JP2965593B2 - Demodulator and data transmission system - Google Patents

Description

The present invention relates to a demodulator for demodulating modulated data, and a data transmission system for modulating and transmitting data.

(Prior art) For example, in a data transmission system or the like, when data is transmitted by modulating data on a transmission side and demodulating a received signal on a reception side to reproduce data, communication at a predetermined modulation rate (baud rate) is performed. Is to do. That is, the conventional modulator and demodulator are configured to support a single baud rate.

Therefore, signals cannot be transmitted / received at a baud rate different from a predetermined baud rate, and only communication between specific partners can be performed. Further, with such a modulator, if the bit rate of the input data changes, the baud rate naturally changes, so that a data terminal or the like that supplies data to the modulator is limited to a specific type. Would.

Therefore, in order to cope with a plurality of types of baud rates, a method of replacing a circuit board or switching modes by manual operation of a plurality of circuits can be considered. However, in these methods, work or operation is extremely troublesome, and when a general user replaces a circuit board in the former method, there is a possibility that the device may be damaged due to a mistake in work. . Furthermore, if the circuit board to be mounted or the mode to be set is wrong, the desired signal cannot be transmitted and received.

In addition, when the above method is used in a demodulation device in a receiving device such as a data transmission system, since the above methods all set a baud rate by a user, the baud rate is fixed to a certain baud rate for a certain period of time. Therefore, it is difficult to apply the present invention to a case where signals of a plurality of types of baud rates are randomly received, that is, a case where the baud rate of a received signal frequently changes.

(Problems to be Solved by the Invention) As described above, the conventional modulator and demodulator are configured to support only a single baud rate.
The data transmission system configured using the modulator and the demodulator has been less versatile.

Further, a method of replacing a circuit board, switching modes by manual operation of a plurality of circuits, and the like have been considered. However, as described above, the method has many problems and is not effective.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a highly versatile modulator and data device that is optically reliable without requiring any extra work or operation. To provide a transmission system.

[Structure of the Invention] (Means for Solving the Problems) In the first invention of the present application, a baud rate detecting means for detecting a baud rate of modulated data obtained by modulating data, and a baud rate detecting means for detecting a baud rate of the modulated data Waveform shaping the modulated data with a preset bandwidth, for example, a digital filter, a waveform shaping means including a bandwidth setting means and a bandwidth table, and demodulating the modulated data waveform-shaped by the waveform shaping means, for example. A demodulator body such as a digital malfunctioner, and a signal of a frequency set in advance corresponding to the detection result of the baud rate detecting means is removed from the data demodulated by the demodulator body, for example, a low-pass filter, An undesired wave removing means comprising an off-frequency setting means and a cut-off frequency table; Rate detection means includes clock recovery means such as the reproducing clock signal synchronized with the modulation data, for example, a clock reproduction unit, first the clock signal reproduced by the clock reproducing means in a predetermined
The level rises at a predetermined rate during a period in which the clock signal is at a predetermined level, and the clock signal is at a predetermined second level different from the first level.
Generating a sawtooth wave signal whose level is reduced at the predetermined rate during a period in which the signal is at a level, for example, a comparator, a NOR,
A gate, an AND gate, a delay, an inverter, an up / down counter, and a D / A converter, and a sawtooth signal generation unit, and full-wave rectification of the sawtooth signal generated by the sawtooth signal generation unit, for example, a bridge connection Rectifying means comprising four diodes and a differential amplifier, a smoothing means comprising, for example, an LC circuit for smoothing a signal which has been full-wave rectified by the rectifying means, and a signal smoothed by the smoothing means. Judge the baud rate of the modulation data based on the level of, for example, A / D
The demodulation device was configured to include a converter and a baud rate determining means including a ROM.

Further, in the second invention of the present application, a modulation device main body such as a digital modulator that modulates data to be transmitted into modulated data, a bit rate detection unit that detects a bit rate of the data to be transmitted, Limiting the bandwidth of the modulated data to a predetermined frequency range corresponding to the detection result of the bit rate detection means by digital processing, for example, a band limitation comprising a digital filter, a bandwidth setting means and a bandwidth table Means for removing a signal of a frequency set in advance corresponding to the detection result of the bit rate detection means from the modulated signal, for example, a low-pass filter, a cutoff frequency setting means and a cutoff frequency table comprising a cutoff frequency table 1 is set in advance corresponding to the detection result of the unnecessary wave removing means and the bit rate detecting means. Generating a clock signal having a frequency, and supplying the clock signal to at least the modulation device main body and the band limiting unit, comprising, for example, a clock generation unit, a clock frequency setting unit and a clock generation unit including a clock frequency table, and Rate detecting means for converting the data to be transmitted into a signal of a predetermined form such as an NRZ signal, for example, a signal converting means such as an interface, and a signal obtained by the signal converting means being a predetermined first level The level of which rises at a predetermined rate during a certain period, and whose level decreases at the predetermined rate during a period when the signal obtained by the signal conversion means is at a predetermined second level different from the first level. Generate signals, eg comparators, NOR gates, A
An ND gate, a delay, an inverter, an up / down counter, and a sawtooth wave signal generating unit including a D / A converter, and full-wave rectifying the sawtooth wave signal generated by the sawtooth wave signal generating unit, for example, a bridge connection is performed. Rectifying means comprising four diodes and a differential amplifier, smoothing means for smoothing a signal which has been full-wave rectified by the rectifying means, for example, an LC circuit, and level of the signal smoothed by the smoothing means. Determine the bit rate of the data to be transmitted based on, for example,
A data transmission system is provided with a modulation device having an A / D converter and a bit rate determination means including a ROM provided on the transmission side, and further provided with the demodulation device of the first invention on the reception side.

(Operation) By taking such a measure, the modulation device applied to the transmission side detects the bit rate of the input data to be transmitted and automatically adjusts the clock frequency to correspond to this bit rate. , Operating conditions such as a band limiting bandwidth and an unnecessary wave removal frequency are set. Accordingly, the modulator changes its corresponding baud rate according to the bit rate of the data to be received, and follows the bit rate of the data to be transmitted.

In the demodulation device applied to the receiving side, the baud rate of the received signal is detected, and the operating conditions such as the bandwidth of the band limitation and the removal frequency of unnecessary waves are automatically set to correspond to the baud rate. Therefore, it automatically follows the received signal and modulates signals of various baud rates.

These enable data transmission at various baud rates.

The bit rate of the data to be received and the baud rate of the received signal are detected by reproducing a clock signal synchronized with the modulated data or converting the data to be transmitted into a signal of a predetermined form.

During a period when the signal obtained above is at a predetermined first level, the level increases at a predetermined rate, and when the signal obtained above is at a predetermined second level different from the first level, the predetermined level is increased. Generation of a sawtooth wave signal whose level decreases at the rate of

 Full-wave rectification of the sawtooth signal generated above.

 Smoothing of the full-wave rectified signal above.

Determination of the baud rate or bit rate based on the level of the signal smoothed above.

This is realized by a combination of such simple processing.

Hereinafter, a data transmission system according to an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of a modulation device in the data transmission system. This modulator comprises an interface 11, a digital modulator 12, a digital filter 1
3, D / A converter 14, low-pass filter (LPF) 15, frequency detector 16, clock frequency setting means 17a, bandwidth setting means 17b
And a ROM 18 in which a clock frequency table 18a, a bandwidth table 18b, and a cutoff frequency table 18c are set, and a clock generation unit 19.

FIG. 2 is a block diagram showing a configuration of a demodulation device in the data transmission system. This demodulator is A /
D converter 21, digital filter 22, digital demodulator 2
3, a clock generator 24, a low-pass filter (LPF) 25, an interface 26, a frequency detection unit 27, a control unit 28 including a bandwidth setting unit 28a and a cut-off frequency setting unit 28b, a bandwidth table 29a And a ROM 29 in which a cutoff frequency table 29b is set.

FIG. 3 shows the frequency detector 16 in FIG.
FIG. 3 is a circuit diagram showing a detailed configuration example of a frequency detection unit 27 in the figure.

This frequency detector consists of a comparator 31, a NOR gate 3
2, AND gate 33, delay 34, inverter 35, up / down counter 36, D / A converter 37, LC circuit 38, amplifier 39, C
R circuit 40, diodes D1, D2, D3, D4, resistors R1, R2, R3, R4,
Differential amplifier 41, LC circuits 42, 43, amplifier 44, A / D converter 45,
It is composed of ROM46.

Thus, in the present frequency detecting section thus configured, the input signal is compared with a predetermined level by the comparator 31 to obtain a complete binary signal. The output signal of this comparator 31 is NOR
Input to each of gate 32 and AND gate 33.
Further, the output signal of the comparator 31 is delayed by a predetermined time by a delay 34, and after its logic is inverted by an inverter 35, is input to each of a NOR gate 32 and an AND gate 33. As a result, the output signal of the NOR gate 32 becomes a signal representing the falling point of the input signal, and the output signal of the AND gate 33 becomes a signal representing the rising point of the input signal.

That is, for example, when the output signal of the comparator 31 is the signal shown by S1 in FIG. 4, the output signal of the inverter 35 becomes S2 in FIG. The AND of S1 and S2 is S3 in FIG. 4, and the NOR is S4 in FIG. Here, as can be seen from the figure, S3 is synchronized with the rise of S1, and
S4 is at "H" level in synchronization with the fall of S1.

The output signals of the NOR gate 32 and the AND gate 33 are input to the up / down counter 36. The up / down counter 36 counts a clock which is n times (for example, about 3 to 5 times) the system clock.
The countdown is performed using the output signal of the NOR gate 32 as a trigger, and the countup is performed using the output signal of the AND gate 33 as a trigger. The count value of the up / down counter 36 is converted into an analog signal by the D / A converter 37. The output signal of the D / A converter 37 is S5 in FIG.

The output signal of the D / A converter 37 is input to a full-wave rectifier circuit having a bridge connection of diodes D1 to D4 via an LC circuit 38, an amplifier 39 having a gain of + A, and a CR circuit 40. Then, the full-wave rectifier circuit, the resistors R1 to R4, the differential amplifier 41, the LC circuits 42 and 43, and the amplifier (gain + A) 44 perform frequency / voltage conversion. At this time, the output signal of the differential amplifier 41 is the fourth signal.
The output signal of S6 in FIG. 4 and the amplifier 44 is S7 in FIG.

The output signal of the amplifier 44 is supplied to the A / D converter 45 and the ROM.
At 46, it is converted into digital data. This digital data becomes frequency information of the input signal.

Next, the operation of the data transmission system configured as described above will be described. First, on the transmitting side, for example, data output from a data terminal (not shown) (hereinafter, referred to as transmission target data) is provided to a modulation device via a digital line (not shown). In the modulator, the interface 11 converts the input transmission target data into an NRZ signal suitable for the modulator. The transmission target data converted into the NRZ signal is input to the digital modulator 12. The digital modulator 12 is composed of, for example, a digital signal processing circuit (DSP), and modulates the input transmission target data to generate modulated data. That is, the digital modulator 12 functions as a modulation device main body.

Since the modulated data thus obtained has components other than the transmission band, spectrum shaping is performed by the digital filter 13 and band limitation is performed. The modulated data whose band has been limited by the digital filter 13 is D / A
The signal is converted into an analog signal for transmission by the converter 14 and becomes a modulated signal. The modulated signal mainly includes the D / A converter 14
And unnecessary waves such as clock components that are mixed in the D / A conversion processing. Therefore, the modulated signal output from the D / A converter 14 is passed through the LPF 15 to remove unnecessary waves. The modulated signal obtained by performing the modulation process in this manner is supplied to, for example, a frequency conversion unit (not shown) and frequency-converted, and then transmitted from, for example, a transmission antenna. Here,
Digital modulator 12, digital filter 13, and D / A
The converters 14 use the clock signal generated by the clock generator 19 as an operation clock.

Incidentally, the transmission target data converted into an NRZ signal by the interface 11 is also input to the frequency detection unit 16. The frequency detector 16 detects the frequency of the transmission target data by the above-described operation. The detected frequency is the same as the bit rate of the data to be transmitted, and the frequency information output by the frequency detection unit 16 is used as the bit rate information by the control unit 17.
Given to.

The control unit 17 includes, for example, a microcomputer as a main control circuit, and includes a clock frequency setting unit 17a, a bandwidth setting unit 17b, and a cutoff frequency setting unit 17c. Here, the clock frequency setting means 17a reads information of a clock frequency preset in the clock frequency table 18a of the ROM 18 (hereinafter, referred to as clock frequency information) corresponding to the input bit rate information,
The read clock frequency information is set in the clock generator 19. The clock generator 19 changes the frequency of the clock signal generated in accordance with the control from the controller 17, and outputs the clock signal of the set frequency. That is, the clock frequency setting unit 17a, the clock frequency table 18a, and the clock generation unit 19 constitute a clock generation unit.

Further, the bandwidth setting means 17b reads out information on the bandwidth of the digital filter 13 (hereinafter referred to as bandwidth information) preset in the bandwidth table 18b of the ROM 18 in accordance with the input bit rate information. The read bandwidth information is set in the digital filter 13. The digital filter 13 changes the bandwidth according to the control from the control unit 17, and outputs only the component of the set bandwidth. That is, the digital filter 13, the bandwidth setting unit 17b, and the bandwidth table 18b constitute a band limiting unit.

Further, the cut-off frequency setting means 17c provides information on the cut-off frequency of the LPF 15 preset in the cut-off frequency table 18c of the ROM 18 corresponding to the input bit rate information (hereinafter referred to as cut-off frequency information).
Is read, and the read cutoff frequency information is
Set to. The LPF 15 changes the cutoff frequency according to the control from the control unit 17, and removes the component of the set cutoff frequency. That is,
The LPF 15, the cut-off frequency setting means 17c, and the cut-off frequency table 18c constitute a first unnecessary wave removing means.

The control unit 17 performs the process according to the following procedure. That is, the fifth
As shown in the figure, first, in step 5a, the process waits until communication occurs, and when communication occurs, step 5b
, The frequency information output by the frequency detection unit 16, that is, bit rate information is input. And then the steps
In 5c, the clock frequency information corresponding to the bit rate information input in step 5b is stored in a clock frequency table.
The data is read from 18a and set in the clock generating means 19 in step 5d. Next, in step 5e, the control unit 17 reads bandwidth information corresponding to the bit rate information input in step 5b from the bandwidth table 18b, and sets this in the digital filter 13 in step 5f.
Further, in step 5g, the control unit 17 reads out the cutoff frequency information corresponding to the bit rate information input in step 5b from the cutoff frequency table 18c, and sets this in the LPF 15 in step 5h. Then control unit 17
Ends the processing.

On the other hand, the receiving side receives the modulated signal transmitted from the transmitting side by, for example, a receiving antenna (not shown) as described above, and further converts the frequency-converted modulated signal by a frequency converting unit (not shown) into a second signal. To the demodulation device. In the demodulation device, the input modulated signal is digitized by the A / D converter 2 to obtain modulated data. The modulated data is input to a digital filter 22, and is subjected to waveform shaping so as to be suitable for demodulation by a digital demodulator. Thereafter, the modulated data is input to the digital demodulator 23 and the clock recovery unit 24,
Demodulation and clock recovery are performed.

The data demodulated and reproduced by the digital demodulator 23 is input to the LPF 25. LPF25 is a digital demodulator 23
Unnecessary waves such as clock components mixed in the data at the time of demodulation in step (1) are removed. The output data of the LPF 25 is input to the interface 26.

The interface 26 receives, in addition to the output data of the LPF 25, the clock signal recovered by the clock recovery unit 24, and outputs the output data of the LPF to, for example, a data terminal (not shown) connected via a digital line (not shown). (Not shown).

Incidentally, the clock signal reproduced by the clock reproducing unit 24 is also input to the frequency detecting unit 27. The frequency detecting section 27 detects the frequency of the modulated data by the above-described operation. The detected frequency is the same as the baud rate at the time of transmission, and the frequency information output by the frequency detection unit 27 is provided to the control unit as baud rate information.

The control unit 28 includes, for example, a microcomputer as a main control circuit, and includes a bandwidth setting unit 28a and a cutoff frequency setting unit 28b. Here, bandwidth setting means 2
8a reads the bandwidth information of the digital filter 22 preset in the bandwidth table 29a of the ROM 29 corresponding to the input baud rate information, and sets the read bandwidth information in the digital filter 22. The digital filter 22 changes the bandwidth under the control of the control unit 28, and outputs only the component of the set bandwidth. That is, the digital filter 22, the bandwidth setting unit 28a, and the bandwidth table 29a constitute a waveform shaping unit.

Further, the cutoff frequency setting means 28b reads out the cutoff frequency information of the LPF 25 preset in the cutoff frequency table 29b of the ROM 29 corresponding to the input baud rate information, and
Set to LPF25. The LPF 25 changes the cutoff frequency according to control from the control unit 28, and removes the component of the set cutoff frequency. That is, the LPF 25, the cut-off frequency setting unit 28b, and the cut-off frequency table 29b constitute a second unnecessary wave removing unit.

The control unit 28 performs processing in the following procedure. That is, the sixth
As shown in the figure, first, in step 6a, the process waits until communication occurs, and when communication occurs, step 6b
, The frequency information output by the frequency detection unit 27, that is, the baud rate information is input. And then step 6c
, The bandwidth information corresponding to the baud rate information input in step 5b is read from the bandwidth table 29a, and is set in the digital filter 22 in step 6d. Next, in step 6e, the control unit 28 reads out the cutoff frequency information corresponding to the baud rate information input in step 5b from the cutoff frequency table 29b, and sets this in the LPF 25 in step 6f. Then control unit 28
Ends the processing.

Thus, according to the present data transmission system, on the transmitting side, the frequency of the operating clock of the modulator, the bandwidth for band control, and the cutoff frequency for removing unnecessary waves are set to values corresponding to the bit rate of the input transmission target data. Is automatically set to In other words, the modulator follows the bit rate of the data to be transmitted by automatically changing its own baud rate.

On the receiving side, the bandwidth for waveform shaping and the cutoff frequency for removing unnecessary waves are automatically set to values according to the baud rate of the received signal. That is, the demodulator follows the baud rate of the received signal by automatically changing its own baud rate.

Therefore, the transmission device can accept data of various bit rates, and can connect various types of data terminals. This greatly increases the versatility of the data transmission system. In addition, since the modulation device and the demodulation device automatically change and set the corresponding baud rate based on the input data to be transmitted and the rate of the received signal, damage due to replacement of the circuit board and mode switching by manual operation are performed. Accurate data transmission can always be performed without causing a mistake or the like.

Further, according to the present embodiment, the detection of the bit rate of the transmission target data and the baud rate of the modulation signal are performed by a simple configuration using a combination of very simple processes, so that the manufacturing cost can be reduced. is there.

Further, the following excellent effects can be obtained only by providing the demodulator shown in FIG. 2 on the receiving side in the existing data transmission system. That is, since the demodulation device can demodulate signals of a plurality of types of baud rates, for example, signals transmitted from a plurality of transmission devices having different baud rates can be received without any trouble. That is, it is possible to cope with transmission devices having various baud rates, and to realize a wider range of data transmission.

Note that the present invention is not limited to the above embodiment,
Various modifications can be made without departing from the spirit of the present invention.

[Effects of the Invention] In the first invention of the present application, a baud rate detecting means for detecting a baud rate of modulated data obtained by modulating data, and a baud rate which is set in advance according to a detection result of the baud rate detecting means. Waveform shaping means for shaping the modulated data; a demodulation device main body for demodulating the modulated data waveform-shaped by the waveform shaping means; and a signal having a frequency set in advance corresponding to the detection result of the baud rate detecting means. An undesired wave removing means for removing from the data demodulated by the demodulation device main body, and the baud rate detecting means comprising: a clock reproducing means for reproducing a clock signal synchronized with the modulated data; and the clock reproducing means. The level increases at a predetermined rate during a period in which the reproduced clock signal is at a predetermined first level, and A sawtooth wave signal generating means for generating a sawtooth wave signal whose level decreases at the predetermined rate during a period in which the signal is at a predetermined second level different from the first level; and a sawtooth wave signal generating means. Rectifying means for full-wave rectifying the sawtooth signal, smoothing means for smoothing the signal which has been full-wave rectified by the rectifying means, and the modulation data based on the level of the signal smoothed by the smoothing means. And a baud rate determining means for determining the baud rate of the demodulator.

Further, in the second invention of the present application, a modulation device main body that modulates data to be transmitted to be modulated data, bit rate detecting means for detecting a bit rate of the data to be transmitted, and a bandwidth of the modulated data Band limiting means for limiting to a preset frequency range corresponding to the detection result of the bit rate detecting means by digital processing,
A first unnecessary wave removing unit for removing a signal of a frequency set in advance corresponding to the detection result of the bit rate detecting unit from the modulated signal, and a first unnecessary wave removing unit corresponding to the detection result of the bit rate detecting unit. A clock signal generating means for generating a clock signal of a set frequency and supplying the clock signal to at least the modulation device main body and the band limiting means, and wherein the bit rate detecting means converts the data to be transmitted to A signal converting means for converting the signal obtained by the signal converting means into a signal of a predetermined form, a level of the signal obtained by the signal converting means being increased at a predetermined rate during a period in which the signal is at a predetermined first level, and A sawtooth wave signal generating means for generating a sawtooth wave signal whose level decreases at the predetermined rate during a period at a predetermined second level different from the first level. Rectifying means for full-wave rectifying the sawtooth signal generated by the sawtooth signal generating means, smoothing means for smoothing the signal rectified by the rectifying means, and smoothing by the smoothing means. A modulation device configured to include a bit rate determining means for determining a bit rate of the data to be transmitted based on the level of the converted signal on the transmission side; and a demodulation device according to the first invention. Was provided on the receiving side to constitute the data transmission system.

As a result, a modulation device and a data transmission system that can be realized with high reliability without requiring any extra work or operation, high versatility, and low manufacturing cost can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a data transmission system according to one embodiment of the present invention. FIG. 1 is a block diagram showing an example of a configuration of a modulation device applied to a transmission side, and FIG. FIG. 3 is a block diagram showing an example of the configuration of a demodulation device according to the present invention.
FIG. 4 is a circuit diagram showing an example of a specific configuration of the frequency detectors 16 and 27 in FIG. 2, FIG. 4 is a diagram showing signal waveforms at various parts of the frequency detector shown in FIG. 3, and FIG. FIG. 6 is a flowchart showing a processing procedure of the control unit 17 in FIG.
4 is a flowchart showing a processing procedure of a control unit 28 in the figure. 11,26 ... Interface, 12 ... Digital modulator, 1
3,22 ... Digital filter, 14 ... D / A converter, 15,25
…… Low-pass filter (LPF), 16,27 …… Frequency detector, 1
7, 28: control section, 17a: clock frequency setting means, 17
b, 28a ... bandwidth setting means, 17c, 28b ... cut-off frequency setting means, 21 ... A / D converter, 23 ... digital demodulator, 24 ... clock recovery unit, 18, 29 ... ROM , 18a ... clock frequency table, 18b, 29a ... bandwidth table, 1
8c, 29b …… Cutoff frequency table.

Claims (2)

(57) [Claims] 1. A baud rate detecting means for detecting a baud rate of modulated data obtained by modulating data, and a waveform for shaping the modulated data with a predetermined bandwidth corresponding to a detection result of the baud rate detecting means. Shaping means, a demodulation device main body for demodulating the modulated data waveform-shaped by the waveform shaping means, and a signal of a frequency set in advance corresponding to the detection result of the baud rate detection means is demodulated by the demodulation device main body. Unnecessary wave removing means for removing from the data, and the baud rate detecting means comprises: a clock reproducing means for reproducing a clock signal synchronized with the modulated data; and a clock signal reproduced by the clock reproducing means. The level rises at a predetermined rate during a predetermined first level, and the clock signal is at the first level. Is a sawtooth wave signal generating means for generating a sawtooth wave signal whose level decreases at the predetermined rate during a period at a different predetermined second level; and the sawtooth wave signal generated by the sawtooth wave signal generating means Rectifying means for rectifying; smoothing means for smoothing a signal which has been full-wave rectified by the rectifying means; baud rate determination for determining a baud rate of the modulated data based on the level of the signal smoothed by the smoothing means And a demodulation device. 2. A modulation apparatus main body that modulates data to be transmitted to generate modulated data, bit rate detection means for detecting a bit rate of the data to be transmitted, and digitally processes a bandwidth of the modulated data. A band limiting unit that limits a frequency range set in advance in accordance with the detection result of the bit rate detection unit; and a signal of a frequency set in advance in accordance with the detection result of the bit rate detection unit. A first unnecessary wave removing unit for removing from the modulated signal, a clock signal of a frequency set in advance corresponding to a detection result of the bit rate detecting unit, and at least the modulation device main body and the band limit A transmitter provided with a clock generating means for supplying the modulated signal to the transmitting means, and a modulation signal output from the modulator. An A / D converter that digitizes the modulated data to obtain modulation data; a baud rate detection unit that detects a baud rate of the modulation data; and a modulation bandwidth that is set in advance according to a detection result of the baud rate detection unit. Waveform shaping means for shaping the waveform; a demodulation device main body for demodulating, by digital processing, the modulated signal whose waveform has been shaped by the waveform shaping means; and a demodulator having a predetermined frequency corresponding to the detection result of the baud rate detecting means. And a second unnecessary wave removing unit for removing a signal from the data demodulated by the demodulation device main body. The demodulation device is provided on the receiving side, and the bit rate detection unit comprises: Signal conversion means for converting data to be transmitted into a signal of a predetermined form; and a signal obtained by the signal conversion means at a predetermined first level. A sawtooth wave signal whose level rises at a predetermined rate during a period, and whose level decreases at the predetermined rate during a period in which the signal obtained by the signal conversion means is at a predetermined second level different from the first level. Means for generating a sawtooth wave signal, rectifying means for full-wave rectifying the sawtooth signal generated by the sawtooth signal generating means, and smoothing means for smoothing a signal which has been full-wave rectified by the rectifying means. And a bit rate determining means for determining a bit rate of the data to be transmitted based on the level of the signal smoothed by the smoothing means, and the baud rate detecting means is synchronized with the modulated data. Clock recovery means for recovering a clock signal; and the clock signal recovered by the clock recovery means is at a predetermined first level. Sawtooth wave signal generation for generating a sawtooth wave signal whose level increases at a predetermined rate during a period and whose level decreases at the predetermined rate during a period when the clock signal is at a predetermined second level different from the first level Means, full-wave rectification of the sawtooth signal generated by the sawtooth signal generation means, smoothing means for smoothing the signal which has been full-wave rectified by the rectification means, A data transmission system comprising: a baud rate determining unit configured to determine a baud rate of the modulated data based on a level of the smoothed signal.