JPH0810537B2 - Digital audio equipment - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital audio device, and more particularly to a digital audio device that corrects an adverse effect of a recording side low pass filter.

<Prior Art> In a digital audio recording / playback system, a low-pass filter for suppressing aliasing distortion is inserted on the recording side, and a low-pass filter for removing unnecessary high frequency components is inserted on the playing side. There is.

Figure 6 shows the recording of conventional digital audio.
It is a block diagram of a reproducing system, and a recording system is on the left side of the dotted line and a reproducing system is on the right side. Reference numeral 1 is a high-order low-pass filter that suppresses aliasing distortion by cutting high-frequency components of fs / 2 or more when the sampling frequency is fs, and has the frequency characteristics shown in FIG. 2 is an AD converter that samples the output signal of the low-pass filter 1 at the frequency fs and outputs the obtained digital data. 3 records the input digital data in the storage medium 4 and reads the digital data from the recording medium. Digital recording / reproducing device for outputting 5
Is a DA converter that converts the digital data read from the storage medium into an analog signal, and 6 is a low-pass filter that removes unnecessary high frequency components from the output of the DA converter to smooth the signal waveform.

<Problems to be Solved by the Invention> In the conventional recording / reproducing system, since the recording system and the reproducing system each have a low-pass filter, the analog output Sa ′ output from the reproducing system is band-limited and ( (See FIG. 7), a phase shift occurs. As a result,
The analog output does not have the high frequency components necessary for music playback,
Moreover, there is a problem that an unnatural vibration component that does not exist in the natural world is added and advanced music reproduction cannot be performed.

By the way, the reproducing low-pass filter can be removed by a high oversampling method, but the recording low-pass filter cannot be removed.

From the above, it is an object of the present invention to provide a digital audio device capable of correcting the adverse effects of a low-pass filter in a recording system.

<Means for Solving the Problems> In the present invention, when the transfer function of the recording-side low-pass filter is G ₁ , behind the reproduction system DA converter,
It is achieved by providing a characteristic compensation circuit having a 1 / transfer function or transfer function of approximately 1 / G ₁ of G _1.

<Operation> If a characteristic correction circuit having a transfer function of 1 / G ₁ or an approximate transfer function of 1 / G ₁ is provided behind the DA converter of the reproducing system, the frequency characteristic of the low-pass filter in the recording system is improved. The correction circuit corrects the adverse effect of the recording-side low-pass filter.

<Embodiment> FIG. 1 is a block diagram of a digital audio apparatus according to the present invention, which is applicable to, for example, a DAT apparatus.
The recording system is on the left side of the dotted line and the reproducing system is on the right side.

11 is a high-order low-pass filter that suppresses aliasing distortion by cutting high-frequency components of fs / 2 or more from the audio signal Sa, where fs is the sampling frequency, and the transfer function G ₁
And has the frequency characteristic shown by the solid line in FIG.

12 is an AD converter that samples the low-pass filter output at frequency fs and outputs the obtained digital data, 13 records the input digital data on a storage medium such as a DAT tape 14, and the digital data from the DAT tape 14 Digital recording / reproducing device for reading and outputting, 15 converts digital data read from tape to analog signal
The DA converter, 16 is a low-pass filter that smoothes the signal waveform by removing unnecessary high-frequency components (system carry frequency components) from the DA converter output, and has a transfer function G ₂ .

Reference numeral 17 is a characteristic correction circuit, which is a recording-side low-pass filter 11
Let G _{1 be} the transfer function of 1 / G ₁ or approximately 1 / G ₁ transfer function, and the frequency characteristic is as shown by the dotted line in FIG. That is, the frequency characteristic of the characteristic correction circuit 17 is line-symmetric with the characteristic of the recording side low-pass filter 11 about 0 dB.

From the above, the audio output of the digital audio device
Sa ′ is the output Sa ・ G ₁・ G ₂ of the low-pass filter 16 of the reproduction system.
Is multiplied by 1 / G ₁ (Sa · G ₂ ), the frequency characteristic of the recording side low-pass filter 11 can be corrected, and the band limitation can be performed only by the reproducing side low-pass filter 16. By increasing the oversampling, it is possible to omit the low-pass filter on the reproducing side, thereby eliminating the adverse effect of the low-pass filter and making the overall transfer function of the system 1.

FIG. 3 shows one example of the characteristic correction circuit 17, which is a characteristic correction circuit 17 configured by a feedback configuration with a differential amplifier 17a and a circuit 17b having a transfer function G _1.
If the gain of 17a is A, the total transfer function G can be expressed by the following equation G = A / (1 + G ₁ · A) = 1 / {(1 / A) + G ₁ }, and if A is very large, G ≈ 1 / G ₁ .

FIG. 4 shows another embodiment of the characteristic correction circuit 17, which is an example of a configuration for simply giving a characteristic of 1 / G ₁ . In the figure, 17c
Is a high-pass filter, 17d is a gain B amplifier, and 17e is an adder. If the transfer function G ₁ of the recording-side low-pass filter 11 in the frequency band higher than the frequency f ₀ ′ is g ₁ (see FIG. 2), the high-pass filter 17c can generate a high frequency band (f ₀ ′).
~ F ₀ ″) has a transfer function G ₃ G ₃ · B = (1-g ₁ ) / g ₁ shown in the following equation (see FIG. 5). That is, Sa ″ = G ₁ · Sa , 'When the G ₁ at ≦ f ≦ f ₀ "and g _1, Sa' f ₀ becomes _{= Sa · g 1 + Sa ·} g 1 · G 3 · B. here, the Sa '= Sa is , G ₃ · B = (1-g ₁ ) / g ₁ is required.

Note that the value of g ₁ at frequency f ₀ ″ is -adB (≈ 1/10000)
Then, the transfer function G ₃ of the high-pass filter 17c is G ₃ = (1/10 ^m ) · (1-g ₁ ) / g ₁ (m = a / 20).

From the above, the total transfer function G in FIG. 4 is G = (1 + G ₃ · B) = 1 + (1-g ₁ ) / g ₁ = 1 / g ₁ = 1 / G ₁ and the gain B of the amplifier is adjusted. By doing so, the sound quality can be controlled.

The case where the present invention is applied to a digital audio device (DAT device) capable of recording and reproducing has been described above, but a digital audio device (for example, CD
It is also applicable to players).

According to the above invention <Effects of the Invention>, recording side when the transfer function of the low-pass filter and G _1, the rear of the reproducing system DA converter, 1 / transfer function in G ₁ or approximately 1 / G ₁ Since the characteristic correction circuit having the transfer function is provided, the frequency characteristic of the low-pass filter in the recording system can be corrected by the characteristic correction circuit, and the adverse effect of the low-pass filter can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram of a digital audio device of the present invention, FIG. 2 is a frequency characteristic diagram of a low-pass filter and a characteristic correction circuit, FIGS. 3 and 4 are embodiments of the characteristic correction circuit, and FIG. FIG. 6 is a characteristic diagram of the high-pass filter shown in FIG. 6, and FIGS. 6 and 7 are explanatory diagrams of a conventional audio device. 11 …… Low-pass filter on recording side 13 …… Recording / reproducing device 15 …… DA converter 17 …… Characteristic correction circuit

Claims (1)

[Claims] 1. A digital audio apparatus for reproducing an audio signal input through a recording-side low-pass filter for cutting high-frequency components of 1/2 or more of a sampling frequency from a medium recorded by sampling at the sampling frequency. , When the transfer function of the recording side low-pass filter is G ₁ ,
The digital data read from the recording medium to the rear of the DA converter for converting an analog signal, and characterized in that a characteristic correction circuit having a transfer function of 1 / G ₁ of the transfer function or approximately 1 / G ₁ Digital audio device.