Communication apparatus

In a communication apparatus has a line control unit that controls connection with a communication line network, a communication unit transmits/receives data through the communication line network. A detection circuit detects one of a line voltage value and a line current value when the communication apparatus is connected to the line network. A memory unit stores multiple voltage-current characteristic curves defining relationships of line voltage values with respect to line current values satisfying a predetermined standard. An adjusting unit adjusts the line voltage value and the line current value so as to satisfy the predetermined standard based on one of the voltage-current characteristic curves stored in the memory unit. A selecting unit selects another voltage-current characteristic curve different from the one of the voltage-current characteristic curves from among the multiple voltage-current characteristic curves stored in the memory unit. A re-adjusting unit adjusts one of the line voltage and a line current.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 from Japanese Patent Application No. 2005-003114, filed on Jan. 7, 2005. The entire subject matter of the application is incorporated herein by reference.

BACKGROUND

1. Technical Field

Aspects of the present invention relate to a communication apparatus and in particular to a communication apparatus that controls to suppress an oscillation in a line communication network when the communication apparatus is connected with a various kind of networks.

2. Description of Related Art

In order to maintain communication quality, a public line network is designed in accordance with predetermined standards. A communication apparatus connected to such a network is configured such that a line voltage or a line current is controlled so as to meet the standards, since a line resistance connected with an exchange installed in a telephone exchange station varies depending on the location of the communication apparatus.

For an example, Japanese Patent Provisional Publication P2004-112490A describes a communication apparatus that detects a line current and a line voltage, calculates line impedance, and adjusts the signal level of the modem based on the calculated impedance. Further, in order to control the line current and the line voltage which meet the standards of public line networks, etc., the apparatus adjusts a relation between the current and the voltage so as to satisfy the standards, based on a predetermined DC mask curve defining a relationship between the line current values and line voltage values.FIG. 10is a graph showing an example of the DC mask curve used in a conventional communication apparatus.

InFIG. 10, a horizontal axis represents the line current value, while a vertical axis represents the line voltage. An area between two dotted lines represents line current values and line voltage values satisfying the public line standards.

A predetermined DC mask curve passing the above area is defined (indicated by solid line inFIG. 10), and by controlling the impedance of the line, the line current and the line voltage are adjusted.

For example, inFIG. 10, when initially a line current value and line voltage value are represented by point A, which is out of the area, point A is located below the DC mask curve and the values do not meet the standards. In such a case, the value of the line impedance is changed to a smaller value. Then, the line current value and the line voltage value are changed to values represented by point B. In this example shown inFIG. 10, point B is also out of the area (i.e., located above the DC mask curve). Therefore, the value of the line impedance is then changed to a larger value, and the line current value and the line voltage value are changed to point C. As shown inFIG. 10, still point C is out of the area (i.e., located below the DC mask curve), and the value of the line impedance is changed again to a smaller value. Then, the line current value and the line voltage value move to point D, which is substantially on the DC mask curve. The line current value and the line voltage value set in this way satisfy the standards.

However, when the line current value and the line voltage value are set in the way described above, an oscillation may occur as the network is in the resonant state with specific frequencies. The oscillation frequency is only within a frequency bandwidth from 20 kHz to 1 MHz, which is higher than an audio frequency range. However, the oscillation generated in the network would cause troubles such as generation of noises, malfunction in echo canceling while calling with a handset.

SUMMARY OF THE INVENTION

Aspects of the invention are advantageous in that there is provided an improved communication apparatus that can prevent an oscillation in a communication line to which the apparatus is connected.

DETAILED DESCRIPTION

General Overview

It is noted that various connections are set forth between elements in the following description. It is noted that these connections in general and unless specified otherwise, may be direct or indirect and that this specification is not intended to be limiting in this respect. Aspects of the invention may be implemented in computer software as programs storable on computer-readable media including but not limited to RAMs, ROMs, flash memory, EEPROMs, CD-media, DVD-media, temporary storage, hard disk drives, floppy drives, permanent storage, and the like.

According to aspects of the invention, there is provided a communication apparatus, which is provided with a line control unit configured to control connection with a communication line network, a communication unit configured to transmit and receive data through the communication line network, a detection circuit configured to detect one of a line voltage value and a line current value when the communication apparatus is connected to the line network, a memory unit configured to store multiple voltage-current characteristic curves defining relationships of line voltage values with respect to line current values, an adjusting unit configured to obtain the line voltage and the line current based on the one of the line voltage value and the line current value, and adjusting the line voltage value and the line current value based on one of the voltage-current characteristic curves stored in the memory unit, a selecting unit configured to select another voltage-current characteristic curve which is different from the one of the voltage-current characteristic curves from among the multiple voltage-current characteristic curves stored in the memory unit, and a re-adjusting unit configured to control the adjusting unit based on the selected voltage-current characteristic curve selected by the selecting unit.

The memory unit may store two voltage-current characteristic curves, and the selecting unit may be configured to switch the selected voltage-current characteristic curve to the other voltage-current characteristic curve when one of the line voltage and line current is adjusted by the adjusting unit.

The communication apparatus may further include an oscillation detection circuit that detects whether an oscillation is generated in the communication line network, and the selecting unit may select another voltage-current characteristic curve if the oscillation detection circuit detects that an oscillation is generated at the line voltage and the line current adjusted by the adjusting unit.

The memory unit may store more than two voltage-current characteristic curves, and the selecting unit may sequentially select one of the voltage-current characteristic curve among more than two voltage-current characteristic curves store in the memory unit if the oscillation detection circuit detects the oscillation in the communication line network after the line voltage and the line current are adjusted by the adjusting unit.

The communication apparatus may further include an oscillation detection circuit that detects whether an oscillation is generated in the communication line network, the oscillation detection circuit obtaining an oscillation frequency if the oscillation is detected. The memory unit may store a voltage-current characteristic curve corresponding to the oscillation frequency if an oscillation is detected in the communication line network, and the selecting unit may select the voltage-current characteristic curve corresponding to the oscillation frequency detected by the oscillation detection circuit from the ones stored in the memory unit.

The memory unit may be configured to store multiple voltage-current characteristic curves corresponding to an oscillation frequency of an oscillation generated in the communication line network, and another voltage-current characteristic curve corresponding to the oscillation frequency detected by the oscillation detection circuit may be selected if the oscillation detection circuit detects that an oscillation is generated in the communication line network at the line voltage value and the line current value previously adjusted by the adjusting unit.

If the oscillation detection circuit detects an oscillation in the communication line network at the line voltage value and the line current value adjusted with the adjusting unit, the selecting unit selects a different one of the multiple voltage-current characteristic curves.

The selecting unit may includes a selected voltage-current characteristic curve memory unit that stores the selected voltage-current characteristic curve, and the selecting unit may select a voltage-current characteristic curve which is not store in the tried voltage-current characteristic curve memory.

The communication apparatus may include an operable member that allows a user to manually select one of the multiple voltage-current characteristic curves stored in the memory unit.

The operable member may allow a user to select one of the multiple voltage-current characteristic curves when a connection with the communication line network is established.

According to aspects of the invention, there is also provided a method of adjusting one of a line voltage and a line current applied to a communication line of a communication apparatus. The communication line is connected to a communication line network. The method includes storing multiple voltage-current characteristic curves, adjusting one of the line voltage and the line current based one on of the multiple voltage-current characteristic curves, detecting whether an oscillation is generated in a communication line network, transmitting and receiving data through the communication line network, and re-adjusting one of the line voltage and the line current based on another one of the multiple voltage-current characteristic curves.

According to aspects of the invention, there is further provided a computer program product comprising computer readable instructions that cause a computer to execute a method of adjusting one of a line voltage and a line current applied to a communication line of a communication apparatus. The communication line is connected to a communication line network. In this case, the method executed by the computer may include storing multiple voltage-current characteristic curves, adjusting one of the line voltage and the line current based on one of the multiple voltage-current characteristic curves, detecting whether an oscillation is generated in the communication line network, transmitting and receiving data through the communication line network, and re-adjusting one of the line voltage and the line current based on another one of the multiple voltage-current characteristic curves.

Illustrative Embodiments

Hereinafter, referring to the accompanying drawings, communication apparatuses according to illustrative embodiments of the invention will be described.

FIG. 1is a perspective view of a communication apparatus1according to aspects of a first embodiment of the invention. The communication apparatus1is an MFP (Multi Function Peripheral) which has multiple functions such as functions of a facsimile machine, a copier, a scanner, a telephone, etc. The main unit2has an upper body portion2band a bottom portion2a. The upper body portion2bis attached to the bottom portion2asuch that the upper body portion2bis moved between open/closed positions with respect to the bottom portion2a. Further, a printer25is built in the bottom portion2aas shown inFIG. 2.

An operation panel4is formed on the front side of the upper body portion2b. The operation panel4includes switches to control multiple functions of the facsimile machine, copier, scanner, etc., numeric keys to input numerals, a cursor key4ato move a cursor displayed on a display unit5, a confirm key (enter key)4bwhich is to be pressed by a user for confirmation of input setting, and the display unit5(hereinafter referred to as an “LCD”).

An original document outlet7and an original document insertion slot8are formed in the upper body portion2b. Original documents inserted through the original document insertion slot8are transmitted to the original document outlet7while the documents are read by a scanner (CIS: Contact Image Sensor) built in the upper body portion2b(not shown) and image data is generated.

In addition to the CIS built in the upper body portion2b, another scanner is built in the bottom portion2a, which reads original documents placed on a platen glass extending horizontally beneath an original document cover6.

A recording paper tray (not shown) is placed on a lower side of the bottom portion2a. The image data generated by the CIS, generated by the scanner22or received by the facsimile is printed by the printer25on the recording paper transported from the record paper tray.

Referring now toFIG. 2, an electric configuration of the communication apparatus1will be described.FIG. 2is a block diagram showing the electronic configuration of the communication apparatus1. The communication apparatus1includes a CPU11, ROM12, EEPROM13, RAM14, image memory15, a line I/F section19, a modem20, a buffer21, a scanner22, an encoding section23, a decoding section24, a printer25, an operation panel4, an LCD5and an amplifier27, which are connected to each other through a bus line30.

The line I/F section19is provided for performing line control, and has a semiconductor DAA (Data Access Arrangement)33(seeFIG. 3), The communication apparatus1is connected, through the line I/F section19, to a telephone line31. The line I/F section19receives various signals such as a call signal and a signal indicating the telephone number of a calling station which are transmitted from an exchange29. The line I/F section19also operate to transmit a dial signal at the calling time responsive to key operations through the operation panel4to the exchange29. An external telephone3is connected via external telephone terminals T1and T2.

The CPU11controls the sections connected by the bus line30in accordance with various signals transmitted and received through the line I/F section19. The ROM12is a read only memory storing control programs executed by the CPU11, and it includes DC mask curve memory12astoring multiple DC mask curves which are used to control a line voltage value and a line current value.

The RAM14is a rewritable memory for temporarily storing various pieces of data while the CPU11executes control programs. The RAM14also includes a temporary memory14astoring identifier (temp) which is referred to for identifying a type of the selected DC mask curve, and tried DC mask identifier memory14bstoring the identifiers of the tried DC mask curves.

The EEPROM13, which is an electrically erasable nonvolatile memory, includes a DC mask curve identifier memory13aand a DC mask curve setting complete flag memory13b. The DC mask curve identifier memory13astores the identifier of the DC mask curve to be used to adjust the communication line among multiple DC mask curves stored in the ROM12. The DC mask curve setting complete flag memory13bstores the flag which indicates whether the identifier of the DC mask curve is set or not.

The image memory15is a memory for storing image data. The received image data is once stored in the image memory15and after the image data is printed on the recording paper by the printer25, the image data is deleted from the image memory15. The image data generated by the scanner22is also stored in the image memory15. When the facsimile transmission function is set, the image data is sent via the line I/F section. When the copier function is set, the image data is printed on the printer15, and then deleted from the image memory15afterward.

The modem20modulates and demodulates image information and communication data for transmission and also transmits and receives various procedure signals for transmission control. The buffer21temporarily stores data containing encoded image information transmitted to and/or received from an opposite station.

The scanner22reads an original document inserted to an original insertion slot8or placed on a platen glass and generates image data The encoding section23encodes the image data generated by the scanner22. The decoding section24reads the image data stored in the buffer21or the image memory15and decodes the image data. The decoded data is printed on recording paper by the printer25.

The printer25according to the illustrative embodiment is an ink jet printer and prints images on the recording papers based on the image data. The amplifier27amplifies electronic signals of ringing tones and voices, and drives a loudspeaker28to output sound (voice).

The communication apparatus I described above is connected to the telephone line31through the line I/F section19. The telephone line31is connected to the exchange29. The exchange29is connected to another exchange via a telephone line32. The other exchange is further connected to another machine at a calling station or at a destination.

FIG. 3is a block diagram showing the configuration of the line I/F section19. The line I/F section19includes the semiconductor DAA33, a CML (Connect MODEM to Line) relay36for switching between the telephone and the facsimile, a rectifier bridge37, and DC loop cut capacitors38a,38b,38cand38d, a transistor45, and an oscillation detection circuit50, etc. InFIG. 3, L1and L2are terminals to the telephone line31, and T1and T2are terminals to the external telephone3.

The semiconductor DAA33and the modem20are insulated in terms of the direct current. Both of them are connected to an insulation section35transmitting data, signals, etc., and a transformer supplying electric power to the semiconductor DAA33.

The semiconductor DAA33is provided with a hybrid network44, a CODEC40, a ring detection circuit39, a transmission amplifier42, a tone amplifier43, a serial I/F41and a voltage detection circuit34.

The hybrid network44establishes connection with the network and disconnection therefrom. The hybrid network44includes a two-wire to four-wire conversion circuit for converting facsimile data via the telephone line31into transmission data and received data, a canceller circuit for suppressing wraparound routing of transmission data to a reception path, a filter circuit, and etc. The hybrid network44is connected to the positive electrode of the rectifier bridge37through a condenser and a resistor. It is also connected to the CODEC40via a transmission amplifier42, and to the base of the transistor45, and to the serial I/F41.

The CODEC40performs A/D conversion and D/A conversion of facsimile reception data and transmission data. The CODEC40is connected to the communication line network and the external telephone3through the tone amplifier43, and is also connected to the hybrid network44trough the transmission amplifier42.

The transmission amplifier42performs gain adjustment of transmission data. The tone amplifier43is a differential amplifier and receives differential input from the terminals L1and L2of the telephone line31.

When the CODEC40receives a tone signal from the telephone line31or the external telephone3, the received tone signal is transmitted through the tone amplifier43, the CODEC40, the serial I/F41, the insulation section35, and the modem20in this order. And the tone detection signal transmitted from the CODEC40is transmitted to the CPU11.

The ring detection circuit39is connected to the communication line network and the external telephone3through the loop cut capacitors38aand38b. The output terminal of the ring detection circuit39is connected to the serial I/F41. When a ring signal reaches the ring detection circuit39, the ring detection circuit39detects the same and outputs the ring detection signal to the serial I/F41.

The serial I/F converts the ring detection signals input from the ring detection circuit39and facsimile reception data input from the CODEC40to serial signals, and transmits the serial signals to the modem20through the capacitor35b. It also receives a control signal to establish the connection to or disconnect from the communication line network and facsimile transmission data from the modem20via the capacitor35a, and separates them respectively. The voltage detection circuit34is connected to the rectifier bridge37via a resistor and obtains the line voltage value.

The transistor45adjusts the line voltage to fit the predetermined voltage current characteristic, based on the value obtained by the voltage detection circuit34. The line impedance can be adjusted by changing the base voltage of the transistor45.

The oscillation detection circuit50obtains the oscillation frequency when a connection with the network through the telephone line31is established and an oscillation occurs in the network. The oscillation detection circuit50includes circuits which can detect the oscillation within a predetermined bandwidth. Specifically, the oscillation detection circuit50applies the line voltage to band-path filters corresponding to, for example, from 20 kHz to 100 kHz, from 100 kHz to 200 kHz, from 200 kHz to 750 kHz, and 750 kHz to 1 MHz, and if the level of the voltage output from a bandpass filter is higher than the predetermined level, the oscillation detection circuit50determines that the oscillation occurs within the frequency range. Further, the oscillation detection circuit50is connected to the communication line network through L1and L2, and the oscillation detection signals are transmitted to the CPU11.

Next, processing of the voltage current characteristic adjustment executed when the communication apparatus1configured as described above starts receiving or transmitting the information, will be discussed with reference to a flowchart ofFIG. 11.FIG. 11is a flowchart showing how the voltage current characteristic adjustment is executed according to the predetermined DC mask curves stored in the communication apparatus1. The process is executed after the connection with the network is established.

First, the line voltage value detected by the voltage detection circuit34is checked (S71). Whether or not the line voltage value exceeds a threshold voltage (e.g., 100 V) is determined. If the line voltage value does not exceeds (No at S72), the line network is in normal state. Then, the line current value is acquired based on the currently set DC impedance (S73). Next, It is determined whether or not the detected line voltage value is within +/−0.5 V of the decided voltage which is on the currently set DC mask curve and corresponding to the acquired line current value. Here, the currently set DC mask curve means the pre-determined DC mask curve to be applied for voltage-current characteristic adjustment process. When multiple DC mask curves are store in ROM12, an identifier is stored in a DC mask curve identifier memory13aof EEPROM13. When trials are done to select an adequate DC mask curve, the identifier is stored in Temp memory14aof RAM14.

If the line voltage is not within +/−0.5 V of the decided voltage value based on the DC mask curve (No at S74), the base voltage of the transistor45is varied to adjust the line impedance (S75), then the process goes back to S71

If the line voltage is within +/−0.5 V of the decided voltage value based on the DC mask curve (Yes at S74), the voltage-current characteristic adjustment process terminates. On the other hand, in the process of S72, if the line voltage value exceeds (Yes at S72), the line network is in abnormal state. Then, the connection with the communication line network is cut off (S76), and the voltage-current characteristic adjustment process terminates.

As described above, when transmitting and receiving begin, the line voltage and the line current are adjusted based on the DC mask curve which is set to meet the required characteristic by the standards.

Next, the DC mask curve selection process will be discussed with reference to a table of DC mask curves shown inFIG. 4, and a flow chart shown inFIG. 5, when the user installs the communication apparatus1, namely, when the user turns on the power.

FIG. 4is a table which shows the DC mask curves stored in the DC mask curve memory12aof the ROM12. The ROM12stores the DC mask curves corresponding to 5 oscillation statuses of the communication network. Each DC mask curve represents 5 points of the voltage-current characteristic. Those points are stored in the memory, the values between two neighboring points being calculated using linear interpolation method.

The DC mask curve shown in the first row of the table corresponds to no oscillation, which has a identifier A. As described above, the voltage-current characteristic is defined by the five points indicated in the table. The first point has the voltage value of 4000 mV(4 V), and the current value of 20 mA. The second point has the voltage value of 4 V and the current value of 40 mA. The third point has the voltage value of 8 V and the current value of 40 mA. The fourth point has the voltage value of 9.5 V and the current value of 60 mA. The fifth point has the voltage value of 15 V and the current value of 150 mA.

For example, a point corresponding the current value 10 mA is not stored in the memory, the point is located between the first point (4 V, 0 mA) and the second point (4 V, 20 mA), therefore, the voltage value is calculated to be 4 V. Similarly, if a current value is 30 mA, the point is located between the second point and the third point. In this case, the voltage value is calculated to be 6 V.

In the second row of the table, the DC mask curve which is applied when an oscillation is generated and its oscillation frequency ranges between 20 kHz and 100 kHz, is shown. The curve is stored in the memory and has an identifier B. The voltage value and the current value of the first point are 5 V and 0 mA. The voltage value and the current value of the second point are 6 V and 21 mA. The voltage values and the current values of the other points are as indicated in the table.

Similarly, in the third row of the table, the DC mask curve which is applied when an oscillation is generated and its oscillation frequency ranges between 100 kHz and 200 kHz. The curve has an identifier C. In the fourth row, the DC mask curve is shown which is applied when an oscillation is generated and its oscillation frequency ranges between 200 kHz and 750 kHz. It has an identifier B. In the fifth row, the DC mask curve is shown which is applied when an oscillation is generated and its oscillation frequency ranges between 750 kHz and 1 MHz, and it has an identifier D. As above, each row of the table shown inFIG. 4indicates 5 pairs of voltage values and current values defining a DC mask curve.

In this example show inFIG. 4, when the oscillation frequency ranges between 20 kHz and 100 kHz, and when the oscillation frequency ranges between 200 kHz and 750 kHz, the same DC mask curve (identifier: B) is used. Further, the DC mask curve (identifier: A) shown in the first row is a default DC mask curve. If the DC mask curve is applied and the oscillation is not generated, this DC mask curve is set to be applied to the communication apparatus1.

Next, the DC mask curve setting process is discussed with reference to the flowchart shown inFIG. 5. The DC mask curve setting process is executed when the communication apparatus1is powered on.

First, the line voltage value detected by the voltage detection circuit34is checked (S1). Then, it is judges whether the line voltage value is equal to or greater than the minimum voltage which allows the communication apparatus1to connect with the communication line network (S2). If the line voltage is equal to or greater than the minimum voltage (Yes at S2), the DC mask curve setting complete flag memory13bis checked, and it is judged whether a DC mask curve has already been set (S3). If the DC mask curve has not been set (No at S3), the identifier A is stored in the Temp memory14a(S4), and the line is disconnected (S5). Next, the voltage-current characteristic adjustment process (seeFIG. 11) is executed (S6). In the voltage-current characteristic adjustment process, the line voltage and line current are adjusted in accordance with the DC mask curve1having the identifier of A that is stored in ROM12.

Subsequently, after the adjustment is done in the voltage-current characteristic adjustment process, it is judged whether the oscillation is generated in the communication line network based on the oscillation detection signal output by the oscillation detection circuit50(S7). If the oscillation is not generated (No at S7), an identifier is set to A as the DC mask curve having identifier A is applied. Here, setting an identifier of the DC mask curve as A, an identifier A is stored in the DC mask curve identifier memory13aof EEPROM13. Thereafter, when transmission/reception process begins, the voltage-current characteristic adjustment process is executed based on the identifier of the DC mask curve set as above,

If it is determined that the oscillation is generated in the communication line network (Yes at S7), it is judged whether the oscillation frequency ranges between 20 kHz and 100 kHz, or between 200 kHz and 750 kHz (S9). If the oscillation frequency ranges between 20 kHz and 100 kHz, or between 200 kHz and 750 kHz (Yes at S9), the identifier of the DC mask curve to be applied is set to B (S10).

If the oscillation frequency does not range between 20 kHz and 100 kHz, or between 200 kHz and 750 kHz (No at S9), it is judged whether the oscillation frequency ranges between 100 kHz and 200 kHz based on the oscillation detection signal output from the oscillation detecting circuit50(S11). If the oscillation frequency ranges between 100 kHz and 200 kHz (Yes at S11), the identifier of the DC mask curve to be applied is set to C (S12). If the oscillation frequency does not range between 100 kHz and 200 kHz (No at S11), the identifier of the DC mask curve to be applied is set to D (S13).

When the DC mask curve setting is completed in S8, S10or S13, the connection with the communication line network is released (i.e., disconnected) (S14), a setting complete flag is set in the DC mask curve setting flag memory (S15), and the DC mask curve setting process terminates. If the line voltage is not equal to or greater than the minimum voltage enabling connection with the communication line network (No at S2), or the DC mask curve has already been set (Yes at S3), the DC mask curve setting process terminates. After termination of this process, the communication apparatus pauses until information is received or a user makes an operation, and a corresponding process is executed according to the received information or the operation the user made. As described above, according to the communication apparatus1according to the fist embodiment, the predetermined (default) DC mask curve and some other DC mask curves which correspond to the oscillation frequencies when the oscillation is generated and can be used to cancel the oscillation. When the communication apparatus is installed first time, the predetermined (default) DC mask curve is applied to adjust the voltage-current characteristic. If the oscillation is generated at the determined characteristic, the adequate DC mask curve is set according to the oscillation frequency. Therefore, the communication apparatus1itself can suppress the oscillation in the communication line network.

Next, the second embodiment will be described with reference toFIGS. 6 and 7. Here, description similar to that of the first embodiment of the communication apparatus1will be omitted for the brevity, and description is made only on the different portions. In the first embodiment, the voltage-current characteristic is adjusted based on the predetermined (default) DC mask curve, then after the adjustment is done, if the oscillation occurs, another adequate DC mask curve is selected according to the oscillation frequency. In the second embodiment, multiple DC mask curves are store corresponding to the oscillation frequency ranges. When the voltage-current characteristic is adjusted based on the selected DC mask curve corresponding to the oscillation frequency, but still an oscillation is generated at the same frequency, then, another DC mask curve can, be selected corresponding to the same oscillation frequency.

FIG. 6shows a table of DC mask curves store in the DC mask curve memory12aof ROM12according to the second embodiment. Similar to the first embodiment, the DC mask curve memory12astores DC mask curves corresponding to 5 oscillation states. Three DC mask curves are stored corresponding to each of the four kinds of oscillation frequency bandwidths.

Each DC mask curve is defined by 5 points of the voltage-current characteristic, the values between two neighboring points are calculated in accordance with the linear interpolation method as in the first embodiment. The DC mask curve labeled as “NO OSCILLATION” (first row) is the same curve which has identifier A in the first embodiment.

Three DC mask curves identified as B1, B2and B3corresponding to the oscillation frequency from 20 kHz to 100 kHz are defined on the second to fourth rows of the table shown inFIG. 6. Three DC mask curves identified as C1, C2and C3corresponding to the oscillation frequency from 100 kHz to 200 kHz are defined on the fifth to seventh rows of the table. Three DC mask curves identified as D1, D2and D3corresponding to the oscillation frequency from 200 kHz to 750 kHz are defined on the eighth to tenth rows of the table. Three DC mask curves identified as E1, E2and E3corresponding to the oscillation frequency from 750 kHz to 1 MHz are defined on the eleventh to thirteenth rows of the table.

Next, the DC mask curve setting process according to the second embodiment will be described with reference to the flowchart shown inFIG. 7. The process is executed when the communication apparatus1is powered on.

First, the line voltage value detected by the voltage detection circuit34is checked (S21). Then, it is judged whether the line voltage value is equal to or greater than the minimum voltage which allows the communication apparatus1to connect with the communication line network (S22). If the line voltage is equal to or greater than the minimum voltage (Yes at S22), the DC mask curve setting complete flag memory13bof EEPROM13is checked, and it is judged whether the DC mask curve has already been set (S23). If a DC mask curve has not been set (No at S23), the identifier A is stored in the Temp memory14a(S24), and the connection with the network is cut (S25). Next, the voltage-current characteristic adjustment process shown inFIG. 11is executed (S26), and the line voltage and the line current are adjusted based on the DC mask curve identified by the identifier stored in the Temp memory14a.

Subsequently, after the adjustment is done in the voltage-current characteristic adjustment process, it is judged whether the oscillation occurs in the communication line network in accordance with the oscillation detection signal output by the oscillation detection circuit50(S27). If the oscillation is generated in the communication line network (Yes at S27), the oscillation frequency is acquired from the oscillation detection signal output by the oscillation detection circuit50, and it is judged whether an untried DC mask curve exists among DC mask curves corresponding to the oscillation frequency stored in ROM12referring to the tried DC mask identifier memory14bof RAM14(S28). The ROM12stores multiple DC mask curves corresponding to respective oscillation frequency bandwidths, and identifiers of the already tried DC mask curves are stored in RAM14in S29, which is described later.

If one or more untried DC mask curve exists (Yes at S28), one of them is selected in the order stored in the table, and the identifier of the selected DC mask curve is stored in the tried DC mask identifier memory14b(S30) as well as in The Temp memory14a, and the process returns to S25. So, thereafter, the voltage-current characteristic adjustment process S26is executed based on the DC mask curve selected in S29.

If it is determined that an oscillation is not generated in the network (No at S27), the DC mask curve identifier stored in the Temp memory14ais set in the DC mask curve identifier memory13a. If an untried DC mask corresponding to the oscillation frequency does not exist (No at S28), an error message is displayed on the LCD5to indicate an oscillating status (S32), and the identifier of the DC mask curve is set to A. After the DC mask curve is set in S31and S32, the connection with the network is cut (S34), a flag stored in the DC mask curve setting complete flag memory of EEPROM13is set (S35), and the DC mask curve setting process terminates.

As described above, with the communication apparatus1according to the second embodiment, the predetermined (default) DC mask curve and multiple DC mask curves corresponding to respective oscillation frequency bandwidths are defined. When the communication apparatus1is installed at the first time, the voltage-current characteristic is adjusted based on the predetermined DC mask curve. If an oscillation is generated in that condition, the DC mask curve corresponding to the oscillation frequency. Further, if an oscillation is still generated in that condition, another DC mask curve corresponding to the oscillation frequency is selected. Because the multiple DC mask curves are stored corresponding to respective oscillation frequency bandwidths, even if an oscillation is generated when the voltage-current characteristic is adjusted based on the DC mask curve corresponding to the oscillation frequency, another DC mask curve can be selected. Therefore, the oscillation in the network can be prevented more surely.

Next, a third embodiment will be described with reference toFIG. 8. Here, description similar to the first embodiment is omitted for brevity, and discussion is made only on the different portions. In the first and second embodiments, the voltage-current characteristic is adjusted based on the predetermined (default) DC mask curve. If the oscillation is generated, another DC mask curve corresponding to the oscillation frequency is used for adjustment to cancel the oscillation. According to the third embodiment, the voltage-current characteristic is firstly adjusted based on the predetermined (default) DC mask curve, and if an oscillation is generated in the network after the adjustment is completed, an untried DC curve is selected sequentially from the stored ones in the memory.

In the third embodiment, the DC mask curves stored in ROM12are the same as those of the first embodiment (seeFIG. 4), and DC mask curves having the identifiers A, B, C and D are stored in this order.

FIG. 8is a flowchart showing the DC mask curve setting process according to the third embodiment. The process is executed when the communication apparatus1is powered on.

First, the line voltage value detected by the voltage detection circuit34is checked (S41). Then, it is judged whether the line voltage value is equal to or greater than the minimum voltage which allows the communication apparatus1to connect with the communication line network (S42). If the line voltage is equal to or greater than the minimum voltage (Yes at S42), the DC mask curve setting complete flag memory13bof EEPROM13is checked, and it is judged whether the DC mask curve has already been set (S43). If the DC mask curve has not been set (No at S43), the identifier A is stored in the Temp memory14a(S44), and the connection with the network is cut (S45). Next, the voltage-current characteristic adjustment process (seeFIG. 11) is executed (S46). In the voltage-current characteristic adjustment process, the line voltage and the line current are adjusted based on the DC mask curve identified as A that is stored in ROM12.

Next, after the adjustment is done in the voltage-current characteristic process, it is judged whether the oscillation is generated in the communication line network based on an oscillation detection signal output by the oscillation detection circuit50(S47). If the oscillation is generated in the communication line network (Yes at S47), it is judged whether an untried DC mask curve exists among the curves stored in ROM12(S48). If an untried DC mask curve exists, the identifier of the DC mask curve is stored in the Temp memory14a(S49), and the process returns to the voltage-current characteristic adjustment process.

If it is determined that the oscillation is not generated in the network (No at S47), the tried DC mask curve currently store the Temp memory14ais set to be applied to the communication apparatus1, while its identifier is stored in the DC mask curve identifier memory of EEPROM13. If an untried DC mask curve does not exist, an error message is displayed on LCD5indicating that an oscillation is generated using any DC mask curve stored in ROM12(S51), and the identifier of the DC mask curve is set to A (S52).

After the DC mask curve is set in S50and S52, the connection with the network is cut (S53), a flag stored in the DC mask curve setting complete flag memory13bof EEPROM13is set (S54), and the DC mask curve setting process terminates. If in the process S42, the line voltage is less than the minimum voltage allowing to connect with the network (No at S42), or in the process S43, a DC mask curve is already set (Yes at S43), the DC mask curve setting process terminates.

In addition, according to the third embodiment, it is not necessary to detect the oscillation frequency when an oscillation is generated in the network, so the oscillation detection circuit50can output an oscillation detection signal when an output level of a high-pass filter is above the predetermined value where the line voltage is input to the high-pass filter of which cut off frequency is, for an example, 20 kHz.

As described above, with the communication apparatus1according to the third embodiment, the ROM12stores multiple DC mask curves. If an oscillation is generated in the network when the voltage-current characteristic is adjusted based on one of them, untried DC mask curves are selected sequentially until the oscillation is suppressed, the DC mask curve is set to be applied to the communication apparatus1so as to prevent an oscillation in the network.

Next, a fourth embodiment is discussed with reference toFIG. 9. Here, repetitive description will be omitted, and description will be made only on different portions. In the first, second and third embodiments, the voltage-current characteristic is adjusted based on the predetermined (default) DC mask curve. Then, after the adjustment, the oscillation detection circuit50detects whether an oscillation is generated, and according to the output of the oscillation detection circuit50, one of DC mask curves stored in ROM12is selected. According to the fourth embodiment, the communication apparatus1has a selection controller with which a user can select any of the multiple DC mask curve. Then the user can select and set the most adequate DC mask curve. In the fourth embodiment, two DC mask curves identified as A and B of multiple curves shown inFIG. 4are employed.

The operational proceeds as follows. First, the DC mask curve selection mode is set with the cursor key4aof the operation panel4, etc. The LCD5displays a message indicating the DC mask curve currently set. For an example, when the DC mask curve identifier A is set, the message is “Level1”, and when the DC mask curve identifier B is set, the message is “Level2”.

Next, when DC mask curve change is directed with the cursor key4a, the message displayed on LCD5is changed. When the confirmation key4bis pressed, the changed DC mask curve is set.

FIG. 9shows a flowchart of the process that is invoked when the DC mask curve selection mode is set with the cursor key4aoperation. The LCD5displays a message indicating a setting screen for adjusting sound quality together with a message indicating the identifier of the DC mask curve currently stored in the DC mask curve identifier memory13aof EEPROM13(S61). Namely, when identifier A is stored in the DC mask curve identifier memory13aof EEPROM13, the message is “Level1”, and when identifier B is stored, the message is “Level2”.

Next, it is judged whether the controller to direct a change of the currently set DC mask curve is operated (S62). When the cursor key4ato change DC mask curves is operated (Yes at S62), it is judged whether the identifier of the currently set DC mask curve is A by referring to the DC mask curve identifier memory13a(S63).

If the stored identifier is A (Yes at S63), LCD5displays the message “Level2” (S64) and the identifier B is stored in the Temp memory14aof RAM14. If the stored identifier is not A (No at S63), the LCD5displays the message “Level1” (S66) and the identifier A is stored in the Temp memory14aof RAM14.

In S64or S65, when the identifier is stored in the Temp memory14a, the voltage-current characteristic adjustment is executed based on the DC mask curve identified by the identifier stored in the Temp memory14a(S68). The voltage-current characteristic adjustment is the process shown in the flowchart inFIG. 11. Next, it is judged whether the confirmation key4bis operated (S69). If the confirmation key4bis operated (Yes at S69), the identifier stored in the Temp memory14ais stored in the DC mask curve identifier memory13aof EEPROM13(S70).

As described above, according to the communication apparatus1of the fourth embodiment, two DC mask curves are stored in ROM12, and a user can select any of the two DC mask curves to be applied to adjustment of the voltage-current characteristic. Therefore, the voltage-current characteristic is adjusted based on the selected DC mask curve. If an oscillation is generated in that condition, the other DC mask curve can be selected.

In particular, when a telephone set is built in the communication apparatus1and the connection with the network is established, the user can select and set the DC mask curve so as to suppress noise caused by the oscillation generated in the network checking with a handset receiver.

In the first embodiment of the communication apparatus1, the communication apparatus is a multiple function processing machine which has multiple functions such as, facsimile, copier, scanner, telephone. However, the invention is not limited to this embodiment. The communication apparatus1may a single function communication apparatus. In the first embodiment of the communication apparatus1, the communication apparatus1has a voltage detection section which detects the line voltage, and the line current is calculated based on the line impedance. The communication apparatus1may have a current detection section instead of a voltage detection section, and the line voltage is calculated based on the line impedance. The communication apparatus1may also have both a voltage detection section and a current detection section. Further, in the first embodiment of the communication apparatus1, when an oscillation is generated in the network, the oscillation frequency is acquired and the voltage-current characteristic is selected corresponding to the oscillation frequency. When an oscillation is generated in the network, a oscillation level may be detected, and the voltage-current characteristic curve is selected corresponding to the oscillation level.