JP2566262B2 - Method for preventing waveform clipping in A / D converter for digital audio - Google Patents

Description

TECHNICAL FIELD The present invention relates to a digital audio A / D converter applied to a digital audio device such as a DAT (digital audio tape recorder), and more particularly,
The present invention relates to a waveform clip prevention method capable of preventing an instantaneous waveform clip.

"Prior Art" Conventionally, an analog input system circuit in a DAT has been configured as shown in FIG. In this figure, an analog input signal supplied from an external device (such as a CD player) to the input terminal Tin is attenuated by a level adjuster (recording level adjusting volume) 1 provided on the operation panel surface, and then A This A / D converter 2 is supplied to the / D converter 2.
After being converted into 16-bit digital output data by the, it is supplied to the digital recording system circuit 3. The output of the level adjuster 1 is also supplied to a digital display type level meter 4 provided on the operation panel surface. The level meter 4 displays the input level (recording level) to the A / D converter 2. It is supposed to be done. Here, when the display of the level meter 4 shows 0 dB, it indicates that the recording level is the upper limit, that is, that the analog input signal has reached the upper limit value of the input range of the A / D converter 2. In this case, all 16 bits of the output data of the A / D converter 2 are full bits that are valid.

"Problems to be solved by the invention" By the way, in the above-mentioned DAT, when recording a music source output from a CD player, for example, it seems that the music source to be recorded has the highest level prior to actual recording. A portion is selected, and while the analog signal of the portion is input to the input terminal Tin, the level adjuster 1 is manually adjusted to set the recording level. In this case, in order to make effective use of the dynamic range, it is necessary to adjust the display of the level meter 4 so as to swing over the full scale from -∞ to 0 dB. However, the level meter 4 display should not exceed 0 dB.
This is because when the recording level exceeds the upper limit of the input range of the A / D converter 2, the output data of the A / D converter 2 becomes full bit, and the output data is distorted by cutting the top of the original analog input signal waveform. This is because the data corresponds to the sub-waveform and so-called waveform clipping occurs. In order to prevent such waveform clipping, the recording level must be adjusted with great care, and there is a problem that extremely complicated operation is required.

Therefore, as shown by the dotted line in the figure, a DSP (digital signal processing processor) is provided between the A / D converter 2 and the digital recording system circuit 3, and this DSP outputs the output data of the A / D converter 2. It has been proposed to constantly monitor and increase the attenuation amount of the level adjuster 1 when the output data becomes a full bit and a waveform clip occurs. However, since there is a considerable response time from the occurrence of the waveform clip to the actual increase of the attenuation amount of the level adjuster 1, there is a problem that the instantaneous waveform clip cannot be prevented.

The present invention has been made in view of the above-mentioned circumstances, and it is of course possible to prevent a waveform clip caused by an excessive analog input signal and also to prevent an instantaneous waveform clip.
An object of the present invention is to provide a method for preventing waveform clipping in an A / D converter. "Means for solving the problem" The present invention converts a digital signal from an analog input signal to a digital data whose input range can be arbitrarily set. In the audio A / D converter, the output data sequentially output from the A / D converter during the conversion operation of the A / D converter is captured as sample data, and based on the captured predetermined number of sample data, the analog Calculate the amount of change of the input signal per unit time,
When the output data exceeds a predetermined value, it is predicted based on the amount of change whether all bits of the output data are valid within a certain period, and all bits of the output data are within a certain period. If it is predicted that will be effective, the A / D
While adjusting the input range of the converter to be large, if it is predicted that all bits of the output data will not be valid within a certain period, do not adjust the input range of the A / D converter. It has a feature.

[Operation] According to the method described above, the A / D for digital audio is used.
When the output data of the converter exceeds a certain value, it is predicted whether all bits of the output data will be valid within a certain period from that time, that is, whether a waveform clip will occur. When all bits of the output data are predicted to be valid, the input range of the A / D converter is adjusted to be large, so that the waveform clip can be dealt with in advance. On the other hand, when it is predicted that all the bits of the output data will not be valid, the input range of the A / D converter is not adjusted.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. In this figure, a music source is supplied from the CD player to the input terminal Tin. 5 is an analog input voltage signal Vin directly supplied from the input terminal Tin 16 bits (DB
_{This is} a successive approximation type A / D converter that sequentially converts the digital output data of _{15 to} DB ₀ ).
An analog comparator 6 that compares the output voltage Vr of the A converter 8, a successive approximation logic 7 that sequentially determines each bit DB _{15 to} DB ₀ of the output data based on the output of the analog comparator 6, and the successive comparison logic 7 And a D / A converter 8 for generating an output voltage Vr corresponding to the weight of each bit DB _{15 to} DB ₀ of the output data output from Then, this A / D converter 5 has an internal D
The analog input voltage range is set according to the reference voltage Vref input to the / A converter 8, and the analog input voltage corresponding to the upper limit value of this input voltage range is set.
When Vin is inputted, the output data becomes full bit, and each bit DB ₁₅ to DB ₀ are all "1".

The 16-bit output data sequentially output from the A / D converter 5 described above is supplied to the digital recording system circuit 3 via the DSP 10, and is supplied to the DAT via the data modulation circuit 13, the recording amplifier 14, and the magnetic head 15. Magnetically recorded on a magnetic tape for use.

Reference numeral 16 is a reference voltage setting circuit, which switches the reference voltage Vref supplied to the A / D converter 5 to 256 (= 2 ⁸ ) steps based on the 8-bit range setting data Dr supplied from the DSP 10. 18 is an automatic setting button for instructing the DSP 10 to start automatic setting processing, which will be described later.

Further, the DS10 is a one-chip microcomputer that executes a digital signal processing algorithm at high speed, and the built-in memory thereof is based on the output data of the A / D converter 5 and has the following automatic setting processing. A program for executing the automatic adjustment process is stored in advance.

[Automatic Setting Process] When the automatic setting button 18 is pressed, predetermined reference voltage setting data Dr is output to the reference voltage setting circuit 16 in order to set the input voltage range of the A / D converter 5 to the specified value. In this case, the specified value is the highest level as a music source.
The value corresponds to the upper limit value (2Vrms is defined by the standard) of the analog signal output from the CD player.

Within a certain period, a predetermined number of data sequentially output from the A / D converter 5 are taken in as sample data, and the maximum value is obtained from the taken in predetermined number of sample data.

From the maximum value obtained above and the specified value of the input voltage range set above, it is determined how many bits are left, and thereby the input voltage range to be newly set is calculated. For example, if the maximum value of the sample data is 13 bits full, there is a margin of 3 bits remaining, and the value obtained by adding the value of 3 bits to the above specified value is the new input voltage range. Become.

In order to set a new input range in the A / D converter 5, the reference voltage setting data Dr required for the reference voltage setting circuit 16 is set.
Is output.

[Automatic adjustment processing] When the A / D converter 5 is performing conversion operation, this A
The output data sequentially output from the / D converter 5 is captured as sample data, and the change amount per unit time of the analog input signal Vin, that is, the slope is calculated based on the captured predetermined number of sample data. The calculation of this inclination is always executed.

At the time when the output data exceeds a certain value, for example, when all 14 bits of each bit DB _{13 to} DB ₀ of the output data become “1”, based on the gradient obtained above, within a certain period, For example, 16-bit DB _{15 to} DB ₀ of the output data are output within a period corresponding to the output data sampling frequency of 5 times or less.
Predict whether all will be "1".

If the output data 16 bits DB _{15 to} DB ₀ all become “1” within a certain period and a waveform clip is predicted, in order to gradually increase the input voltage range of the A / D converter 5,
The required reference voltage setting data Dr is sequentially output to the reference voltage setting circuit 16. However, if the waveform clip is not predicted within a certain period, the adjustment of the input voltage range is not executed.

Next, in the above-described embodiment, when recording the music source output from the CD player, prior to the actual recording, select the part of the music source that seems to have the highest level and select that part. Input the analog signal to the input terminal Tin and press the automatic setting button 18. Then DSP
Reference numeral 10 sets the input voltage range of the value corresponding to the supplied analog input voltage Vin in the A / D converter 5 in the procedure from each step to the above-mentioned automatic setting processing. As a result, prior to the actual recording, the input / output characteristic of the A / D converter 5 is automatically adjusted so that waveform clipping does not occur by inputting only the portion of the analog input signal that is supposed to have the highest level. As a result, the level adjustment of the analog input signal, which was conventionally required to prevent the waveform clipping, is completely unnecessary.

Next, the analog signal of the music source to be actually recorded is input to the input terminal Tin from the CD player. Then D
The SP10 appropriately adjusts the input voltage range of the A / D converter 5 while predicting whether or not a waveform clip may occur in the procedure from each step to the above-described automatic adjustment processing. That is, when the output data of the A / D converter 5 exceeds a certain value (14-bit full), it is predicted whether or not a waveform clip will occur after a certain period (within 5 sampling times) from that point. When it is determined that the waveform clip is predicted, the required reference voltage setting data Dr is sequentially output to the reference voltage setting circuit 16 in order to gradually increase the input voltage range of the A / D converter 5. This allows you to deal with waveform clips in advance,
Instantaneous waveform clipping, which could not be prevented by the conventional configuration shown in FIG. 2, can also be prevented.

[Advantages of the Invention] As described above, according to the present invention, when the output data of the digital audio A / D converter exceeds a certain value, all bits of the output data are valid within a certain period from that time. Or not, that is, whether or not a waveform clip will occur. When all bits of the output data are predicted to be valid, the input range of the A / D converter is adjusted to be large, so that the waveform clip can be dealt with in advance. On the other hand, if it is predicted that all bits of the output data will not be valid, the A / D
No adjustment is made to the converter input range.

Therefore, while it is possible to prevent a momentary waveform clip that could not be prevented by the conventional technique, the input range is not adjusted when the waveform clip is not likely to occur, and the dynamic range of the device is effective. The effect of being able to utilize it is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention,
FIG. 2 is a block diagram showing a configuration of a conventional DAT analog input system circuit. 5 ... Successive approximation type A / D converter, 10 ... DSP, 16 ... Reference voltage setting circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-58-47327 (JP, A) JP-A-57-67322 (JP, A) JP-A-56-94515 (JP, A) JP-A-57- 190416 (JP, A) Actually opened 59-187221 (JP, U) Actually opened 57-46330 (JP, U)

Claims (1)

(57) [Claims] 1. A digital audio A / D converter capable of converting an analog input signal into digital data and optionally setting an input range, wherein (B) the A / D converter is operating during a conversion operation of the A / D converter. The output data sequentially output from the / D converter is taken in as sample data, and (C) the change amount of the analog input signal per unit time is calculated based on the taken in predetermined number of sample data, (D) When the output data exceeds a predetermined value, based on the change amount, predict whether all bits of the output data will be valid within a fixed period, and (E1) output the output data within a fixed period. If all bits are predicted to be valid, the input range of the A / D converter is adjusted to be large, while (E2) all bits of the output data are adjusted within a certain period. A method for preventing waveform clipping in an A / D converter for digital audio, wherein the input range of the A / D converter is not adjusted when it is predicted that the input will not be effective.