JP2870120B2 - FM demodulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention demodulates a signal recorded by FM modulation.
The present invention relates to an FM demodulation circuit suitable for equipment in a VTR or the like.

[Summary of the Invention]

The present invention demodulates a signal recorded by FM modulation.
Regarding FM demodulation circuit suitable for equipment for VTR etc., input F
The M-wave or variable equalizer takes in the demodulation circuit, and the frequency characteristics of the variable equalizer change according to whether the inversion is white inversion or black inversion. Suitable demodulation can be appropriately performed according to the type, recording state, and the like.

[Conventional technology]

For example, in a VTR reproducing system, as can be understood from FIG. 2, an FM modulation signal (analog FM wave) recorded on a magnetic tape (T) is converted into an A / D signal via a head (H) and an amplifier (3). The signal is taken into the converter (5) and converted into a digital FM wave.

The output of the A / D converter (5) is input to the demodulation circuit (4) via the equalizer (2) and demodulated, so that the characteristics of the head (H) and the like are corrected by the equalizer (2). You.

[Problems to be solved by the invention]

However, since the filter (frequency) characteristics of the equalizer (2) are fixed, if the type of the magnetic tape (T), the recording state changes, or the like, the reproduction state changes, the correction by the equalizer (2) is not always appropriate. In some cases, white reversal and black reversal occur, and good demodulation cannot be performed.

An object of the present invention is to provide an FM demodulation circuit that can perform good demodulation even when a reproduction state or the like fluctuates.

[Means for solving the problem]

In order to achieve the above object, an FM demodulation circuit according to the present invention comprises: an FM demodulation circuit (11) for demodulating an FM wave input via a variable equalizer (7) having a variable frequency characteristic; A high-pass filter (11) that receives a first signal output from the variable equalizer (7) and outputs a second signal in which black inversion is suppressed; and a white inversion that receives the first signal and receives the first signal A low-pass filter (15) that outputs a third signal in which the first signal, the second signal, and the third signal are suppressed; When the detection circuits (19), (21), (23) and the detection circuits (19), (21) detect inversion of the first signal and the second signal, it is determined that white inversion is performed. Detection circuits (21) and (23) detect inversion of the first signal and the third signal. Determination circuit determines that black inverted when the (25)
(27), (29), (31) and (35), wherein the frequency characteristic is changed according to the judgment output signal of the judgment circuit (25) (27) (29) (31) (35). It is characterized by.

[Action]

In the FM demodulation circuit according to the present invention, the frequency characteristics of the variable equalizer are changed according to the judgment output signal of the judgment circuit.

Embodiment A preferred embodiment of the FM demodulation circuit according to the present invention will be described below with reference to the drawings.

In FIG. 1, an FM demodulation circuit (1) to which the present invention is applied is provided in, for example, a VTR reproduction system, and an analog FM modulation signal such as a video signal recorded on a magnetic tape (T) is transmitted to a head (H). ), Taken into the A / D converter (5) via the amplifier (3), converted into a digital FM wave, and input to the variable equalizer (7).

The output signal of the variable equalizer (7) is input to a separation circuit (9) such as a Hilbert transformer, and is separated into two-component waves having phase shift angles of 0 and 90 degrees.

The separated two-component wave is converted to a high-pass filter (HPF) (1
1), delay circuit (DL) (13), low-pass filter (L
PF) (15) and a demodulation circuit (17), the output signal of the demodulation circuit (17) is subjected to a predetermined process (Y process), D / A converted, and outputted to an output terminal (1
0) is output.

In addition, as a technique in which a digital FM wave is separated into two component waves, and these two component waves are subjected to complex number operation and demodulated,
IEEE Transactions on Consumer Electronics.Vol.CE-3
1, No. 3, August 1985, and the like are known.

On the other hand, the output signals of the HPF (11), the DL (13), and the LPF (15) (the second signal, the first signal, and the third signal, respectively) are inverted detection circuits (19) and (21), respectively. , (23)
White inversion, black and white inversion, and black inversion are respectively detected.

That is, the white inversion and the black inversion do not occur at the same time, and the black inversion is suppressed by the HPF (11).
LPF (15) suppresses white inversion and DL (13)
In this case, both white inversion and black inversion can occur.

Therefore, when the output of the HPF (11) and the output of the DL (13) are taken into the AND gate (25), it is determined that white inversion occurs, and conversely, the output of the LPF (15) and the output of the DL (13) are determined. When the output is taken into the AND gate (27), it is determined that black inversion has occurred (determination circuit).

When the inversion phenomenon occurs, the input level value decreases. Therefore, as a circuit for detecting the inversion, it is also preferable to use a circuit for detecting the level value.

The output signals of the AND gates (25) and (27) are input to integrating circuits (29) and (31), respectively.
The output signal of (31) is input to the comparator (35) and fed back to the variable equalizer (7).

Here, the variable equalizer (7) is an A / D converter (5)
Delay element (7a), adder (7c) that receives the output signal of
And a delay element (7b) to which an output signal of the delay element (7a) is input, and an adder (7e).

The output signal of the delay element (7b) is input to the adder (7c), and the output signal of the adder (7c) is input to the amplitude adjustment circuit (7d).
Is input to the adder (7d), and the polarity of the input signal is inverted and input to the adder (7e) in the circuit (7d).

The characteristic (frequency characteristic) of the circuit (7d) is changed according to the signal fed back from the comparator (35), and functions as an HPF when K is positive, as a flat delay when K = 0, and as an LPF when K is negative. Is set to

That is, the balance is set such that the occurrence of white inversion and the occurrence of black inversion occur at substantially equal rates.

As described above, in the present embodiment, while the first signal and the second signal are input to the AND gate (25),
The first signal and the third signal are input to the AND gate (27), and white inversion and black inversion are determined.

Then, the frequency characteristic of the circuit (7d) is changed according to the determination result.

Therefore, even if the type of the magnetic tape (T), the recording state, and the like, the reproduction state and the like fluctuate, it is possible to appropriately perform good demodulation, and always maintain the optimum image quality.

〔The invention's effect〕

As understood from the above description, in the FM demodulation circuit according to the present invention, the frequency characteristic of the variable equalizer is changed according to the judgment output signal of the judgment circuit.

Therefore, for example, the type of magnetic tape played back by a VTR,
The optimum frequency characteristic can be adjusted according to the recording state and the like, and good demodulation can be performed appropriately corresponding to the reproduction state and the like.

[Brief description of the drawings]

FIG. 1 is a block diagram of a preferred embodiment of an FM demodulation circuit according to the present invention, and FIG. 2 is a block diagram of a conventional example. (1) is an FM demodulation circuit, (7) is a variable equalizer, (7d)
Is an amplitude adjuster, (19), (21) and (23) are inversion detection circuits, (25) and (27) are AND gates, and (35) is a comparator.

Claims (1)

(57) [Claims] An FM demodulation circuit for demodulating an FM wave input through a variable equalizer having a variable frequency characteristic, wherein a first signal output from the variable equalizer is input to suppress black inversion. A high-pass filter that outputs a second signal that has been output, a low-pass filter that receives the first signal and outputs a third signal that suppresses white inversion, A detection circuit in which the inversion of the second signal and the third signal is detected by a change in the input level value of each signal; and when the detection circuit detects the inversion of the first signal and the second signal. A determination circuit that is determined to be a black inversion when the detection circuit detects the inversion of the first signal and the third signal, according to a determination output signal of the determination circuit. The above frequency characteristics change A FM demodulation circuit characterized by: