JPH0443451B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a PCM (pulse code modulation) transmission system and its apparatus,
In particular, the linear data used in normal PCM, that is, linear PCM, and the DPCM (differential PCM)
This invention relates to a technique for improving the deviation of the DC component in PCM transmission, which involves alternately switching and selecting differential data used in differential PCM to be used for transmission, and including data compression/expansion of the transmitted data. In this case, "transmission" is not limited to transmission via wireless or wired communication circuits, but simply means transmission in a broad sense, including transmission through modulation/demodulation systems and recording/reproduction systems. The term "transmission line" means a system including a communication/signal line, a modulation/demodulation system, a recording/reproduction system, etc., or a combination thereof.

[Conventional technology]

NI- is a PCM method that quasi-instantaneously compands PCM data that is simply A/D (analog-digital) conversion of an analog signal, that is, linear data.
PCM（near instantaneous companding PCM〜
It is called quasi-instantaneous companding (PCM). On the contrary,
A PCM method for quasi-instantaneous companding of differential data, which is PCM data of difference values between samples, is called differential quasi-instantaneous companding PCM (hereinafter referred to as "NI-
(hereinafter referred to as ``DPCM'') is considered.
In NI-PCM, the S/N is determined by the level of the original signal. On the other hand, in DPCM, the S/N is determined by the frequency and level of the original signal. Therefore, if the size of the original signal is smaller than the value corresponding to the transmission compression bit length, NI−
The S/M is the same for either PCM or NI-DPCM, but if the original signal is larger than the value corresponding to the transmission compression bit length, the NI-DPCM
In PCM and NI-DPCM, the S/N depends on the frequency.
There is a difference. That is, at a low frequency, even if the amplitude of the original signal is large, the value of the difference data is small, and therefore, there is little loss of information when data is compressed, so the S/N of NI-DPCM is good.
However, in this NI-DPCM, if there is a reduction in information due to compression and the above-mentioned data loss, a shift in the DC component occurs. On the other hand, the value of linear data simply corresponds to the amplitude of the original signal. Therefore, if the amplitude of the original signal is small, there is less information loss when compressing the data, and the S/N of NI-PCM is good. becomes. Furthermore, since NI-PCM basically transmits linear data, no deviation occurs in the DC component.

Therefore, we have developed a PCM that utilizes compressed transmission of differential data and can substantially prevent adverse effects such as deviations in the DC component caused by transmission errors, thereby realizing highly efficient PCM transmission with a good S/N ratio. As a transmission method, a combination of NI-PCM and NI-DPCM is being considered.

In such a PCM transmission system, on the transmitting side, linear data and differential data are selected and compressed in accordance with the original data, and are transmitted together with information indicating the selection state. Then, on the receiving side, the received data is expanded, and when differential data is being transmitted, the expanded received data is further integrated and reproduced according to the information indicating the selection state, and linear data is being transmitted. When the received data is expanded, the expanded received data is directly reproduced.

On the other hand, in compressed transmission of PCM data in which transmission data of a predetermined bit length is obtained by giving priority to the upper effective bits of the original data, the transmission side It has been considered to accumulate the lower truncated bits and include this accumulated data in the compressed data.

In such a system, the truncated bits due to compression are added and accumulated to an accumulator provided on the transmitting side, and when the accumulated value of this accumulator reaches a value that corresponds to the transmission digit due to compression, it is added to the transmitted data. It is included in the calculation. The cumulative value of this accumulator can be included in the transmitted data, for example, by directly adding the cumulative value to the original data before compression, by adding the cumulative value to the compressed data, or by transmitting the cumulative value by compression. One possible method is to increment the transmitted data appropriately when the value reaches a value within the digit, and the value of the accumulator after addition or increment is the value obtained by subtracting the amount equivalent to the addition or increment from the original cumulative value. Become.

In particular, when differential data is compressed and transmitted, the transmitted data is integrated for demodulation on the receiving side, so the accumulation of truncation errors becomes a large DC error. The effect is great.

Therefore, it is conceivable to apply this cumulative error prevention method to a PCM transmission method in which linear data and differential data described above are selected adaptively to the original data, compressed, and transmitted.

[Problem that the invention seeks to solve]

When applying the cumulative error prevention method to a PCM transmission method in which the linear data and differential data described above are selected adaptively to the original data, compressed, and transmitted, the following problem occurs.

The meaning of the truncation error is different when linear data is compressed and when differential data is compressed. Therefore, the cumulative value held in the accumulator at the time of switching from linear data to differential data or from differential data to linear data is not an appropriate value.

For example, a case will be described in which the original data is 16-bit data, and this linear data and differential data corresponding to successive differences between the linear data are selected and compressed into 8-bit data and transmitted.

In FIG. 4, 16-bit original data, ie, linear data, is applied to an input terminal 1, and this linear data is applied to an adder 2, a latch circuit 3, and a discriminator 4. The discriminator 4 determines whether to compress linear data or differential data based on predetermined discrimination conditions from linear data, that is, NI-
It determines whether to transmit using PCM or NI-DPCM and outputs a corresponding determination signal. This determination is performed in units of predetermined data blocks. The latch circuit 3 latches the input linear data, delays it by one sample period, and supplies it to the multiplier 5. Multiplier 5 multiplies the data provided from latch circuit 3 by a difference coefficient α corresponding to the output of discriminator 4 and provides the result to adder 2 . The adder 2 subtracts the output of the multiplier 5 from the original data and sends it to the data compressor 6.
give to A data compressor 6 compresses the output of the adder 2 and sends 8-bit compressed data from an output terminal 7 to a transmission line. Data compression by the data compressor 6 is performed for each predetermined data block.
Data at bit positions corresponding to the 8 most significant bits of the maximum value (absolute value) in the data block is extracted. When data is compressed by the data compressor 6, the truncated bits accompanying the compression, that is, the data of lower digits than the 8 bits to be transmitted, are cumulatively added in the accumulator 8, and the cumulative value is the 8 bits transmitted. If the value falls within the digits, it is included in the transmission data. A discrimination signal indicating the discrimination result of the discriminator 4 is sent to the transmission line via the output terminal 9.

When the value of the difference coefficient α in multiplier 5 is zero (α=0), the output of multiplier 5 is zero.
When NI-PCM transmission is performed and α>0, NI-DPCM transmission is performed. Normal NI−DPCM
In the case of transmission, α=1.

During NI-DPCM transmission, the truncated bits of the differential data are cumulatively added to the accumulator 8, so the contents of the accumulator 8 always contain the difference between the original data and the demodulated data demodulated by the receiver. It shows.

On the other hand, during NI-PCM transmission, the accumulator 8 uses the truncation bits of the linear data, that is,
Since the difference between the original data and the demodulated data is cumulatively added, the value stored in the accumulator 8 is the cumulative value of the difference between the original data and the demodulated data. The cumulative value is sequentially added to the transmission data within a plurality of sampling periods, but the instantaneous value itself of the value held in the accumulator 8 in this case is not directly related to the value of the error due to companding.

Therefore, if the value of accumulator 8 is left unchanged when switching between NI-PCM and NI-DPCM 8, the contents of accumulator 8 immediately after switching will have no meaning in each transmission method (method after switching). value, the continuity of cumulative addition is lost, and a DC shift occurs. This DC shift occurs every time the transmission system is switched, and therefore, under conditions where the switching is frequently performed, it causes further deterioration of the S/N ratio, increase in audible noise, and the like.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a PCM transmission system which compresses and transmits linear data and differential data by alternately selecting them in accordance with the original data, and which employs the cumulative error prevention method by cumulative addition of truncated bits. It is an object of the present invention to provide a PCM transmission method that can solve the problem of discontinuity of cumulative addition values when switching between linear data and differential data, and can prevent the occurrence of DC deviation.

[Means for solving problems]

The present invention achieves the above objects,
Linear data corresponding to the analog original signal and difference data corresponding to successive difference values of the linear data are alternately selected and compressed to obtain digital transmission data, and the transmission data and substantially the above switching selection are performed. , receives these transmission data and selection information, expands the received transmission data, and if linear data is selected on the sending side based on the reception selection information, expands the expanded data. In the PCM transmission method, when differential data is selected on the transmitting side, the expanded data is integrated and converted to an analog playback signal.
In accumulating the truncation error due to the data compression described above and reflecting it in subsequent transmission data generation, when the data selected for data compression is switched from linear data to differential data, the previous truncation error This is a PCM transmission system characterized in that, instead of the cumulative value, the immediately preceding truncation error is used as the initial value of the cumulative error after switching, and subsequent error accumulation is performed.

〔Example〕

Before specifically describing embodiments of the present invention,
First, the principle of the present invention will be explained.

For example, when switching from NI-DPCM to NI-PCM, the contents of the accumulator, ie, the results of cumulative addition in the previous NI-DPCM, lose meaning. However, since linear data is compressed in NI-PCM, the contents of the accumulator are discarded and
In other words, even if zero-clearing occurs, the error based on zero-clearing will not exceed the size of the truncation error due to compression, and there will be little actual damage.

However, conversely, when switching from NI-PCM to NI-DPCM, the receiving side demodulates by adding the expanded received data of NI-DPCM to the last demodulated data of NI-PCM.
The compression error immediately after switching is as shown in Figure 5.
The NI-PCM compression error and the NI-DPCM compression error are added together, resulting in a large error exceeding the size equivalent to the truncated bit length (corresponding to the compression scale value). This error results in a sudden DC shift in the reproduced signal. Since such an error occurs every time there is a switch from NI-PCM to NI-DPCM, the difference coefficient α is often switched at frequencies where switching between NI-PCM and NI-DPCM occurs. and/or under high level conditions, the S/N ratio deteriorates and noise increases significantly in terms of audibility.

Therefore, in the present invention, from NI-PCM to NI
-When switching to DPCM, the contents of the accumulator where the previous NI-PCM truncation error has been accumulated are
The value is modified to be suitable for demodulating the NI-DPCM, and the modified content is used to encode the NI-DPCM data immediately after switching (data compression and cumulative addition of truncation errors). The appropriate value for the accumulator immediately after switching from NI-PCM to NI-DPCM is the difference between the demodulated data of the NI-PCM data (last sample) immediately before switching and the original data (linear data), that is, the difference between the demodulated data of the NI-PCM data (last sample) immediately before switching, This is the truncation error (single value, not cumulative value) in the immediately preceding NI-PCM data (last sample). This value is the value immediately before switching.
It can also be said to be the difference between the value of the accumulator when compressing NI-PCM data (the last sample) and the value of the accumulator when compressing the NI-PCM data one sample before that.

Note that when switching from NI-DPCM to NI-PCM, the value of the accumulator may be cleared to zero as described above.

Therefore, in the first embodiment of the present invention, when switching from NI-PCM to NI-DPCM, the
The truncation error in the NI-PCM data (last sample) is latched, and when switching from NI-PCM to NI-DPCM, the contents of the accumulator are
- Replace the accumulated value of the PCM truncation error with the above-mentioned latched value, and accumulate the NI-DPCM truncation error after switching using this corrected content as an initial value.
Then, when switching from NI-DPCM to NI-PCM, the accumulator is cleared to zero.

Therefore, the configuration of the transmitter according to the first embodiment of the present invention is as shown in FIG.

In FIG. 1, input terminal 1, adder 2, latch circuit 3, discriminator 4, multiplier 5, data compressor 6, output terminal 7, and output terminal 9 are the same as in FIG. , a detailed explanation of these will be omitted.

The accumulator 20 has the following function added to the function of the accumulator 8 shown in FIG. 4, that is, the function of adding and accumulating truncation errors. In response to the discrimination signal output from the discriminator 4, the NI-PCM to NI-
When switching to DPCM, from NI-PCM to NI-
The truncation error in the NI-PCM data (last sample) immediately before switching to DPCM is latched, and the NI-PCM data (last sample) is latched.
When switching from PCM to NI-DPCM, the contents of the accumulator are replaced from the accumulated value of the previous NI-PCM truncation error to the latched value. and,
When switching from NI-DPCM to NI-PCM,
Clear the accumulator to zero.

Specifically, the accumulator 20 is, for example, a second
Constructed as shown in the figure. data compressor 6
The truncation error output from the adder 21 and the latch 22 is supplied to the adder 21 and the latch 22. The addition result output from the adder 21 is input to the selector 23. Data delayed by one sample by the latch 22 is also supplied to the selector 23. The selector 23
Either the output of adder 21 or the value latched by latch 22 is selected. The data selected by the selector 23 is latched by the latch 24.
The data latched by the latch 24 and delayed by one sample is provided to the adder 21, where it is added to the truncation error provided from the data compressor 6. The operations of the selector 23 and the latch 24 are controlled at predetermined timing by a control section 25 that responds to the discrimination signal output from the discriminator 4. The addition result of the adder 21 is the accumulated value in the accumulator 20, and is given to the data compressor 6, and if there is a value that falls within the transmission digit, it is included in the compressed data.

That is, when NI-PCM is being transmitted, the selector 23 selects the addition result of the adder 21 by the operation of the control unit 25 in response to the discrimination signal, and this value is selected by the latch 24 for one sample. The signal is delayed and provided to the adder 21. In the adder 21, the newly inputted truncation error data is added to the addition result of one sample before, and cumulative addition of the truncation error is performed.

When the NI-PCM transmission is switched to the NI-DPCM transmission, the discrimination signal of the discriminator 4 changes, and the control unit 25 that responds to this changes the selector 23, which selects the output of the latch 22, i.e. Select the truncation error delayed by one sample,
Furthermore, a latch command is given to the latch 24 by the control section 25, and the truncation error of the previous sample is latched by the latch 24. This is added by the adder 21 to the truncation error of the NI-DPCM immediately after switching.
Thereafter, under the control of the control section 25, the selector 23
selects the output of adder 21, which is connected to latch 2
It is latched at 4 and used for addition. In this way, from now on, the accumulator 20 accumulates the NI-DPCM truncation error until switching to NI-PCM transmission.

Next, when switching to NI-PCM transmission, the control unit 25 responds to the output of the discriminator 4, and the latch 24
A clear command is given to the latch 24, and the contents held in the latch 24 are cleared to zero. Therefore, in the adder 21, zero is added to the truncation error of the NI-PCM immediately after switching. Thereafter, under the control of the control section 25, the selector 23 selects the output of the adder 21, which is latched by the latch 24 and used for addition. In this way, from now on, in this accumulator 20, NI
- Truncation error of NI-PCM is accumulated until switching to DPCM transmission.

In this way, even if switching between NI-PCM and NI-DPCM is performed, the continuity of cumulative addition is maintained. As a result, even if both methods are switched alternately, no errors greater than those due to truncation will occur, and there will be no direct current deviation or S/N error caused by switching the transmission method.
deterioration etc. are effectively prevented.

Next, a second embodiment of the present invention provides at least
A demodulator that performs demodulation equivalent to that on the receiving side is installed during the NI-PCM transmission period, and the difference between the demodulated data immediately after switching obtained by this demodulator and the current linear data is calculated and used as the NI-DPCM after switching. be the initial value of the cumulative truncation error. And from NI−DPCM to NI−
When switching to PCM, clear the accumulator to zero.

The configuration of such a second embodiment of the present invention is shown in FIG.

In FIG. 3, the same parts as in FIG. 1 are designated by the same reference numerals. In this case, upon receiving the outputs of the data compressor 6 and the discriminator 4, at least
It has a demodulator 30 that performs demodulation equivalent to that on the receiving side during NI-PCM transmission, and the output of this demodulator 30 is given to an adder 31. The adder 30 takes the difference between the original linear data input from the input terminal 1 and the output of the demodulator 30, and supplies the difference to the latch 32. Data delayed by one sample by the latch 32 is input to the accumulator 33. In this accumulator 33, when transmitting NI-PCM,
NI−PCM
When switching from NI-DPCM to NI-DPCM, the accumulated value is replaced with the value given from latch 32, that is, the difference between the demodulated data one sample before and the original linear data, and when transmitting NI-DPCM, the normal truncation error is also used. Perform cumulative addition and from NI-DPCM
When switched to NI-PCM, the cumulative value is cleared to zero.

Even in this case, substantially the same results as in the first embodiment can be obtained.

It should be noted that the present invention is not limited to the embodiments described above and shown in the drawings, but can be implemented with various modifications without changing the gist thereof.

For example, when switching from NI-PCM to NI-DPCM, find the difference between the cumulative value at that time and the cumulative value one sample before, and use this as the initial value of the cumulative value after switching. This is similar to the first embodiment described above.

〔Effect of the invention〕

According to the present invention, in a PCM transmission system that compresses and transmits linear data and differential data by alternately selecting them in accordance with the original data, and employing the cumulative error prevention method by cumulatively adding the switching bits, It is possible to provide a PCM transmission system that can solve the problem of discontinuity of cumulative addition values when switching between linear data and differential data, and can prevent DC deviation from occurring.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a schematic configuration of a first embodiment of the present invention, FIG. 2 is a schematic block diagram showing a detailed configuration of main parts of the same embodiment, and FIG. FIGS. 4 and 5 are a block diagram showing a schematic configuration of an embodiment, and FIGS. 4 and 5 are a block diagram and an explanatory diagram, respectively, for explaining the problems to be solved by the present invention. 2, 21, 31... Adder, 3, 22, 24,
32... Latch, 4... Discriminator, 5... Multiplier,
6... Data compressor, 20, 33... Accumulator, 23... Selector, 25... Control unit, 30
...Demodulator.

Claims (1)

[Claims] 1. Switching and selecting linear data corresponding to the analog original signal and difference data corresponding to successive difference values of the linear data based on the analog original signal or a signal obtained from the analog original signal, and Compressing data to a predetermined compression bit length to obtain digital transmission data, and transmitting the transmission data and selection information substantially corresponding to the switching selection,
Upon receiving these transmission data and selection information, the received transmission data is data decompressed, and based on the reception selection information, if linear data is selected on the transmitting side, the decompressed data is used as is, and the differential data is processed on the transmitting side. If selected, the expanded data is integrated and converted into an analog playback signal.In the PCM transmission method, the transmitting side accumulates the truncation error due to the data compression and performs subsequent transmission. In addition to being reflected in data generation, when the data selected for data compression is switched from linear data to differential data, the previous truncation error is used as the initial value of the cumulative error for subsequent error accumulation. PCM transmission method. 2. In the PCM transmission method described in claim 1, when the selected data to be subjected to data compression is switched from differential data to linear data, the cumulative value of the previous truncation error is cleared to zero, and then It is characterized by performing error accumulation of
PCM transmission method. 3. In the PCM transmission method according to claim 1, at least during the linear data transmission period,
The transmitting side performs the same demodulation as the receiving side, finds the difference between the previous demodulated data obtained by this demodulation and the original data, and switches the data selected for data compression from linear data to differential data. A PCM transmission method that is characterized by obtaining the truncation error immediately before the input. 4. In the PCM transmission method according to claim 1, the cumulative value of the data compression of the immediately preceding sample is held at least during the linear data transmission period;
The data selected for data compression is switched from linear data to differential data by determining the difference between the cumulative value of the last sample of linear data during data compression and the cumulative value of the immediately preceding sample during data compression. A PCM transmission method characterized by obtaining the last rounding error when 5. In the PCM transmission method described in claim 1, the truncation error at the time of data compression of the last sample of linear data is retained, and the data selected for data compression is changed from linear data to differential data. A PCM transmission method characterized by using the truncation error immediately before switching.