JP2508498B2 - Code converter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a linear PC having a predetermined number of bits.
The present invention relates to a code converter that encodes an M signal into a differential PCM signal .

[0002]

2. Description of the Related Art In a coded transmission apparatus or a coded recording / reproducing apparatus, a sample value of a PCM signal and a PC adjacent to the PCM signal are used for the purpose of lowering the transmission bit rate.
Difference P that quantizes and transmits the difference value from the predicted value of the M signal
The CM method may be adopted. This system is suitable for transmitting a signal with strong redundancy such as a voice signal or a TV signal. However, this system shows a tendency for S / N to deteriorate in the high frequency range, and in order to obtain a dynamic range (or input / output linearity) above a certain level, increase in distortion in the high frequency range cannot be avoided.

[0003] The present invention is, in view of the above problems, when the transmission of the signal (or recording) code of the code transmission system so as to change the instantaneous dynamic range of the transmission system in dependence on the frequency spectrum of the original analog signal conversion
It is the one that proposes a container .

The code converter according to the present invention has an input terminal for supplying a linear PCM signal of a predetermined bit, a subtracting circuit for supplying the linear PCM signal supplied to the input terminal, and an output signal of the subtracting circuit. A bit reduction circuit that is supplied and converts the word length from the predetermined bit to a shorter bit, and an output signal of the bit reduction circuit as an input signal.
And, an adder circuit for the output signal is supplied from the output of the adder circuit, at least a means for suppressing high frequency level of the output signal of the bit reduction circuit, the signal which the high-range level is suppressed in , The subtraction circuit and the addition circuit
To the linear PCM signal into shorter bits.
It is characterized in that the code is converted into.

According to the code converter of the present invention thus constructed, the code transmission system is constructed in combination with the adaptive encoding device to adaptively adjust the instantaneous dynamic range of the transmission system. By changing it, it is possible to transmit a high frequency signal with a high S / N even in a low bit rate transmission system.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of a code converter according to the present invention will be described below with reference to the drawings.

First, the frequency characteristics of the dynamic range of various code transmission systems will be described with reference to FIG.

In FIG. 1, the standard bit rate is 1 bit × 1.
The S / N-frequency characteristic about PCM, delta modulation ((DELTA) M), and differential PCM (DPCM) in a 28-KHz transmission system is shown, and each straight line shows the upper limit (dB value) of the dynamic range of the signal which can be transmitted.
The straight line A shows a case where the sampling frequency is 128 KHz and PCM transmission is performed with 1 bit for each sampling point, and the S / N is 6 dB in the transmission band 64 KHz range. The straight line B shows a case where the sampling frequency is 32 KHz and PCM transmission is performed with 4 bits at each sampling point, and the dynamic range of 24 dB is 16 K.
Transmission in the Hz band is possible. The straight line C is for ΔM transmission of 1-bit quantization at a sampling frequency of 128 KHz, and the S / N is zero dB at a point (64 KHz) at a sampling frequency of ½ (a state where a signal and noise cannot be distinguished). Then, the S / N ratio increases with a slope of 6 dB / OCT in the lower frequency range.

The straight line D is the case of DPCM transmission of 4-bit quantization at a sampling frequency of 32 KHz, and since it takes 1 bit to transmit the positive / negative of the difference, P of 4 bits × 32 KHz is used.
Compared with CM transmission, 1/2 the sampling frequency (16KH
In z), the 4-bit DPCM is equivalent to the 3-bit PCM, and the S / N is 18 dB (-6 dB: 1 bit lower). Below, the S / N increases with a slope of 6 dB / OCT. That is, as shown by the shaded area in FIG.
Below Hz, DPCM transmission is more S than PCM transmission.
/ N, which is advantageous, but at 8 KHz to 16 KHz,
On the contrary, the S / N is deteriorated and the dynamic range is narrowed.

As described above, in the PCM transmission system, the frequency characteristic of the dynamic range is flat without depending on the frequency. This is a redundant coding method in the case of a voice signal, considering that the probability that the spectrum is spread over the entire frequency region is small in a state where the level is high. On the other hand, the DPCM method has the highest transmission efficiency because the dynamic range in the low frequency band is improved when compared at the same transmission bit rate, but on the other hand, the S / N ratio in the high frequency band is high.
There is an inconvenience that must be sacrificed.

[0011]

[0012]

[0013]

[0014]

[0015]

Next, FIG. 2 shows an embodiment of a decoding device corresponding to the encoding device. In FIG. 2, a digital signal (differential PCM signal) encoded and transmitted by the encoding device is applied to the input terminal 10. This input digital signal is converted into an analog signal by the D / A converter 12 via the adder 11. The output of the D / A converter 12 is converted into a digital signal again by the A / D converter 13, and then added to the input digital signal by the adder 11. Since the conversion operation of the A / D converter 13 causes a delay of one sample interval, the adder 11 adds the transmitted difference value to the sample value of one sample interval before, and the DPCM is added.
To PCM (encoding) is performed. Therefore, the original analog signal is decoded and obtained from the output terminal 14 connected to the output of the D / A converter 12.

The output of the D / A converter 12 is applied to the high pass filter 15 to extract high frequency components. Further, envelope information of high frequency components is detected by the detection circuit 16 and the smoothing circuit 17, and this information signal is given as a control signal to the gain control input (reference current input) of the A / D converter 13 to cause A / D conversion. The conversion gain is controlled.
As a result, in the adder 11, the calculation of a '+ kb' is performed on the decoded original signal b'one sample period before and the transmitted current value a'of the differential signal. When the amount of high frequency components is small, the gain k of the A / D converter 13 is almost 1, and the adder 11 calculates a ′ + b ′ to obtain DPCM → P.
Performs CM encoding processing. As the high frequency component increases,
The control signal increases and the gain k of the A / D converter 13 approaches 0, and the transmitted PCM signal a ′ is directly decoded by the D / A converter 12 into an analog signal.

It should be noted that the decoding device of FIG.
Adder 1 if used only as M encoding
A PCM signal output of 1 is derived from the output terminal 18.

[0019]

[0020]

[0021]

[0022]

[0023]

[0024]

[0025]

FIG. 7 shows an embodiment of a code converter (encoder) for encoding a linear PCM signal into a differential PCM signal. In FIG. 7, for example, a 13-bit linear PCM signal is applied to the input terminal 27. This PC
The M signal is subjected to difference calculation by the subtractor 28, and is then output to the output terminal 30 as a 4-bit difference PCM signal through the bit reduction circuit 29. The differential PCM signal is supplied to the D / A converter 32 through the adder 31, converted into an analog signal, and further converted into a digital signal by the A / D converter 33. The output of the A / D converter 33 is the adder 3
1 and added with a 4-bit differential PCM signal. In the D / A converter 32, a delay of one sample period occurs due to the conversion process, so that the output of the adder 31 can obtain digital information obtained by converting the differential PCM signal into a linear PCM signal, and further, this information. The output terminal 3
The quantization error component of the 4-bit differential PCM signal (signal actually transmitted) occurring at 0 is included. This information is supplied to the subtractor 28 via the A / D converter 33 as a kind of predicted value.
And the linear PCM of the input in this subtractor 28
The signal is subtracted and encoded into a differential PCM signal.

Further, in the code converter shown in FIG. 7, the code converter shown in FIG.
Similarly to the encoding device and the decoding device of FIG. 2, the high frequency level information of the transmission signal is detected through the high pass filter 34, the detection circuit 35 and the smoothing circuit 36, and the A / D is detected by the detected high frequency level information. The gain of the converter 33 is controlled. As a result, for the high level large level signal, the subtraction input of the subtractor 28 becomes substantially zero, and P
The conversion from CM to DPCM will not be performed.

The code converter (encoder) of FIG. 7 is originally for changing the transmission bit rate,
It can also be used as an encoding device or a decoding device. In the case of the encoding device, an A / D converter is provided in front of the input terminal 27. In the case of the decoding device, a D / A converter is provided after the output terminal 30.

FIG. 8 shows a block diagram of an embodiment in which the amplitude companding system is introduced into the code transmission system according to the present invention which is equipped with the adaptive encoding device and the decoding device.

In FIG. 8, an analog waveform signal to be transmitted is given to the input terminal 39. This waveform signal is
After being pre-emphasized through the high pass filter 40, it is sampled by the sample hold circuit 41. The sampled value is converted into a PCM signal by the A / D converter 42. The A / D converter 42 is of a successive approximation type and includes a comparator 43, a successive approximation register 44 and a D / A converter 4.
It is composed of 5. The comparator 43 outputs the sample value of the output of the sample hold circuit 41 and the D / A converter 4
The output of "5" is compared with the output of "5" to generate the output "1" or "0" according to the magnitude relation. This output is the successive approximation register 44.
Are sequentially stored from the MSB of the register, and the output of the register 44 is D /
After being converted to an analog level by the A converter 45, it is given to the comparator 43 as a comparison signal. The successive approximation loop of the A / D converter 42 operates by the number of digits of the register 44, and thereby the work of converting into a PCM signal having the required number of bits is performed.

The output of the A / D converter (42) is given to a dynamic range variable encoder 46 similar to that shown in FIG. 7, which performs conversion between PCM and DPCM, and the differential PCM signal in which the number of bits is reduced. Is converted to. The constituent elements of the encoder 46 are designated by the same reference numerals as those in FIG. 7.

PCM and DPCM in the encoder 46
The output of the high-frequency level detection unit (the output of the smoothing circuit 36) that performs conversion control between the
It is also used to control the gain of the A converter 45.
By this gain control, when there are many high frequency components in the transmission signal, the output width of the D / A converter 45 increases, and therefore the sampled value of the output of the sample hold circuit 41 is amplitude-compressed to a PCM signal. To be converted. That is, the A / D converter 42 operates as an adaptive nonlinear quantizer whose resolution is variable according to the high frequency level of the transmission signal. The quantization step for the high level high level input is equivalently fined by the compression, and the transmission performance is further improved in combination with the S / N improvement of the high frequency (enlargement of the dynamic range) by the encoder 46.

The differential PCM signal output from the encoder 46 is transmitted from the terminal 30 through the transmission line 47 to the receiving side or demodulating side. The transmission path 47 is, for example, a magnetic recording / reproducing system such as a VTR or a wired or wireless communication line.

The signal passing through the transmission path 47 is given from the terminal 10 to the decoding device 48. This decoding device 48 is almost the same as that of FIG. 2, and the same elements are denoted by the same reference numerals. In FIG. 8, the D / A converter 12 also operates as an expander, so that the D / A converter 12 ′ having the same function as in FIG. 2 is used.
2 is different from that of FIG. DPM → P
The operation of the CM decoding device is the same as that in FIG. 2, and the gain of the A / D converter 13 is controlled by the control signal representing the high frequency level of the output of the smoothing circuit 17. At the same time, the output of the smoothing circuit 17 is also given to the gain control input of the D / A converter 12, whereby the gain of the D / A converter 12 becomes large and the transmission input becomes high for a high level large level signal. The amplitude expansion corresponding to the compression on the side is performed. The output of the decoding device 48 is led to an output terminal 50 through a low-pass filter 49 for de-emphasis.

In FIG. 8, the adaptive non-linear A / D converter 42 on the transmission input side and the D / A converter 12 on the transmission output side may have the functions of instantaneous compression and expansion. In this case, for example, as the compressor, the double-wave rectified signal of the output of the high-pass filter 34 or the D / A converter (32) of FIG. 8 is used as the gain control signal in the D / A converter (4
Given in 5). As a result, the D / A converter (4
5) operates as a multiplier (square calculator), and the A / D converter (42) is 1/100 for the entire band or high band of the transmission signal.
Square compression is performed. Further, the instantaneous decompressor on the transmission output side can also be constructed by operating the D / A converter as a square calculator.

[0036]

According to the above-described code converter of the present invention,
It is possible to perform code conversion from linear PCM to differential PCM . That is, normally, the differential PCM is used for the low-frequency S / N.
Good transmission, wide dynamic range transmission and high signal power in the high range , conversion from PCM to DPCM is performed.
I won't forget. As a result, the high frequency S / N deterioration peculiar to the differential PCM is compensated. Therefore, when transmitting an audio signal, it is possible to reduce a sense of distortion when a high frequency signal is received.

[Brief description of drawings]

FIG. 1 is a graph showing frequency characteristics of a dynamic range of various code transmission systems.

FIG. 2 is a block diagram of a decoding apparatus for adaptively coded signals according to an embodiment of the present invention.

FIG. 3 is a block diagram of an adaptive encoding device and a code converter that can be used as a decoding device thereof in the present invention.

FIG. 4 is a block diagram of an embodiment in which a swing companding system is introduced into a code transmission system according to the present invention, which is equipped with an adaptive encoding device and its decoding device.

[Explanation of symbols]

 27 Input Terminal 28 Subtractor 29 Bit Reduction Circuit 31 Adder 32 D / A Converter 33 A / D Converter 34 High Pass Filter 35 Detection Circuit 36 Smoothing Circuit

Claims (1)

(57) [Claims] 1. An input terminal for supplying a linear PCM signal of a predetermined bit, a subtraction circuit to which the linear PCM signal supplied to the input terminal is supplied, and an output signal of the subtraction circuit and the predetermined signal. a bit reduction circuit for converting a word length shorter bits from the bit, the output signal of the bit reduction circuit as an input signal, and a summing circuit for the output <br/> force signal is supplied, the output of the addition circuit from a means for suppressing high frequency level of the output signals of at least the bit reduction circuit, the high frequency signal level is suppressed, said subtraction circuit and
The linear PCM signal supplied to the adder circuit is
A code converter that performs code conversion into shorter bits .