Voice-coding-and-transmission system

Abstract: In a voice coding-and-transmission system using a differential coding, a degradation of voice quality caused by a silent period transmission network and an ATM network is prevented without improving a silent-period elimination transmission network and existing transmission networks such as a STM network. In a relay node (104), an encoder (114) codes a voice signal from a decoder (108) again for a transient period immediately after a voice is started, and a transmission node (100) and a reception node (102) are tandem-connected. The encoder (114) and a decoder (122) at a reception node (102) are given respectively same reference values of a differential processing by memories (118, 128) when the voice is started, thereby preventing an abnormal sound generation due to a mismatch of inner statuses thereof when the voice is started. During the transient period, the internal statuses of an encoder (106) at a reception node (100) and the decoder (122) are closed each other. After the transient period is elapsed, switching a switch in a silent-period eliminator (112) to a digital-one-link, thereby preventing the degradation of the voice quality caused by quantization errors.

Description: BACKGROUND OF THE INVENTION The present invention relates to a voice coding-and-transmission system for compressing and transmitting a voice signal at a high efficiency, with particularly improved voice quality. In today's age of multimedia communication, communication networks are used not only for voice, as exemplified by the telephone, but also for transmission of images and computer data. Transmission of large amounts of information such as images and computer data is realized by the digital art. That is, information to be transmitted is digital-coded and the switching system is also improved from circuit switching to packet switching. In the future, communication by ATM (Asynchronous Transfer Mode) will be the mainstream technology used to efficiantly transmit-such varied information. To more efficiently perform transmission and correspondingly increase the transmitted information content, data to be transmitted is divided into units such as packets or cells which are transmitted by time division multiplexing. Voice transmission has hitherto used a high-efficiency voice coding art for efficiently coding a voice signal by removing redundant components from the signal by differential coding or a similar art. The documents US-A-3 836 719 and US-A-3 832 493 disclose coding and transmission systems in which redundant components are removed. High-efficiency voice coding systems for performing coding by using a difference include predictive differential coding system such as the ADPCM (Adaptive Differential Pulse Code Modulation) coding system. The predictive differential coding system predicts present signals based on past signals and quantizes differences between values of the predicted signal and values of the actual signal. Because a difference generally has a value smaller than the original data, the number of bits of a code obtained by quantizing the difference is smaller than the number of bits of a code not depending on a difference. A coding part and a decoding part of this system have respective internal states, which are used as a reference value for a differential processing. The internal state consists of a set of parameters which represent the past voice signal. In a transmission by an ATM network, multiple transmission lines are used by digital-coding information sources such as voice, image, and computer data, dividing the sources into a unit, called a cell, and transmitting asynchronously in a burst mode to improve an efficiency of utilizing the transmission lines. In communication with the ATM network, the above-mentioned high efficiency voice coding technology can be used in combination therewith. As the majority of traffic is due to voice information, applying high efficiency voice coding technology to voice information will reduce transmission amount and achieve higher efficiency transmission. Moreover, the voice coding system includes the ITU (International Telecommunication Union) Recommendation G.728 coding system (LD-CELP system: Low-Delay Code-Excited Linear Prediction) in addition to the above ADPCM. This coding system is described in Draft CCITT Recommendation G.728 "Coding of Speech at 16 Kbits/s using Code Excited Linear Prediction (LD-CELP)" in detail. This coding system is based on the backward adaption for performing adaptation of a synthesizing filter and excitation gain in accordance with past voice signals. This system also has an aggregate of parameters of the past voice signal as an internal state, which is used as a reference for a differential processing of a synthesis filter coefficient, an adaptive gain coefficient, or the like. Recently, because of a request for higher efficiency as described above, the silent-period elimination art of excluding a silent part when transmitting a voice signal has been used. It is known that the silentperiod elimination art can decrease the total quantity of voice signals to be transmitted to a transmission line with a small voice-quality degradation and realizes higher-efficiency voice transmission according to a statistical multiplication effect. In the case of the silent-period-eliminated voice transmission system, however, operations of a decoding part for receiving and decoding a differential-coded voice signal become indefinite because there is no voice information transmitted during silent periods. That is, when a silent state (this may be referred to as a state with no talk spurt) changes to a voiceful state (this may be referred to as a state with a talk spurt), the internal state of an coding part for generating a voice code does not coincide with that of a decoding part. Therefore, the decoding part is not always able to decode a correct voice signal, even if the part is given a correct high-efficiency code with no transmission line error. This phenomenon frequently appears as uncomfortable abnormal sounds, such as a click or oscillation sound, in a regenerated sound at a reception node. Figure 16 is a block diagram of a conventional voice coding-and-transmission system for solving the above problem. This diagram is based on the block diagram shown in Japanese Patent Laid-Open No. Hei 2-181552. This voice transmitting system forms a set of structures by a transmission node 2 and a reception node 4. Under a state with a talk spurt, that is, at a voicefilled period, the transmission node 2 codes a voice signal using a high-efficiency voice encoder 6 and transmits the signal to a transmission line 10 via a changeover switch 8. Because the changeover switch 8 of the transmission node 2 is switched so as to transmit no data to the transmission line 10 with no talk spurt, that is, at a silent time, a silent-period-eliminated voice code is transmitted from the transmission node 2. A voice detector 12 detects a voice or silence of a voice signal and switches the changeover switch 8. The reception node 4 decodes a voice code sent from the transmission line 10 to a voice signal by a decoder 14 and outputs the signal. While silent period elimination is performed, the changeover switch 16 is switched to the pseudo-background-noise signal generator 18 side and artificial noises are output from the reception node 4. A voice/silence information extractor 20 detects voice or silence in accordance with a voice code and switches the changeover switch 16. In this system, the transmission node 2 is provided with a memory 22 storing a predetermined internal state of the encoder 6, while the reception node 4 is provided with a memory 24 storing the same content with the memory 22. Moreover, at the transition which a voice signal changes from a silent state to a voiceful state and causes the above problem, the voice detector 12 and the voice/silence information extractor 20 synchronously detect the transition, a reference value for differential processing is set from the memory 22 to the encoder 6 as an internal state in the transmission node 2, and the same reference value for differential processing as that of the encoder 6 is sent from the memory 24 to the decoder 14 as an internal state in the reception node 4. Thus, the timing in which a talk spurt is detected synchronizes between the transmission node 2 and the reception node 4 and, at this point, both internal states are reset to the same state. Therefore, the internal state of the encoder 6 always coincides with that of the decoder 14 in a voice period and thereby, it is possible to avoid abnormal sound at the head of a talk spurt. In the future, as described above, a silent-period-eliminating transmission network or an ATM network will mainly be constructed using the above arts. However, transmission networks that do not eliminate silent periods and STM (Synchronous Transfer Mode) networks have already been constructed. These transmission networks were constructed as an infrastructure, in many cases using a great deal of capital. Therefore, it is economically difficult to immediately replace them with silent-period-eliminating transmission networks or ATM networks, or otherwise improve them. Therefore, to construct a large network including a range covered by these conventional transmission networks, it is necessary to allow networks eliminating silent periods and networks not eliminating silent periods, or ATM network and STM networks to coexist respectively. For the time being, it is possible to realize coexistence of both networks by connecting two types of networks with a relay node. There are two methods for connecting the silent-period-eliminating network and the silent period network, as shown in Figs 18 and 19. These Figures illustrates a transmission from the silent-period-eliminating network to the silent period network. In addition, there are two methods for connecting the ATM network and the STM network as shown in Figs. 20 and 21. These Figures illustrates a transmission from the ATM network to the STM network. Figure 18 is a block diagram of a transmission system consisiting of tandem-connecting networks eliminating silent periods and of networks not eliminating silent period connected through a relay node. In Fig. 18 components having corresponding functions as those in Fig. 16 are provided with the same symbol, and their description is omitted. An encoder 32 of a transmission node 30 of this system performs the coding, not eliminating silent periods, and transmits a generated voice code to a transmission line 34 (transmission line B). A relay node 36 receives the voice code from the transmission line B, silent-period-eliminates the voice code, and transmits the silent-period-eliminated voice code to the reception node 4 through a transmission line A. The relay node 36 decodes the voice code from the transmission node 30 as a voice signal by a decoder 38 and, thereafter, codes the voice signal as a silent-period-eliminated voice code and transmits it to the reception node 4. The processing, after decoding by the decoder 38, uses the silent-period-eliminated transmission system using the synchronous resetting described for Fig. 16. Therefore, in the case of this transmission system, because the relay node 36 performs decoding once and then coding again, the transmission lines A and B are from the viewpoint of coding greatly independent from each other and, this system is therefore referred to as a tandem connection. Figure 19 is a block diagram of a transmission system constituted by connecting networks eliminating silent periods and networks not eliminating silent periods by digitalone-link through a relay node. In Fig. 19, components having corresponding function as those in Fig. 18 are provided with the same symbol and their description is omitted. A voice code with no silent period eliminated that is transmitted to the transmission line 34 from the transmission node 30 is silent-period-eliminated by a relay node 50 and transmitted to a reception node 54 through a transmission line 52 (transmission line A). In the relay node 50, a decoder 56 decodes a voice code sent from a transmission line B to restore a voice signal. A voice detector 58 detects voice or silence (presence or absence of a talk spurt) in accordance with the voice signal and controls a changeover switch 60. The changeover switch 60 connects the transmission line B to the transmission line A only when a voice code with no silent period eliminated from the transmission line B has a talk spurt. When the voice code does not have any talk spurts, it is abandoned and no data is output to the transmission line A. Thereby, a silent-period-eliminated voice code is transmitted to the transmission line A. In this connection, a processing delay unit 62 delays the voice code from the transmission line B by the processing time in the decoder 56 and the voice detector 58 and realize the synchronization between the operation of the changeover switch 60 and the voice code. The reception node 54 decodes a silent-period-eliminated voice code transmitted from the relay node 50 to the reception node 54 through the transmission line A as a voice signal by a decoder 64 corresponding to the encoder 32 of the reception node 30 and outputs the decoded voice code. When no voice code is input from the transmission line A, that is, while silent period elimination is performed, a voice/silence information extractor 66 switches a changeover switch 68 toward a pseudo-backgroundnoise signal generator 70 to output artificial noise from the reception node 54. Thus, the relay node 60 only performs switching. Therefore, though a voice code transmitted to the reception node 54 is silent-period-eliminated, the voice code itself is transmitted from the transmission node 30. Therefore, in the case of this transmission system, the transmission lines A and B are well combined with each other and this is thus referred to as a digital-one-link. Figure 20 is a block diagram of a conventional transmission system constituted by tandem-connecting the ATM network and the STM network through a relay node. An encoder 73 of a transmission node 72 in the system digitizes a voice signal and performs the coding at a high compression rate. A cell composer 74 assorts a sequential voice code coded with the encoder 73 and transmits the code to a transmission line A. The transmission line A is the ATM network. The voice code is transmitted through the transmission line A in cell units in a burst mode. In the relay node 75, a buffer 76 absorbs a transmission fluctuation of the cell, and then a cell decomposer 77 decomposes the received cell to produce the sequential voice code. An vanished cell detector 78 detects a dead cell due to a disuse or a delay in the ATM network, and controls operations of each portion in the relay node 75. A decoder 79 decodes a voice code extracted from the cell to an original digital sampling voice signal, for example a PCM (Pulse Code Modulation) voice signal. A synchronous incoming unit 80 mates an operation timing between the decoder 73 and the decoder 79. An vanished cell compensator 81 compensates a voice signal for the vanished cell. A memory 82 stores a latest voice signal for compensating the cell. A selector switch 83 is a switch for selecting either the voice signal decoded in the decoder 79 or the voice signal compensated the vanished cell. An encoder 84 is same as the encoder 73. A transmission line B is the STM network. A reception node 85 has a decoder 86 corresponding to the decoder 79. For voice communication, a real time ability is required. Therefore, a retransmission procedure that a data communication utilizes cannot be applied thereto, if a cell disuse occurrs which is a specific cause of degrading of the ATM network. Especially, in an ATM voice communication combining with the high-efficiency coding, cell size is fixed at 53 bytes. With a more efficienct coding method, more information can be accommodated in one cell, resulting in greater damage in regenerated voice due to cell disuse. Consequently, to realize a high quality voice transmission with the ATM, a processing for regenerating a natural voice is necessary for interpolating assuming the information included in the vanished cell. The system as shown in Fig. 20 utilizes the following method as one countermeasure against cell vanishing. The vanished cell detector 78 monitors cells reaching the relay node 75, detects disappeared cells in the ATM network or those not reaching the relay node 75 within a predetermined period, and sends a control signal based on the detection results to the vanished cell compensator 81 and the selector switch 83. As a method for detecting the vanished cell, the cell composer 74, for example, adds an index representing a sending order to a pay load portion of the cell, and the vanished cell detector 78 monitors whether or not the index is lost. Once the vanished cell detector 78 notifies the vanished cell compensator 81 of an elimination of the cell, the vanished cell compensator 81 interpolates / extrapolates or mutes the lost voice signa, 1 based on a past voice signal stored in the memory 82. In addition, the selector switch 83 chooses between an output of the decoder 79 and an output signal of the vanished cell compensator 81 based on a control signal from the vanished cell detector 78. Chosen signal is reapplied the high efficiency coding with the encoder 84, and is sent to the transmission line B (STM network). Thereby, a voice code with reduced cell vanishing damage is sent from the relay node 75. In the relay node 75, coding is performed again after the voice code is decoded. Therefore, the transmission system has mutually highly independent transmission lines A and B in view of coding. For this reason the system is called the tandem connection system. As a voice high efficiency coding algorithm used in the encoders 73, 84 and the decoders 79, 86, ITU-T Recommendation G.726/727 ADPCM (Adaptive Differential Pulse Code Modulation), ITU-T Recommendation G.728 LD-CELP (Low-Delay Code-Excited Linear Prediction), and ITU-T Recommendation G.729 CS-ACELP (Conjugate Structure Algebraic Code Excited Linear Prediction) or the like is well known. Figure 21 is a block diagram of a conventional transmission system consisting of digital-one-linking the ATM network and the STM network through a relay node. Components in Fig. 21 having corresponding functions as those in Fig. 20 are provided with the same symbol and their description is omitted. A cell including high efficiency voice code which is sent from the transmission node 72 to the transmission line A (ATM network) is decomposed by the relay node 90, remounted to a synchronous frame, and then transmitted to the reception node 85 through the transmission line B (STM network). The reception node 85 decodes the voice code, which is transmitted from the relay node 90 through the transmission line B, using the decoder 86 corresponding to the encoder 73 at the transmission node 72, and outputs the decoded voice code. Thus, the relay node 90 only performs a switching. The voice code for transmitting to the reception node 85 is a signal sent from the transmission node 72 itself. Therefore, the transmission system has mutually highly integrated transmission lines A and B in view of encoding. This is a reason that the system is called the digital-one-link system. Connecting the transmission lines A and B according to a tandem connection or digital-one-link has the following problems. In the case of tandem-connecting a network eliminating silent period and a network not eliminating silent period as shown in Fig. 18 a voice code from the transmission node 30 is once decoded to a voice signal and then transmitted in accordance with the silent period elimination using synchronous resetting. Therefore, the internal state of the encoder 6 of the relay node 36 coincides with that of the reception node 4 and abnormal sound is avoided as described above. However, because the processing of decoding and coding a voice code is performed in a relay node, a voice signal input to a transmission node is coded and decoded twice before it is output from a reception node. Therefore, a problem occurs that quantization errors are accumulated and the quality of a voice signal output from the reception node 4 deteriorates. It is known that the above quality degradation becomes more remarkable as an elimination rate increases, though the quality degradation is almost inconsequential at a high bit rate (16 Kbit/s or more). Because a voice transmission system uses a low bit rate, it is impossible to ignore the above voice quality degradation. This is entirely applicable to the transmission system combined with the high efficiency coding where the ATM network and the STM network is tandem-connected as shown in Fig. 20. However, in the case of connecting a network eliminating silent period and a network not eliminating silent period according to digital-one-link as shown in Fig. 19, the conditions are completely reversed. In this case, because a voice code corresponding to presence of a talk spurt transmitted to the reception node 54 is the same as a voice code generated in the transmission node 30, voice-signal quality degradation due to accumulation of quantization errors is prevented. However, the internal state of the encoder 32 of the transmission node 30 does not generally coincide with that of the decoder 64 of the reception node 4 at the timing of change from a silent state to a voiceful state. That is, because reference values of the differences in coding/decoding are different, though the voice codes are same, a problem again occurs that abnormal sound is produced. This abnormal sound is not only unpleasant to a user, but it also causes the problem of extreme degradation of speech content clarity because the abnormal sound is generally produced at the head of a talk spurt. For a transmission system combining high efficiency coding technology in which the ATM network and the STM network are connected in digital-one-link as shown in Fig. 21, the voice code for transmitting to the reception node 85 is the same as the voice code generated at the transmission node 72. Therefore, voice-signal quality degradation due to an accumulation of quantization errors is prevented. However, in the relay node, only switching is performed and extracting voice information from the voice code is not performed. Normally, it is difficult to directly compensate for the vanished voice code by a simple method such as interpolation / extrapolation / assumption without decoding the voice code applied the high efficiency coding. Accordingly, it is extremely difficult to remove the impact of the cell vanishing in the relay node of the transmission system, although the cell vanishing itself can be detected. As a result, the voice information transmitted to the reception node 85 is discontinuous to induce an abnormal sound at the reception node 85 making a listener uncomfortable. In addition, a missing phoneme remarkably lowers speech comprehension. Nevertheless, to remove the impact due to the cell vanishing at the reception node 85 nevertheless in the digital-one-link connection, the information about the cell vanishing detected in the relay node may be transmitted to, for example, the STM network by providing a signal line separately, and other mechanism for a countermeasure of the cell vanishing may be provided at the reception node 85. However, connecting the ATM network and the STM network is required in case that the STM network and the reception node 85 are existing systems, as described above. Consequently, the solution of removing the impact due to the cell vanishing at the reception node 85 needs an improvement or alternation of the existing system, and lacks reality. As described above, conventionally, problems have been existed in housing the transmission network in the silent period transmission network or in the ATM network without improving the voice communication system at a side of existing silent-period-vanished transmission network or a side of existing STM network. SUMMARY OF THE INVENTION An object of the present invention is to provide a voice coding-and-transmission system solving the above problems and realizing a high-quality voice transmission at a realistic cost. This problem is solved by the voice coding-and-transmission system according to claim 1. Further improvements of the inventive coding-and-transmission system are provided in the respective dependent claims. A voice coding-and-transmission system according to the present invention is characterized in that a relay node includes a relay decoder for extracting voice information included in a voice signal from an original voice code, a relay control circuit for discriminating between a voice period and a silent period of said voice signal in accordance with said voice information and outputting a relay control signal for controlling operations of a relay node in accordance with a discrimination result, an coding reference value determination circuit for determining a reference value for differential coding at the start of voicing which is the timing of change from said silent period to said voice period in accordance with said relay control signal, a relay encoder for starting said differential coding of said voice information in accordance with said reference value and generating relay voice codes for at least a certain change period, and a silent-period elimination circuit for receiving said original voice code and said relay voice code and outputting said relay voice code to said second transmission line during said change period and said original voice code to the second transmission line during a voice period after said change period in accordance with said relay control signal to synthesize a silent-periodvanished voice code; and a reception node includes a reception control circuit for deciding the start of said voicing in accordance with said silent-period-vanished voice code and outputting a reception control signal for controlling operations of a reception node in accordance with a decision result, a decoding reference-value determination circuit for determining a reference value for said decoding corresponding to said reference value for differential coding at the start of said voicing in accordance with said reception control signal, and a reception decoder for starting said decoding of said silent-period-vanished voice code in accordance with the reference value for said decoding at the start of said voicing and outputting said voice signal. According to the present invention, a relay encoder and a reception decoder obtain a differential-coding reference value (referred to as a reference value at start of voicing) from respective coding reference-value determination or decoding reference-value determination circuits. The differential coding is a method for fetching and coding a difference between reference values given by past coding or decoding. The number of reference values is not limited to one, but it is possible to use a reference value for each of various parameters showing a voice signal. A reference value at start of voicing to be determined by an coding-reference value determination circuit and that to be determined by a decoding reference-value determination circuit respectively are made to correspond to each other so that a reception decoder can regenerate voice information input to a relay encoder and the reference values are generally equal to each other. Hereafter, in the case of the encoder and decoder in which their reference values are made to correspond to each other, it is assumed that their internal states coincide. If internal states do not coincide with each other, abnormal sound may be output from a reception node. However, because the internal states of the relay encoder and reception decoder are synchronized and initialized to coincide with each other, no abnormal sound is produced. In this case, however, it is not assured that the internal state for coding in a transmission node coincides with the internal state of a reception decoder. Therefore, a silent-period elimination circuit transmits a relay voice code which is an output of a relay encoder to a reception decoder via the second transmission line within a predetermined change period from the start of voicing. In this change period, the internal state for coding in the transmission node approximates the internal state of the reception decoder. Therefore, the silent-period elimination circuit directly transmits an original voice code transmitted from the transmission node to the reception decoder during a voiceful period after the change period. That is, after the change period, a voice signal is differential-coded by the transmission node and then regenerated through decoding in the reception node without undergoing the coding/decoding in the change period by the relay node. Therefore, the coding/decoding frequency is smaller than that in the change period and the number of quantization errors decreases. Therefore, voice quality degradation due to abnormal sound is prevented by tandem connection when the internal state of the transmission node dissociates from that of the reception decoder, and voice quality degradation due to accumulation of quantization errors such as in tandem connection is prevented by digital-one-link when their internal states approximate each other. In this case, the degree of approximation between the internal state of the transmission node and that of the reception decoder is further improved as the time after the start of voicing increases and the abnormal-sound suppression effect is improved. However, the period of degradation due to quantization errors by tandem connection also increases. The transient period is determined in accordance with the balance between suppression of abnormal sounds and lengthening of the period in which voice quality degradation due to quantization errors is suppressed. Therefore, according to the voice coding-and-transmission system of the present invention, voice quality degradation due to abnormal sound at the head of a talk spurt is prevented by tandem connection during only a short change period until the difference between the internal state for coding in a transmission node and the internal state of a decoder of a reception node converge immediately after the talk spurt is detected, and voice quality degradation due to accumulation of quantization errors such as in the tandem connection is prevented by performing digital-one-link during most voice period after the difference between these internal states completely converges. That is, there are advantages that abnormal sound produced at the head of a talk spurt is suppressed and moderated, rugged feeling due to abnormal sound is vanished, degree of voice comprehension is improved, and, moreover, voice quality degradation due to continuous tandem connection is prevented. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of the voice coding-and-transmission system of the first embodiment; Figure 2 is a waveform diagram of a voice signal for explaining operation modes related to the first to seventeenth embodiments; Figure 3 is a state change diagram showing change between operation modes; Figure 4 is a block diagram of a relay node related to the second embodiment; Figure 5 is a block diagram of a relay node related to the third embodiment; Figure 6 is a block diagram of an encoder of a relay node related to the third embodiment; Figure 7 is a block diagram of the voice coding-and-transmission system of the fourth embodiment; Figure 8 is a block diagram of a relay node related to the fifth embodiment; Figure 9 is a block diagram of the voice coding-and-transmission system of the sixth embodiment; Figure 10 is a block diagram of the voice coding-and-transmission system of the seventh embodiment; Figure 11 is a block diagram of a relay node related to the eighth embodiment; Figure 12 is a block diagram of the voice coding-and-transmission system of the ninth embodiment; Figure 13 is a block diagram of the voice coding-and-transmission system of the tenth embodiment; Figure 14 is a block diagram of the voice coding-and-transmission system of the eleventh embodiment; Figure 15 is a block diagram of the voice coding-and-transmission system of the twelfth embodiment; Figure 16 is a block diagram of a conventional voice coding-and-transmission system; Figure 17 is a block diagram of the ITU Recommendation G.728 coding system which is an example of the differential coding system; Figure 18 is a block diagram of a conventional voice coding-and-transmission system tandem-connected a silentperiod-eliminating transmission network and a silent-period-not-eliminating transmission network; Figure 19 is a block diagram of a conventional voice coding-and-transmission system digital-one-link connected a silent-period-eliminating transmission network and a silent-period-not-eliminating transmission network; Figure 20 is a block diagram of a conventional transmission system tandemconnected an ATM network and a STM network; and Figure 21 is a block diagram of a conventional transmission system digital-one-linked the ATM network and the STM network. DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment 1] The first embodiment of the present invention is described below by referring to the accompanying drawings. Figure 1 is a block diagram of the voice coding-and-transmission system of this embodiment. In the case of the voice coding-and-transmission system, a transmission node 100 outputs an original voice code obtained by coding a voice signal. Though the original voice code is a differential-coded high-efficiency voice code, it is not silent-period-vanished. The original voice code is transmitted to a transmission line B. That is, the transmission line B represents a transmission network in which silent period elimination is not performed. However, a transmission line A to which a reception node 102 is connected represents a transmission network in which silent period elimination is performed. A relay node 104 connects these two transmission networks, receives an original voice code from the transmission node 100 through the transmission line B, and converts the voice code to a silent-period- vanished voice code to output it to the transmission line A. The reception node 102 decodes the silent-period-vanished voice code and outputs a voice signal. The transmission node 100 has an encoder (coding unit) 106 for differential-coding an input voice signal. The encoder 106 generates an original voice code which is a high-efficiency voice code. The high-efficiency voice code transmitted from the transmission node 100 to the relay node 104 through the transmission line B is decoded as a voice signal by a decoder (relay decoder) 108. A voice detector 110 detects presence or absence of a talk spurt in accordance with the voice signal, that is, discriminates between a voice period and a silent period and outputs a signal (relay control signal) for controlling operation modes of the relay node in accordance with a discrimination result. The relay node has three operation modes switched by the voice detector 110. These operation modes are described below by referring to Fig. 2. Figure 2 is a waveform diagram of a voice signal output from a decoder 108. The y axis represents signal level and x axis represents time. The voice detector 110 divides the voice signal into three periods (sections) corresponding to operation modes and controls operations of the relay node 104. First, the period in which no talks part is detected from a high-efficiency voice code input to the relay node 104 is assumed as mode 1. Second, the period for some tens to hundreds of milliseconds after a talk spurt is detected (this period is referred to as a change period or a transient period) is assumed as mode 2. Third, the period in which talk spurts are continuously detected after mode 2 is assumed to be mode 3. The voice detector 110 supplies a control signal reflecting the above-described operation-mode decision results to a silent period elimination circuit 112. The relay node 104 has two routes for connecting the transmission lines B and A. The first route comprises the decoder 108 and the encoder (relay encoder) 114 and the second route passes a processing delay unit 116. The silent period elimination circuit 112 has a built-in switch for switching three states of voice code outputting to the transmission line A through either of the first and second routes or outputting no voice code by not connecting the transmission line A to any object. The processing delay unit 116 has a delay time equal to a signal delay produced in the route comprising the decoder 108 and the encoder 114 and arranges the signal timing between the first and second routes. As described later, the silent period elimination circuit 112 eliminates a voice code during silent periods by outputting no data to the transmission line A in mode 1 with "no" talk spurt. Moreover, the silent period elimination circuit 112 adds the information necessary for the mode decision (mode information) in the reception node 102 to a voice code. The mode information shows the start or end of a voice period. Thus, a silent period eliminator 112 synthesizes a silent-period-vanished voice code and transmits the code to the transmission line A. A memory 118 is described later. In the reception node 102, a voice/silence information extractor 120 extracts mode information from a silent-period-vanished voice code and outputs a signal for controlling operation modes of a reception node (reception control signal). The reception node 102 includes a decoder (reception decoder) 122 and a pseudo background noise generator (pseudo-background-noise signal generator) 124 for generating artificial noises. A changeover switch 126 directs output of a signal from the decoder 122 or generator 124. A memory 128 is described later. Operations in each mode are described below mainly on a relay node and a reception node. First, in mode 1, the relay node 104 connects the changeover switch in the silent period eliminator 112 to a terminal 112b. Because the terminal 112b is not connected to either of the first or second routes, no high-efficiency voice code is output to the transmission line A in this case. The voice detector 110 constantly operates because it is necessary to continuously monitor the change of modes. Because the voice detector 110 performs mode decision by using a voice signal output from the decoder 108 as its input, the voice signal must always be supplied. Therefore, the decoder 108 also operates constantly. However, the encoder 114 need not be operated because it is unnecessary to supply a high-efficiency voice code output from the encoder 114 to other block or transmit it to a reception node in this mode. Moreover, in the reception node 102, the voice/silence information extractor 120 decides mode 1 in accordance with a silent-period-vanished voice code transmitted from the transmission line A. This decision is made by obtaining the information showing the end of a voice period added to the last of a group of silent-period-vanished voice codes (the last packet or cell when silent-period-vanished voice codes are transmitted by being divided into a plurality of packets or cells) and thereby deciding mode 1 after the final code. By receiving a control signal reflecting the information of being mode 1, the changeover switch 126 is switched to the terminal-126a side, pseudo-background noises of the pseudo background noise generator 124 are output from the reception node 102, and a natural silent state is transferred to a receiver. In the relay node 104, when the voice detector 110 detects that operation modes change from 1 to 2, it transmits to the encoder 114 a control signal form notifying that a silent state changes to a voice state. The encoder 114 responds to the control signal, loads the data stored in a memory 118 in a memory inside of the encoder 114 as a reference value for the differential coding of various voice parameters, and starts coding a voice signal output from the decoder 108 in accordance with the reference value. That is, the memory 118 determines a reference value of the encoder 114. Moreover, by receiving the same control signal, a changeover switch of the silent period eliminator 112 is switched to the terminal-112c side. Moreover, in the reception node 102, the voice/silence information extractor 120 extracts mode information from a silent-period-vanished voice code transmitted from the transmission line A and detects that operation modes change from 1 to 2. The voice/silence information extractor 120 transmits a control signal for notifying that a silent state changes to a voice state to the decoder 122. The decoder 122 responds to the control signal to load the data stored in the memory 128 in a memory inside of the decoder 122 as a reference value for the differential coding or decoding of various voice parameters. Moreover, the voice/silence information extractor 120 transmits the same signal to the changeover switch 126. The changeover switch 126 is switched to the terminal-126b side in accordance with the control signal. That is, the memory 128 determines a reference value of the decoder 122. When the voice detector 110 decides mode 3, a changeover switch in the silent period eliminator 112 is switched to the terminal-112a side and a high-efficiency voice code sent from the encoder 106 of the transmission node 100 is output directly to the transmission line A. Also, in this case, the voice detector 110 is continuously operated because it is necessary to monitor the change of modes. Because the voice detector 110 performs mode decision by using a voice signal output from the decoder 108 as its input, the voice signal must be supplied to the voice detector 110. Therefore, the decoder 108 also continuously operates. However, the encoder 114 need not be operated because it is unnecessary to supply a high-efficiency voice code generated by the encoder 114 to other block or transmit the code to a reception node in this mode. Operations of the reception node 102 are the same as those in mode 2. In this case, if the state of mode 2 is not prepared, the following trouble occurs. That is, though it is assured that internal state of the encoder 106 of the transmission node 100 coincides with that of the decoder 108 of the relay node 104 and the internal state of the encoder 114 of the relay node 104 coincides with that of the decoder 122 of the reception node 102, it is not at all assured that the internal state of the encoder 106 coincides with that of the decoder 122. Therefore, when operation modes suddenly change from 1 to 3, abnormal sound due to the incoincidence between the internal states is produced similar to the case of the conventional system. However, by setting a change period defined by mode 2, abnormal sound can be avoided because the present operation mode changes to operation mode 3 when the internal state of the decoder 122 approaches that of the encoder 106 and their internal states completely coincide with each other. About the setting of internal memories of the encoder 114 and decoder 122 shown above, it is the necessary minimum condition of the present invention to delete the memory contents reflecting the processing results of the past indefinite operations by equalizing the data stored in the memory 118 with the data stored in the memory 128 and setting the same reference value for differential coding to the encoder 114 and decoder 122 when assuming that prevention of abnormal sound is the final object. However, the data values stored in the memories 118 and 128 are used only when the mode changes from 1 to 2. Therefore, by using a value corresponding to the state of the mode change, it is possible to obtain a higher-quality decoded voice. For example, when ITU Recommendation G.728 is used for a high-efficiency coding system, a higher-quality decoded voice can be obtained by using a previously-calculated predictive filter factor and memory belonging to predictive filter adaptive means (e.g. auto-correlation function) or memory belonging to adaptive gain or gain adaptive means. Moreover, when ITU Recommendation G.728 is applied to the high-efficiency coding system, the data calculated and stored when coding/decoding background noises is the most suitable from the viewpoint of the acoustic quality. However, it can easily be imaged that this value depends on the coding system used. Moreover, an advantage almost equal to that of the above embodiment can be obtained even if using other value. That is, it is the essence of the present invention that the timings of control signals generated by the voice detector 110 and voice/silence information extractor 120 coincide each other and thereby, the same internal state is sent to the encoder 114 and decoder 122 and indefinite components due to past data are vanished. The voice coding-and-transmission system of this embodiment makes it possible to avoid quality degradation by limiting the period for performing tandem connection for which it is known to cause voice quality degradation to a short time of a transient period in which a silent state changes to a voiceful state and connecting most talk spurts by digital-one-link and fully bring out the performances of the high-efficiency voice coding system. Moreover, it is possible to decrease the processor processing load and the hardware scale of the relay node 104. As described above, the value of tens to hundreds of milliseconds is shown as the continuous time (change time) of mode 2. However, the base of this value conforms to the following empirical rule. First, as a prerequisite, it is assumed that the internal state of the encoder 106 is completely different from that of the decoder 122 when using G.728 as the high-efficiency coding system. Under the prerequisite, coding/decoding is performed by the encoder 106 and decoder 122 through a transmission line. Because the stability of every filter used for G.728 is assured, the internal states for transmission and reception gradually converge to become equal. While coding and decoding are continued, the internal states completely coincide with each other, up to a degree in which there is to possibility that abnormal sound is produced. the time required from a mode change up to a complete coincidence between internal states is some tens or hundreds of milliseconds. It is obvious that it is predicted that the above value changes depending on the high-efficiency coding system used. Therefore, it is important to set a change period corresponding to each coding system. Figure 3 is a state change diagram showing the change between the modes described above. Only the directions shown by arrows are allowed for the change between three modes and a change other than the above change is an inhibited change or a change which cannot physically be considered. In the case of this embodiment, a system is described in which ITU Recommendation G.728 is applied to a high-efficiency coding system. However, the present invention is not restricted to the coding system. The present invention can be applied to every voice coding system using past coding/decoding result referred to as the differential coding system in this case. [Embodiment 2] Figure 4 is a block diagram of a relay node for explaining the second embodiment of the present invention. This embodiment is obtained by improving the relay node of the voice coding-and-transmission system of the embodiment 1. As a result of improving the relay node, the processing load and hardware scale of the relay node can be decreased. In Fig. 4, the transmission node 100 and the reception node 102 are not illustrated because they are the same as those of the embodiment 1; only a relay node is shown. Moreover, in Fig. 4, a component having the same function as that of the component described for the embodiment 1 is provided with the same symbol as in Fig. 1 and its description is not repeated. For a modified component, the character B is added to its symbol in Fig. 1 so that how the component corresponds to the component of the embodiment 1 can easily be understood. A decoder 108B decodes a voice signal and outputs some of the adaptive parameters. An adaptive parameter is generated in high-efficiency coding such as ADPCM, which is a voice parameter for constituting a voice signal. An encoder 114B receives the voice signal and adaptive parameters. In the case of the encoder 114B, it is possible to omit the processing for generating input adaptive parameters. Most operations of this voice coding-and-transmission system are the same as those of the embodiment 1 except that some of adaptive parameters are supplied to the encoder 114B from the decoder 108B. Thereby, some adaptive differential processings can be omitted for the encoder 114B. However, supply of some parameters to the encoder 114B from the decoder 108B may result in partially admitting the incoincidence between internal states of the encoder 114B and the decoder 122 of the reception node. Therefore, it is necessary to carefully select parameters to be supplied in order to not correspondingly cause abnormal sound to a high-efficiency coding system. For example, to use G.728 for a high-efficiency coding system, there is a synthesizing filter factor as a backward-type parameter which can be supplied to the encoder 114B from the decoder 108B. A synthesizing filter takes charge of a sound adjusting mechanism equivalent to the throat or palate of the to generate a vowel. However, a consonant part or background noise part frequently appears in the period of mode 2. Therefore, the sound adjusting mechanism does not greatly contribute to voice synthesis. Moreover, abnormal sounds such as "gya" or "bu" (phonetic) are in most cases caused by an unsuited gain value. From the above viewpoint, even if some troubles occur in adaptation of a synthesizing-filter factor, it is considered that no abnormal sound is produced in this period. Supply of backward-type parameters is described above and it is pointed out that the parameters must carefully be selected. In the case of forward-type parameters, however, it is needless to say that there is no problem on the supply of the parameters from the decoder 108B to the encoder 114B because the parameters are not provided with past influences at all. [Embodiment 3] Figure 5 is a block diagram of a relay node for explaining the third embodiment of the present invention. This embodiment is obtained by improving the relay node of the voice coding-and-transmission system of the embodiment 1 or 2. As the result of improving the relay node, the processing load and hardware scale of the relay node can be decreased. In Fig. 5, the transmission node 100 and the reception node 102 are not illustrated because they are the same as those of the embodiment 1 and only the relay node is shown. Moreover, in Fig. 5, a component having the same function as that explained in the embodiment 1 is provided with the same symbol and its description is not repeated. For a modified component, the character C is added to its symbol in Fig. 1, so that how the component corresponds to the components of the embodiment 1 and embodiment 2 can easily be understood. A parameter separator 108C is constituted by omitting some processings of the decoder 108B in Fig. 4. The parameter separator 108C is not provided with a function for decoding a voice signal in a complete form, but it is provided with a parameter extracting function. The parameter separator 108C outputs an excitation signal and a parameter to the encoder 114C and outputs pitch information (or excitation signal information) to a voice detector 110C. The voice detector 110C detects voices in accordance with the pitch information (or excitation signal information). Other operations of this voice coding-and-transmission system are the same as those of the embodiment 2. It is pointed out in the description of the embodiment 2 that parameters causing abnormal sound due to incoincidence can be specified to a certain extent. In the case of this voice coding-and-transmission system, an encoder and a decoder omit the adaptive processings for some parameters in a relay node. In the case of the parameter separator 108C, if even some of the adaptive processings performed by the decoder 108B are omitted, every voice decoding function is lost and no voice signal cannot be output. Because a relay node 104C does not require a voice signal, which is an output signal, there is no macroscopic problem. However, because the voice detector 110B and the encoder 114B in Fig. 4 require a voice signal input, the relay node 104C uses the voice detector 110C and encoder 114C having a structure requiring no voice signal input instead of the detector 110B and encoder 114B. First, the structure of the encoder 114C is described below. As an example, a case is described in which ITU Recommendation G.728 is used for a high-efficiency coding system (see Fig. 28). It is described for the embodiment 2 that a slight incoincidence between synthesizing-filter factors does not greatly influence abnormal sound in G.728. When omitting the synthesizing-filter processing, the parameter separating section 108C can only decode up to an excitation vector. Figure 6 is a block diagram of the encoder 114C for performing coding in accordance with an excitation vector without using any voice signal input. By constituting the encoder 114C as shown in Fig. 6, it is possible to realize an encoder requiring no voice signal input. In the case of the encoder 114C, a vector to be referenced is only shifted from a voice signal to an excitation signal and the structure is the same as that of the original encoder, except that a synthesizing filter and its adaptive processing are omitted. Therefore, the compatibility with the original ITU Recommendation G.728 coding system is assured. Also, it is easy to change the structure of the voice detector 110C to a structure based on an excitation signal because voice power is strongly reflected on excitation gain. Moreover, it is possible to improve the accuracy by extracting pitch information from an excitation signal. [Embodiment 4] Figure 7 is a block diagram of the voice coding-and-transmission system of the fourth embodiment of the present invention. This embodiment is obtained by improving the relay node and reception node of the voice coding-and-transmission system of the embodiment 1. In Fig. 7, a component having the same function as that described for the embodiment 1 is provided with the same symbol and its description is not repeated. In the case of a modified component, the character D is added to the symbol in Fig. 1 so that how the component corresponds to that of the embodiment 1 can easily be understood. A relay node 104D has a pseudo background noise generator 140. An input from the encoder 114 is connected to either the pseudo background noise generator 140 or the decoder 108 by a changeover switch 142. In a reception node 102D, an output from a pseudo background noise generator 144 is coded by an encoder (noise encoder) 146. An input for the decoder 122 is connected to either the encoder 146 or the transmission line A by a changeover switch 148. Operations of the fourth embodiment are described below by referring to Fig. 7. A voice code from the transmission line B is once decoded as a voice signal by the decoder 108 in the relay node 104D. The voice detector 110 detects presence or absence of a talk spurt in accordance with the voice signal and decides an operation mode of the relay node 104D in accordance with a detection result. An coding/decoding system of the present invention has three operation modes. However, description of these operation modes is omitted because the operation modes are the same as those described for the embodiment 1. The operation in mode 3 (voiceful state) is completely the same as the operation in mode 3 shown in the embodiment 1. In this case, it is possible to stop the encoder 146 at the reception node. In the relay node 104D, when it is detected that the voice detector 110 changes from mode 3 to mode 1, the changeover switch 142 is connected to a terminal 142a and a changeover switch 112 is connected to the terminal 112b. Therefore, a pseudo background noise output from the pseudo background noise generator 140 is input to the encoder 114. The encoder 114 receives the input of the pseudo back ground noise and codes the noise. As a result, a signal obtained by high-efficiency-coding of the pseudo background noise is output from the encoder 114 and, moreover, internal variables of a filter factor and the like are adaptively updated. This operation is previously shown by taking ITU Recommendation G.728 as an example. In this case, because the high-efficiency-coded signal output from the encoder 114 is not connected to the changeover switch 112c, it is not output to the transmission line A. The voice detector 110 is always operated because it is necessary to continuously monitor the mode changes. Moreover, in the reception node 102D, the voice/silence information extractor 120 fetches mode information from a silent-period-vanished voice code transmitted from the transmission line A, extracts the information showing that the decision result of the voice encoder 110 is switched from mode 3 to mode 1, and outputs a control signal according to the information to the changeover switch 148 and the encoder 146. The changeover switch 148 is switched to the terminal-148a side in accordance with the control signal. Moreover, the encoder 146 loads the internal variables of the decoder 122 (e.g. synthesizing filter memory and adaptive gain) in a predetermined area of the encoder 146 by responding to the control signal and also makes its internal state coincide with that of the decoder 122. Thereafter, the encoder 146 starts coding by using a pseudo background noise output from the pseudo background noise generator 144 as its input. The decoder 122 operates by using a high-efficiency background noise code output from the encoder 146 as its input. In this case, to continuously keep the same internal state of the encoder 114 and of the decoder 122, a high-efficiency background noise code output from the encoder 114 (the code is not actually output to a transmission line) must be completely the same as that output from the encoder 122. Because the internal state of the encoder 146 and that of the decoder 122 are kept so that both internal states are equal, a pseudo background noise output from the pseudo background noise generator 144 must be the same as a pseudo background noise output from the pseudo background noise generator 140 in the relay node 104D. As described above, by setting the pseudo background noise generator 144 and the encoder 146 to the reception node 102D, it is possible to avoid an indefinite state during the silent periods described for the prior art because setting of the generator 144 and the encoder 146 is equivalent to setting of a pseudo transmission node to the reception node 102D. Therefore, the pseudo background noise generator 140 supplies a reference value for differential coding to the encoder 114 when changing from mode 1 to mode 2 (that is, at start of voicing) and the pseudo background noise generator 144 and the encoder 146 supply a reference value for differential coding to the decoder 122 at start of voicing). Therefore, the incoincidence between the internal states of the encoder 114 of the relay node 104D and the decoder 122 of the reception node 102D does not occur and abnormal sound at the change of operation modes from 1 to 2 can be avoided. However, it is necessary to consider that the internal states of the encoder 106 and the decoder 122 still do not coincide with each other. Operations of the relay node 104D in mode 2 are described below. When the voice detector 110 detects the head of a talk spurt, it transmits a control signal to the changeover switch 142 and the silent period eliminator 112. By responding to the control signal, the changeover switch 142 is switched to the terminal-142b side and a changeover switch in the silent period eliminator 112 is switched to the terminal-112c side. Thereby, in the relay node 104D, a voice signal decoded by the decoder 108 is coded as a high-efficiency voice code again by the encoder 114 and the high-efficiency voice code is output to the transmission line A from the relay node 104D. In the reception side 102D, when the voice/silence information extractor 120 detects the change to operation mode 2, it outputs a control signal to the changeover switch 148. The changeover switch 148 is switched to the terminal-148b side by the control signal. The decoder 122 decodes an output of the encoder 114 input from the transmission line A. When the period of mode 2 continues, the internal states of the encoder 106 and the decoder 122 of the transmission node 100 approach each other. Therefore, no abnormal sound is produced, even if operation modes are thereafter changed from 2 to 3. As described above, it is possible to avoid quality degradation and fully realize the performance of a high-efficiency voice coding system. Moreover, it is possible to decrease the processor processing load and the hardware scale of the relay node 104D. [Embodiment 5] Figure 8 is a block diagram of a relay node for explaining the fifth embodiment of the present invention. This embodiment is obtained by applying the same improvement as that shown in the embodiment 2 to the relay node of the embodiment 4. That is, a relay decoder and relay encoder use the decoder 108B and the encoder 114B having the same function as that of the embodiment 2 respectively. The decoder 108B decodes a voice signal and outputs some adaptive parameters. In the case of the encoder 114B, it is possible to omit the processing for generating these adaptive parameters. This improvement decreases the processing load and hardware scale of the relay node. The decoder 108B decodes a voice signal and outputs some adaptive parameters. An adaptive parameter is a voice parameter to be generated in high-efficiency coding such as ADPCM to form a voice signal. The encoder 114B receives the voice signal and adaptive parameters. In the case of the encoder 114B, it is possible to omit the processing for generating input adaptive parameters. Most operations of this voice coding-and-transmission system are the same as those of the embodiment 4, except that the decoder 108B fetches adaptive parameters and the encoder 114B uses them similar to the case of the embodiment 2. [Embodiment 6] Figure 9 is a block diagram of the voice coding-and-transmission system of the sixth embodiment of the present invention. This embodiment is obtained by further applying the same improvement as that shown in the embodiment 3 to the relay node of the embodiment 5. That is, a relay decoder, relay encoder, and voice detector use the parameter separator 108C, encoder 114C, and voice detector 110C having the same function as the embodiment 3 respectively. The parameter separator 108C fetches only some of the adaptive parameters included in a voice signal while the encoder 114 generates a voice code instead of a complete voice signal in accordance with some of the adaptive parameters. This improvement further decreases the processing load and hardware scale of the relay node. [Embodiment 7] Figure 10 is a block diagram of the voice coding-and-transmission system of the seventh embodiment of the present invention. This embodiment is obtained by improving the relay node and the reception node of the voice coding-and-transmission system of the embodiment 1 of the present invention. In Fig. 10, a component having the same function as that described for the embodiment 1 is provided with the same symbols as in Fig. 1 and its description is not repeated. In the case of a modified component, the character G is added to the symbol in Fig. 1 so that how the component corresponds to that of the embodiment 1 can easily be understood. A relay node 104G and a reception node 102G have respective task controllers 160 and 162. The task controller 160 controls operations of the encoder 114 in accordance with a control signal output from the voice detector 110. The task controller 162 controls the decoder 122 in accordance with a control signal output from the voice/silence information extractor 120. Then, operations of the embodiment 7 are described below by referring to Fig. 10. In the relay node 104G, the decoder 108 once decodes a voice code sent from the transmission node 100. The voice detector 110 detects presence or absence of a talk spurt in accordance with the voice signal and decides an operation mode of the relay node in accordance with a detection result. The coding/decoding system of the present invention has three operation modes. However, description of the operation modes is omitted because the operation modes are the same as those described for the embodiment 1. The operation in mode 3 is completely identical to that mode 3 shown in the embodiment 1. In this case, however, the encoder 114 of the relay node 104G codes a voice signal output from the decoder 108. In the relay node 104G, when the voice detector 110 detects the change of operation modes from 3 to 1, it transmits a control signal to the silent period eliminator 112. A changeover switch in the silent period eliminator 112 responds to the control signal to connect with the terminal 112b and stop the output of a voice code from the relay node 104G. Moreover, the control signal is sent to the task controller 160. The task controller 160 responds to the control signal and sends a control signal for stopping the coding operation of the encoder 114 to the encoder 114. The encoder 114 responds to the control signal to stop the coding operation while holding the contents (e.g. synthesizing filter factor and adaptive gain) in its internal memory. The encoder 114 does not perform any coding while holding the contents of the internal memory as long as the state of mode 1 continues since the mode change. In the reception node 102G, the voice/silence information extractor 120 fetches mode information from a silent-period-vanished voice code transmitted from the transmission line A and sends a control signal corresponding to the change of operation modes from 3 to 1 to the changeover switch 126 and the task control section 162. The changeover switch 126 is switched to the terminal-126a side. The task controller 162 responds to the control signal to stop the decoding operation of the decoder 122 while holding the contents of the internal memory. The decoder 122 does not perform decoding at all while holding the contents of the internal memory as long as the state of mode 1 continues since the mode change. In the relay node 104G, when the voice detector 110 detects the change of operation modes from 1 to 2, it switches a changeover switch in the silent period eliminator 112 to the terminal-112c and sends a control signal for notifying the change of operation modes from 1 to 2 to the task controller 160. The task controller 160 responds to this control signal and outputs a control signal for restarting coding to the encoder 114. The encoder 114 responds to the control signal to restart coding by using the contents (e.g. synthesizing filter factor and adaptive gain) held in the internal memory since the change of operation modes from 3 to 1 without initializing the contents as reference values for differential coding/decoding. A high-efficiency voice code output from the encoder 114 is output to the transmission line A from the relay node and transmitted to the reception node 102G. Moreover, the voice/silence information extractor 120 fetches mode information from a silent-period-vanished voice code transmitted from the transmission line A and transmits a control signal corresponding to the change of operation modes from 1 to 2 to the changeover switch 126 and the task control section 162. The changeover switch 126 is switched to the terminal-126b side in accordance with the control signal. The task controller 162 responds to the control signal and outputs a control signal for restarting decoding to the decoder 122. The decoder 122 responds to the control signal to restart decoding by using the contents held in the internal memory since the change of operation modes from 3 to 1 as the reference values for differential coding/decoding without initializing the contents. The decoder 122 decodes an output of the encoder 114 of the relay node 104G and outputs a voice signal. As described above, it is possible to avoid an indefinite state of the decoder described for the prior art by setting the task controllers 160 and 162 to the relay node 104G and the reception node 102G respectively and synchronizing the processing schedule of the encoder 114 with that of the decoder 122. Thus, the task controller 160 determines a reference value for differential coding at the change of the encoder 114 to mode 2 (that is, at start of voicing) and the task controller 162 determines a reference value for differential coding at start of voicing for the decoder 122. Therefore, the incoincidence between the internal states of the encoder 114 of the relay node 104G and the decoder 122 of the reception node 102G does not occur and it is possible to avoid abnormal sound at the change of operation modes from 1 to 2. However, it is necessary to consider that the internal states of the encoder 106 and the decoder 122 still do not coincide with each other. Operations of this embodiment in mode 2 are basically the same as those of the embodiment 1. In the relay node 104G, when the voice detector 110 detects the head of a talk spurt, it sends a control signal to the silent period eliminator 112. By responding to the control signal, a changeover switch in the silent period eliminator 112 is switched to the terminal-112c side. Thereby, in the relay node 104G, a voice signal decoded by the decoder 108 is coded as a high-efficiency voice code again by the encoder 114 and the high-efficiency voice code is output to the transmission line A from the relay node 104G. In the reception side 102G, when the voice/silence information extractor 120 detects the change to operation mode 2, it outputs a control signal to the changeover switch 126. The changeover switch 126 is switched to the terminal-126b side in accordance with the control signal. The decoder 122 decodes an output of the encoder 114 input from the transmission line A. When the period of mode 2 continues, the internal states of the encoder 106 and the decoder 122 of the transmission node 100 adequately approach as described for the embodiment 1. Thereafter, no abnormal sound is produced, even if operation modes are changed from 2 to 3. As described above, it is possible to avoid quality degradation and fully realize the performance of a high-efficiency voice coding system by limiting the period for performing tandem connection, which is known to cause voice quality degradation, to the short time of a transient period for the change from a silent state to a voice state and connecting most talk spurts by digital-one-link. Moreover, it is possible to decrease the processor processing load and hardware scale of the relay node 104G. [Embodiment 8] Figure 11 is a block diagram of a relay node for explaining the eighth embodiment of the present invention. This embodiment is obtained by applying the same improvement as shown in the embodiment 2 to the relay node of the embodiment 7. That is, a relay decoder and a relay encoder use the decoder 108B and the encoder 114B having the same respective functions as those of the embodiment 2. The decoder 108B decodes a voice signal and outputs some of adaptive parameters. In the case of the encoder 114B, it is possible to omit the processing for generating the adaptive parameters. This improvement decreases the processing load and hardware scale of the relay node. [Embodiment 9 ] Figure 12 is a block diagram of the voice coding-and-transmission system of the ninth embodiment of the present invention. This embodiment is obtained by further applying the same improvement as that shown in the embodiment 3 to the relay node of the embodiment 7. That is, a relay decoder, relay encoder, and voice detector use the parameter separator 108C, encoder 114C, and voice detector 110C having the same respective functions as in embodiment 3. The parameter separator 108C fetches only some of the adaptive parameters included in a voice signal and generates, instead of a complete voice signal, a voice code in accordance with the fetched adaptive parameters. This improvement further decreases the processing load and hardware scale of the relay node. As described above, the embodiments 1 to 9 basically perform synchronous resetting between a relay encoder of a relay node and a reception decoder of a reception node at start of voicing. Figure 10 is a block diagram of the voice coding-and-transmission system of the tenth embodiment. This embodiment is not an embodiment of the invention but helpful in understanding certain aspects of the invention. In Fig. 13, a component having the same function as that described for the embodiment 1 is provided with the same symbol as in Fig. 1. This embodiment uses the high-efficiency voice coding system according to ITU Recommendation G.728. However, a high-efficiency coding system applicable to the present invention is not restricted to the above voice coding system. This embodiment is described below by referring to Fig. 13. In a relay node 404, coding/decoding related to gain is performed by using a gain codebook. The gain codebook makes one gain correspond to every several ranges provided for the gain value of a voice signal as a quantized value. A gain code is made to correspond to the quantized value. In Fig. 13, standard gain codebooks 408 and 410 are the same normally used codebooks. Specifically, he standard gain codebooks 408 and 410 are memories storing gain codebooks specified by ITU Recommendation G.728. Suppressed gain codebooks 412 and 414 are memories storing gain codebooks having only quantized gain values causing no divergence, even for an unstable coding/decoding system, by attenuating the quantized values of he standard gain codebooks 408 and 414. That is, a suppressed gain codebook and a standard gain codebook have the same range section (gain value range) for gain values. In the same range, for example, the quantized gain value of the suppressed gain codebook is given a value further attenuated than that of the standard gain codebook, that is, a smaller value. An attenuation degree is set to a larger value for a gain value range at a higher position. It is also possible to use a suppressed gain codebook having a gain value range different from that of a standard gain codebook. For example, it is possible that the lower limit of the highest gain value range of a suppressed gain codebook is smaller than that of a standard gain value codebook. In this case, it is possible to set the quantized gain value corresponding to the highest gain value range of the suppressed gain codebook to a quantized gain value attenuation degree higher than the above case of having the same gain value range and thereby, obtain a suppressed gain codebook having a high abnormal-sound suppression effect, as will be mentioned later. A decoder 416 performs decoding by using the standard gain codebook 408, an encoder 418 performs coding by using the suppressed gain codebook 412, and a decoder 420 performs decoding by switching the standard gain codebook 410 and the suppressed gain codebook 414. Gain codebooks to be connected to the decoder 420 are switched by a changeover switch 422. The changeover switch 422 is switched by a control signal sent from a voice/silence information extractor 424. The coding/decoding system of the present system has three operation modes described for the embodiment 1. The voice/silence information extractor 424 outputs control signals corresponding to these three operation modes in the same way as the voice/silenoe information extractor 312 of the embodiment 12. Operations of this embodiment are described below by referring to Fig. 13. In the relay node 404, the decoder 416 once decodes a high-efficiency voice code sent from the transmission node 100 as a voice signal and the voice detector 110 detects presence or absence of a talk spurt in accordance with the voice signal to decide an operation mode of the relay node 404 in accordance with a detection result. The operation in mode 3 (voice state) is completely identical to that in mode 3 shown in the embodiment 1. When the voice detector 110 in the relay node 404 detects the change of operation modes from 3 to 1, it sends a control signal to the silent period eliminator 112. A changeover switch in the silent period eliminator 112 is switched to the terminal-112b side by responding to the control signal but no data is output to the transmission line A. That is, the line A is silent-period-vanished. It is permitted that the encoder 418 is in an indefinite state. In the reception node 402, the voice/silence information extractor 424 fetches mode information from a silent-period-vanished voice code transmitted from the transmission line A, extracts the information for notifying the change of operation modes from 3 to 1, and sends a control signal reflecting the information to the changeover switch 126. The changeover switch 126 is switched to the terminal-126a side in accordance with the control signal and a pseudo-background noise is output to a receiver. In this case, it is permitted that the decoder 420 is in an indefinite state. In the relay node 404, when the voice detector 110 detects the change of operation modes from 1 to 2, it generates a control signal and a changeover switch in the silent period eliminator 112 is switched to the terminal-112c side in accordance with the control signal. A high-efficiency voice code output from the encoder 418 is output to the transmission line A from the relay node 404 and transmitted to the reception node 402. In the reception node 402, the voice/silence information extractor 424 detects the change of operation modes from 1 to 2 and generates a control signal. In accordance with the control signal, the changeover switch 126 is switched to the terminal-126b side. Moreover, the changeover switch 422 is switched to the terminal 422b to connect the decoder 420 with the suppressed gain codebook 414. The decoder 420 decodes a silent-period-vanished voice code sent from the transmission line A by using the suppressed gain codebook 414 and outputs a voice signal to a receiver. In this case, the internal state of the decoder 420 of the reception node 402 is different from the internal state of the encoder 418 of the relay node 404. However, abnormal sound can be avoided because the selected suppressed-gain codebook 414 is optimized so that no divergence occurs, even in an unstable coding/decoding system. In the period of mode 2, a voice signal output from the decoder 420 is not very faithful to the original voice signal input to the encoder 106 because the encoder 418 and decoder 420 are different in internal state. That is, the S/N ratio tends to get lower than the normal S/N ratio. However, a voice signal coded/decoded in mode 2 is in many cases a consonant part at the head of a talk spurt. If the voice waveform of a consonant part is very noisy, the acoustic property of an original voice signal is not lost, even for a low S/N ratio. Therefore, even in the case of the simple structure shown in Fig. 13, no abnormal sound is produced and it is possible to reproduce voices with a relatively small degradation of voice quality. The incoincidence between the internal states of the encoder 106 and the decoder 420 tends to converge under the condition of mode 2 as described for the embodiment 1. Therefore, no abnormal sound is thereafter produced, even when switching the changeover switch 112 to the terminal 112a and the changeover switch 422 to a terminal 422a and thereby changing the operation mode from 2 to 3. Therefore, to suppress abnormal sound, the present voice coding-and-transmission system uses a method of changing coding tables used for a transient period, so that a voice code causing divergence of the system is not output instead of using a method of readapting a voice code output, from the encoder 106. This embodiment has an advantage preferable for practical use that the embodiment can easily be executed because the embodiment requires a fewer control signals be added and has few units for performing complex processing as compared to the above embodiments. (Embodiment 11] Figure 14 is a block diagram of the voice coding-and-transmission system of the eleventh embodiment. This embodiment is not an embodiment of the invention but helpful in understanding certain aspects of the invention. This embodiment is obtained by applying the same improvement as that shown in the embodiment 2 to the relay node of the embodiment 10. In Fig. 14, a component having the same function as that described for the embodiment 10 is provided with the same symbol as in Fig. 13. This system is slightly different from the embodiment 10 in its relay decoder and relay encoder. A decoder 416B decodes a voice signal and outputs some of the adaptive parameters. An adaptive parameter is generated in high-efficiency coding such as ADPCM and serves as a voice parameter for constituting a voice signal. An encoder 418B receives the voice signal and adaptive parameters. In the case of the encoder 418B, it is possible to omit the processing for generating adaptive parameters input. In this case, it is necessary to select parameters to be supplied causing no abnormal sound in accordance with a high-efficiency coding system because supply of some adaptive parameters from the decoder 416B to the encoder 418B results in the partial admittance of incoincidence between the internal states of the encoder 418B and the decoder 420 of the reception node as described for the embodiment 2. This improvement decreases the processing load and hardware scale of the relay node. [Embodiment 12] Figure 15 is a block diagram of the voice coding-and-transmission system of the twelfth embodiment. This embodiment is not an embodiment of the invention but helpful to understand certain aspects of the invention. This embodiment is obtained by applying the same improvement as that shown in the embodiment 3 to the relay node of the embodiment 10. In Fig. 15, a component having the same function as that described for the embodiment 10 is provided with the same symbol as in Fig. 13. This system is slightly different from the embodiment 10 in relay decoder, relay encoder, and voice detector. A parameter separator 416C is constituted by omitting some processing from the decoder 416B in Fig. 14. The parameter separator 416C is not provided with a function for decoding a voice signal in the complete form and is only provided with a parameter extracting function. The parameter separator 416C outputs an excitation signal and an coding parameter to the encoder 418C and excitation signal information to a voice detector 440. The voice detector 440 detects voice in accordance with the excitation signal information. Other operations of this voice coding-and-transmission system are the same as those of the embodiment 10. This improvement further decreases the processing load and hardware scale of the relay node.

Claims: A voice coding-and-transmission system comprising: a transmission node (100) for outputting an original voice code which is a voice code obtained by coding a voice signal to a first transmission line; a relay node (104) for performing silent period elimination by selecting only a voice code corresponding to a voiceful period of a voice signal in accordance with an original voice code received from said first transmission line and outputting it to a second transmission line; and a reception node (102) for decoding a silent-period-eliminated voice code received from said second transmission line and outputting a voice signal; wherein said relay node (104) includes: a relay decoder (108) for extracting voice information included in a voice signal from said original voice code; a relay control circuit (100) for discriminating between a voice period and a silent period of said voice signal in accordance with said voice information and outputting a relay control signal for controlling operations of a relay node in accordance with a discrimination result; a silent period elimination circuit (112) for receiving said original voice code and said relay voice code and outputting said relay voice code during said transient period and said original voice code during a voice period after said transient period to said second transmission line in accordance with said relay control signal to synthesize said silent-period-eliminated voice code; wherein said reception node (102) includes: a reception control circuit (120) for deciding the start of said voicing in accordance with said silent-period-eliminated voice code and outputting a reception control signal for controlling operations of the reception node in accordance with the discrimination result; characterized in that said relay node (104) further includes: a coding reference value determination circuit for determining a reference value for said voice coding at the start of voicing which is the timing of the change from said silent period to said voiceful period in accordance with said relay control signal; a relay encoder (114) for starting said coding of said voice information in accordance with said reference value at the start of voicing and generating relay voice codes during at least a certain transient period; and said reception node (102) further includes: a decoding reference value determination circuit for determining a reference value for said decoding corresponding to said reference value for coding in accordance with said reception control signal at the start of said voicing; and a reception decoder (122) for starting said decoding of said silent-period-eliminated voice code in accordance with said decoding reference value at the start of said voicing and outputting said voice signal. The voice coding-and-transmission system according to claim 1, characterized in that said coding reference value determination circuit includes a memory (118) storing a predetermined reference value for said coding; and reads said reference value and uses it to set said relay encoder (114) at the start of said voicing; and said decoding reference value determination circuit includes a memory (128) storing a predetermined reference value for said decoding; and reads said reference value and uses it to set said reception signal decoder (122) at the start of said voicing. The voice coding-and-transmission system according to claim 1, characterized in that said coding reference value determination circuit includes a pseudo-background-noise signal generator (140) for outputting artificial noise; and an coded input switching unit (142) for switching an input terminal of said relay encoder (114) from said relay decoder (108) to said pseudo-background-noise signal generator (140) during said voice-signal silent period; and said decoding reference-value determination circuit includes: a pseudo-background-noise signal generator (114); a noise encoder (146) for coding an output of said pseudo-background-noise signal generator; and a decoded input switching unit (148) for switching an input terminal of said reception decoder (122) from said second transmission line to said noise decoder. The voice coding-and-transmission system according to claim 1, characterized in that said coding reference value determination circuit has a task controller (160) for controlling said relay encoder (114); said decoding reference-value determination circuit has a task controller (162) for controlling said reception decoder (122); and each of said task controllers (160 and 162) stops said coding of each control object or decoding corresponding to said coding when said voice signal changes from a voice period to a silent period while making the coding or decoding hold its latest reference value, and restarts the processing of each control object when said voice signal changes from a silent period to a voice period. The voice coding-and-transmission system according to one of claims 1, 3 or 4, characterized in that one of said relay encoder (114B, 418B) performs said coding by using voice parameters calculated by one of said relay decoders (108B, 416B). The voice coding-and-transmission system according to one of claims 1, 3 or 4, characterized in that one of said relay decoders (108C, 416C) extracts only some of the voice parameters included in said voice signal, one of said relay encoders (114C, 418C) performs said coding in accordance with an output of one of said relay decoders (108C, 416C), and said relay control circuit (110C or 440) discriminates between a voice period and a silent period of said voice signal in accordance with an output of one of said relay decoder (108C, 416C).

Inventors: KAWANO NORIAKI, NAITO YUSHI, SUZUKI SHIGEAKI, YAJIMA HISASHI, KAWANO, NORIAKI, NAITO, YUSHI, SUZUKI, SHIGEAKI, YAJIMA, HISASHI
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1997
Main classification: G10L15
IPC codes: G10L19, H03M3, H03M7, H04Q3, G10L15, H04L12, H04B1