JPH01105365A - Digital magnetic recording and reproducing device - Google Patents

Abstract

PURPOSE:To prevent the noise reduction from being erroneously expanded against an error generated at the time of reproducing and to obtain a sound of good reproducibility by lowering a detecting band when the number of errors is generated more than a set value and changing high band exaggeration characteristics. CONSTITUTION:The number of uncorrectable data is detected out of data from a PCM demodulation circuit 18 by an ERR detector 21, and the error generating frequency is counted by an ERR counter 17. The information showing the error frequency in reading the present demodulated data is sent from the counter 17 to a coefficient controller 15, and coefficient values of a high band attenuation circuit EM II 52 in a noise reduction circuit NR 14 and to a high band exaggeration circuit WE 54 respectively are changed by the controller 15. By this method, a cutoff of the EM II 52 is lowered, and a detecting level of the WE 54 is also lowered. Then, a signal brought back into its original dynamic range by the NR 14 is converted by a D/A converter 12 and outputted via an LPF 11.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention compresses the dynamic range of an input signal by digital processing noise reduction and then records it as a digital signal, and when playing back, the reproduced digital signal is again subjected to the digital processing noise reduction described above. This invention relates to a digital magnetic recording and reproducing device that expands the original dynamic range.

[Conventional technology]

When an audio signal is recorded by magnetic recording and playback, the dynamic range is compressed, and when played back, it is expanded using noise reduction (NR), which pseudo-expands the dynamic range originally possessed by magnetic tape. Such methods have been known for some time.

In particular, when the present digital NR is applied when the signal recorded on the magnetic tape is of the pulse code quality pl (hereinafter abbreviated as PCM) system, it becomes possible to record audio with a wide dynamic range using a smaller number of bits than before.

As an example of this, Japanese Patent Laid-Open No. 59-66208 has been proposed. This is a method in which an analog signal is converted into a digital signal, the dynamic range is compressed by digital signal processing, the dynamic range is expanded by digital signal processing during playback, and then the signal is converted into an analog signal.

The basic operation of the above method will be explained below with reference to FIGS. 2 and 3.

FIG. 2 shows a dynamic range compression circuit using digital signal processing. Reference numeral 25 denotes a terminal into which an audio signal converted into a PCM signal is input, 26 a division circuit, and 27 a detection circuit which detects the output signal of the division circuit 26 and outputs an amplitude information signal. 28 is a terminal to which the compressed PCM signal is output. The operation of obtaining the transfer function shown in FIG. 2 and compressing the signal input from the input terminal 25 logarithmically will be described below.

The amplitude value of the PCM signal now input from terminal 25 is X,
If the amplitude value of the PCM signal output from the division circuit 26 is y, the value y will be output from the detection circuit 27, so the output terminal 28 of the compression circuit will have the following formula: y=x/y... ......(1) Therefore, a signal with an amplitude of "f=n-...+21" will be output.If we take the logarithm of both sides of equation (2), we get --!-pupil

Next, as shown in FIG. 3, the detection circuit includes an absolute value circuit 30 that takes the absolute value of the value represented by the digital signal, and a low-pass filter that converts the output of the absolute value circuit 30 into amplitude information. (hereinafter abbreviated as LPF) 31.

FIG. 4 shows the basic configuration of a dynamic range expansion circuit using digital signal processing. In FIG. 4, 33 is a terminal to which a PCM signal with a compressed dynamic range is input, and 35 is a multiplication circuit.

The PCM signal inputted from the terminal 33 is inputted to the multiplication circuit 35 after its amplitude is detected by the detection circuit 34 . The multiplier circuit multiplies the signal from the terminal 63 by the output signal of the detection circuit 33 and outputs the signal to the terminal 36.

The operation of calculating the transfer function shown in FIG. 4 and expanding the signal inputted from the input terminal 33 by a factor of two logarithmically will be described below. Now, if the amplitude value of the PCM signal input from the terminal 33 is X, and the amplitude value output from the multiplier circuit 35 is y, the value y will be output from the detection circuit 34, so it is sent to the output terminal 36 of the expansion circuit. A signal with an amplitude of y=x2...old...(4) will be output. Taking the logarithm of both sides of equation (4), kg 7 = 2 kg x −−−・・・−・−+
51, so this expansion circuit operates to logarithmically expand the amplitude of the input signal by two times. The conventional example described above has the advantage that, compared to analog type NR, external circuits can be significantly reduced, and there is no characteristic deterioration due to element variations or changes over time, and the same characteristics can always be obtained.

[Problem that the invention seeks to solve]

In the conventional digital magnetic recording/reproducing apparatus described above, 4. Digital NR expansion operation when an error occurs is not described.

Generally, in digital magnetic recording and reproducing devices, data reading errors occur due to dust, scratches, etc. on the tape during reproduction.

For those with few errors, error correction processing 3m
However, if the amount of error is large, the error cannot be corrected completely. Therefore, if the data is reproduced as it is, noise will occur as shown in FIG. 17. Therefore, in order to reduce the noise, as shown in FIGS. 18 and 19, methods have been devised to hold the previous value only during the error occurrence period, and means to directly approximate the values before and after the error occurrence period.

However, the correction waveform obtained by holding the previous value and performing linear approximation contains harmonic components, and when expanded by digital NR, a problem arises in that an erroneous detection wave occurs in the detection and erroneous expansion occurs.

Detection here refers to detecting the input signal waveform as shown in Figure 5.
As shown in the figure, the absolute value is converted to a detection level corresponding to the peak value of the waveform, and the NR performs an expansion operation in accordance with this detection level.

However, when the above-mentioned correction waveform shown in FIG. 18 is detected, the high frequency band is emphasized by the WB circuit 54 shown in FIG. 15, and linking occurs as shown in FIG. Next, the detection circuit 56 detects the peak value of the detection level of the high-frequency emphasized signal, and performs an NR expansion operation in accordance with the peak level. However, when peak detection is performed on a waveform that causes ringing, the NR will be erroneously expanded.

SUMMARY OF THE INVENTION An object of the present invention is to provide a digital magnetic recording and reproducing apparatus having a digital NR that does not erroneously expand even if an uncorrectable code error occurs during reproduction.

[Means to solve the problem]

In order to achieve the above object, the present invention has a configuration in which the detection band of the amplitude detection circuit is switched according to the number of errors, and when the number of errors occurs beyond a set value, the detection band is lowered. Furthermore, the digital NR including an emphasis circuit, which is a high-frequency emphasis circuit, is configured to change the high-frequency emphasis characteristic by switching the coefficient value that determines the emphasis characteristic according to the number of errors.

[Effect]

With such a configuration, the detection level of digital NR can be adjusted according to the frequency of error occurrence.

Furthermore, the high-frequency emphasis characteristic of digital NR is switched depending on the frequency of error occurrence.

Therefore, even if an error occurs during playback, it is possible to obtain sound with good reproducibility without digital NR error expansion.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 shows an embodiment of a digital magnetic recording/reproducing apparatus using digital noise reduction according to the present invention. 1 is an audio input terminal.

2 is a low-pass filter (hereinafter abbreviated as LPF), and 3 is an analog-to-digital converter (hereinafter abbreviated as AD converter).
, 4 is a digital LPF, 5 is a digital 7. Noise reduction by signal processing (hereinafter referred to as digital NR)
It is abbreviated as ), 6 is a bit compressor, which compresses a 10-bit digital audio signal into an 8-bit digital audio signal. 7 is a 20M modulator, 8 is a recording amplifier, 9 is a reproduction amplifier, 1o is an audio output terminal, 11 is an LPF, 12 is a digital-analog converter (hereinafter abbreviated as DA converter), 16
is a digital LPF, 14 is a digital NR (expander), 1
5 is a coefficient controller for changing the coefficients of the digital LPF of the 14 digital audio signals; 16 is a bit expander for expanding the 8-bit digital audio signal into a 10-bit digital audio signal; 21 is for changing the coefficients of the digital LPF of the digital audio signal; 18 is a PCM demodulator, 19 is a clock regenerator, 2o is a waveform equalizer, 21a and 21b are recording/reproducing heads, 22 is a magnetic tape, and 17 is an error counter.

Now, the signal flow in diagram gg1 will be explained.

The audio signal input from the terminal 1 is input to the AD converter 3 after being band-limited by an LPF 2 for preventing aliasing noise. The AD converter 3 requires a quantization bit number of approximately 16 bits in order to handle signals with a dynamic range of 90 dB or more. The signal obtained by sampling and quantizing the analog signal by the AD converter 3 passes through the digital LPF 4, has its dynamic range compressed by the digital NR 5, and is output as a 10-bit signal.

The signal output from the digital NR5 is sent to the bit compressor 6.
The 10-bit data is compressed to 8-bit data and P
The signal is input to the CM modulator 7. The digital audio signal supplied to the PCM modulator 7 is subjected to processing such as encoding and modulation for error detection and correction, and then the recording signal is sent to a recording amplifier, and is recorded on a magnetic tape by magnetic heads 21a and 21b. A signal is recorded to 22.

On the other hand, the reproduction signal is input to the reproduction amplifier 9 from the magnetic heads 21a and 21b. The output from the regenerative amplifier circuit 9 is input to the waveform equalizer 2o to eliminate code amount interference, the clock regenerating circuit 19 extracts PCM data, and the PCM demodulator 18 demodulates the data.

After error correction is performed on the demodulated data, a bit expander 16 expands the bits, and a digital NR 14 expands the dynamic range.

Furthermore, if the demodulated data cannot be completely error corrected, the data is interpolated and the dynamic range is expanded in the same manner.

The ERR detector 21 detects the number of uncorrectable data from the data transmitted from the PCM demodulation circuit 18, and
The RR kakunta 17 counts the frequency of error occurrence.

The ERR counter 17 sends information indicating the current read error frequency of demodulated data to the coefficient controller 15. Since the ERR counter 17 described above is processed in units of a certain fixed period, the ERR counter is cleared after the above processing.

The coefficient controller 15 switches the respective coefficient values of the EM152 of the high-frequency attenuation circuit and the W3Si of the high-frequency emphasis circuit of the digital NR 14. For example, when the amount of uncorrectable data exceeds a certain set value, the coefficient unit is switched so that the EMI 52 cutoff of the high frequency attenuation circuit is lowered. Also, WFi54
The detection output level of is switched so that the detection level decreases.

Next, the signal restored to its original dynamic range by the digital NR 14 is converted into an analog signal by the DA converter 12, and the harmonic component is removed by the LPFll, and an analog signal is output from the terminal 10.

Next, the most important part of the present invention, digital NR (compression system) 5
The configuration of (expansion system) 14 will be explained using FIGS. 15 and 16.

FIG. 19 is a block diagram of the digital NR expansion section.

51 EM-1 has 82.84 adders, 83 delayers,
It consists of 86.87 coefficient multipliers. 52 BM-
1 has an adder of 88.89, a delayer of 90, 91, 92 .
It consists of 93 variable coefficient multipliers. 54 WE is
It consists of 95.96 adders, 97 delay units, 98.99 coefficient units, and 100 variable coefficient units. 56 D
ET has an absolute value converter of 101, an adder of 102, 105, a delay of 104, and a delay of 10! It consists of i, 106 coefficient multipliers.

-11·94 is a multiplier, 107, 108, 109°111 is an input terminal for coefficient control, and 11G is an expanded audio output terminal.

In the above digital NR expansion system, FtM-1゜EM
-1 is composed of a digital filter with high-frequency attenuation characteristics, and the signal input from the input terminal 81 is output from the terminal 110 with its dynamic range and frequency characteristics restored to their original values.

In particular, in the digital and NR expansion section, in order to change the emphasis characteristic of EM-1, the coefficient units 91 to 93 are made variable coefficient units, and the coefficient values are switched at coefficient control terminals 107 to 109. In addition, in order to prevent false detection when an uncorrectable error occurs, the coefficient 100 is made into a variable coefficient unit so that the detection sensitivity can be changed, and the coefficient value is switched using the coefficient control terminal 111.

Next, in the digital NR compression system block diagram shown in FIG.
It consists of 14 delay units, 115, 11S, and 117 coefficient units. The EM-1 of 120 is composed of adders 121 and 125, delay devices 123, and coefficient multipliers 12, 122, and 124, 126.

1130DH'F is an adder of 128, 129, 130
It consists of a delay unit 127, a coefficient unit 131, and an absolute value converter 132.

140 WE'l', 135, 136 adders, 13
It consists of 7 delay units and 134, 138, and 139 coefficient units. 11B is a divider, 111 is an audio input terminal,
114 is a compressed audio output terminal.

119 of KM-1, 120 of EM-1, 14 of WE
0 are filter circuits each having high-frequency emphasis characteristics. EM-1120 and WE140 have the same frequency characteristics, and since WE140 is in the feedback path, the 120 of EM-11 and the 140 of WE are canceled out in response at constant amplitude, resulting in EM-1 Only 119 frequency characteristics appear in the output.

When a sound source containing more mid-range components than the input terminal and whose amplitude changes is input, noise fluctuations (breathing) in the high-range components depending on the amplitude changes of the mid-range components become noticeable. If the amplitude changes quickly, W15
The characteristic of 1 of FiM-1 is no longer transmitted to the division circuit.
It has the characteristics of both IM-19 and IM-1t120, and high frequencies are emphasized where praising occurs.

However, by passing the signal through a compression circuit equipped with W]4140, high-frequency quantization noise can be relatively reduced.

Next, the reason why the correction waveform at the time of a read error in the reproduced signal affects the digital NR error expansion will be explained using FIGS. 5 to 8.

FIG. 5 shows audio data when there is no reading error, and FIG. 6 shows the output of the detector 56.

The detector 56 is of a peak detection type that detects the peak signal of the absolute value of the input signal, holds it for a detection hold period tH, and outputs a detected signal.

The multiplier 35 performs an expansion operation according to the detected signal level. However, as shown in Figure 7, a reading error occurred.
When the previous value holding correction is performed, the output of the WE54 before detection generates ringing as shown in FIG. 8 because the corrected portion contains harmonic components. Therefore, if peak detection is performed, a detection error will occur and the digital NR expansion operation will not be performed properly.

Therefore, in this embodiment, if an error occurs during playback and ringing as shown in FIG.
can be changed to reduce erroneous expansion of digital NR.

Next, there is a detection circuit shown in FIG. 9 as a second embodiment of the detection circuit for reducing the erroneous expansion of the digital NR.

The detection circuit shown in FIG. 9 is equipped with a detection holding circuit 45 for holding the detection level and a changeover switch 47. The detection level is controlled by the detection hold control terminal. The switch 47 is an ER switch shown in FIG.
The switch 47 is switched to the detection hold 45 only when the RGT signal is input and an error occurs. Usually, it is connected to the LPF43 side.

FIG. 10 shows a time chart of the detection section. Detection 15: If the detection level is held at time t2 when a reading error occurs as shown in the waveform, the detection level will become as shown in ■, and the digital NR will be expanded incorrectly.

Therefore, the ERRG'l' signal indicating the error occurrence period goes high.
Detection hold is not performed only during the I level period, and the previous detection hold value is substituted.

Through the above-described processing, the detection error due to the influence of the WE 41 when an error occurs is reduced as shown in the detection level (■) in FIG. 10, and NR error expansion can be reduced.

Figure 13 shows a flowchart of the above processing steps. First, a detection level is detected in process 150, and the detection level is held in process 151. Next, in process 152, an expansion operation is performed according to the above-mentioned detection level, and in process 153, the error occurrence status is checked. If the amount of error occurrence exceeds a certain set value, the detection level is expanded using the previously held value. If no error occurs, the process returns to the initial state.

Next, as a third embodiment of the detection circuit for reducing the above-mentioned digital NR erroneous expansion, there is a detection circuit shown in FIG.

In the detection circuit shown in FIG. 11, 50 is an input terminal, 51 is EM-1, 52 is EM-1[, 55 is a multiplier, and 54 is W
E, 55 is an absolute value exchanger, and 56 is an LPF.

57 is a detection holding device, 58 is a coefficient switching device, 59 is an error detector, 60 is an ERRG'[' signal input terminal, and 61 is an output terminal.

In this embodiment, in order to suppress the ringing level that occurs in the correction waveform during the error occurrence period, the high frequency emphasis level performed by 54 WEs is lowered.

The error pulse input from the terminal 60 is read by an error detector 59 to count the frequency of error occurrence, and the frequency of error occurrence is transmitted to the coefficient switching device 58 to switch the coefficient value of 540WH to suppress high frequency emphasis.

FIG. 12 shows waveforms of the detector of the above embodiment.

As shown at time t2 of the detected waveform, it is actually a dotted waveform, but since an error has occurred, the coefficient value is switched from A to B, the high frequency emphasis is suppressed, and the detection level is set to ■.

FIG. 14 shows a flowchart of the above processing process. In process 154, the detection level is detected, and in process 155, the detection level is held. Next, in process 156, an expansion operation is performed according to the detection level, and in process 157, the error occurrence frequency is investigated, and if the error occurrence frequency exceeds the set level, 540
Switch the WE coefficient to lower the high frequency emphasis.

As explained above, by switching the digital filter coefficient value in the digital NR according to the frequency of reading errors in the reproduced data, it is possible to eliminate aliasing noise caused by correction waveform distortion when an error occurs, which has been a problem in the past, and to eliminate errors in the NR. Stretching can be prevented by adding a simple additional device to the digital NR.

〔Effect of the invention〕

As detailed above, according to the present invention, even if a code error occurs during reproduction, by switching the digital filter coefficient in the digital NR, aliasing noise generated due to correction distortion, which has been a problem in the past, can be eliminated. Erroneous expansion of NR can be prevented, and the effect is great.

・19・

[Brief explanation of the drawing]

The figure is the detection circuit diagram, Figure 10 is the waveform diagram of the ERRG'[' signal, Figure 11 is the detection circuit diagram, Figure 12 is the waveform diagram of the ERRGT signal, Figure 13 is the processing process R expansion system FIG.

Claims (1)

[Claims] 1. An arithmetic circuit that receives a pulse-encoded audio signal as input and compresses or expands the dynamic range of the input signal according to a control signal, and detects the level of the input or output signal of the arithmetic circuit to output the control signal. In a digital magnetic recording and reproducing apparatus having noise reduction using a digital signal processing method, which is composed of an output detection circuit and a filter circuit that emphasizes or attenuates high frequency components of the input or output signal of the arithmetic circuit, a demodulated digital an error detection circuit for detecting code errors in a signal; a counter for counting the number of error detections using the output of the error detection circuit as input; and means for varying the frequency characteristics of the noise reduction filter circuit in accordance with the counted value of the counter. A digital magnetic recording and reproducing device characterized by having.