JPH0713089Y2 - Frequency characteristic correction circuit for demodulation circuit - Google Patents

Description

[Detailed Description of the Invention] (a) Field of Industrial Application This invention relates to a frequency characteristic correction circuit of a demodulation circuit of a digital device using a sample hold circuit, and particularly, a sampling frequency of a digital audio tape recorder (DAT). It is suitable for a frequency characteristic correction circuit of a demodulation circuit in which a plurality of demodulation signals corresponding to respective sampling frequencies are mixed and mixed.

(B) Prior Art The frequency characteristic correction circuit of the conventional demodulation circuit is configured as shown in FIG. 3 in order to correct the frequency characteristic of the aperture effect of the sample hold circuit.

The demodulated signal from the digital-to-analog converter is a pulse train with an amplitude corresponding to the digital data, that is, PAM.
A signal (pulse amplitude modulation signal) is input to the input terminal 1. In the two-system demodulation circuit, which is sample-held by the sample-hold circuit 2, but the sampling frequency is different, a control signal for setting the target sampling frequency is sent from the sampling frequency switching signal 3 to the sample-hold circuit 2. .

The sample and hold circuit 2 samples and holds the peak level of the PAM signal at this sampling frequency, but the sample and hold circuit 2 has an aperture effect due to the sampling pulse width, that is, a phenomenon in which a high frequency band is attenuated.

The output signal of the sample hold circuit 2 is supplied to the low pass filter 4 through the switch S1.

Now, assuming that the sampling frequency is a 2 system demodulation circuit of 44.1 KHz and 38 KHz, at the sampling frequency of 44.1 KHz, it is connected to the contact a side of the switch S1,
It is assumed that z is connected to the b contact side of the switch S1.

Sample-hold circuit 2 with sampling frequency 44.1KHz
Output signal is supplied to the low-pass filter 4 from the contact a of the switch S1. Since the cut-off frequency fc ₁ = 20 KHz is set in the low-pass filter 4, unnecessary high frequency components of 20 KHz or higher are cut and an audible frequency band signal up to 20 KHz is obtained.

Since the audible frequency band signal up to 20 KHz has a high frequency band attenuated by the aperture effect as described above, the aperture correction circuit 6 corrects the high frequency band.

The aperture correction circuit 6 is a correction circuit having a frequency characteristic in which the gain in the high frequency band is increased immediately before the cutoff frequency of the low pass filter 4, which is 20 KHz, that is, up to about 20 KHz.

The demodulated signal whose frequency characteristic has been corrected by the aperture correction circuit 6 is amplified to a specified output voltage by the amplifier 8 and is output to the switch S2.
Is output to the output terminal 10.

On the other hand, the demodulated signal with the sampling frequency of 38KHz is supplied to the b contact of the switch S1, and the cutoff frequency fc ₂ = 16KHz.
Through the low-pass filter 5 set to 44.1 above.
Similar to the case of KHz, it is corrected by the aperture correction circuit 7 and the amplifier 9, amplified and output to the output terminal 10 from the switch S2.

The aperture correction circuit 7 is a frequency correction circuit that increases the gain in the high frequency range up to about 16 KHz.

Conventionally, demodulation circuits having different sampling frequencies as described above constitute a multi-system (two systems in the conventional example) low-pass filter circuit and an aperture correction circuit corresponding to the sampling frequency, and demodulate by switching according to the sampling frequency. I was going.

(C) Problems to be solved by the invention However, the above-mentioned conventional one requires a multi-system low-pass filter circuit and an aperture correction circuit matching the sampling frequency, which increases the circuit configuration and makes it complicated and expensive. There was a flaw.

Further, since the low-pass filter circuit is independent for each sampling frequency, there is a drawback that the order of the low-pass filter circuit must be set large.

The frequency characteristic correction circuit of the demodulation circuit according to the present invention uses a sample hold circuit for sampling in accordance with the sampling frequency, and in the frequency characteristic correction circuit of the demodulation circuit, which is composed of a low-pass filter circuit and an aperture correction circuit, The first low-pass filter set to the cut-off frequency and the first low-pass filter having the frequency characteristic in which the gain is increased in the high-frequency region immediately before the cut-off frequency of the first low-pass filter to raise the high-frequency band. The aperture correction circuit is set to a cutoff frequency lower than that of the first low-pass filter, and the second low-pass filter whose order is lower than that of the first low-pass filter and the portion immediately before the cut-off frequency of the second low-pass filter are set. The second with the frequency characteristic which raised the gain in the high region and raised the high region.
Of the first aperture correction circuit and the output of the second aperture correction circuit in accordance with the sampling frequency, and switching means for switching the output of the first aperture correction circuit and the output of the second aperture correction circuit. A frequency characteristic correction circuit of a demodulation circuit, which is configured to be amplified and output.

(E) Action In the demodulation circuit composed of the sample-hold circuit that samples and holds the PAM signal of the output signal of the digital-to-analog converter and the low-pass filter and the aperture correction circuit according to the sampling frequency, A first low-pass filter and a first aperture-correction circuit demodulation circuit corresponding to different sampling signals are constituted by a second low-pass filter and a second aperture-correction circuit that are connected in cascade, and the first aperture-correction circuit is provided. It is a demodulation circuit that switches and outputs the output of the circuit and the output of the second aperture correction circuit according to the sampling frequency.

The second low-pass filter is connected in cascade to reduce the order of the filter and to obtain a steep attenuation characteristic, thereby simplifying the circuit configuration.

(F) Embodiment An embodiment of the frequency characteristic correction circuit of the demodulation circuit according to the present invention will be described with reference to FIGS. 1 and 2.

FIG. 1 is a block diagram of a digital audio signal demodulation circuit using a sample hold circuit. FIG. 2 is a frequency characteristic diagram of this demodulation circuit and a characteristic diagram of each corrected portion thereof.

The same parts as those of the conventional example (FIG. 3) are designated by the same reference numerals and the description thereof will be omitted.

The PAM signal applied to the input terminal 1 is sampled and held by the sample and hold circuit 2 and supplied to the first low pass filter circuit 4. The cutoff frequency fc _{1 of} the first low-pass filter circuit 4 is designed to be 20 kHz.

The frequency characteristic of the output signal of the first low-pass filter 4 is
It becomes like the m characteristic of FIG. 2 (A). With a cutoff frequency of 20 KHz, a sharp cutoff characteristic is obtained above 20 KHz. Further, due to the aperture effect of the sample and hold circuit 2, high frequency attenuation characteristics are exhibited from around 14 to 15 KHz.

The output signal of the first low-pass filter 4 is subjected to frequency characteristic correction by the first aperture correction circuit 6 and is supplied to the second low-pass filter 20 (fc ₂ = 16 KHz).
It is also connected to the contact a of the changeover switch S2.

When the control signal from the sampling frequency switching signal 3 has a sampling frequency of 44.1 KHz, the switch S2 is connected to the a contact, so the output terminal 10 has the sampling frequency.
The output signal of the first aperture correction circuit 6 is output as a 44.1 KHz demodulated signal.

In the aperture correction circuit 6, the frequency characteristic of the output signal of the first aperture correction circuit 6 rises from around 14 to 15 KHz as in the K characteristic of FIG. 2 (B), and the gain is increased in the high range of 20 KHz or less. 2A, the m-characteristic of FIG. 2A is cut off by the first aperture correction circuit 6 like the P-characteristic of FIG. 2C.
It realizes a near-ideal low-pass characteristic that sharply attenuates above KHz.

That is, the demodulated signal can be taken out from the output terminal 10 by completely cutting unnecessary high frequency components of 20 KHz or higher.

Switch S2 when sampling frequency is connected to 38KHz
Is connected to the B contact. The output signal of the first aperture correction circuit 6 is supplied to the second low-pass filter 20, high-frequency components having a cutoff frequency fc ₂ = 16 KHz or higher are cut, and the output signal is supplied to the second aperture correction circuit 21. The frequency characteristic is corrected by the same method as that of the first aperture correction circuit 6 described above.

The second low-pass filter 20 is simple in structure and has a smaller filter order than the low-pass filter (FIG. 3) described in the conventional example. This is connected in cascade with the first low-pass filter 4, and since the filter characteristics exert multiple effects, it is possible to realize a frequency characteristic with a small order, which is equal to or higher than that of the conventional example having a large order. There is.

The demodulated signal with the sampling frequency of 38 KHz is output to the output terminal 10 from the b contact of the switch S2 through the second low pass filter 20 and the second aperture correction circuit 21 with respect to the demodulated signal circuit with the sampling frequency of 44.1 KHz.

The frequency characteristics of each of these circuits are shown in FIGS. 2 (A) to (C).
L, n, g characteristics of. (Sampling frequency 44.1K
The same as the k, m, p characteristics of Hz) In the cascade-connected lowpass filter, a lowpass filter having a high cutoff frequency is located in the first lowpass filter 4, and a lowpass filter having a low cutoff frequency is located in the second lowpass filter. The low pass filter 20 of FIG.

(G) Effect of the invention The frequency characteristic correction circuit of the demodulation circuit according to the invention is the first
Since the second low-pass filter and the second low-pass filter are cascade-connected to each other, the order of the second low-pass filter can be reduced, so that the configuration of the second low-pass filter circuit is simplified, and the switching by the sampling frequency is further performed. And the amplifier on the output side are common, and are configured by one, which has the effect of simplifying the circuit configuration.

In addition, the circuit can be downsized, and since it can be constructed at a low cost, it is easy to implement and has other excellent features.

[Brief description of drawings]

1 and 2 show an embodiment of a frequency characteristic correction circuit of a demodulation circuit according to the present invention, and FIG. 1 is a block diagram and FIG.
Figures (A), (B), and (C) are characteristic diagrams showing the frequency characteristic of each part. FIG. 3 is a block diagram of a conventional example. Explanation of main numbers and symbols 2 …… Sample hold circuit, 4,5,20 …… Low pass filter, 6,7,21 …… Aperture correction circuit, 8,9 …… Amplifier, S1, S2, S3 …… Switch switch

Claims (1)

[Scope of utility model registration request] 1. A frequency characteristic correction circuit for a demodulation circuit, which comprises a low-pass filter circuit and an aperture correction circuit, using a sample-hold circuit for sampling in accordance with a sampling frequency, and a first cutoff frequency is set. A first low-pass filter, and a first aperture correction circuit having a frequency characteristic in which the gain is increased in a high-frequency region from a portion immediately before the cutoff frequency of the first low-pass filter to increase the high frequency.
The cutoff frequency is set to be lower than that of the first lowpass filter, and the gain is set in the high frequency region from the second lowpass filter having a lower order than the first lowpass filter and the portion immediately before the cutoff frequency of the second lowpass filter. And a second aperture correction circuit having a frequency characteristic in which the high frequency band is raised is connected in a cascade manner, and the output of the first aperture correction circuit and the output of the second aperture correction circuit are A frequency characteristic correction circuit of a demodulation circuit, comprising a switching means for switching in accordance with a sampling frequency, and configured to amplify and output by an output amplifier.