JPH039421Y2 - - Google Patents

Description

[Detailed description of the invention] "Field of industrial application" This invention relates to a video audio signal transmitter used in VTRs and the like.

"Prior Art" FIG. 5 is a system diagram of a conventional video audio signal transmitter, and is shown together with a video transmission circuit. The video signal input to the video signal input terminal 1 is given to the video transmitting circuit 2, where a synchronizing signal etc. are added, and as shown by curve 2a in FIG. 6, for example, _v =
The band is limited to 4.3MHz, amplified to an appropriate level, and sent to the transmission line from the output terminal 3. Audio signal input to audio input terminal 4 (e.g. 0.2~
20KHz) is applied to the pre-emphasis circuit 5,
In order to improve the S/N ratio on the high frequency side, higher frequency components are given a larger gain and then supplied to the limiter 6. In order to prevent the sideband bandwidth of the frequency modulation signal from expanding more than necessary in the limiter 6,
Particularly large peak voltages of the audio signal are sliced and their amplitude is limited to within a predetermined value ±E _s .

When the audio signal is sliced by the limiter 6, a high frequency noise component is generated, so this noise component is removed through the low frequency filter 7.

A carrier wave (for example, 4.5 MHz) supplied from the carrier wave oscillator 10 is frequency-modulated by the FM modulator 9 using the audio signal provided from the low frequency wave generator 7, and an output modulated wave is applied to the band wave generator 11. If the maximum frequency deviation of the instantaneous frequency of the modulated wave is Δ _s , then Δ _s = k ₁ E _s (1). k ₁ is a constant. The maximum frequency deviation Δ _s is set to 600 KHz, for example. Frequency modulated waves generally have frequencies above and below the carrier wave frequency _c ,
If the frequency is _c ±n _p ( _p is the frequency of the audio signal, n=
1, 2, 3, ...), the amplitude is Betzel function Jn (m)
This results in countless sideband waves expressed as , and the occupied bandwidth becomes infinitely wide. Here, m is called the modulation index and is given by m=Δ _s / _p (2). Curve A in FIG. 7 shows the envelope of the frequency spectrum of this modulated wave. However, there is no practical problem even if the bandwidths B _u and B _l of the upper and lower sidebands around the carrier wave are limited to B _u =B _l Δ _s + _p (3) for transmission. For example, when Δ _s =60 KHz and _p =20 KHz (therefore m=3), B _u =B _l =60+20=80 KHz. The modulated wave is band-limited mainly to frequencies _l to _u by the bandpass filter 11, as shown by curves 11l and 11u in FIG.

Here, _l = _c − B _l , _u = _c + B _u (4). A sideband wave having a frequency in the video signal band (0 to _v ) of the lower sideband of the modulated wave is superimposed on the video signal and is attenuated, for example, by AldB or more by the bandpass filter 11 so that the S/N ratio does not decrease. .

The limiter 6 is configured as shown in FIG. 8, for example. Buffers 21 and 27 are connected to the input terminal 20 and output terminal 29, respectively, a resistor 22 is connected in series between them, and a diode 23 and a reference power source 24 are connected between the input terminal of the buffer 27 and a common potential point. A series circuit with the diode 25 and a series circuit with the reference power supply 26 are connected, respectively. The polarities of the diodes are opposite to each other, and the polarities of the reference power supplies are also opposite to each other. Each buffer has a payoff of 1
Assume that the input impedance is extremely large and the output impedance is almost zero. If the magnitude of the voltage of each reference power source is E _s , then during the period when the audio signal output voltage of the buffer 21 exceeds E _s , the diode 2
A forward voltage is applied to diode 3, so it turns on, and a reverse voltage is applied to diode 25, so it turns off. Therefore, the voltage + _Es of the reference power supply 24 is applied to the input terminal of the buffer 27. Batsuhua 2
Similarly, during a period in which the output voltage of 1 is below _-Es , the diode 23 is turned off and the diode 25 is turned on, and the voltage _-Es of the reference power supply 26 is applied to the input terminal of the buffer 27. During the period when the output voltage of the buffer 21 is in the range of -E _s to +E _s , both diodes are turned off, and the output voltage of the buffer 21 is directly applied to the input terminal of the buffer 27 . Eventually, a voltage whose amplitude is sliced by ± _Es and whose amplitude is limited is output to the output terminal 29.

The low frequency converter 7 is configured, for example, as a ladder type circuit as shown in FIG. 9, and its cutoff frequency is, for example, 20K.
Set to Hz.

The band wave transmitter 11 is, for example, as shown in FIG.
Consists of LC circuit.

``The problem that the invention attempts to solve'' VTRs have become more and more popular recently, and the production quantity has increased, but on the other hand, their distribution prices have been decreasing year by year, and the question of how to reduce the cost of manufacturing is This is a big problem for manufacturers. The purpose of this invention is to realize economicalization of the audio signal transmission circuit of a VTR transmitter.

"Means for solving the problem" The video audio signal transmitter of this invention has an instantaneous frequency _c + kV (t) ( _c is the carrier wave) corresponding to the instantaneous value V (t) (t is time) of the audio signal. frequency, k is a constant)
an FM modulator that outputs a modulated wave having
is larger than the negative reference level (-E _s ), that reference level (-E s ) is output and the reference level (-E _s ) is output.
E _s ) and is expressed as V(t) = -{E _s + υ(t)}, there is a clip circuit that outputs the instantaneous value V(t) as it is, and a clip circuit that outputs the instantaneous value V(t) as it is, and When the output is at the above reference level (-E _s ), the above carrier frequency _c is oscillated, and the output of the clip circuit is smaller than the reference level (-E _s ), and the above V(t) = -
When {E _s +υ(t)}, a carrier wave oscillator consisting of a voltage controlled oscillator that oscillates at a frequency _c +kυ(t) according to its amplitude is provided. In this way, the limiter and low-frequency amplifier that were conventionally required are eliminated.

``Embodiment'' Let us consider a case where the limiter 6 and the low frequency converter 7 are omitted in order to reduce the cost of the conventional circuit shown in FIG. If the maximum amplitude ±V _a of the output voltage of the pre-emphasis circuit 5 is ±V _a = ±(E _s + υ (5), then the maximum frequency deviation ±Δ _s ′ of the output modulated wave of the FM modulator 9 is (1) , from equation (5), it is expressed as ±Δ _s ′z = ±k ₁ V _a = ±k ₁ (E _s + υ) = ± (Δ _s + k ₁ υ)} (6), and k ₁ is lower than the conventional Δ _s becomes larger by υ.

Therefore, the modulation index m in equation (2) becomes m'Δ _s ′/ _p = Δ _s / _p + k ₁ υ/ _p = m + k ₁ υ/ _p } (7) and increases by k ₁ υ/ _p . For this reason, the amplitude J _o (m') of the sideband distributed in the video band (0 to _v ) increases as shown by curve B in Figure 7. Wave device 1
The attenuation amount at frequency _v of 1 is expressed as curve 1 in Figure 6.
At frequency _v as shown by 1'l, 11'u
must be increased to A′ldB. For this reason, the number of sections in the band wave transmitter 11 must be increased significantly (for example, by about twice) compared to the conventional one, resulting in an increase in cost. Therefore, if things continue as they are, the cost of the device as a whole will not be reduced.

Therefore, in this invention, without changing the bandpass filter 11, as shown in FIG.
A) is branched and applied to the clip circuit 12. As shown in FIG. 2B, the clip circuit 12 outputs the reference voltage -Es during the period when the waveform amplitude is above the reference voltage _-Es , which is the same as in the conventional limiter, and outputs the reference voltage _-Es during the peak waveform period when the amplitude is below _-Es . outputs the input waveform as it is and applies it to the control terminal of the voltage controlled oscillator 10'. The voltage controlled oscillator 10' generates an AC signal having a frequency proportional to the magnitude of the input voltage applied to the control terminal and supplies it to the FM modulator 9. That is, the voltage controlled oscillator 10' is used in place of a conventional carrier wave oscillator,
When the control voltage is -E _s , the same frequency _c as before is generated, and when it exceeds -E _s and reaches the peak of -(E _s + υ),
Generate _c ′ expressed by the following equation.

{ _c ′=k ₂ (E _s +υ)=k ₂ E _s +k ₂ +υ = _c +k ₂ υ} (8) In other words, a frequency higher than _c by k ₂ υ is generated.
Here, k ₂ is a proportionality constant, which is set as one of the characteristics of the voltage controlled oscillator. this k ₂
The constant k ₁ in equation (6), which is one characteristic value of the FM modulator 9
, the instantaneous frequency of the modulated wave becomes equal to , if the frequency shift to the negative side by the modulator increases by k ₁ υ according to equation (6), at the same time the frequency of the carrier wave increases according to equation (8). It increases by k ₂ υ, and since k ₁ = k ₂ , the instantaneous frequency of the modulated wave becomes a constant value _c − Δ _s at the negative peak of the audio voltage, as shown in Figure 3, and becomes smaller than this. Never. As a result, the frequency spectrum of the modulated wave is prevented from spreading on the low frequency side as shown by curve C in FIG. There is no need to increase the amount of attenuation at the frequency _v , and the conventional circuit can be used as is.

A part of the upper sideband of the spectrum of the modulated wave selected by the bandpass filter 11 is lost, causing distortion in the positive peak waveform exceeding E _s of the audio signal demodulated on the receiving side, and -E of the audio signal. The waveform in which the negative peak exceeding _s is sliced is demodulated, but it is only instantaneous, so this is not a practical problem. (Even in the conventional example, the peak waveform is sliced by a limiter.) The clip circuit 12 can be easily constructed as shown in FIG. 4, for example. A buffer 31 and a resistor 32 are connected in series between the input terminal 30 and the output terminal 35, and a diode 33 and a reference power source 34 are connected in series between the output terminal 35 and a common potential point. The voltage of the reference power supply 34 is made equal to the reference voltage E _s of the conventional limiter. As already explained with reference to FIG. 2, if the output voltage of the buffer 31 is greater than _-Es , a forward voltage is applied to the diode 33, which turns it on, and the reference voltage _-Es is applied to the output terminal 35. When the output voltage of the buffer 31 becomes -E _s or less, a voltage in the opposite direction is applied to the diode 33, turning it off, and the output voltage of the buffer 31 is applied to the resistor 32.
is applied to the output terminal 35 through.

In the voltage controlled oscillator 10', for example, one of the plurality of capacitors of a tuning circuit that determines the oscillation frequency of the oscillation circuit is configured with a variable capacitance diode, and the output voltage of the clip circuit 12 is applied to both ends of the capacitor, and the voltage is The oscillation frequency may be changed in proportion to the magnitude of .

In the explanation so far, the video signal is transmitted as is. Although the case of so-called baseband transmission has been explained as an example, this invention is not limited to this case.
It is clear that the method can also be applied to the case where the signal is converted into a frequency (frequency) and transmitted. It goes without saying that not only wired transmission lines but also wireless transmission lines can be used.

``Effects of the invention'' As stated above, this invention changes the carrier wave oscillator of the conventional device to a voltage controlled oscillator, branches the input audio signal and guides it to a simple clip circuit.
By clipping the large peak voltage on the negative side and using the clipped voltage to control the oscillation frequency of the voltage controlled oscillator, it is possible to eliminate the limiter and low-frequency amplifier required in conventional devices. This contributes to reducing the cost of the device.

[Brief explanation of the drawing]

Figure 1 is a system diagram showing an embodiment of this invention;
Figure 3 is a waveform diagram to explain the operation of the clip circuit of this invention, Figure 3 is a waveform diagram showing the instantaneous frequency of the modulated wave in this invention together with the input audio signal, and Figure 4 is an example of the clip circuit of this invention. 5 is a system diagram of a conventional video audio signal transmitter, and FIG. 6 is a diagram showing the attenuation characteristics of the bandpass filter of the video audio signal transmitter and the video transmission circuit of the video transmitter. Fig. 7 is a diagram showing the frequency spectrum of a frequency modulated wave, Fig. 8 is a circuit diagram showing an example of a conventional limiter, Fig. 9 is a circuit diagram showing an example of a conventional low frequency filter, and Fig. 10 is a circuit diagram showing an example of a conventional limiter. FIG. 2 is a circuit diagram showing an example of a band wave generator. 1: Video signal input terminal, 2: Video transmission circuit, 3: Video, audio output terminal, 4: Audio input terminal, 5: Pre-emphasis circuit, 6: Limiter,
7: Low frequency generator, 9: FM modulator, 10: Carrier wave oscillator, 10': Voltage controlled oscillator (VCO), 1
2: Clip circuit.

Claims (1)

[Claims for Utility Model Registration] In a video audio signal transmitter that modulates the carrier wave of a carrier wave oscillator with an audio signal using an FM modulator to obtain a frequency modulated output in an adjacent frequency band above the video signal frequency band, Instantaneous frequency _c +kV(t) according to instantaneous value V(t) of signal (t is time) ( _c is carrier frequency,
an FM modulator that outputs a modulated wave with a constant value (k is a constant), and a branch input of the above audio signal, whose instantaneous value V(t)
is larger than the negative reference level (-E _s ), that reference level (-E s ) is output and the reference level (-E _s ) is output.
E _s ) and is expressed as V(t) = -{E _s + υ(t)}, there is a clip circuit that outputs the instantaneous value V(t) as it is, and a clip circuit that outputs the instantaneous value V(t) as it is, and When the output is at the above reference level (-E _s ), the above carrier frequency _c is oscillated, and the output of the clip circuit is smaller than the reference level (-E _s ), and the above V(t) = -
A carrier wave oscillator comprising a voltage controlled oscillator that oscillates a frequency _c + kυ (t) according to the amplitude when {E _s +υ (t)}.