JP3777680B2 - Frequency modulation circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a frequency modulation (FM) circuit, and more particularly to a frequency modulation circuit that frequency-modulates an audio signal when the audio signal is recorded on a tape in 8 mm video or the like.
[0002]
[Prior art]
In the 8 mm video standard, the tape running speed is reduced in order to make the cassette smaller. In general, when the tape running speed is slow, it becomes difficult to record high-frequency sound, or it becomes difficult to feed the tape at a constant speed, and the sound shakes. Therefore, in 8 mm video, an FM audio (AFM: Audio Frequency Modulation) system in which an audio signal is frequency-modulated in the same way as a video signal and recorded together with the video signal with a rotating head so that it can be recorded with high quality even in a small size. Is adopted.
[0003]
A conventional example of this type of frequency modulation circuit is shown in FIG. In FIG. 5, an audio signal to be recorded on the tape is input by an input circuit 51, subjected to noise removal processing by a noise reduction circuit 52, and then input to an AF (Audio Frequency) limiter 53. The AF limiter 53 has a certain limit level and limits the amplitude of the input level of the audio signal so that the signal level supplied to the subsequent stage does not exceed the limit level.
[0004]
The audio signal whose amplitude is limited by the AF limiter 53 is supplied to a VCO (voltage controlled oscillator) 55 through a V (voltage) / I (current) converter 54. The VCO 55 modulates the audio signal by changing the oscillation frequency in accordance with the signal level of the audio signal from which the noise is removed by the noise reduction circuit 52 and the amplitude of which is limited by the AF limiter 53.
[0005]
In this frequency modulation, when an audio signal, which is a modulated wave, is input, in order to adjust the deviation of the frequency of the modulated wave from the carrier in proportion to its amplitude, that is, the deviation (frequency shift) to the standard width, A deviation adjustment circuit 56 is provided. The deviation adjustment circuit 56 adjusts the maximum deviation width by supplying an adjustment current i corresponding to the adjustment voltage V given from the outside to the AF limiter 53.
[0006]
[Problems to be solved by the invention]
By the way, in the frequency modulation circuit according to the conventional example having the above configuration, if there is a variation in characteristics of circuit elements in the circuit system from the input circuit 51 to the input end of the AF limiter 53, the AF limiter is caused by the variation in the circuit. The input level of 53 varies. This change in the input level is equivalent to a relative change in the limit level of the AF limiter 53.
[0007]
As a result, the maximum deviation width becomes wider than the standard, and as shown by the broken line in FIG. 6, it may be applied to the C (color signal) system or the Y (luminance signal) system, or Lch and Rch overlap to crosstalk. Becomes worse. On the other hand, as shown by the one-dot chain line in FIG. 6, when the maximum deviation width is narrower than the standard, it is impossible to record a loud sound, and as a result, a phenomenon occurs that the sound is crushed when reproduced. become.
[0008]
The present invention has been made in view of the above problems, and an object of the present invention is to provide a frequency modulation circuit that always limits amplitude at a constant level even if the input level fluctuates due to circuit variations. There is to do.
[0009]
[Means for Solving the Problems]
The frequency modulation circuit according to the present invention includes an input circuit that performs input processing of an audio signal, a noise reduction circuit that performs noise removal processing on the audio signal input by the input circuit, and a signal level input from the noise reduction circuit. and AF limiter for performing an amplitude limitation to the audio signal amplitude limited by the AF limiter V (voltage) / I (current) conversion and V / I converter for signal level passing through the V / I converter A voltage-controlled oscillator whose oscillation frequency changes according to the deviation, and a deviation adjustment that supplies an adjustment current to the AF limiter according to an adjustment voltage set externally based on the input level while monitoring the input level of the AF limiter And the deviation adjustment circuit adjusts the input level of the AF limiter. It has a configuration which controls the limit level of the AF limiter in Zui.
[0010]
In the frequency modulation circuit configured as described above, by controlling the limit level of the AF limiter that limits the amplitude of the signal level of the audio signal input via the input circuit and the noise reduction circuit based on the input level, Even if the input level fluctuates due to variations in the input circuit or noise reduction circuit , the amplitude is always limited at a constant level.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0012]
FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, an audio signal to be recorded on a tape is input by an input circuit 11, subjected to noise removal processing by a noise reduction circuit 12, and then input to an AF limiter 13. This AF limiter 13 has a certain limit level, so that the signal level supplied to the subsequent stage does not exceed the limit level, in other words, it does not exceed a certain level when recording on the tape. Limit the amplitude. This constant level is the maximum deviation width.
[0013]
The audio signal whose amplitude is limited by the AF limiter 13 is supplied to a VCO (voltage controlled oscillator) 15 via a V (voltage) / I (current) converter 14. The VCO 15 modulates the audio signal by changing the oscillation frequency according to the signal level of the audio signal from which noise is removed by the noise reduction circuit 12 and whose amplitude is limited by the AF limiter 13.
[0014]
In this frequency modulation, a deviation adjustment circuit 16 is provided for adjusting the deviation (frequency shift) to a standard width. Here, the deviation means an amount by which the frequency of the modulated wave deviates from the carrier wave in proportion to the amplitude when an audio signal that is a modulated wave is input. The deviation adjustment circuit 16 monitors the input level of the AF limiter 13 and, when an adjustment voltage V set externally based on the input level is applied, the deviation adjustment circuit 16 outputs an adjustment current i1 corresponding to the adjustment voltage V to the AF limiter. 13 to adjust the maximum deviation width.
[0015]
In this example, the deviation adjustment circuit 16 is also used as a control unit that controls the limit level of the AF limiter 13 based on the input level of the AF limiter 13. That is, the adjustment current i1 output from the deviation adjustment circuit 16 is supplied to the V / I converter 14 and also to the AF limiter 13. The AF limiter 13 is configured such that the limit level changes in accordance with the adjustment current i1 supplied from the deviation adjustment circuit 16.
[0016]
FIG. 2 shows an example of the circuit configuration of the AF limiter 13. In FIG. 2, one ends of resistors R1 and R2 that determine the upper limit and the lower limit of the limit level are connected in common, and a predetermined fixed potential Eo is applied to the common connection point O. Between the other end of the resistor R1 and the power source Vcc, a transistor Q1 and a resistor R3 constituting the current source 21 are connected in series. Between the other end of the resistor R2 and the ground, the transistor Q2 constituting the current source 22 and the resistor R4 are connected in series. The voltage at the other end of the resistor R2 is applied to the lower limit unit 23 as a lower limit level, and the voltage at the other end of the resistor R1 is applied to the upper limit unit 24 as an upper limit level.
[0017]
The lower limit unit 23 includes a differential pair of transistors Q3 and Q4 having common emitters connected thereto, a resistor R5 that provides an input level to the base of the transistor Q3, a resistor R6 that provides a lower limit level to the base of the transistor Q4, and a transistor Q3 , Q4 are connected in series between the common emitter connection point of Q4 and the ground, the transistor Q5 and the resistor R7 constituting a current source, and the resistors R8, R8 connected between the collectors of the transistors Q3 and Q4 and the power source Vcc, R9 and transistors Q6 and Q7, each collector being connected to each emitter of transistors Q3 and Q4, each base being connected to each collector and each emitter being connected to power supply Vcc.
[0018]
On the other hand, the upper limit unit 24 is connected in series between the differential pair transistors Q8 and Q9 having emitters connected in common and the emitter common connection point of the transistors Q8 and Q9 and the power supply Vcc to form a current source. Transistor Q10 and resistor R10, resistors R11 and R12 connected between the collectors of transistors Q8 and Q9 and ground, collectors connected to emitters of transistors Q8 and Q9, and bases connected to collectors, respectively. Each of the emitters is composed of transistors Q11 and Q12 connected to the ground.
[0019]
A transistor Q13 and a resistor R13 constituting a current source are connected in series between the base of the transistor Q9 and the ground, and a transistor Q14 is connected between the base of the transistor Q9 and the power supply Vcc. The base of the transistor Q14 is connected to the other end of the resistor R1, thereby giving an upper limit level to the base of the transistor Q9. Further, since the base of the transistor Q8 is connected to the collector of the transistor Q5, a signal whose lower level is limited by the lower limit unit 21 is given to the base of the transistor Q8.
[0020]
Further, when the maximum deviation width is set based on the input level so that the limit level of the AF limiter 13 can be set based on the input level, the current source according to the adjustment current i1 supplied from the deviation adjustment circuit 16 Current control units 25 and 26 for controlling currents 21 and 22 are provided. When the currents of the current sources 21 and 22 change according to the input level, the upper limit level and the lower limit level obtained at the other ends of the resistors R1 and R2 also change according to the input level.
[0021]
The current control unit 25 includes a base of the transistor Q1 constituting the current source 21 and transistors Q15, Q16, Q17 connected in common, and between the emitters of these transistors Q15, Q16, Q17 and the power supply Vcc. The connected resistors R14, R15, and R16, the base of the transistor Q15 is connected to the collector of the transistor Q15, the emitter is connected to the collector of the transistor Q15, and the collector is grounded. The base of the transistor Q16 is connected to the collector. Current mirror circuit configuration.
[0022]
In the current control unit 25 having such a configuration, the collector of the transistor Q15 and the base of the transistor Q18 serve as control inputs, and the adjustment current i1 is supplied from the deviation adjustment circuit 16, so that a current i2 proportional to the adjustment current i1 is a current source. The current flows to the base of the transistor Q1 constituting the transistor 21, and the current of the current source 21 is thereby controlled.
[0023]
On the other hand, the current control unit 26 is connected between the base of the transistor Q2 constituting the current source 22 and the transistors Q19 and Q20 in which the respective bases are connected in common, and between the emitters of these transistors Q19 and Q20 and the ground. Resistors R17 and R18, and a transistor Q21 having a base connected to the collector of the transistor Q19, an emitter connected to the collector of the transistor Q20, and a collector connected to the power source Vcc, and a current with the base of the transistor Q20 connected to the collector It has a mirror circuit configuration.
[0024]
In the current control unit 26 configured as described above, the collector of the transistor Q19 and the base of the transistor Q21 are commonly connected to the collector of the current control unit 25, so that the same current as that of the current control unit 25 also flows to the current control unit 26. . As a result, the same current i2 that flows to the base of the transistor Q1 that constitutes the current source 21 also flows to the base of the transistor Q2 that constitutes the current source 22, whereby the current of the current source 22 is controlled.
[0025]
An equivalent circuit of the AF limiter 13 configured as described above is shown in FIG. As shown in the figure, the limit level does not vary and is constant, and the amplitude is limited, for example, at −10 dBm (+5 dB up) regardless of the input level, and this level becomes the maximum deviation width.
[0026]
Here, if the input level of the AF limiter 13 fluctuates due to circuit variations, the limit level fluctuates relatively depending on the case. As an example, if the limit level is −10 dBm, the input level becomes −17 dBm when the fluctuation is −2 dB, the amplitude is limited by +7 dB up, and the input level becomes −13 dBm when the fluctuation is +2 dB. The amplitude is limited by +3 dB up.
[0027]
However, the deviation adjustment circuit 16 adjusts the deviation width to the standards of Lch = ± 60 kHz and Rch = ± 30 kHz regardless of the input level of the AF limiter 13, for example. Therefore, for example, when the input level of the AF limiter 13 is −17 dBm, the amplitude limit is applied when the maximum deviation width is increased by +5 dB, but the amplitude is limited when increased by +7 dB. As shown by a broken line in FIG. 5, the AFM signal is intruded into the C system and the Y system. Conversely, when the input level of the AF limiter 13 is −13 dBm, the amplitude is limited by +3 dB up.
[0028]
However, in the present embodiment, the external adjustment voltage V is changed based on the input level of the AF limiter 13, and the adjustment current i1 corresponding to the adjustment voltage V is supplied from the deviation adjustment circuit 16 to the V / I converter 14. Thus, when the deviation adjustment is performed so that the deviation width of the output of the VCO 16 becomes, for example, Lch = ± 60 kHz and Rch = ± 30 kHz, the adjustment current i1 is also supplied to the AF limiter 13 so that the AF limiter 13 The limit level is controlled based on the input level.
[0029]
In this way, the adjustment current i1 corresponding to the input level of the AF limiter 13 is also supplied to the AF limiter 13, and the limit level of the AF limiter 13 is controlled based on the input level, thereby setting the limit level to the input level. On the other hand, the amplitude can be limited at a constant level (for example, +5 dB up). As a result, even if the input level varies due to circuit variations, the amplitude is always limited at a constant level.
[0030]
FIG. 4 shows a comparison result between the conventional example and this embodiment when the limit level of the AF limiter 13 is examined after setting the external adjustment voltage V so that the deviation width becomes ± 60 kHz in Lch, for example. As is clear from the figure, in the case of the conventional example, the limit level is constant regardless of the input level, whereas in the case of this embodiment, the limit level changes in a form almost proportional to the input level. I understand that
[0031]
In the above-described embodiment, the case where the deviation circuit 16 is configured to be used as a control unit that controls the limit level based on the input level of the AF limiter 13 has been described. However, the present invention is not limited to this configuration. Of course, a control circuit for controlling the limit level based on the input level of the AF limiter 13 may be provided separately. However, when the deviation circuit 16 is also used, there is an advantage that it is not necessary to prepare a new adjustment terminal when making an IC.
[0032]
In the above embodiment, a single frequency modulation circuit has been described. However, when applied to 8 mm video, for example, as is clear from the frequency allocation of FIG. , Rch, two frequency conversion circuits are used.
[0033]
【The invention's effect】
As described above, according to the present invention, in the frequency modulation circuit that modulates the frequency of the audio signal, the AF limiter that limits the amplitude of the signal level of the audio signal input through the input circuit and the noise reduction circuit . Since the limit level is controlled based on the input level, even if the input level fluctuates due to variations in the input circuit or noise reduction circuit , the amplitude can always be limited at a constant level. The circuit variation from the circuit input to immediately before the AF limiter can be absorbed, and the AFM signal does not bite into the C system and Y system, and no loud sound can be recorded.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an embodiment of the present invention.
FIG. 2 is a circuit diagram illustrating an example of a circuit configuration of an AF limiter.
FIG. 3 is an equivalent circuit diagram of an AF limiter.
FIG. 4 is a characteristic diagram showing a relationship between an input level of an AF limiter and its limit level.
FIG. 5 is a block diagram showing a conventional example.
FIG. 6 is a frequency allocation diagram for explaining a problem;
[Explanation of symbols]
12 Noise Reduction Circuit 13 AF Limiter 15 Voltage Control Oscillator 16 Deviation Adjustment Circuit

Claims (1)

An input circuit for processing an audio signal;
A noise reduction circuit for performing noise removal processing on the audio signal input processed by the input circuit;
An AF limiter that limits the amplitude of the signal level input from the noise reduction circuit ;
A V / I converter for converting a signal level whose amplitude is limited by the AF limiter to V (voltage) / I (current);
A voltage controlled oscillator whose oscillation frequency changes according to the signal level passed through the V / I converter ;
A deviation adjustment circuit for monitoring the input level of the AF limiter and supplying an adjustment current corresponding to an adjustment voltage set externally based on the input level to the AF limiter ;
The deviation adjustment circuit controls a limit level of the AF limiter based on an input level of the AF limiter .

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