JPH0244572A - Demodulating circuit - Google Patents

Abstract

PURPOSE:To remove a rasping tone at the time of switching by supplying a series of FM signals through a PLL, which traces the change of a modulating wave and does not trace the raid change, to a demodulating circuit. CONSTITUTION:A series of the FM signals, which pass through a switch circuit 1, a buffer amplifier 2, a BPF3 and a limiter amplifier 4, to be successively switched and outputted are processed by a PLL5, which traces the frequency change by the modulating wave and does not trace the rapid change, of loop gain and damping factor. A signal to be locked by the FM signal, which is outputted from the PLL, is inputted to an FM modulator 6. Thus, an output signal, which is smoothly changed even during a switching period between the respective FM signals, is obtained and the rasping tone can be removed.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a demodulation circuit, and in particular, the present invention relates to a demodulation circuit, and in particular, when demodulating Hi-Fi audio of a VTR, the FM signal generated when switching between FM signals from two audio heads attached to a cylinder head. The present invention relates to a demodulation circuit including a circuit for removing noise.

[Conventional technology]

Conventionally, this type of demodulation circuit, as shown in FIG.
and the switch circuit 1 under the control of the switching control signal Vs.
to create a series of FM signals, and buffer amplifier 2
, input to the FM demodulator 6 via the bandpass filter 3 and limiter amplifier 4, and after demodulating a series of FM signals in the FM demodulator 6, input to the hold circuit 8 via the buffer amplifier 7,
The hold circuit 8, together with the capacitor C2, uses the hold pulse VH from the hold pulse generation circuit 10 to maintain the level immediately before switching between the first and second FM signals IN, IN2 when the switch circuit switches between the first and second FM signals IN, IN2. was held and outputted via a buffer circuit 9.

FIG. 4 is a waveform diagram of signals of various parts of this demodulation circuit.

When the first and second FM signals IN, IN2 are switched by the switch circuit 1 to become a series of FM signals V, they are discontinuous in the switching period T1 and the level rapidly decreases. The output signal of the demodulator 6, ie, the demodulated signal V. Noise (C) occurs during the switching period T1.

In order to remove this noise, the hold circuit 8 and the capacitor C1 control the first and second FM signals IN1. IN
The level immediately before switching between 2 and 2 is held for a switching period T1 (D).

[Problem to be solved by the invention]

The above-described conventional demodulation circuit is configured to convert the first and second FM signals I
In order to remove the noise that occurs when switching IN2, the level immediately before switching is held for the switching period T1, so there is a step in the output waveform (OUT'), which makes it difficult to listen to this output signal. The drawback is that the sound can be very harsh at times.

An object of the present invention is to provide a demodulation circuit that can eliminate harsh sounds during switching.

[Means to solve the problem]

The demodulation circuit of the present invention includes a switch circuit that sequentially switches a plurality of FM signals respectively inputted from a plurality of input terminals and outputs them as a series of FM signals, and a switch circuit that at least follows frequency changes due to modulated waves of each of the FM signals. A PLL circuit that has a predetermined loop gain and damping coefficient that does not follow sudden changes in the circuit, inputs a series of FM signals from the switch circuit, and outputs a signal locked to this FM signal, and this PLL circuit. and an FM demodulator that demodulates the output signal of the circuit.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention.

This embodiment inputs a first FM signal IN from a first audio head of the cylinder head and a second FM signal IN2 from a second audio head from the first and second input ends, These FM signals I under the control of the switching control signal Vs
A switch circuit 1 that switches NI and IN2 to produce a series of FM signals, and a buffer and amplifier 2 that band-amplifies and limits the amplitude of this series of FM signals. A bandpass filter 3, a limiter amplifier 4, and a phase comparator 51. It includes a loop filter 52 and a voltage controlled oscillator 53, and includes at least a first and a second F
It has a loop gain and damping coefficient that follows the frequency changes caused by the modulated waves of the M signals IN, IN2, but does not follow steep changes. Locked signal (V2
), an FM demodulator 6 that demodulates the output signal 2 of this PLL circuit 5, and this demodulator 6.
The configuration includes a buffer amplifier 7 that amplifies the output signal of.

Next, the operation of this embodiment will be explained.

FIG. 2 is a waveform diagram of various signals for explaining the operation of this embodiment.

The switch circuit 1 to the limiter amplifier 4 operate in the same manner as before, and from the limiter amplifier 4, at the time of switching (TI),
A series of discontinuous FM signals (1) whose level rapidly drops is output.

The PLL circuit 5 receives the output signal V2 of the voltage controlled oscillator 53.
(also the output signal of the PLL circuit 5) is locked with a phase shift of 90° to the series of FM signals V from the limiter amplifier 4, the loop gain is relatively low, and the damping coefficient is 1.0. As described above, the convergence time is made a little longer, and the frequency changes caused by the modulated waves of each FM signal IN, . . . IN2 are followed, but steep changes are not followed.

Therefore, during the switching period T of the FM signals IN, IN2, even if the level suddenly drops or the signal disappears and the operation returns to the free-running frequency, it takes time to converge, so the next FM signal is immediately switched on. When the signal (IN2) is input, it operates so as to follow the next FM signal (IN2) without becoming discontinuous (part (A)).

The output signal V2 of the PLL circuit 5 obtained in this way is demodulated by the FM demodulator 6, and becomes an output signal OUT that changes smoothly as shown in the bottom row of FIG.

If the transient response characteristic of PLL circuit N5 is made to be a characteristic that can follow sudden changes, the waveform will include a step as shown in (B) at the bottom of Figure 2, and as with the conventional example, it will produce a noise that does not disturb the ears. Become.

〔Effect of the invention〕

As explained above, the present invention transmits a series of FM signals through a PLL circuit that follows at least frequency changes caused by modulated waves of the FM signal, but does not follow sudden changes.
By adopting a configuration in which the signal is transmitted to the IN modulator, it is possible to obtain an output signal that changes smoothly even during the switching period between each FM signal, and when listening to this output signal, it does not produce a harsh sound like the conventional one. It has the effect of being able to remove.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a waveform diagram of various signals for explaining the operation of the embodiment shown in FIG. 1, and FIGS. 3 and 4 are conventional diagrams. FIG. 2 is a block diagram showing an example of a demodulation circuit and waveform diagrams of signals of various parts. 1... Switch circuit, 2... Buffer amplifier, 3.
... Bandpass filter, 4... Limiter amplifier, 5...
PLL circuit, 6...FM demodulator, 7...buffer amplifier, 8...hold circuit, 9...buffer amplifier,
DESCRIPTION OF SYMBOLS 10... Hold pulse generation circuit, 51... Phase comparator, 52... Loop filter, 53... Voltage controlled oscillator.

Claims (1)

[Claims] A switch circuit that sequentially switches a plurality of FM signals respectively input from a plurality of input terminals and outputs them as a series of FM signals, and a switch circuit that at least follows frequency changes due to modulated waves of each FM signal, but does not follow sudden changes. a PLL circuit that has a predetermined loop gain and damping coefficient, and that inputs a series of FM signals from the switch circuit and outputs a signal locked to this FM signal; and an FM demodulator that demodulates the output signal of this PLL circuit. A demodulation circuit comprising: