Note: Descriptions are shown in the official language in which they were submitted.
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ATM TRANSMISSION SYSTEM
BACKGROUND OF THE INVENTION
The present invention relates to ATM
transmission systems for assembling and transmitting
information into data cells having a predetermined
bit number and, more particularly, to an ATM
transmission system which assembles only speech part
of a speech signal into cells to be transmitted.
Up to date, As a speech coding method in speech
coders there have been used a CELP (Code-Exited
Linear Prediction) system which is one of speech
analysis and synthesis methods or an LD-CELP
(Low-Delay Code-Exited Linear Prediction) system
which is an improvement of the CELP system. The
LD-CELP system is a typical vector quantization
system, in which successive excitation vector
candidates from excitation vector codebooks are
passed through a synthesis filter to transmit an
index of a codebook with which decoded speech
closest to the input signal can be obtained. The
index is a parameter to be transmitted. A problem
here is that burying a voice/no-voice bit through
bit steal of the index independently of the data
coding process, results in different code vectors
between on the coding and decoding sides. That is,
difficulty is encountered when burying the
voice/no-voice bit in the coded data.
A prior art ATM speech transmission system
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utilizes such high efficient speech codec. A CLAD
(Cell Assembling and Disassembling) apparatus for
assembling and disassembling the transmission cells
includes a speech detector for detecting
voice/no-voice and the speech decoder with a high
efficient coding of the speech signal. Only when
the speech detector detects the voice, the coded
data are assembled into cells and thus obtained cell
data are transmitted without addition of any code
bit or the like directly by a mode switcher.
In a prior art ATM speech codec adopting the
CELP system (as disclosed in Japanese Patent
Laid-Open Publication Heisei 5-22153), the voice and
no-voice is detected, and when the no-voice is
detected the power supply is held "off" during the
frame period which is irrelevant to the coding in
order to save power. In this system, no
voice/no-voice bit is multiplexed in the cell. In
addition, it has been difficult to transmit the CELP
coded data by multiplexing the voice/no-voice data,
and therefore only the voice part of the signal is
converted to cells without decoding the coded data.
This gives rise to a problem if it is intended to
introduce the ATM transmission line locally into an
existing CBR (Constant Bit Rate) network constituted
by CBR transmission lines. To this end, it is
necessary that the coded data is decoded once and
then coded again at the inter-connection point
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between CBR transmitter adopting the CELP coding
system and ATM transmitter. The speech quality,
therefore, is deteriorated by repeated coding and
decoding of the data. A further disadvantage is
that for the repeated coding and decoding of data it
is necessary to store once cells having a
predetermined bit number, and this increases the
propagation delay. A still further disadvantage is
that in order to m; n; m; ze the speech quality
deterioration it is necessary to replace all the
existing network CBR transmitters with ATM
transmitters, which spoils the economy of the
network.
SUMMARY OF THE INVENTION
An object of the invention is therefore to
provide an ATM speech coding system, capable of
minimizing the speech quality deterioration and the
signal propagation delay generated due to the data
storage for assembling and disassembling the cells
and to high efficient speech codec and decoding,
without requiring the change of the transmission
rate of the existing CBR network and the necessity
of repeated coding and decoding of data in case of
its inter-connection to an ATM transmission line.
According to one aspect of the present
invention, there is provided an asynchronous
transmission mode speech coding system comprising:
a high efficient speech codec unit provided between
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an asynchronous transmission mode transmission line
along which digital data is transmitted and received
in an asynchronous transmission mode, and a
switchboard for switching telephone frequency band
speech signals and for high efficient coding and
decoding the telephone frequency band speech signals
and FAX signals; and a cell assembling and
disassembling unit including means for dividing
coded data from the high efficient speech codec unit
into frames of a first fixed bit number, adding a
he~er of a second fixed bit number to each of the
frames, producing a cell having a third fixed number
and sending out the cells thus produced to the
asynchronous transmission mode transmission line,
and means for separating data having been coded in
the high efficient speech codec unit from cells
transmitted from the asynchronous transmission mode
transmission line and transmitting the separated
data to the high efficient speech codec unit; the
high efficient speech codec unit including: speech
coding means for coding a telephone frequency band
speech signal received from the switchboard into
digital data of a low bit rate and outputting the
coded data as a digital speech signal of a first
- 25 format; speech detecting means for outputting
voice/no-voice information of input speech by
checking input signal power from the telephone
frequency band speech signal received from the
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switchboard; and a facsimile signal coding means for
checking, from the telephone frequency band speech
signal received from the switchboard, whether the
input signal is a modulated signal from a facsimile
input terminal and, if the input signal is a
modulated signal from the facsimile input terminal,
sending out a facsimile signal detection signal
indicative of the reception of the modulated signal
while sending out a digital speech signal of a
second format for a predetermined period of time and
then sending out a digital signal of a third format
obtained through demodulation of the modulated
signal; the cell assembling and disassembling unit
further including: facsimile transmission start data
detecting means for detecting the second format
digital speech signal for a predetermined period of
time; and cell producing means for producing cells
of the third fixed bit number by adding a header of
the second fixed bit number to each frame of a
digital speech signal of a fourth format or of the
digital speech signals of the second and third
formats and sending out the cells thus produced to
the asynchronous transmission mode transmission
line; the cell assembling and disassembling unit
still further including: means for deleting the
header of the second fixed bit number from each cell
received from the asynchronous transmission mode
transmission line and transmitting the resultant
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data as a digital signal of a seventh format to the
high efficient speech codec unit; and means for
transmitting a digital signal of a sixth format to
the high efficient speech codec unit in the absence
of cell received from the asynchronous transmission
mode transmission line: the high efficient speech
codec unit further including: means for checking
whether digital signal of the sixth format is
received from the cell assembling and disassembling
unit and, if the signal is received, outputting
noise to the switchboard; and facsimile signal
decoding means for detecting, when no digital signal
of the sixth format is received from the cell
assembling and disassembling unit, the digital voice
signal of the second format from the digital signal
of the seventh format for a predetermined period of
time and when it is detected modulating the
subsequent digital signal of the third format and
transmitting the modulated signal to the
switchboard; the high efficient speech codec unit
still further including: means for receiving the
digital speech signal of the first format and result
data of the check in the voice detecting means and
producing the digital speech signal of the fourth
format by multiplexing the two received signals
through bit steal of one bit for each frame; and
means for receiving the digital speech signal of the
fourth format and the facsimile signal detection
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signal as well as the second format digital speech
signal and third format digital signal from the
facsimile signal coding means, selecting the fourth
format digital speech signal if the facsimile signal
detection signal indicates detection of no facsimile
signal while selecting the second format digital
voice signal and third format digital signal from
the facsimile signal coding means if the facsimile
signal detection signal indicates detection of a
facsimile signal, and transmitting the selected
signal as a digital signal of a fifth format to the
cell assembling and disassembling unit; the cell
assembling and disassembling unit yet further
including: voice/no-voice bit detecting means for
extracting the result data, having been bit steal
multiplexed, of the voice detecting means from the
digital signal of the fifth format: and cell
producing means controlled according to the
voice/no-voice bit so as to assemble cell data of
the afore-the frame configuration from the second
and fourth format digital speech signals if the bit
indicates voice while assembling no cell data if the
bit indicates no-voice, and also controlled
according to the result of detection of the
facsimile transmission start data such that if the
digital speech signal of the second format has been
detected for a predetermined period of time,
subsequently received data is not subjected to the
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cell production control according to the
voice/no-voice bit but is all converted to cell data
to be sent out to the asynchronous transmission mode
transmission line; the high efficient speech codec
unit yet further including: speech decoding means
for d~coA~g the digital speech signal of the
seventh format and transmitting the decoded signal
to the switchboard if no digital signal of the sixth
formula is detected as digital signal received from
the cell assembling and disassembling unit or if no
digital speech signal of the seventh format is
detected from the digital signal of the seventh
format.
According to another aspect of the present
invention, there is provided a asynchronous speech
coding system for inputting and outputting a
telephone frequency band signal with respect to a
switchboard through a high efficient speech codec
unit and for inputting and outputting cells with
respect to an ATM transmission line through a CLAD
unit, the high efficient speech codec unit
comprising on a transmission side, an LD-CELP speech
coder for coding the speech signal, a speech
detector for detecting voice and no-voice by
monitoring the input speech power, a FAX signal
coder for detecting a modulation signal inputted
from a FAX terminal, a multiplexer for receiving a
first signal from the voice detector and a second
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signal from the LD-CELP speech coder, and a selector
for receiving a third signal of the multiplexer, a
fourth signal indicative of FAX start data which is
transmitted for a predetermined period of time upon
detection of the FAX signal, and a fifth signal
which is demodulated data of a modulated signal from
actual FAX terminal of the FAX signal coder and
output a sixth signal indicative of whether the FAX
signal has been detected and selecting, based on the
sixth signal, one of the fourth, sixth signals and
the third signal of the multiplexer as a seventh
signal of the transmitting side of the high
efficient speech codec unit, the CLAD unit
comprising, on the transmission side a
voice/no-voice bit detector for receiving the
seventh signal from the transmitting side of the
high efficient speech codec unit and extracting the
voice/no-voice bit from the seventh signal, a FAX
start data detector for detecting the fourth signal
from the seventh signal, and a cell assembler which,
in case of failure of detection of the fourth signal
from the seventh signal, assembles cells and sends
out the assembled with a header data to the ATM
transmission line according to the result of
detection in the voice/no-voice discrimination bit
detector, and on the receiving side a voice cell
data receiver which, when receiving the cell data
from the ATM transmission line, deletes the header
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data that has been added on the transmitting side
and outputs the resultant data in the frame format
of an eighth signal, and a no-voice cell data
receiver which, in the absence of cell received,
outputs a ninth signal having a frame configuration,
the high efficient speech codec unit further
including on the receiving side, an LD-CELP speech
decoder for dec-o~ing the eighth signal received from
the CLAD unit to reproduce a speech signal, a
no-voice compensation noise generator for outputting
a first control signal for controlling the switching
of the output signal to the switchboard and,
no-voice compensation white noise at a level close
to the environmental noise of the speech, the input
signal from the CLAD unit is of the ninth signal
format and, a FAX signal decoder for outputting a
second control signal for controlling the switching
of the output signal to the switchboard and
modulating the subsequent fifth signal to output the
modulated signal as the FAX signal if the fourth
signal is received, and a selector for selectively
outputting, according to the first and second
control signals, either one of the outputs of the
LD-CELP speech decoder, no-voice compensation noise
generator and FAX signal decoder to the switchboard.
According to other aspect of the present
invention, there is provided an asynchronous speech
coding system comprising: a first coder for coding n
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input signal supplied through a telephone line based
on the low-delay code-excited linear prediction; a
first detector for detecting voice/no-voice data of
the input signal; a FAX terminal input detector for
detecting a FAX signal of the input signal; a
multiplexer for multiplexing the coded data and the
voice/no-voice data; a second detector for detecting
a speech data frame and the voice/no-voice data from
the multiplexed data; a conversion means for
converting the coded data to cell data to be sent
out to a transmission line if the voice/no-voice
information indicates voice and FAX terminal input
data to cell data and transmitting the converted
cell data to the transmission line independently of
the voice/no-voice data.
Other objects and features of the present
invention will be clarified from the following
description with reference to attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram showing an embodiment
of the ATM speech coding system according to the
present invention;
Fig. 2 is a block diagram showing an example of
the LD-CELP speech coder 211 for coding the speech
signal with five successive input samples as one
vector, and
Figs. 4 to 8 show the signals Sl, S4, S2, S3,
S6 and S7 in Fig. 1.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Fig. 1 is a block diagram showing an embodiment
of the ATM speech coding system according to the
present invention.
Referring to Fig. 1, there is shown the ATM
speech codec system 10, which comprises terminals
101 and 102 for inputting and outputting a telephone
frequency band signal with respect to a switchboard,
terminals 103 and 104 for inputting and outputting
cells with respect to an ATM transmission line, a
high efficient speech codec unit 200 connected
between the terminals 101 and 102, and a CLAD unit
300 connected between the terminals 103 and 104 and
serving to assemble and disassemble the cells. The
high efficient speech codec unit 200 and CLAD unit
300 are connected to each other.
The high efficient speech codec unit 200
includes an LD-CELP speech coder 211 for coding the
speech signal for the transmitting side, a speech
detector 212 for detecting voice and no-voice by
monitoring the input speech power, a FAX signal
coder 213 for detecting a modulation signal inputted
from a FAX terminal, a multiplexer 214 for receiving
output result signal Sll from the voice detector 212
and output signal Sl from the LD-CELP speech coder
211, and a selector 215 for receiving output signal
S4 of the multiplexer 214, FAX signal coded output
signals S2 and S3 of the FAX signal coder 213 and
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output signal S10 indicative of whether a FAX signal
has been detected and selecting, based on the signal
S10, one of the FAX signal coded output signals S2,
S3 and the output signal S4 of the multiplexer 214
as an output signal S5 of the transmitting side of
the high efficient speech codec unit 200. The
output signal S11 of the voice detector 212 is
supplied to the LD-CELP speech coding section 211
for controlling the codebooks. The FAX signal coder
output signal S2 is FAX start data which is
transmitted for a predetermined period of time upon
detection of the FAX signal, and the signal S3 is
demodulated data of a modulated signal from actual
FAX terminal. The signals Sl and S2 have a 320-bit
frame configuration. The signal S4 has a frame
configuration obtained bit steal multiplexing the
signal S11, indicative of a voice/no-voice bit
signal, on the frame forefront. The signal S2 has a
structure that the frame forefront bit is fixed to a
code indicative of voice.
The CLAD unit 300 includes a voice/no-voice bit
detector 311 for receiving the output signal S5 from
the transmitting side of the high efficient speech
codec unit 200 and extracting the voice/no-voice bit
from the output signal S5, a FAX start data detector
312 for detecting the FAX start data S2 from the
signal S5, and a cell assembler 313 which, in case
of failure of detection of the FAX start data S2
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from the signal S5, assembles cells and sends out
these cells through the output terminal 105 to the
ATM transmission line according to the result of
detection in the voice/no-voice discrimination bit
detector 311, i.e., when and only when the voice is
detected. When the FAX start data S2 is detected
from the signal S5, the cell assembler 313 converts
all the subseguent data of signal S3 into cells to
be outputted through the output terminal 103 to the
ATM transmission line regardless of the detection
result in the voice/no-voice bit detector 311. The
cell assembler 313 adds a 40-bit header to every 320
bits of data as one frame and outputs the result
data as one cell through the output terminal 103 to
the ATM transmission line. The receiving side of
the CLAD unit 300 includes a voice cell data
receiver 321 which, when receiving the cell data
from the ATM transmission line, deletes the 40-bit
header data that has been added on the transmitting
side and sends out the resultant data in the frame
format of signal S7 to the LD-CELP speech decoder
221, and a no-voice cell data receiver 322 which, in
the absence of cell received from the input terminal
104, transmits a signal S6 having a frame
configuration to the LD-CELP speech decoder 221.
The high efficient speech codec unit 200
includes an LD-CELP speech decoder 221 for decoding
a signal S7 received from the CLAD unit 300 to
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reproduce a speech signal, a no-voice compensation
noise generator 222 for always checking whether its
input signal from the CLAD unit 300 is of the signal
S6 format and, if so, i.e., in case of the signal S6
format, outputting a control signal S12 for
controlling the switching of the output signal to
the switchboard and, no-voice compensation white
noise at a level close to the environmental noise of
the speech, a FAX signal decoder 223 for always
checking a input signal S7 from the CLAD unit 300
and, if the FAX start signal S2 is received,
outputting a control signal S13 for controlling the
switching of the output signal to the switchboard
and modulating the subsequent signal S3 to output
the modulated signal as the FAX signal, and a
selector 224 for selectively outputting, according
to the signals S12 and S13, either one of the
outputs of the LD-CELP speech decoder 221, no-voice
compensation noise generator 222 and FAX signal
decoder 223 through the output terminal 102 to the
switchboard.
Fig. 2 is a block diagram showing an example of
the LD-CELP speech coder 211 for coding the speech
signal with five successive input samples as one
vector. The LD-CELP speech coder 211 includes a
codebook 401 which is constituted by 1,024 exciting
source vector candidates, a synthesis filter 402 for
successively receiving the exciting source vector
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candidates having coefficients for synthesized
signal calculation obtained by the linear prediction
analysis result, a comparator 403 for comparing
successive the input vectors with the synthesis
filter output and outputting an index of the
exciting vector which permits obtaining reproduced
speech closest to the input, and a codebook address
controller 404 for reducing the number of exciting
source vector candidates to one half, i.e., 512,
with the timing of the bit steal according to the
output signal S11 from the voice detector 212.
In operation, a telephone frequency band signal
from the switchboard, when supplied to the input
terminal 101, is inputted simultaneously to the
LD-CELP speech coder 211, voice detector 212 and FAX
signal coder 213.
The LD-CELP speech coder 211 divides the input
signal into five successive input sample vectors and
executes a coding process. The coding process is
executed irrespective of whether the result from the
voice detector 212 shows voice or no-voice. In the
codebook 401 used in the coding process, the number
of exciting vector candidates is limited to one
half, i.e., 512, with the timing of the bit steal
multiplexing of the output signal S11 of the voice
detector 213 on the coded data. This control is
executed at the time of the retrieval for the first
index in one frame, and upon provision of the output
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_,
signal Sll of the voice detector 212 the codebook
address controller 404 outputs only the codebook
addresses of 512 exciting source vector candidates.
The exciting source vector candidates of the
designated addresses are successively inputted to
the synthesis filter 402 for the synthesized signal
calculation. The synthesis filter 402 has
coefficients obtained by the uses the result of
linear prediction analysis result. The input
vectors are successively inputted along with the
synthesis filter output to the comparator 403, which
outputs the 9-bit index of an exciting vector which
permits obtaining reproduced speech closest to the
input.
For an index other than the first one in one
frame, whenever samples for one frame are inputted,
exciting source vector candidates are inputted
successively from the codebook 401 constituted by
1,024 exciting source vector candidates to the
synthesis filter 402 in the LD-CELP speech coder 211
for synthesizing signal calculation. The input
speech vectors are inputted successively along with
the synthesis filter output to the comparator 403.
The comparator 403 outputs, for each vector, the
10-bit index of an exciting vector which permits
obtaining reproduced speech closest to the input.
The signal Sl has a format as shown in Fig. 3.
Referring to the Fig.3, labeled Index 01, Index 02,
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... are 10-bit output data from the LD-CELP speech
coder 211.
The voice detector 212 is always checking the
input signal power and outputs, in comparison to a
threshold level, a signal S11 indicating voice when
the input signal power is higher than the threshold
level and indicating no-voice otherwise. Actually,
the signal S11 is provided as a result of data check
for 20 milliseconds (a time length corresponding to
32 vectors in the LD-CELP speech coder) before the
bit steal multiplexed bits. If a portion of the
data is judged to correspond to voice during this
time, the pertinent frame is regarded to be a voice
frame, and the multiplexer 213 multiplexes a bit
indicative of voice. The signal S4 is shown in Fig.
4. Referring to Fig. 4, labeled V0 and V1 are
forefront bits in the frame (MSBs of index) that
indicate the check result of the preceding frame as
outputted from the voice detector 212. V0 indicates
that the preceding frame is a voice frame, and V1
indicates that the preceding frame is a no-voice
frame.
The FAX signal coder 213 is always checking the
input signal, and upon detection of a V.21 modulated
signal from the FAX terminal, outputs a control
signal S10 and also outputs the FAX start signal S2
and following demodulated signal S3 to be
transmitted through a selector 215 to the CLAD unit
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300. When the control signal S10 is provided, the
selector 215 selectively transmits the signals S2
and S3 from the FAX signal coder 213 as serial data
to the CLAD unit 300. Without provision of the
control signal S10 it selectively transmits the
signal S4 from the multiplexer 214 as serial data to
the CLAD 300. The control signal S10, once
outputted, is held until arrival of a call end
signal from the switchboard. The signals S2 and S3
are shown in Figs. 5 and 6, respectively.
Referring to Fig. 5, labeled "Fax Start Data"
is a bit pattern in which all 319 bits but the
forefront bit of the frame are "0". This bit series
usually can not occur with the index data outputted
from the LD-CELP speech coder 211. The frame
forefront bit is fixed as a voice bit indicative of
the voice frame like the bit V0 in Fig. 4.
Referring to Fig. 6, labeled "FAX Modulate
Data" is demodulated data from the FAX signal
demodulator 223. The frame of this data is free
from the voice/no-voice bit.
In the CLAD unit 300, the input signal S5 from
the high efficient speech codec unit 200 is
simultaneously inputted to the voice/no-voice bit
detector 311, FAX start data detector 312 and cell
assembler 313. While the FAX start data detector
312 does not detect the signal S2, the cell
assembler 313 operates according to the result of
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check in the voice/no-voice bit detector 311. The
cell assembler 313 is normally storing the signal S5
by MSB first storing in a 320-bit buffer, and
determines whether it is necessary to form cells
upon reception of a voice/no-voice bit. When the
received voice/no-voice bit indicates no-voice, the
320-bit data that has been stored is discarded, and
no cell is transmitted to the ATM transmission line.
Upon appearance of a voice/no-voice bit indicative
of voice, the forefront bit of the 320-bit data
having been stored is made to be a voice bit, and
then the 40-bit header is added. The resultant data
is transmitted as a cell through the output terminal
103 to the ATM transmission line.
Upon detection of the signal S2 by the FAX
start data detector 312, the cell assembler 313
produces cell data from all the received data by
adding a 40-bit header for every 320 bits
irrespective of the check result in the
voice/no-voice detector 311, the cells thus produced
being transmitted through the output terminal 103 to
the ATM transmission line. The operation of the
cell assembler 313 subsequent to the detection of
the signal S2, is continued until arrival of a call
end signal from the switchboard.
Cell data which is supplied from the ATM
transmission line to the input terminal 104, is
inputted to the voice cell data receiver 321. The
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voice cell data receiver 321 deletes the 40-bit
h~A~er data having been added on the transmitting
side and transmits sole necessary 320-bit data as
serial data to the high efficient speech codec unit
200.
Without input of cell from the ATM transmission
line, a no-voice bit is added as voice/no-voice bit
to the forefront of the 320-bit data for
transmission of the resultant data as serial data.
At this time, the 319 bits among the 320 bits and
excluding the voice/no-voice bit are ineffective
data, and it is possible to make all the 319 bits
"0", for instance. The processes of the 40-bit
header addition and voice/no-voice bit change that
are involved in the cell assembling and
disassembling in the CLAD unit 300, substantially
has no effect on the propagation delay because of
the facts that the cell configuration in the ATM
transmission format is of 424 bits inclusive of the
header and that the data transmission on the ATM
transmission line is at a very high rate, i.e.,
155.52 Mbits/sec.
On the receiving side of the high efficient
speech codec unit 200, the signals S6 and S7 from
the CLAD unit 300 are inputted to the LD-CELP speech
decoder 221 which decodes the signal S7 to reproduce
the speech signal, to the no-voice compensation
noise generator 222 which checks whether the input
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signal from the CLAD unit 300 is of the signal S6
format and, in case of the signal S6 format, outputs
the control signal S12 for controlling the switching
of the output signal to the switchboard and, at the
same time, the input signal S7 from the CLAD unit
300, and to the FAX signal decoder 223 which always
checks the input signal S7 from the CLAD unit 300
and, upon reception of the FAX start data S2,
outputs the control signal S13 for controlling the
switching of the output signal to the switchboard
while subsequently modulating the signal S3 to
output the modulation as the FAX signal.
The selector 224 which transmits the output
through the output terminal 102 to the switchboard,
operates under control of the signals S12 and S13,
and outputs either one of the outputs of the LD-CELP
speech decoder 221, no-voice compensation noise
generator 222 and FAX signal decoder 223 to the
switchboard. The signals S6 and S7 are shown in
Figs. 7 and 8, respectively.
Referring to Fig. 7, labeled V1 is the
forefront bit of the frame (i.e., MSB of index) and
indicates that the prevailing frame is an no-voice
frame. This bit V1 starts the no-voice compensation
noise generator 222. Labeled "UV Data" is 319-bit
data other than the bit V1. This data is
ineffective data, so that all the bits other than
the bit V1 may be "0", for instance.
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Shown in Fig. 8 is the format of the signal S7
when the index cell data from the LD-CELP speech
coder 211 is received. Labeled V0 is the forefront
bit of the frame (i.e., MSB of index). This bit
indicates that the prevailing frame is of voice
data, and it starts the LD-CELP speech decoder 221.
As has been described in the foregoing,
according to the present invention input/output
coded data of high efficient speech codec unit and
voice/no-voice data can be multiplexed together by
an ATM speech coding system. It is also possible to
separate the high efficient speech codec unit and
CLAD unit from each other by the provision on the
CLAD unit side of voice/no-voice bit extraction
means and FAX start data detection means. With this
structure, it is possible in a network, which is
provided by inter-connecting an existing CBR network
and an ATM network, to introduce CLAD as a
constituent element of the present invention to a
point of connection to the ATM network without
changing the transmission rate of the existing CBR
network by replacing a high efficient speech codec
in the CBR network with the high efficient speech
codec as a constituent element of the ATM speech
coding system according to the present invention.
This eliminates the need for repeated decoding and
coding of data in the prior art inter-connection
system and permits minimizing the speech quality
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deterioration.
A further effect obtainable according to the
present invention is that the signal propagation
delay that is generated due to data storage for cell
assembling and disassembling in the CLAD and also to
high efficient speech codec and decoding, can be
minimized with an arrangement that the output of the
high efficient speech codec to the CLAD is
multiplexed on the last of cell data and conversely
the output of the CLAD to the high efficient speech
codec is multiplexed on the forefront of cell data.
Changes in construction will occur to those
skilled in the art and various apparently different
modifications and embodiments may be made without
departing from the scope of the present invention.
The matter set forth in the foregoing description
and accompanying drawings is offered by way of
illustration only. It is therefore intended that
the foregoing description be regarded as
illustrative rather than limiting.
24
