Moden signal detecting system for adaptive differential PCM CODEC

A modem signal detecting system for an adaptive differential code modulation system, which is intended for detecting a modem training signal, comprises a comparison and decision circuit which receives an input signal from a signal power calculator and compares a signal power level during a silent period and a training period of a modem training signal. The comparison and decision circuit then decides whether or not the transmission quality of the transmission line is suitable for the modem communication.

BACKGROUND OF THE INVENTION 
The present invention relates to a modem signal detecting system for an 
adaptive differential PCM CODEC providing a coding characteristic suitable 
for modem signals by detecting the start and the end of modem 
communication. 
An adaptive differential pulse code modulation coding system (herein-after 
referred to as ADPCM coding system) is intended to effectively transmit 
voice signals with lower transmission rate (bandwidth) as compared with 
the conventional PCM system by utilizing the statistic characteristic of 
the power spectrum of voice signals shown in FIG. 1. 
In application of such ADPCM coding system in an existing communication 
network, it is desirable to provide data transmission of a voice band 
modem signal in addition to the voice signal, but the power spectrum of 
the voice band modem signal shows less fluctuation, as shown in FIG. 2, 
apparently indicating different statistic characteristic, while the signal 
power of the voice signal shows fluctuation of about 40 dB. 
For the ADPCM coding system, the CCITT recommends (G721) a coding algorithm 
for realizing the voice signal transmission with the bit rate of 32 
kbits/sec by reducing the bandwidth of the voice signal to 1/2. This 
coding algorithm utilizes, as explained previously, the statistic 
characteristic of the power spectrum of the voice signal shown in FIG. 1. 
Accordingly, such algorithm is not suitable for coding the modem signal 
having the power spectrum as shown in FIG. 2 and it only compensates for 
the coding of the modem signal with a maximum bit rate of 4,800 bit/sec. 
Therefore, the modem signal of 9,600 bit/sec which is most widely used 
under the regulation GIII for the facsimile communication cannot be 
transmitted with this coding algorithm. 
To avoid the above disadvantage, a system has been proposed in U.S. Pat. 
No. 4,788,692, the disclosure of which is incorporated herein by 
reference, which system switches the coding algorithm to the algorithm for 
the modem communication only when the modem communication is decided by 
detecting a modem training signal. The modem training signal is detected 
in this system by monitoring the initial modem training signal of 9600 
bit/sec. Namely, when an input signal drops to a value exceeding the 
decided level, the end of the modem communication is detected. Such modem 
communication should be always detected even in case a transmission line 
noise level is comparatively high. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to ensure detection of 
the moden training signal without the influence of network noise in order 
to detect the start of the modem communication. 
It is another object of the present invention to reliably and quickly 
switch the modem coding mode to the voice coding mode without the 
influence of network noise by detecting the end of the modem 
communication. 
These and other objects of the invention are attained by a modem signal 
detector for detecting a modem training signal for use in an adaptive 
differential coding system, the detector comprising: means for detecting 
and deciding a first signal pattern of a modem training signal; means for 
detecting a second signal pattern inverting a phase of the first signal 
after detection of the first signal pattern of the modem training signal; 
timer means for counting a time passage of a predetermined time period 
after detecting and deciding said first signal pattern; and reset means 
for resetting said detection and decision means of said first signal 
pattern when said second signal pattern is not detected within the 
predetermined time period counted by said timer means after detection and 
decision of said first signal pattern.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The start and the end of the modem communication are generally conducted as 
follows. The start of the modem communication is detected by monitoring a 
modem training signal, while the termination of the modem communication is 
detected by detecting a drop of the receiving signal in the modem 
communication, exceeding the decision level. 
In the case of the modem communication signal of 9600 bit/sec (V29 modem 
communication) recommended by CCITT, data is transmitted by a 16-level QAM 
modulation with the carrier frequency of 1700 Hz and the training signal 
is transmitted for a constant period of time for initiation of the 
internal condition of the modem system prior to the transmission of data. 
This training signal is formed, as shown in FIG. 3, by four kinds of 
signals in time periods identified here as segments 1-4. The segment 1 
designates the silent period of 20 ms; the segment 2 designates an A/B 
pattern period of 53 ms; the segment 3 designates a C/D pattern period of 
160 ms and segment 4 designates a random pattern period of 20 ms. 
The segments 2 and 3 are modulation periods of A/B, C/D patterns, 
respectively, and include the sine wave signal components of 500 Hz, 1700 
Hz and 2900 Hz and the signal phase changes of 180 degrees (reverses) at 
the boundary of segments 2 and 3. 
Therefore, after the silent period of 20 ms, when an input signal level is, 
for example, -35 dBmO or higher, output signal levels of band rejection 
filters of 500 Hz, 1700 Hz and 2900 Hz are lower than the predetermined 
level and the period, during which an output signal level of a band-pass 
filter of 2900 Hz indicates 1/3 of all signal levels, continues for 25 to 
48 ms or longer, preparation or switching from the voice signal coding 
mode to the modem signal coding mode is started. Thereafter, an output 
signal of the phase lock loop circuit (PLL) synchronized with the carrier 
frequency of 1700 Hz of the modem communication is monitored. At the 
boundary of segments 2 and 3, since the phase of the sine wave signal is 
inverted by 180 degrees, it is detected by PLL and the start of the modem 
communication is decided. Thereby, the voice signal coding mode is 
switched to the modem signal coding mode and the modem communication is 
initiated. 
After the start of the modem communication, when the receiving signal 
becomes, for example, -35 dBmO or less, the end of the modem communication 
is detected. 
FIG. 4 is a block diagram of an embodiment of a modem training signal 
detector, where numeral 10 denotes an analog to digital converter A/D and 
reference numerals 11 and 13 denote signal power calculators. The modem 
signal detector further includes a band reflection filter 15 connected to 
the power calculator 12, bandpass filters 16, 17 and a comparison and 
decision circuit 18 receiving signals from power calculators 11, 12, 13 
and bandpass filter 17. As explained previously, in the 9600 bit/sec modem 
system, the sine wave signals of three kinds of frequencies (500 Hz, 1700 
Hz, 2900 Hz) within the voice frequency band are used as training signals. 
The band reflection filter 15 has a structure adapted to reflect the 
frequencies of 500 Hz, 1700 Hz, and 2900 Hz. Bandpass filter 16 has a 
structure adapted to transmit the frequency of 2900 Hz and a bandpass 
filter 17 has a structure adapted to transmit the frequency of 1700 Hz. 
The power of receiving signal S converted to a digital signal by the A/D 
converter 10 is calculated in the signal power calculator 11. The power of 
the frequency components other than the frequencies 500 Hz, 1700 Hz and 
2900 Hz reflected by the band-reflection filter 15 is also calculated by 
the signal power calculator 12. Moreover, the power of signals transmitted 
by the bandpass filter 16 is calculated by the signal power calculator 13. 
A signal power calculator can also be connected between bandpass filter 17 
and circuit 18. The start and the end of the decision circuit 18 using 
output signals S.sub.0, R.sub.0, W.sub.2 of the signal power calculators 
11 to 13 and the output signal Y of the bandpass filter 17. 
The output signals of the signal power calculators 11 to 13 and the output 
signal of bandpass filter 17 are referred to hereinbelow as the receiving 
output signal S.sub.0, band reflection output signal R.sub.o, 2900 Hz 
receiving output signal W.sub.2 and 1700 Hz receiving output signal Y. 
Conditions for detecting the start of the modem communication are as 
follows: 
1. The level of the receiving output signal S.sub.0 &gt;the level during the 
silent period. 
2. The level of the band-reflection output signal R.sub.0 &gt;constant level. 
3. The period during which the level of 2900 Hz receiving output signal 
W.sub.2 takes about 1/3 of the level of receiving output signal S.sub.0, 
continues for a constant period of time or longer. 
When all three above conditions are satisfied, the preparation for 
switching to the modem signal coding mode is started. 
Thereafter, when the phase inversion of sine wave signals between the 
segments 2 and 3 of modem the training signal shown in FIG. 3 is detected, 
the start of the modem communication is decided and the voice signal 
coding mode is switched to the modem signal coding mode. 
Whether the above listed conditions are satisfied or not is decided in the 
comparison and decision circuit 18 shown in FIG. 4. 
FIG. 5 illustrates a block diagram of the comparison and decision circuit 
18. This circuit comprises comparators 21, 22, 24, 25, 26, a delay circuit 
including delay elements 29, a flip-flop circuit 36 (FF), an A/B pattern 
detector 37, a phase lock loop circuit 38 (PLL), a polarity detector 39, a 
training signal detector 40, and a modem end detector 42. 
The A/B pattern detector 37, training signal detector 40 and modem end 
detector 42 form a signal processor 50. The receiving output signal 
S.sub.o is applied to the flip-flop circuit 36 via a delay circuit 
consisting of a plurality of series-connected delay elements 29. The 
flip-flop circuit 36 is cleared when the modem communication end detection 
signal MED is applied to a clear terminal CL. The flip-flop circuit 36 
stores the receiving output signal S.sub.0 when the modem communication 
start detection signal MSD is applied to a clock terminal CK thereof. 
Therefore, a delay time of the delay circuit is set so that the level of 
the receiving output signal S.sub.0 in the silent section of the preceding 
segment 1 is stored when the start of the modem communication is detected 
by phase reverse of the training signal between the segments 2 and 3. As 
will be understood from FIG. 3, it is recommended that this delay time is 
set to 53 ms or longer and 73 ms (53+20 ms) or less. 
The set output signal level (level in the silent section of segment 1) of 
the flip-flop circuit 36 is considered as a threshold level for detection 
of the end of modem communication and it is then compared with the 
receiving output signal S.sub.0 in the comparator 21. Moreover, the 
receiving output signal S.sub.0 is compared, in the comparator 22, with a 
constant level, for example, -35 dBmO corresponding to the silent period. 
The output signals of the comparators 21 and 22 are input to the modem end 
detector 42. 
The receiving output signal S.sub.0 is input to the comparators 25, 26. In 
the comparator 25, the receiving output signal S.sub.0 is compared with a 
constant level, -33 dBmO which is higher than that corresponding to the 
silent period and a comparison output signal X.sub.1 is then input to A/B 
pattern detector 37. Namely, the signal for deciding the condition 1 
mentioned above is generated. In addition, the band-reflection output 
signal R.sub.0 is compared, in the comparator 24, with a constant level 
(0.09) and a comparison output signal X.sub.2 is input to the A/B pattern 
detector 37. Namely, the signal for deciding the condition 2 explained 
above is generated. 
The 2900 Hz receiving output signal W.sub.2 is compared, in comparator 26, 
with a signal level obtained by multiplying the receiving output signal 
S.sub.0 by 0.28 and a comparison output signal X.sub.3 is input to the A/B 
pattern detector 37. Namely, the signal for deciding the condition 3 is 
generated. 
Here, the values, -35 dBmO, -33 dBmO, 0.09 and 0.28 are set based on the 
following: 
When the carrier signal transmitted is turned OFF in the modem, a signal 
level in the transmission line is indicated by dBmO. It is generally known 
that the modem is influenced by noise when the carrier signal transmitted 
is turned OFF and the line level is not zero and becomes about -45 dBmO. 
However, in a FAX system, since the modem signal is transmitted or 
received using an ordinary telephone line, noise level becomes rather high 
when the line condition is bad. According to the experiments conducted by 
the inventors, it is apparent that when the telephone line is used, noise 
level is about -35 dBmO. However, this value -35 dBmO is difficult to 
determine, principally due to the condition of the line used. Therefore, 
the level in the silent period to be applied to the condition 1 is set to 
-33 dBmO, by adding a decided margin (2 dB). 
Moreover, the level of the band-reflection output R.sub.0 must be "0" 
during the modem communication. Howcer, the band-reflection filter 15 
cannot perfectly reject the in-band signal and allows such in-band signal 
to pass therethrough to a certain degree. The level of the in-band signal 
passing the band-reflection filter during the modem communication is set 
to 0.09 and it is applied to the condition 2, explained previously. 
In addition, the bandpass filter 16 cannot transmit the sine wave signal of 
2900 Hz without any attenuation, which results in a small amount of the 
signal being attenuated. Therefore, the receiving output signal S.sub.o 
must certainly be multiplied by 1/3(0.33) but such value is set to 0.28, 
considering a loss generated when the sine wave signal of 2900 Hz passes 
through the bandpass filter 16. 
These values are all based on the results of experiments conducted by the 
inventors of the present invention. 
As explained above, the A/B pattern detector 37 decides to receive the A/B 
pattern when the receiving output signal S.sub.0 has the level higher than 
-33 dBmO (condition 1), the level of the band-reflection output signal 
R.sub.o is lower than 0.09 (condition 2) and the level of the receiving 
output signal W.sub.2 of 2900 Hz is higher than the receiving output 
signal S.sub.o multiplied by 0.28. 
The deciding and detecting signal transmitted from the A/B pattern detector 
37 is applied to the training signal detector 40. 
Upon application of the modem start detecting signal MSD, the modem end 
detector 42 starts the detecting operation and detects the end of the 
modem communication with a comparison output signal from comparators 21, 
22. Detector 42 detects the end of the modem communication with the 
comparison output signal of comparator 21 when the level of the receiving 
output signal S.sub.o becomes lower than the noise level during the silent 
period of segment 1 and the comparison output signal from the comparator 
22 when the level of receiving output signal S.sub.o becomes lower than 
-35 dBmO indicating the silent period and outputs the modem end detecting 
signal MED, as shown in FIG. 5. With this modem end detecting signal MED, 
the modem signal coding mode is returned to the voice signal coding mode. 
Now, operations of A/B pattern detector 37 and training signal detector 40 
will be explained in greater detail with reference to a flow chart diagram 
of FIG. 6. 
The A/B pattern detector 37 monitors an output signal X.sub.1 of comparator 
25 (step S1). When the output signal X.sub.1 indicates that the receiving 
output signal S.sub.o is &lt;-33 dBmO, the detection of the silent period 
(segment 1) is decided (step S2). Thereafter (step S3), when it is 
detected that the output signal X.sub.1 of comparator 25 indicates that 
the level of receiving output signal S.sub.o is &gt;the level of the silent 
period (-33 dBmO) and the output signal X.sub.2 of comparator 24 indicates 
the level of reflecting output signal R.sub.o &lt;than the predetermined 
level (0.09), the operation moves to the step S4. 
It is checked whether the detected output of the PLL circuit 38 makes a 
zero-cross (polarity decision) or not due to the phase change from A/B 
pattern to C/D pattern by monitoring an output of the polarity detector 
39. 
If zero-cross is not executed, it is checked whether the A/B pattern 
detecting conditions 1, 2 are detected or not in the step S3 with the 
output signal X.sub.2 of comparator 24 and output signal X.sub.1 of 
comparator 25 (step S6). In case conditions 1 and 3 are detected, 
detection of the A/B pattern is decided and a counter is set (step S7). 
Establishment of the A/B pattern is decided (steps S8, S9) when the 
condition that the level of receiving output signal W.sub.2 of 2900 Hz is 
higher than the receiving output signal S.sub.o multiplied by 0.28 and 
continued for 40 msec. This is detected by the output X.sub.3 of 
comparator 26 and counted. When the A/B pattern is established, "1" is set 
to the A/B pattern detection set flag SET (step S10) and operation shifts 
to the next step S18. In case the A/B pattern is not detected or not 
established, the operation also shifts to step S18. 
In step S18, the condition of the out flag OUT, as will be explained later, 
is checked. Since the out flag OUT is "0" when the zero-cross is not 
detected in step S4, the modem communication start detecting signal MSD is 
set to "0", the operation returns to the start of detecting a sequence 
flow and the same steps are repeated. 
In case the polarity detector 39 detects a zero-cross in the output of PLL 
circuit 38 after the next repeated cycles in step S4, a change from the 
A/B pattern to the C/D pattern is decided and the operation shifts to step 
S14. Condition of the out flag OUT is checked in step S15. When the out 
flag is "0", the condition of set flag SET is checked (step S16). In case 
the set flag SET is "1" (namely, A/B pattern has been already detected), 
"1" is set to the out flag OUT and the flag SET is reset to "0" (step 
S17). Thereafter, operation shifts to step S18. In this case, since the 
out flag OUT is "1" in step S18, the modem communication start detecting 
signal MSD is set to "1". 
The modem training signal can be detected by repeating such processing 
sequence. 
Here, after the set flag SET indicating establishment of the A/B pattern is 
set from "0" to "1", zero-cross of the output of the PLL circuit 38 
indicating detection of the C/D pattern is detected. Therefore, it is 
essential to accurately switch the mode to the modem mode by detecting the 
change from the A/B pattern to the C/D pattern. 
For this purpose, the A/B pattern detector 37 detects the A/B pattern to 
count the time period from the detection time for the decided time with 
another timer means. If the C/D pattern is not detected by the polarity 
detector means 39 before the decided time is counted by the timer 
(zero-cross is not generated), it is estimated that the input of the C/D 
pattern is ceased for some reasons. Therefore, the result of the A/B 
pattern detection by the A/B pattern detector 37 is reset and detection of 
the A/B pattern is repeated. 
Detailed explanation will now be made with reference to FIG. 6. The flow 
chart of FIG. 6 also shows step S11 in which the decided time (20 msec) is 
counted by a timer after setting the set flag SET to "1", step S12 in 
which a time period or passage of 20 msec is decided and step S13 for 
resetting the set flag SET to "0" after 20 msec has passed. 
After the set flag SET has been changed from "0" to "1" after establishment 
of the A/B pattern, a zero-cross is to be detected, thereby indicating 
that a C/D pattern has been detected. However, in step S10, the time until 
detection of the C/D pattern from the time "1" is set to set flag SET, is 
counted. If zero-cross does not appear within the decided time (20 msec), 
the set flag SET is reset to "0" and the A/B pattern is queued. 
In this case, since the pattern duration for establishment of the A/B 
pattern is 40 msec, as explained above, and the zero-cross queuing time 
after establishment of the A/B pattern is 20 msec, the total time is 60 
msec. Meanwhile, since the A/B pattern duration is 53 msec as shown in 
FIG. 3, if zero-cross is not detected even after the zero-cross queuing 
time of 20 msec, it is decided that zero-cross will no longer be detected. 
Thereby, even if the C/D pattern is not input after the A/B pattern, it can 
be prevented that the apparatus intermittently suspends operation waiting 
for the C/D pattern. 
Another embodiment for detection of A/B pattern will be explained 
hereunder. 
FIG. 7A indicates a signal level at the leading portion of the modem 
training signal to be input to the detector. When the voice signal is 
switched to the modem signal, the silent period (segment 1) of the modem 
training signal appears and then the A/B pattern (segment 2) appears. 
However, the signal level does not become zero since transmission line 
noise appears during the silent period. 
Therefore, here rises a problem that in case a line noise which is similar 
to the A/B pattern of the modem training signal is input, a line noise may 
be erroneously detected in place of the A/B pattern, before the actual A/B 
pattern appears, resulting in influence on the subsequent processings. 
The signal component of the modem training signal during the silent period 
is almost equal to the line noise component. Therefore, a mean value of 
the line noise component can be represented by a mean value of the signal 
level during the silent period. When such a mean value of the transmission 
line noise component is subtracted from the input signal, the line noise 
component in the silent period can be almost eliminated. Thereby, 
erroneous detection of the modem signal pattern resulting from line noise 
during the silent period can be prevented by detecting the modem training 
signal detection in the A/B pattern detector 37 with the input signal 
after eliminating a line noise component (FIG. 7B). 
FIG. 8 is a block diagram of this embodiment and FIG. 9 is a flow chart of 
operation steps thereof. The operation sequence of the A/B pattern 
detector 37 and training signal detector 40 in FIG. 8 will be explained 
below. 
First, a mean value of the total voltage of input signals is obtained from 
the receiving signal output S.sub.o in the A/B pattern detector 37 (step 
S10). A threshold value for identifying the silent period is decided based 
on the mean value of the voltage of signal S.sub.o applied to the 
comparator 25 to detect the silent period (segment 1) (step S20). Next, 
operation shifts to step S3 similarly to step S3 of FIG. 6. 
Switching from the A/B pattern to the C/D pattern is made when the detected 
output of the PLL circuit 38 detects zero-cross with the output from the 
polarity reverse detector 39 due to a phase reverse from the A/B pattern 
to the C/D pattern (step 40). 
When zero-cross is not detected in the step (step S4) for detecting 
zero-cross, a line noise component is eliminated (step S50) and thereafter 
operation shifts to the A/B pattern detection process (steps S6 and S10). 
Namely, when the silent period of the modem training signal is detected, a 
mean value of the signal level in the silent period is calculated from the 
receiving signal output S.sub.o in the A/B pattern detector 37. In step 
S50, a mean value S1 of the silent period signal (corresponding to a mean 
value of a line noise component) is subtracted from the receiving signal 
output S.sub.o in a conventional subtracter. Therefore, as shown in FIG. 
7B, the input signal does not include a line noise component. 
Here, the A/B pattern has the level three times as high as that for the 
line noise signal component and therefore the A/B pattern detection can be 
reliably obtained even when the line noise component is subtracted. 
On the other hand, since the line noise signal component in the silent 
period becomes very low because a mean value thereof is subtracted from 
the receiving signal, in case a line noise component, which is similar to 
the A/B pattern, is input, it does not satisfy the signal level detecting 
condition for the A/B pattern after step S6 and therefore erroneous 
detection never takes place. 
The processing for subtracting the line noise component is not carried out 
for the signal which is actually transmitted to the modem system. Other 
sequence steps in FIG. 9 are similar to those of FIG. 6. 
FIG. 10 is a block diagram of an embodiment of the system for detecting the 
start of the modem communication (detecting training signal). 
In this embodiment, communication quantity (SN) of transmission path is 
monitored at the time of detecting the start of the modem communication. 
Only when SN is higher than a predetermined level, the start of the modem 
communication is decided and the modem communication start detecting 
signal MSD is output. When SN is lower than a predetermined level, SN is 
insufficient and communication is impossible, the modem start detecting 
signal MSD is reset, the modem communication end signal is transmitted to 
the end detector 42 and the operational sequence is returned to the A/B 
pattern detecting process by the modem training signal. 
Usually, when considering modem data transmission, a modem data error rate 
should not exceed 10.sup.-5. The theoretical value of line quality (SN) 
required for acquiring such service quality is 21 dB or less. Therefore, 
the SN of the training signal is monitored for the start of the modem 
communication. When SN becomes 21 dB or less, the modem communication 
start detecting signal MSD is output as a non-detecting condition even 
when the modem signal is received. Actually, the threshold value of SN is 
set to 15 dB, adding a margin of 6 dB to the theoretical SN. A signal 
power level for calculating SN is calculated from the signals of segment 1 
and segment 2. 
Explanation will be now made referring to FIG. 10. When the training signal 
detector 40 detects switching from the A/B pattern to C/D pattern with an 
output from the polarity detector 39, it outputs the modem communication 
start detecting signal MSD. Thereby, a level comparison result (output of 
comparator 52) between the signal receiving output S.sub.o and output of 
delay circuit 29 is set to the flip-flop circuit 53 (FF). 
The delay circuit 29 adjusts a delay time so that the level of the signal 
in segment 1 (SEG1) which is a level in the silent period, corresponding 
to noise level, is output when the training signal detector 40 detects 
switching from the A/B pattern to the C/D pattern. Moreover, a multiplier 
51 is provided in this embodiment, which multiplies the value of the 
current receiving signal output S.sub.o by 0.178. Namely, as explained 
above, the output S (noise level) of delay circuit 29 and receiving signal 
output S.sub.o should be in the following relation: 
EQU S.sub.o /S&gt;15 dB 
Therefore, it is detected whether the SN of the training signal is larger 
or smaller than 15 dB by comparing S.sub.o /15dB and S in the comparator 
52 and then the result is applied to the flip-flop circuit 53. 
Accordingly, the receiving signal output S.sub.o is multiplied by 1/15 
dB=0.178 in the multiplier 51. 
The modem communication end detector 42 decides whether the SN of the 
training signal is larger than 15 dB or not with the output of the 
flip-flop circuit 53. When it is larger than 15 dB, it is decided that the 
modem communication is possible, the modem communication start detecting 
signal MSD of training signal detector 40 is validated and the voice 
coding mode is switched to the modem coding mode. 
When it is detected that SN is smaller than 15 dB, the line quality is 
non-satisfactory, it is decided that the modem communication is disabled, 
the modem communication start detecting signal MSD is invalidated and the 
modem communication end detecting signal MED is output for resetting the 
flip-flop circuit 53 (FF). Therefore, the mode switching is not carried 
out, and operational sequence returns to the detection of the silent 
period of the modem training signal and detection of the A/B pattern. 
For detection of a modem signal, the SN of receiving signal output S.sub.o 
is monitored to decide whether or not it satisfies the value required for 
the modem communication. Only when the SN is satisfactory, the modem 
detection result is considered to be significant. Therefore, if the line 
quality is not satisfactory, the end of the modem communication can be 
executed immediately. Thus, when the modem communication is disabled, the 
modem coding mode can be switched back to the voice coding mode. 
Now, the detection of the end of the modem communication will be explained 
in detail. 
In the embodiments of FIG. 5 and FIG. 8, the end of the modem communication 
is detected by the modem end detector 42 when the receiving signal output 
S.sub.o drops to a value exceeding a predetermined level such as -35 dBmO 
by the comparator 22. 
However, when the line quality is non-satisfactory and a noise level is 
high, the receiving level becomes sometimes higher than -35 dBmO. In this 
case, the end of the modem communication cannot be detected and therefore 
the possibility exists that the modem signal coding mode will be switched 
back to the voice signal coding mode. When the signal is transmitted using 
the 32 Kbits/sec line, it cannot be transmitted if the end of the modem 
communication cannot be detected. Thereby, a fault disabling disconnection 
of the line may be generated. 
For this reason, in the embodiment of FIG. 10, the end of the modem 
communication is reliably detected even in case the noise level is high. 
Namely, a threshold level is set by adding a margin of a constant rate to 
the receiving signal level in the silent period of segment 1 of the modem 
training signal; this threshold level is compared with the level of the 
receiving output signal S.sub.o after the start of the modem 
communication, thereby the end of the modem communication is decided when 
the level of receiving output signal S.sub.o becomes lower than the 
threshold level. 
FIG. 11 is a block diagram of yet another embodiment of the comparison and 
decision circuit 18, which includes comparators 21 through 28, delay 
circuit elements 29 and 30, multipliers 31 through 35, a flip-flop circuit 
36 (FF); an A/B pattern detector 37, a phase lock loop (PLL) circuit 38; a 
polarity detector 39; a training signal detector 40; an OR circuit 41 and 
a modem end detector 42. 
The receiving output signal S.sub.o is applied to the flip-flop circuit 36 
via the delay circuit consisting of a plurality of delay elements 29. The 
flip-flop circuit 36 is cleared when the modem communication end detecting 
signal MED is applied to the clear terminal CL and stores the receiving 
output signal S.sub.o when the modem communication start detecting signal 
MSD is applied to the clock terminal CK. Therefore, a delay time of the 
delay circuit is set so that when the detection of the start of the modem 
communication is conducted through a phase inversion between segments 2 
and 3 of the modem training signal, the level of the receiving output 
signal S.sub.o in the silent period of segment 1 is stored. 
The threshold level for the detection of the end of the modem communication 
is obtained by multiplying 2.0 (margin for end detection) by the value of 
the output signal of the flip-flop circuit 36 in the multiplier 31 and 
this threshold level is compared with the receiving output signal S.sub.o 
in the comparator 21. A constant level corresponding to the silent period, 
for example, -35 dBmO is compared with the level of receiving output 
signal S.sub.o in the comparator 22. The comparison output signals of 
comparators 21, 22 are input to the end detector 42. 
The receiving output signal S.sub.o is input to the comparator 26 via 
multiplier 33 and to comparator 25. In the comparator 25 it is compared 
with a constant level, -33 dBmO higher than the level corresponding to the 
silent period. The comparison output signal is input to the A/B pattern 
detector 37. The receiving output signal S.sub.o is multiplied by 0.28 in 
the multiplier 33, the obtained level is compared with the receiving 
output signal W.sub.2 of 2900 Hz and the comparison output signal is input 
to the A/B pattern detector 37. 
The band reflection output signal R.sub.o is compared with 0.09 in the 
comparator 24 and the comparison output signal is input to the A/B pattern 
detector 37. The receiving output signal W.sub.2 of 2900 Hz is multiplied 
by 2.sup.-3, or 0.125, the multiplied output signal is compared with the 
band reflecting output signal R.sub.o in the comparator 23 and the 
comparison output signal is input to the A/B pattern detector 37. 
Accordingly, the A/B pattern detector 37 has a logical structure adapted to 
decide reception of the A/B pattern when: the receiving output signal 
S.sub.o has a level higher than -33 dBmO; the level of band reflection 
output signal R.sub.o is lower than 0.09; 0.125 times (1/8) of the level 
of the receiving signal W.sub.2 of 2900 Hz is higher than the level of the 
band reflection output signal R.sub.o and the level of receiving output 
signal W.sub.2 of 2900 Hz is higher than 0.28 times of the level of 
receiving output signal S.sub.o, and then the decided and detected signal 
is applied to the training signal detector 40. 
The phase lock loop (PLL) circuit 38 is constructed for locking the phase 
to 1700 Hz signal Y. After the phase locking operation has been carried 
out in the segment 2 of the modem training signal, the phase of 1700 Hz 
signal Y is inverted in the segment 3 of the C/D pattern and thereby 
polarity of the phase control voltage of PLL circuit 38 is reversed. It is 
detected in the polarity detector 39 and a polarity reverse detection 
signal is applied to the training signal detector 40. Therefore, the 
training signal detector 40 outputs the modem communication start 
detection signal because the polarity reverse detection is carried out 
after detection of the A/B pattern. This start detection signal is applied 
to the end detector 42. 
The receiving output signal Y.sub.o of 1700 Hz is input to the comparators 
27, 28 via the delay circuit elements 29 and is compared with the 
receiving output signal Y.sub.o of 1700 Hz multiplied by the constants a, 
b in the multipliers 34, 35. The constants a, b are set in correspondence 
with the receiving level of the training signal of 1700 Hz. When the delay 
time of the delay circuit is set to .DELTA., such constants are selected 
to satisfy the relation, a Y.sub.o (n)&lt;Y.sub.o (n-.DELTA.)&lt;Y.sub.o (n). 
After comparison in the comparator 27, 28, the comparison output signal is 
applied to the modem end detector 42 via the OR circuit 41. 
According to experiments conducted by the inventors of the present 
invention, level fluctuation of the carrier signal (1700 Hz signal) during 
the modem communication is very low and fluctuation range is about +3 dB. 
Therefore, values of a and b are set to satisfy the following relations: 
EQU a Y.sub.o (n)=Y.sub.o (n)-3 dB, b Y.sub.o (n)=Y.sub.o (n)+3 dB. 
Namely, even when the level of receiving output signal Y.sub.o of 1700 Hz 
is increased or decreased by 3 dB or more, the carrier signal of 1700 Hz 
is decided to be not received (not during the modem communication). 
The modem end detector 42 has a logical structure to start the detecting 
operation when the start detecting signal is applied and to execute the 
modem end detection with the comparison output signal transmitted from 
comparators 27, 28 via the comparators 21, 22 and OR circuit 41. Detector 
42 also outputs the modem communication end detecting signal with the 
comparison output signal of the comparator 21 when the level of receiving 
output signal S.sub.o becomes lower than the receiving output signal 
S.sub.o indicating noise level in the silent period of segment 1, or with 
the comparison output of comparator 22 when the level of receiving output 
signal S.sub.o drops, exceeding -35 dBmO indicating the silent period, or 
with the comparison output of comparator 27 when the receiving output 
signal Y.sub.o of 1700 Hz multiplied by "a" becomes higher than the level 
of a preceding receiving output signal Y.sub.o of 1700 Hz (delayed signal 
Y.sub.o) or with the comparison output signal of comparator 28 when the 
receiving output signal Y.sub.o of 1700 Hz multiplied by value "b" becomes 
lower than the level of a preceding receiving output signal Y.sub.o of 
1700 Hz (delayed signal Y.sub.o). 
Moreover, the following method is also effective for detection of the end 
of the modem communication, in addition to the method of multiplying an 
output of the flip-flop circuit (FF) 36 by 2.0. When the A/B pattern is 
switched to the C/D pattern in the training signal, circuit 36 outputs the 
silent level of segment 1. Here, since a noise level in the modem 
communication is so far lower than the signal level, the difference of at 
least 10 dB or more exists between the level stored in the flip-flop 
circuit 36 and the level during the modem communication. The level 
(S.sub.o) when the modem communication ends is almost similar to the level 
stored in the flip-flop circuit 36 (level in the silent period) and the 
comparator 21 outputs a comparison output signal. When this condition is 
continued for 16 msec or longer, the end of the modem signal is decided. 
The duration of 16 msec is set, taking into consideration momentary 
intermission of the modem communication. When the signal level at the time 
of starting the modem communication becomes equal to that at the time of 
ending the modem communication, the end of the modem signal is detected. 
FIGS. 12A, 12B, 12C show modem signal waveforms for explaining operations 
of this embodiment. 
As shown in FIG. 12A, the silent period appears prior to rising of MSD 
where the voice signal is switched to the modem signal. Thereafter, data 
communication starts after the end of training. Upon termination of the 
modem signal communication, the signal level drops to the level which is 
almost the same as the silent period of the training signal. However, here 
a problem arises that when noise appears on the line as shown in FIG. 12B, 
even if the modem signal is detected by the training signal detector and 
MSD changes from "0" to "1", it does not become lower than the switch-back 
threshold from the modem coding mode to the voice signal coding mode at 
the time of ending the modem signal communication and thereby a modem 
signal cannot be detected. 
According to this embodiment, since the switchback threshold value of 
comparator 21 is set to the silent level of segment 1 of the modem 
training signal, the relation, where the level in the silent period is&lt;the 
level during communication level 2, can be set during the modem 
communication. Since the level 3 after the end of the modem communication 
becomes almost equal to the level in the silent period 1, the modem 
communication or the end of the modem communication can be detected 
accurately from an output of the comparator 21. 
FIG. 13 is a modification of the embodiment of FIG. 11. In FIG. 13, 
reference numeral 45 denotes a power calculating unit supplying a 
receiving signal Y.sub.o to a delay circuit 30. The circuit of FIG. 13 
further comprises a comparison and selection circuit 47 and a comparator 
48. The receiving output signal S.sub.o is applied to the delay circuit 29 
and comparator 48, and the delayed output signal is multiplied by a value 
larger than 1 as the margin k in the multiplier 46 and is then applied to 
the comparison and selection circuit 47. The comparison and selection 
circuit 47 compares the level of the output signal of the multiplier 46 
with the level -35 dBmO in the silent period and selects a larger level 
than the threshold level. This threshold level is compared with the 
receiving output signal S.sub.o in the comparator 48. 
Therefore, when the level of receiving output signal S.sub.o is lowered at 
the time when the modem communication ends, since it drops, exceeding a 
value obtained by the level of the receiving output signal S.sub.o 
multiplying the margin k or -35 dBmO, the end of the modem communication 
can be detected by the comparator 48. 
FIG. 14 is a flow chart showing operation steps of the end detector 42 
shown in FIG. 11 and FIG. 13. Power of segment 1, namely the level of 
receiving output signal S.sub.o in the silent period of segment 1 of the 
modem training signal is obtained (step S11). Here, the offset value is 
added by multiplying a constant value by such level in the multiplier 31 
(step S12). The resulting value is set as the end threshold (step S13) and 
it is compared with the signal power (step S14). Namely, the threshold 
level is obtained by adding a margin to the receiving output signal 
S.sub.o indicating the noise level in the silent period and it is compared 
with the level of receiving output signal S.sub.o. When the level of 
receiving output signal S.sub.o is lower than the threshold level, the end 
of the modem communication is decided. 
FIG. 15 is a diagram for explaining operation of an embodiment shown in 
FIG. 11. In FIG. 15, "a" denotes a modem signal including the training 
signal; "b" is the fixed threshold level (-35 dBmO); "c" is the line noise 
level; "d1" is a threshold level adding margin M to the line noise level. 
SEG 1 to SEG 4 are segments 1 to 4 of the modem training signal. 
Since the level of receiving output signal S.sub.o in the segment 1 of the 
modem training signal corresponds to the line noise level "c", the 
threshold level "d1" is set by adding the margin "M" to the receiving 
output signal S.sub.o. The modem communication is started by phase 
inversion between the segments 2 and 3 of the training signal. When the 
level of the modem signal "a" drops to a value exceeding the threshold 
level "d1" due to the end of the modem communication, the end of the modem 
communication is decided and the voice communication may be started. In 
this case, when the fixed threshold level "b" is set as the threshold 
level for detecting the end of the modem communication, since the line 
noise level "c" is higher than the fixed threshold level "b", the modem 
communication end cannot be detected. However, in this embodiment, the end 
of the modem communication can be reliably detected by setting the 
threshold level "d1" based on the line noise level "c". 
FIG. 16 is a diagram for explaining operations of the embodiment shown in 
FIG. 13. The like elements in FIGS. 15 and 16 are denoted by the like 
reference numerals. In addition, "d2" denotes a threshold level. This 
threshold level "d2" delays the segments 2, 3, 4 in the modem training 
signal or the modem signal "a" during the modem communication and sets a 
value of delayed output signal 1/k. In this case, "k" is set so that the 
threshold level "d2" during the reception of the modem signal "a" becomes 
higher than the line noise level "c" and lower than the level of modem 
signal "a". 
Therefore, the threshold level "d2" becomes "1/k" of line noise level "c" 
in the segment 1 (S1) of the modem training signal and becomes "1/k" of 
the level of the modem signal "a" after the delay time during the modem 
signal "a" after the segment 2 (S2) of the training signal. When the level 
of the modem signal "a" becomes lower than the threshold level d2, the end 
of the modem communication is detected and the modem communication mode is 
switched to the voice communication mode. 
It should be further indicated with reference to FIGS. 11 to 16 that the 
threshold level for detecting the end of the modem communication is set 
by: (a) adding the margin of a constant rate to the receiving level of the 
line in the first silent period (segment 1) of the training signal period; 
(b) setting a value of 1/k of the threshold value by delaying the modem 
signal "a"; (c) multiplying the receiving output signal Y.sub.o of 1700 Hz 
by "a" and "b". However, such a threshold value may also be set with other 
means. For example, since the A/B pattern or C/D pattern of the segments 2 
or 3 of the training signal may be identified easily, the threshold level 
may be set by multiplying a predetermined constant by any one of both 
signal levels. Moreover, the final detection result can be obtained by 
combining the results of the detection of the end of the modem 
communication based on various threshold levels explained above. 
FIG. 17 is a block diagram illustrating yet another embodiment of the 
circuit for detection of the end of the modem communication. In this 
embodiment, a signal power detector 49 detects power of the modem signals 
as a whole (S.sub.o) and a carrier signal power calculator 45 provides a 
power signal by extracting the carrier frequency component Y (the signal 
of 1700 Hz in the case of CCITT recommendation, V.29) which always exists 
in the modem signal. 
The comparator 50 compares the power of carrier frequency component from 
power calculator 45 with the power of the modem signal as a whole from 
detector 49 and transmits the result to the end detector 42. 
Moreover, comparator 22 compares the level of receiving signal output 
S.sub.o with the level during the silent period (-35 dBmO) and sends the 
result to the modem end detector 42. 
The modem end detector 42 decides that the modem communication is still 
continued when the level of receiving signal output S.sub.o is -35 dBmO or 
higher and the power of the carrier component (Y) is higher than a 
predetermined value (for example, 1/2) in comparison with the power of the 
receiving signal output S.sub.o. However, when the power of the carrier 
component Y is lower than the predetermined value in comparison with the 
power of signal output S.sub.o, the end detector 42 decides the end of the 
modem communication and outputs the modem communication end detecting 
signal MED. 
As explained above, the end of the modem communication is decided when the 
threshold level is set in the setting circuit based on the signal level 
received through the line, the receiving signal level after the start of 
the modem communication is compared with the threshold level in the 
comparison and decision circuit and the receiving signal level becomes 
lower than the threshold level. In the case of comparison based on the 
fixed threshold level, the end of the modem communication cannot be 
detected in some cases when the line noise level is high. However, in this 
embodiment, since the threshold level is set based on the receiving signal 
level received through the line including the line noise level, the end of 
the modem communication can be reliably detected. 
The receiving level of the line in the first silent period (segment 1) in 
the training signal corresponds to the line noise level, and the threshold 
level is obtained by adding the margin of a constant rate to the receiving 
level. Thereby, if the line noise level is high, the threshold level 
corresponding to such noise level may be set and therefore the end of the 
modem communication may be reliably detected. 
Moreover, the signal level of segment 2 or segment 3 transmitted before the 
start of the modem communication of the training period is usually 
stabilized and, since the threshold level corresponding to the receiving 
signal level may be set by obtaining the threshold level by multiplying 
such stabilized level by a predetermined constant value, the end of the 
modem communication can be reliably detected. 
Moreover, the level of the carrier signal (1700 Hz signal) during the modem 
communication is almost constant during the modem communication and the 
end of the modem communication can be detected reliably by setting the 
threshold level in accordance with the receiving level of the carrier 
signal. In this case, the condition for detecting the end of the modem 
communication may be set depending on whether the receiving level of the 
carrier signal is within the range of two kinds of high and low threshold 
levels. 
In addition, since the level of the modem signal after the start of the 
modem communication is almost constant, the end of the modem communication 
can be reliably detected by setting the threshold level based on such 
receiving modem signal and then by comparing the threshold level with the 
level of the receiving modem signal. 
There has been disclosed heretofore the best embodiment of the invention 
presently contemplated. However, it is to be understood that various 
changes and modifications may be made thereto without departing from the 
spirit of the invention.