JPS61193584A - Input circuit for fm demodulator - Google Patents

Abstract

PURPOSE:To prevent inversion phenomenon by making high- and low-pass filters into high order filter constitution having plural different poles and preventing the occurrence of phase dislocation between a signal on the high-pass filter side and a signal on the low-pass filter side. CONSTITUTION:The high-pass filter 2 and the low-pass filter on which 4 poles are mounted together have the frequency characteristics as shown in the diagrams and their characteristic curves are more sharp than conventional ones. When comparing phase respective characteristics 12 and 13 of the high-pass filter 2 and the low-pass filter 3, change ratios of phase angle against a fre quency change are roughly coincident. Moreover the frequencies of respective two poles (ie, omega1=omega2, omega2=omega4), which are corresponding to complex planes of respective high- and low-pass filters 2 and 3, conicides each other and accord ingly the output phase of the high-pass filter 2 can be always dislocated 180 deg. against the low-pass filter 3 in a total frequency band. consequently, a mixed phase can be changed to be an in-phase by subtracting, ie, adding signals picked up from respective filters 2 and 3 in reverse phase. Thus it enhances more preventive effect for inversion phenomenon than before.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to an input circuit for an FM demodulator, and relates to an input circuit for an FM demodulator, for example, in a magnetic recording/reproducing device, in which demodulated output is generated due to an imbalance between an upper sideband and a lower sideband of a reproduced FM video signal. This is designed to reduce the impact on

[Technical Background of the Invention] Conventionally, in a magnetic recording/reproducing device (hereinafter referred to as a VTR), a video signal is converted into an FM signal in a frequency range that can tolerate various losses of a video head and then recorded. As a result, the reproduced FM signal has an upper sideband as opposed to a lower sideband due to limitations imposed by the recording wavelength of the head (e.g. tape self-demagnetization loss, thickness loss, recording demagnetization loss, head gap loss, etc.). It has the property of being attenuated. therefore,
If this is demodulated as is, distortion due to the AM component will be mixed in, so an amplitude limiter is used to restore the upper side wave before demodulation. However, simply by limiting the amplitude in the reproduction system as described above, it is not possible to reduce the difference in the reproduction level between the signal containing the upper side wave and the signal not including the upper side wave to 6J8 or less, resulting in attenuation of the high frequency range due to the reproduction frequency characteristic. Therefore, this high frequency deterioration is
To compensate without inducing signal-to-noise ratio degradation,
In recording systems, the amount of pre-emphasis of the video signal is increased, or the lower side wave level of the modulated signal is increased relative to the carrier wave level.

However, if the electromagnetic conversion characteristics of the tape-head system deteriorate, spurious noise enters the head, or the video signal suddenly changes from black to white, the carrier wave level will drop below the lower side wave level. The average carrier frequency may shift from its original position to the lower waveform. When such a reproduced FM signal is demodulated, the FM
The demodulator regards the lower side wave as a carrier wave, causing a so-called inversion phenomenon in which the reproduced video signal changes from white to black.

FIG. 4 is a circuit block diagram showing the configuration of a conventional demodulation input stage designed to prevent the above-mentioned inversion phenomenon.

In this figure, reference numeral 21 is an input terminal to which a reproduced FM signal is supplied, 22 is a high-pass (pass) filter, and 24 is a low-pass (pass) filter.
These filters have damping characteristics with a relatively gentle slope similar to those of ordinary differentiating circuits and integrating circuits. The high-pass filter 22 mainly extracts carrier wave components, and the low-pass filter 23 extracts lower side wave components. The signal passing through the high-pass filter 22 is transmitted to the first amplitude limiter 2
4, the carrier wave component is corrected to eliminate level fluctuations due to fluctuations in the average carrier frequency, and is further mixed with the lower side wave component from the low-pass filter 23 via the amplifier 26 via the harmonic removal circuit 25. be done. This mixing is performed in the mixer 27, and its output is sent to the second amplitude limiter 2 in the next stage.
8 to the FM demodulator 29, and a demodulated output from which the inversion phenomenon has been removed is obtained. In other words, in this system, in addition to the second amplitude limiter 28 for correcting the upper side wave, a first width limiter 24 is specially provided as a level fluctuation correction means dedicated to the carrier wave, and the DL-FM (double limiter) FM) method.

[Problems with the background art] By the way, as mentioned above, the reproduced FM signal is passed through the high-pass filter 2.
In a circuit that branches to the 2 side and the low-pass filter 23 side and mixes again, it is required that the signals from both paths are not out of phase with each other. However, the high-pass filter 22 and the low-pass filter 24 in the conventional circuit use filters whose transmission characteristics are expressed as (at, bl are positive coefficients, and ωH1ω is the cutoff angular frequency), so each filter There is a problem in that the output phase of 22.23 is shifted and the resulting phase modulation distortion causes overshoot and smear at the edge portion of the waveform after FM demodulation (waveform distortion in the case of a sine wave).

That is, FIGS. 5 and 6 correspond to (1) above.

The phase characteristics and amplitude characteristics of the high-pass filter and low-pass filter expressed by equation (2) are shown. The horizontal axis in each figure is the frequency [MHz] axis, and the vertical axis in Figure 5 is the phase table. [deo] axis, the vertical axis in Figure 6 is the standardized amplitude axis,
30 is a phase characteristic curve of a high-pass filter, 31 is a phase characteristic curve of a low-pass filter, 32 is an amplitude characteristic curve of a high-pass filter, and 33 is an amplitude characteristic curve of a low-pass filter.

As is clear from these figures, when characteristics 30 and 31 are compared, it can be seen that the rate of change of the phase angle with respect to frequency is different. Therefore, according to the conventional circuit, a circuit that complementarily corrects the phases of the two FM signals input to the adder 27 is required. Moreover, ωC indicates the carrier frequency, and the frequency below this ωC (lower sideband) is transmitted by the high-pass filter 2.
Since the cut-off characteristics of 2 are not good, the signal leaks and is mixed into the lower sideband from the low-pass filter. Therefore, if the phase of the lower side wave from the high-pass filter deviates from the phase of the lower side wave from the low-pass filter, it appears as a decrease in the output amplitude of the FM demodulation output, and the lower side wave from the low-pass filter side is mixed. Therefore, the purpose of not deteriorating the high frequency range cannot be fully achieved.

[Object of the invention] The present invention has been made in view of the above-mentioned points.
In a demodulator adopting the M method, distortion of the demodulated output due to a phase shift between the high-pass filter side and the low-pass filter side,
It is an object of the present invention to provide a means for preventing an inversion phenomenon without causing deterioration of the SN ratio or deterioration of reproduction frequency characteristics.

[Summary of the Invention] In order to achieve the above object, the present invention provides a high-pass filter and a low-pass filter with a high-order filter configuration having a plurality of different poles, and in which each pole frequency is made to match each other. This setting is made so that no phase shift occurs between the signal on the high-pass filter side and the signal on the low-pass filter side.

[Embodiments of the invention] Hereinafter, the present invention will be described in detail with reference to illustrated embodiments.

FIG. 1 is a circuit block diagram showing one embodiment of an input circuit according to the present invention, and FIGS. 2 and 3 show amplitude characteristics and phase characteristics of a high-pass filter and a low-pass filter used in the present invention. It is a characteristic diagram.

In FIG. 1, 1 is an input terminal to which a reproduced FM signal from a video head (not shown) is supplied;
For example, the reproduced FM signal is introduced into a high-pass filter 2 and a low-pass filter 3 each having a second-order filter configuration. These high-pass and low-pass filters 2 and 3 are set to have two poles each, using, for example, a differential circuit with CR as an element and a cascade of integrated circuits, and in this embodiment, a total of four poles. Make sure the frequencies match.

The main component of the output of high-pass filter 2 is the carrier frequency,
The first amplitude limiter 4 corrects the level fluctuation. This amplitude limiter 4 is constructed as an inverted output type, and its output is introduced into a mixer 6 via a harmonic removal circuit 5 at the next stage.

On the other hand, the low-pass filter 3 extracts the lower side wave component and introduces it into the amplifier 7, and the signal amplified by the amplifier 7 is supplied to the mixer 6. This amplifier 7 is used to match the carrier level of the signal component derived by the low-pass filter 3 to the output of the harmonic removal circuit 5 (the fundamental wave level of the first amplitude limiter 4), and is not necessarily necessary. . The harmonic removal circuit 5 attenuates the odd-order harmonics generated in the first amplitude limiter 4 to obtain a mixing effect with the lower side waves from the low-pass filter 3 side, and changes the phase characteristics of the fundamental wave. A simple low-pass filter may be used as long as it does not significantly affect the

Next, since the first amplitude limiter 4 is of a phase inversion type, the mixer 6 functions as a subtracter and introduces the output to the second amplitude limiter 8. The output of the second amplitude limiter 8 is introduced into an FM demodulator 9 and demodulated.

According to the above configuration, the high-pass filter 2 and the low-pass filter 3, whose four poles are matched, have amplitude characteristics 10 and 11 as shown in FIG. 2, which are steeper than the characteristics 32 and 33 in FIG. 4, respectively. It becomes. Note that ωC represents a carrier wave frequency, and WC represents its average frequency band. Further, WL indicates a lower side wave band. FIG. 3 shows the phase characteristics, in which 12 is a high-pass filter 2 and 13 is a low-pass filter 13.

Comparing these phase characteristics 12 and 13, it can be seen that the rate of change in phase angle with respect to frequency change is approximately the same.

That is, in the case of the embodiment, the second-order high-pass filter 2 has an ω of O−■.
180° to O for change.

The phase angle of the low-pass filter 3 changes from Oo to 180° with respect to the same change in ω. 2
By matching the frequencies of the two poles (ω1 = ω2. ω2 - ω), the output phase of the high-pass filter 2 can always be shifted by 180° in the entire frequency band with respect to the low-pass filter 3. . Therefore, each filter 2.3
By subtracting the signals extracted from, that is, adding them in opposite phases, the mixed phase can be brought into the same phase. In general, when each filter 2.3 is configured with (4n-2) orders, the mixing may be subtractive, and when it is configured with 4n orders, it may be an additive type. This allows the mixed phases to be in phase. Note that n is a positive integer.

Here, ω1. ω2 and ω3. If ω is the two pole frequencies of each of the high-pass filter 2 and the low-pass filter 2.3, the transmission characteristics of the high-pass filter 2 and the low-pass filter 3 are expressed by the following equations.

...(4) Here, a2. b2 and ωj, ttr2, ω3
, ω is a positive constant. Further, the phase angle is expressed by the following equation.

ω arQGH = π-(Tan-1(-)+Tan-1(
))... (5) aro GL = -(Tan -1(? )
+Tan-1(7))... (6) Here, Tan-1 is the upper value of tan-i.

In this example, in equations (3) and (4), ω1−ω2
Since it is set as = ω3 = ω, the respective phase characteristics match except for a deviation of π, that is, 18°. However, in the above embodiment, ω1-ω3 and ω2=
It can also be used as a cow.

In addition, each filter 2 and 3 when all four poles are matched
The gains IGH1 and IGL1 are expressed as by appropriately adjusting the carrier level of the low-pass filter 3 at the output end of the amplifier 7 with respect to the level of the fundamental wave of the first amplitude limiter 4 (for example, (just adjust the gain of amplifier 7)
, the transmission characteristics of the sidebands can be flattened.

The phase angle at that time is aro GH-7C-2tan -1 (1) 11. (
9) ωC arQ GL = yc-2tan -1 (Sat), (1G
) ωC. Here, ω0 is preferably set to approximately the average carrier wave angular frequency ωC.

Furthermore, in the present invention, since the mixing level of the lower side wave from the low-pass filter 3 side can be increased or decreased with respect to the lower side wave level from the high-pass filter 2 side, the reproduction frequency characteristics can be extended. .

As described above, the present invention eliminates the need to provide adjustment elements for phase correction on each of the high-frequency side and the low-frequency side.
．． The signals from 3 can be mixed in phase.

Note that if the first amplitude limiter 4 is not configured with an inverted output type, the mixer 6 can be an addition type, but as another method of making it a subtractive type, the amplifier 7 can be configured with an inverted output type. You can do it like this.

Furthermore, regarding the group delay time caused by passing through the filter according to the present invention, flatness in the FM transmission system and the amount of delay between the color signal and the color signal are problematic, but these problems are related to the total phase compensation and the delay amount in the FM transmission system, respectively. Adjustment can be made using the Y-C time difference correction circuit after demodulation.

[Effects of the Invention] As explained above, according to the present invention, the high-pass and low-pass filters have a second-order configuration, and the eight poles of the filters are made to coincide with each other, so that the signals transmitted on the high-frequency side and the low-pass side can be transmitted. This has the effect that the phase can be matched to the input phase.

This eliminates the need to provide a phase correction circuit on each of the high-frequency side and the low-frequency side as in the past, and the lower side waves from the high-frequency side can be sufficiently attenuated, making the inversion prevention effect more effective than in the past. It has the advantage of being more expensive.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a circuit showing an embodiment of an input circuit according to the present invention, FIG. 2 is a characteristic diagram showing an example of amplitude characteristics of high-pass and low-pass filters according to the present invention, and FIG. 3 is a diagram showing an example of the amplitude characteristics of the above-mentioned filter. Fig. 4 is a block diagram showing an example of the configuration of a conventional input stage, Fig. 5 is a characteristic diagram showing a conventional phase characteristic, and Fig. 6 shows a conventional amplitude characteristic. It is a characteristic diagram. 2...High-pass filter, 3...Low-pass filter, 4...
・First amplitude limiter, 5... Harmonic removal circuit, 6...
Mixer, 7... Amplifier, 8... Second amplitude limiter, 9
...-FM demodulator, ωC...carrier angular frequency. Agent: Patent Attorney Noriyuki Chika (and 1 other person) 3rd
Figure 4 Figure 5

Claims (5)

[Claims] (1) Two filter circuits to which frequency-modulated video signals are respectively supplied, each having a plurality of poles on the complex plane, and at least the frequencies of the corresponding poles are set to be approximately equal. a high-pass filter and a low-pass filter; a first amplitude limiter that corrects level fluctuations in the signal that has passed through the high-pass filter; and an output of the amplitude limiter and an output of the low-pass filter. A mixing means for mixing, and a second amplitude limiter for correcting level fluctuations of the signal from the means, and adjusting the phases of the signals output from the high-pass and low-pass filters to a predetermined phase relationship. An input circuit for an FM demodulator, characterized in that the input circuit is configured to supply data to an FM demodulator in this manner. (2) When the order of each of the filters is (4n-2) (n is a positive integer), the mixing means is of a subtractive type in which the phases of the input signals are opposite to each other. An input circuit for an FM demodulator according to claim 1, characterized in that the input circuit is configured as follows. (3) The first amplitude limiter is configured such that its output is inverted in phase with respect to the input, and the output thereof is used as one input of the mixing means. The input circuit of the FM demodulator according to item 2. (4) The means for inverting the phase of one input to the mixing means is an amplifier that inverts the phase of and amplifies the output of the low-pass filter. The input circuit of the FM demodulator described in . (5) When the order of each of the filters is 4n, the mixing means is of an additive type in which the input signals are in phase with each other. FM demodulator input circuit according to range 1.