JP3142728B2 - Signal input circuit for 1-bit analog / digital converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to a signal input circuit for an analog / digital converter that converts a modulated signal into a 1-bit digital signal.

[0002]

2. Description of the Related Art A technique for converting an analog input signal into a digital signal and performing recording / reproduction and signal processing is suitably used particularly in audio equipment. As the audio equipment, for example, a digital audio tape recorder (abbreviated as DAT) is configured to quantize an analog input signal at a sampling frequency of 48 kHz, convert it into 16-bit digital data, and record it on a magnetic tape. I have.

In such analog / digital conversion, the sampling frequency must determine a frequency band in which the conversion is possible, and must be at least twice the frequency of the analog input signal to be converted. In the DAT, 48 kHz is selected as described above in order to enable conversion up to 22000 Hz including a surplus in the audible frequency band. The number of quantization bits is a factor that determines a dynamic range. For example, in the DAT, in order to set the dynamic range to 90 dB or more,
The number of quantization bits is selected to be 16 bits.

On the other hand, when a digital signal is converted into an analog signal, in addition to the audio signal component up to 22000 Hz,
The signal component of the upper sideband and the lower sideband having the frequency deviation of 22000 Hz is included on the high frequency side and the low frequency side around the sampling frequency of 48 kHz. Therefore, in making the sound, since the frequency between the audio signal component and the lower sideband component is very close, the filtering characteristic of a low-pass filter (LPF) for filtering only the audio signal component is sharply changed. There is a need to.

However, when such a filter having a sharp filtering characteristic is used, a phase distortion called a group delay, in which a delay time changes with respect to each frequency of an audio signal, occurs. Therefore, in recent years, sampling has been performed by a technique called oversampling in which the sampling frequency is set to, for example, four times.

Also, quantization noise due to analog / digital conversion is distributed from DC to 1/2 of the sampling frequency.
It is known that the overall noise power is determined by the resolution of the analog / digital converter. Therefore, when the sampling frequency is increased as described above, the noise distribution is widened, and the noise power in the audio signal band can be reduced. For this reason, along with the progress of semiconductor technology,
A so-called ΔΣ conversion for converting an analog input signal into a digital signal with one bit has been performed.

The ΔΣ converter converts an analog input signal as shown in FIG. 10A into a signal as shown in FIG.

The binary signal of [0] or [1] is output. FIG. 10 shows a case where the frequency of the analog input signal shown in FIG. 10A is 1500 Hz and the sampling frequency is 768 kHz. As described above, the 1-bit analog / digital converter derives an output having a pulse density corresponding to the waveform of the analog input signal.

However, in the analog / digital conversion by Δ に よ る modulation as described above, as shown in FIG. 11, a spectrum analysis of the converted data shown in FIG. 10B shows that the quantization noise is low on the low frequency side. Although it is a level, it tends to increase as the frequency becomes higher. That is, as apparent from FIG. 11, even within the audible frequency band of 20 kHz, the dynamic range is affected by -10.
It is understood that quantization noise of 0 dB or more exists.

Therefore, in the DAT, when the number of quantization bits is set to 16 and the sampling frequency is set to 48 kHz, the sampling frequency for converting into a 1-bit signal is set to ensure a sufficient dynamic range. , 3
It is necessary to use a very high frequency of about 12 MHz. However, in a newly proposed apparatus for recording / reproducing a 1-bit signal as it is, in order to match the transmission capacity used for existing digital audio, the sampling frequency is 768 kHz of 48 kHz × 16 bits.
It is set to 1.536 MHz from z. In order to suppress the increase in quantization noise, a typical prior art 1-bit signal recording / reproducing apparatus is configured as shown in FIG.

That is, the 1-bit signal recording / reproducing apparatus 1 converts the analog input signal to the input terminal 2 in advance into a pre-emphasis circuit 3 to enhance the high-frequency component, and then converts the analog / digital converter 4 by 1-bit ΔΣ modulation. It is configured to input to. The 1-bit digital data from the analog / digital converter 4 passes through an interface circuit 5 and is then provided to a deck 6 and recorded on a magnetic tape.

The data reproduced from the magnetic tape is:
After passing through the interface circuit 7, the digital / analog converter 8 converts the signal into an analog signal having an amplitude level corresponding to the pulse density. Thereafter, the analog signal is output from the output terminal 10 after the high frequency component is suppressed by the de-emphasis circuit 9 by a value corresponding to the amount of enhancement in the pre-emphasis circuit 3. In this way, by performing analog-to-digital conversion by enhancing the high-frequency component of the audio signal in advance and suppressing the enhancement after digital-to-analog conversion, the quantization noise can be reduced even at the sampling frequency as described above. It is configured to suppress.

[0012]

However, the analog / digital converter 4 oscillates when it receives an excessive input. For example, the analog / digital converter 4 shown in FIG.
The input signal is a sine wave of 00 Hz, and the sampling frequency is 76
Assuming that the frequency is 8 kHz, the data after the ΔΣ conversion periodically as shown in FIG.

[0] and [1] are repeated. The frequency spectrum when such data is converted from digital to analog is shown in FIG.
The original signal component is not retained.

Therefore, in the 1-bit signal recording / reproducing apparatus 1 described above, the frequency spectrum of the audio signal input from the input terminal 2 is 0d as shown in FIG.
When the line α1 of B is input at a level having the peak height limit value, as the high-frequency component is increased in the pre-emphasis circuit 3, as shown by the reference numeral α2 in FIG. It will be relatively suppressed. Therefore, when the de-emphasis circuit 9 suppresses the signal by the amount corresponding to the enhancement of the high-frequency component, the maximum level of the audio signal indicated by reference numeral α3 in FIG.
There is a problem that the dynamic range is narrowed near the quantization noise level shown in FIG.

An object of the present invention is to provide a signal input circuit for a 1-bit analog / digital converter capable of performing a stable conversion operation while securing a wide dynamic range without excessively increasing a sampling frequency. It is to be.

[0015]

The invention according to claim 1 is
A signal input circuit for an analog / digital converter for converting an analog input signal into a 1-bit digital signal, wherein a pre-emphasis circuit for enhancing the analog input signal toward a higher frequency side of a frequency band to be converted; A limiter circuit for limiting the output of the pre-emphasis circuit within a predetermined peak height limit value; and a low-pass filter for filtering a component of the output of the limiter circuit below the frequency band to be converted. The sampling frequency is relatively low with respect to the frequency band to be converted.

According to a second aspect of the present invention, the peak value is selected to be slightly lower than an oscillation limit value of an analog input signal to the analog / digital converter.

According to a third aspect of the present invention, the peak value of the analog input signal to be input to the pre-emphasis circuit is less than the oscillation limit value of the analog input signal to the analog / digital converter. I do.

[0018]

According to the first aspect of the present invention, in a signal input circuit for an analog / digital converter for converting an analog input signal such as an audio signal into a 1-bit digital signal by Δ 、 conversion or the like, first, a pre-emphasis circuit is used. The analog input signal is increased as the frequency band to be converted becomes higher, for example, from 0 to 22000 Hz. Then, the enhanced analog input signal is limited to a predetermined peak height limit value by a limiter circuit. /
The digital converter is restricted so as not to be over-input by the pre-emphasis. Subsequently, from the output of the limiter circuit, a component below the frequency band to be converted is filtered by the low-pass filter to remove a distortion component such as a harmonic generated by the restriction to the peak height limit value.

Therefore, even if the sampling frequency of the analog / digital converter is set to about 2 MHz or less, which is suitable for the transmission capacity and is relatively low as compared with the frequency band to be converted, the high-frequency component in which the noise increases increases in harmonics. Since the generation of waves is suppressed to be within a predetermined wave height limit value by suppressing the generation of waves, low-frequency components are not relatively suppressed even when the de-emphasis processing is performed. In this way, even if the sampling frequency is relatively low, it is possible to suppress the generation of noise in high frequency components and secure a wide dynamic range. In addition, it is possible to reliably suppress excessive input to the analog / digital converter and perform a stable operation.

Preferably, according to the second aspect of the present invention, the peak value is selected to be slightly lower than a peak value of an analog input signal to an analog / digital converter. Therefore, oscillation due to excessive input of the analog / digital converter can be suppressed, and the allowable input range of the analog / digital converter can be used effectively.

Still more preferably, according to the invention of claim 3, the peak value of the analog input signal to be input to the pre-emphasis circuit is selected to be less than the oscillation limit value of the analog input signal to the analog / digital converter. It is. Therefore, the entire frequency band is set to be less than the oscillation limit value in advance, and only the high-frequency component enhanced by the pre-emphasis circuit temporarily exceeds the oscillation limit value, and is thereafter limited by the limiter circuit. The value is constant over the entire frequency band, and the narrowing of the dynamic range due to the suppression of the level of the low-frequency component as described in the related art can be prevented.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a block diagram showing an electrical configuration of a 1-bit signal recording / reproducing apparatus 12 using a signal input circuit 11 according to one embodiment of the present invention. An analog audio signal, which is an input signal input from the input terminal 13, is subjected to pre-emphasis, peak value limitation, and the like in the signal input circuit 11, and then to the analog / digital converter 14. The analog / digital converter 14 converts the input audio signal into a 1-bit digital audio signal by ΔΣ modulation using 768 kHz to 1.536 MHz as a sampling frequency.

The digital audio signal from the analog / digital converter 14 is output to the deck 16 after passing through the interface circuit 15. The deck 16 records the input digital audio signal on the magnetic tape by a rotating head.

The digital audio signal reproduced by the deck 16 passes through the interface circuit 17 and
The digital / analog converter 18 converts the signal into an analog audio signal. The analog audio signal is supplied to the signal input circuit 1 by a de-emphasis circuit 19 as described later.
After the de-emphasis processing corresponding to the pre-emphasis processing in 1 is performed, the signal is output from the output terminal 20 to a power amplifier or the like.

The signal input circuit 11 includes a pre-emphasis circuit 21 for performing a pre-emphasis process for increasing the input analog audio signal as the frequency of the analog audio signal increases, and an output from the pre-emphasis circuit 21 to a predetermined peak height limit value. And a LPF 23 for filtering only the audible frequency band component from the output of the limiter circuit 22.

FIG. 2 is a circuit diagram showing a specific configuration of the pre-emphasis circuit 21. The analog audio signal from the input terminal 13 is supplied to the pre-emphasis circuit 21.
To the input terminal P1. Voltage dividing resistors R1 and R2 are interposed between the input terminal P1 and the ground line. The input analog audio signal is divided by these voltage dividing resistors R1 and R2, and is supplied to the amplifier circuit 24. Is entered. A high-frequency component bypass capacitor C1 is connected in parallel to the voltage dividing resistor R1. For example, the capacitance of the capacitor C1 is 0.001 μF, and the resistance values of the voltage dividing resistors R1 and R2 are 100K, respectively.
Ω, 1 KΩ.

Therefore, the analog audio signal input to the input terminal P 1 is amplified and input to the amplifier circuit 24 as the frequency increases. The gain of the amplifier circuit 24 is flat over the entire audible frequency band, so that the pre-emphasis circuit 21 has a frequency characteristic as shown in FIG. Therefore, the output terminal P2
In the above, an output is derived in which the middle and low frequency components are limited to less than the peak value limit, the high frequency components are enhanced as the frequency becomes higher, and the level includes a level exceeding the peak value limit value.

FIG. 4 is a circuit diagram showing a specific configuration of the limiter circuit 22. The limiter circuit 22 generally includes a differential amplifier 25, resistors R3 to R5, and zener diodes D1 and D2. The audio signal whose high frequency component has been enhanced by the pre-emphasis circuit 21 is input from the input terminal P3 to the inverting input terminal of the differential amplifier 25 via the input resistor R3. This differential amplifier 25
Are connected to ground via a resistor R4. The output from the differential amplifier 25 is output from the output terminal P4 to the LPF 23, and is also fed back to the inverting input terminal via a feedback resistor R5. Also, a series circuit of zener diodes D1 and D2 connected in opposite polarities to each other is connected in parallel to the resistor R5.

Therefore, when the input level to the input terminal P3 is indicated by the X axis in FIG. 5, the output level to the output terminal P4 is within the peak height limit value W1 of a predetermined amplitude as indicated by the Y axis. The peak value is limited.

The LPF 23 has a resistor R as shown in FIG.
6 and a capacitor C2, the input signal to the input terminal P5 is changed to 22000 which is the upper limit of the audible frequency band, as shown in FIG.
After filtering with a filtering characteristic in which Hz is a cutoff frequency fc, the signal is output from the output terminal P6 to the analog / digital converter 14.

Therefore, in the signal input circuit 11 configured as described above, as shown in FIGS. 8A and 9A, the analog audio signal to the input terminal 13 has a low frequency component and a high frequency component. If the component also has a very large amplitude near the peak height limit values W1 and α12, the pre-emphasis circuit 21
8 (b) and 9
In (b), as indicated by reference numeral α13, the amplitude of the high frequency component exceeds the peak height limit value W1. However, in the limiter circuit 22, FIG.
As shown in FIG. 9 (c) and FIG. 9 (b), components exceeding the peak height limit values W1, α12
Within. At this time, harmonic components included in the edge generated as indicated by the reference numeral α11 are removed by the LPF 23 as illustrated in FIG.

As described above, by providing the limiter circuit 22 and the LPF 23 for preventing the harmonic distortion accompanying the limiter circuit 22, the increase in the high frequency component by the pre-emphasis circuit 21 and the increase in the high frequency component in FIG. As shown, the amplitude of the low-frequency component is not suppressed relatively, and a sufficient dynamic range can be ensured with respect to the noise level.

The peak height limit value W1 is set to be equal to or slightly smaller than the input amplitude oscillation limit value of the analog / digital converter 14, so that the allowable input range of the analog / digital converter 14 is valid. And the oscillation of the analog / digital converter 14 can be prevented.

As described above, the signal input circuit 11 according to the present invention
By using the analog / digital converter 14
The sampling frequency of the input signal to be converted,
That is, it is not a relatively high frequency of about 3 to 12 MHz that can secure a sufficient S / N of a high frequency component with respect to the audible frequency band, but to the interface circuit 15 in the subsequent stage and other digital audio equipment. Even if the frequency is as low as 2 MHz or less corresponding to the transmission capacity, a stable conversion operation can be performed without generating oscillation due to excessive input while securing a wide dynamic range.

[0036]

According to the signal input circuit for the 1-bit analog / digital converter according to the first aspect of the present invention, as described above, after the high frequency component of the analog input signal is enhanced by the pre-emphasis circuit, the signal is predetermined by the limiter circuit. Limiting is performed within the wave height limit value so that the analog / digital converter is not over-input. Further, a distortion component such as a harmonic component caused by the restriction is removed by the low-pass filter.

Therefore, the sampling frequency of the analog / digital converter is set to 0 to 22000H suitable for the transmission capacity and the like.
2 MHz relatively lower than the frequency band to be converted of z
Even below, the high-frequency component where noise increases is reasonably enhanced, and even if de-emphasis processing is performed, the low-frequency component is not relatively suppressed and a wide dynamic range can be secured. it can. Further, it is possible to reliably suppress excessive input to the analog / digital converter, prevent oscillation and the like, and perform a stable operation.

In the signal input circuit according to the second aspect of the present invention, as described above, the crest limit value is selected to be slightly lower than the crest limit value to the analog / digital converter.
Therefore, oscillation due to excessive input of the analog / digital converter can be suppressed, and the allowable input range of the analog / digital converter can be used effectively.

In the signal input circuit according to the third aspect of the present invention, as described above, the peak value of the analog input signal to be input to the pre-emphasis circuit is determined by the oscillation of the analog input signal to the analog / digital converter. Selected below the limit. Therefore, the entire frequency band is set to be less than the oscillation limit value in advance, and only the high-frequency component enhanced by the pre-emphasis circuit temporarily exceeds the oscillation limit value and is thereafter limited by the limiter circuit. The limit value is constant over the entire frequency band, and it is possible to prevent the dynamic range from being narrowed due to the suppression of the level of the low-frequency component as described in the related art.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an electrical configuration of a signal input circuit according to one embodiment of the present invention.

FIG. 2 is a circuit diagram showing a specific configuration of a pre-emphasis circuit 21 in the signal input circuit.

FIG. 3 is a graph showing a frequency characteristic of the pre-emphasis circuit 21;

FIG. 4 is a circuit diagram showing a specific configuration of a limiter circuit 22 in the signal input circuit.

FIG. 5 is a graph showing input / output characteristics of the limiter circuit 22;

FIG. 6 is a pneumatic circuit diagram showing a specific configuration of the LPF 23 in the signal input circuit.

FIG. 7 is a graph showing a frequency characteristic of the LPF 23;

FIG. 8 is a waveform chart for explaining the operation of the signal input circuit.

FIG. 9 is a diagram showing a frequency spectrum for explaining the operation of the signal input circuit.

FIG. 10 is a waveform chart for explaining the operation of 1-bit analog / digital conversion.

FIG. 11 is a graph showing a frequency spectrum after a conversion operation of a predetermined input signal during a normal operation of the 1-bit analog / digital converter.

FIG. 12 shows D using typical prior art input signal processing.
FIG. 3 is a block diagram illustrating an electrical configuration of the AT.

FIG. 13 is a diagram showing converted data for explaining an oscillation operation due to excessive input of the 1-bit analog / digital converter.

FIG. 14 is a graph showing a frequency spectrum after the conversion operation at the time of oscillation as shown in FIG. 13 for the input signal of the predetermined frequency of the 1-bit analog / digital converter.

FIG. 15 is a waveform diagram for explaining an input signal processing operation in the conventional DAT.

[Explanation of symbols]

 Reference Signs List 11 signal input circuit 12 1-bit signal recording / reproducing device 14 analog / digital converter 15 interface circuit 16 deck 17 interface circuit 18 digital / analog converter 19 de-emphasis circuit 21 pre-emphasis circuit 22 limiter circuit 23 low-pass filter

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03M 3/02

Claims (3)

(57) [Claims] 1. A signal input circuit for an analog / digital converter for converting an analog input signal into a 1-bit digital signal, wherein the analog input signal is pre-enhanced as it becomes higher in a frequency band to be converted. An emphasis circuit, a limiter circuit for limiting an output of the pre-emphasis circuit to a predetermined peak height limit value, and a low-pass filter for filtering a component of an output of the limiter circuit which is lower than a frequency band to be converted, A signal input circuit for a 1-bit analog / digital converter, wherein the sampling frequency of the digital / digital converter is relatively low with respect to the frequency band to be converted. 2. A 1-bit analog / digital converter according to claim 1, wherein said peak value is selected to be slightly lower than an oscillation limit value of an analog input signal to said analog / digital converter. Signal input circuit. 3. A peak value of an analog input signal to be input to said pre-emphasis circuit is less than an oscillation limit value of an analog input signal to said analog / digital converter. 2. A signal input circuit for a 1-bit analog / digital converter according to claim 1.