. .