Equalizing circuit and method for data communication terminal equipment

An equalizing circuit and method for data communication terminal equipment includes: a level detecting unit for detecting respective levels of a high frequency signal and a low frequency signal which are frequency shift keying signals inputted through a telephone line during a handshake and for outputting the detected respective levels; a control unit for comparing the levels of the high and low frequency signals detected by the level detecting unit and for generating a frequency compensation amplification control signal according to a level difference between the high and low frequency signals; and an equalizing unit for inputting a level detection signal outputted from the level detecting unit and for compensating any attenuation and distortion of received signals in response to the frequency compensation amplification control signal of the control unit, so as to effect equalization.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an equalizing circuit and method for 
communication terminal equipment, and more particularly, to an equalizing 
circuit and method for compensating for the attenuation and distortion of 
signals received through a public telephone network. 
2. Description of the Related Art 
In general, data communication terminal equipment includes a key phone 
system and a facsimile system, etc., and among the rest, the facsimile 
system is used for transmitting/receiving a document. In this facsimile 
system, the transmission and reception of image data of the document are 
performed according to a process of a protocol after a document is 
inserted in the document feeding cassette and a receiving party's 
telephone number is then dialed. The protocol for transmitting and 
receiving the document is prescribed in a CCITT recommendation, and the 
protocol in accordance with CCITT.41420 is performed through five phases 
as shown in the following Table &lt;1&gt;. 
TABLE 1 
__________________________________________________________________________ 
CALLING TRANSMITTER 
CALLED RECEIVER 
__________________________________________________________________________ 
PHASE A 
CALLING(CNG) CALLED STATION 
IDENTIFICATION(CED) 
PHASE B 
COMMAND CAPABILITIES IDENTIFICATION 
INFORMATION (NSF + CSI + DIS) 
(NSS + TSI + DCS) 
RECEPTION CONFIRMATION 
TRAINING (HIGH SPEED) 
(CFR) 
(9600/7200/4800/2400) 
PHASE C 
MESSAGE 
HIGH SPEED 
(9600/7200/4800/2400) 
PHASE D 
END OF MESSAGE RECEPTION CONFIRMATION 
(EOP/MPS/EOM/RPI-Q) 
(MCF/RTP/RTN/PIP/PIN) 
PHASE E 
DISCONNECT THE LINE 
(DCN) 
__________________________________________________________________________ 
As shown in Table &lt;1&gt;, in phase A, which is a call setup phase, the calling 
transmitter and the called receiver exchange the CNG and CED signal with 
each other. In phase B, which is a premessage procedure phase, the calling 
transmitter and the called receiver exchange the NSS+TSI+DCS and 
NSF+CSI+DIS and CFR signals with each other. The phase C is a phase for 
transmitting a fax message (image data). Furthermore, the phase D is a 
postmessage procedure phase and in this phase, the calling transmitter and 
the called receiver exchange the EOP/MPS/EOM/RPI-Q and the 
MCF/RTP/RTNIP/PIN with each other, and the phase E is a phase of 
transmitting the DCN signal and releasing the call. 
However, in the facsimile system as described above, as a cable length of 
the public telephone network is extended, signals transmitted through this 
cable are attenuated as shown in FIG. 1A. Particularly, signals of a high 
frequency are attenuated more than signals of a low frequency due to a 
capacitance factor of the cable as shown in FIG. 1B. Accordingly, in order 
to compensate for the attenuation of the signals as mentioned above, data 
communication terminal equipment is generally provided with an 
equalization function. The aforesaid equalization function is an optional 
function and therefore, the equalization function appropriate to the 
status set by the user is effected. For example, when the user sets a 
cable length that is one of the optional items in the facsimile system as 
0.0 Km, the equalization function is not effected. On the other hand, if 
the user sets the cable length as 1.8 Km or 3.6 Km, the equalization 
function appropriate to this cable length is effected. As discussed 
previously, in the conventional data communication terminal equipment, the 
user has to set the equalization function appropriate to the corresponding 
cable length in a setup menu and therefore, there is a problem in that in 
case that the equalization function inappropriate to the corresponding 
cable length is set or the set equalization function is not designated in 
setup menu, the communication is not completely established. 
Ukegawa and Iizuka, et al., U.S. Pat. Nos. 5,150,400 and 4,736,249 each 
disclose facsimile apparatus which include training procedures prior to 
the normal transmission of documents. However, neither of these references 
teach or suggest the use of the handshake process signals to determine the 
equalization necessary for the telephone line as in the present invention. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide an 
equalizing circuit and method of detecting status of signals received 
through a public telephone network to effect an equalization function so 
that the signals are not attenuated below a given level in data 
communication terminal equipment. 
It is another object of the present invention to provide an equalizing 
circuit and method of detecting a reception signal upon performance of a 
protocol to thereby compensate for the attenuation and distortion of a 
received image data signal in a facsimile system. 
To achieve this and other objects, in data communication terminal equipment 
in accordance with the present invention, an equalizing circuit comprises: 
a level detecting unit for detecting the respective levels of a high 
frequency signal and a low frequency signal which are inputted as 
frequency shift keying signals through a telephone line during a 
handshake; a control unit for comparing the levels of the low and high 
frequency signals detected by the level detecting unit and for generating 
a frequency compensation amplification control signal according to a level 
difference between the two signals; and an equalizer for inputting a level 
detection signal outputted from the level detecting unit and for 
compensating for the frequency attenuation and distortion of a received 
signal in response to the frequency compensation amplification control 
signal of the control unit, thereby effecting an equalization. 
To achieve this and other objects, the present invention provides an 
automatic gain control method for data communication terminal equipment 
including the steps of detecting a level of a received protocol signal 
during a communication in accordance with a process of a protocol; 
detecting a two tone frequency shift keying FSK signal that is a handshake 
signal after detecting a level of the detected received protocol signal to 
check a status of a cable; setting a compensation parameter in accordance 
with a level difference of the two tones of the FSK signal forming the 
handshake signals; and effecting an equalization in accordance with the 
set compensation parameter prior to receiving data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning now to the drawings and referring to FIG. 2, which is a block 
diagram illustrating a facsimile system constructed according to the 
principles of the present invention, a central processing unit CPU 10 
controls the facsimile system according to a given program and compares 
detected levels of two low and high frequency signals occurring during the 
handshake so as to generate an equalization control signal in order to 
compensate for the attenuation and distortion of received signals 
according to a level difference of the two signals. A memory 20 stores the 
program including protocol data and character data, and accesses data 
under the control of the CPU 10. An operation panel OPE 30 has a plurality 
of keys for generating various key data, such as a dial key, a density 
setting key, a resolution setting key and a function setting key, etc., 
and a display unit for displaying data. A scanner 40 scans an image from a 
document to change it into the digital image data and then provides the 
digital image data to the CPU 10. A MODEM 50 converts output data of the 
CPU 10 into analog data and outputs it, and simultaneously converts 
received analog data into digital data and outputs it, under the control 
of the CPU 10. A line interface unit LIU 60 connects a speech loop of a 
telephone line and interfaces the signal of the MODEM 50 and the signal of 
the telephone line, under the control of the CPU 10. A transformer 61 is 
connected to the LIU 60 and matches an impedance characteristic between an 
exchange and a facsimile system. A first amplifier 62 amplifies a 
frequency shift keying signal inputted through the transformer 61. A level 
detector 63 detects the respective levels of the high and low frequency 
signals which are inputted as amplified frequency shift keying signals 
from the first amplifier 62 during a handshake. An equalizer 64 inputs the 
level detection signal outputted from the level detector 63 and 
compensates for the frequency attenuation and distortion of the reception 
signal in response to the equalization control signal of the CPU 10, so as 
to effect the equalization. A second amplifier 65 amplifies an analog 
signal outputted from the MODEM 50 and outputs the amplified analog 
signal. A printer 70 prints the received data according to the control 
signal of the CPU 10. 
FIG. 3 is a flowchart illustrating the control procedure for compensating 
for the attenuation and distortion of the received signals according to 
the present invention. The aforesaid control procedure comprises the steps 
of: detecting a level of a received protocol signal in phase B during a 
communication in accordance with a process of a protocol; checking an FSK 
signal that is a handshake signal after detecting the level of the 
detected received protocol signal to check the status of the cable; 
setting the compensation parameter in accordance with a level difference 
of the handshake signals corresponding to the status of the cable; and 
effecting the equalization according to the set compensation parameter 
prior to receive data. 
FIG. 4 shows a characteristic graph for use in describing the compensation 
gain for each frequency, and FIG. 5 shows a waveform for describing the 
frequency shift keying method in accordance with the CCITT V.21 
recommendation. 
With reference with FIGS. 2 through 5, the preferred embodiment of the 
present invention is explained below in detail. 
In the data communication terminal equipment, the protocol is performed in 
order to form the unified communication status between the terminal 
equipment, that is, the handshake of the FSK signal is performed in order 
to establish the identical communication condition between two terminals 
in phase B during the protocol process. At this time, in the facsimile 
system, as prescribed in the CCITT V.21 recommendation, the FSK signal 
includes signals of 1650 Hz and 1850 Hz during the handshake while being 
divided into a low (0) signal and a high (1) signal as shown in FIG. 5. 
These handshake signals are interfaced through the LIU 60 and 
impedance-matched in the transformer 61 and are supplied to the first 
amplifier 62. The first amplifier 62 amplifies the received handshake 
signals by a given amount and outputs the amplified signals to the level 
detector 63. The handshake signals are composed of two signals of 1650 Hz 
and 1850 Hz, and the level detector 63 detects the levels of the two 
signals of 1650 Hz and 1850 Hz and outputs the detected levels to the CPU 
10. However, in these signals of 1650 Hz and 1850 Hz which are the 
received handshake signals, the frequency thereof is distorted and also 
the levels thereof are attenuated according to the status of the cable. 
Particularly, as the cable length is extended, the high frequency signal 
(i.e., 1850 Hz) is attenuated more than the low frequency (i.e., 1650 Hz). 
Therefore, the data communication terminal equipment compares the levels 
of the 1650 Hz and 1850 Hz signals to check the status of the cable, and 
compensates according to the level difference value between the two 
signals, and thus the equalization is effected. Referring now to FIG. 3 
for explaining the equalization function, the CPU 10 detects whether or 
not a ring signal is received in step 21, and then if received, proceeds 
to step 22 to thereby perform the protocol. And then, the received 
protocol signal is interfaced through the LIU 60 and impedance-matched in 
the transformer 61 and is supplied to the first amplifier 62. The first 
amplifier 62 amplifies the received protocol signal by the given level and 
outputs the amplified signal to the level detector 63. The level detector 
63 detects the level of the protocol reception signal to provide the 
detected level to the CPU 10. In step 23, the CPU 10 detects the 
respective levels of 1650 Hz and 1850 Hz signals that are the received 
signals of the premessage procedure phase B. After then, the CPU 10 in 
step 24 compares the level difference values of the 1650 Hz and 1850 Hz 
signals to check the status of the cable. In step 25, the CPU 10 sets the 
parameter for compensating the received signal according to the level 
difference between the 1650 Hz and 1850 Hz signals. The high frequency 
factor is compensated more than the low frequency factor. The level 
difference value is expressed as .DELTA.G and can be obtained by the 
following equation &lt;1&gt;. 
EQU .DELTA.G=1650 Hz level-1850 Hz level &lt;1&gt; 
As shown in FIG. 4, each frequency band has a different compensation gain 
according to the compensation amplification rate in order to compensate 
the high frequency factor more than the low frequency factor, and the 
compensation amplification rate can be obtained by the following equation 
&lt;2&gt;. 
##EQU1## 
wherein, .DELTA.f represents a frequency difference. 
When the frequency compensation parameter is set in this manner, the CPU 10 
controls the equalizer 64 by the equalization control signal in accordance 
with the set compensation amplification rate in step 26 to compensate for 
the attenuation and distortion of the received signal detected by the 
level detector 63, and outputs the compensated signal to the MODEM 50. And 
then, the CPU 10 proceeds to step 27 to receive data. 
The data communication terminal equipment as discussed previously detects 
the handshake level of the premessage procedure phase B according to a 
process of the protocol, and checks the status of the cable according to 
the handshake level to set the frequency compensation parameter. Then, the 
aforesaid data communication terminal equipment compensates for the 
attenuation and distortion of the received signal upon reception of image 
data, thereby enhancing the efficiency of the data reception. 
It should be understood that the present invention is not limited to the 
particular embodiment disclosed herein as the best mode contemplated for 
carrying out the present invention, but rather that the present invention 
is not limited to the specific embodiment described in this specification 
except as defined in the appended claims.