CA1230162A

CA1230162A - Data transmission system

Info

Publication number: CA1230162A
Application number: CA000462602A
Authority: CA
Inventors: John Fitzgerald
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-09-10
Filing date: 1984-09-06
Publication date: 1987-12-08
Also published as: JPS60197036A; GB2146509B; EP0139405A3; US4799213A; GB8324310D0; GB2146509A; EP0139405A2

Abstract

DATA TRANSMISSION SYSTEM
Abstract of the Disclosure Voice and data signals can be transmitted simultaneously yet independently over a subscriber telephone line. The speech and data use the same cable pair as far as the local exchange but thereafter they may be routed to the same destination or to different destinations, as desired. Each terminal unit (A or B) has a low pass filter (2) to isolate the voice and data.
A data switch (5) switches an oscillator (6) by FSK.
Switch (7) locks onto the Data Ready Interface (8).
Bandpass filter (9) feeds the signal to the telephone line. A loop back switch LBS provides for testing from the exchange.

Description

123()1~;2 03 This invention relates to data transmission 04 systems, in particular to data transmission over 05 subscriber telephone cables.
06 It is already known to transmit data signals 07 over twisted wire pairs and systems are already known 08 which enable data and voice transmission to carry on 09 simultaneously over the same physical pair of conductors.
11 However these present systems tend to be 12 expensive and cannot operate at line losses much above 13 25dB. It is an object of the present invention to 14 provide a system for transmitting voice and data independently but simultaneously over a pair of wires 16 in an economical way, and with line losses up to 40dB.
17 According to one aspect of the present 18 invention there is provided a transmission system in 19 which data and voice signals can be transmitted independently yet simultaneously over a telephone 21 line, the system comprising first and second 22 terminals, the terminals being connectible bet~een a 23 data terminal unit or a telephone exchange and the 24 telephone line, each terminal comprising: first and second bandpass filters for selecting first and second 26 carrier signals in respective transmit and receive 27 circuits of the terminal; the receive passband of the 28 first terminal being centered on the same carrier 29 frequency as the transmit pa~sband of the second terminal and the transmit passband of the first 31 terminal being centered on the same carrier frequency 32 as the receive passband of the second terminal; the 33 transmit circuit including a modulator for modulating 34 the first carrier signal with data to be transmitted from the transmit circuit to the line; a demodulator 36 in the receiver circuit for recovering received data 37 from the second carrier received from the line; and 38 amplifier in the receive circuit for amplifying the lZ30162 02 received data; and a low pass LC passive filter 03 connected to receive both voice and carrier signals 04 from a telephone line and having a passband effective 05 to pass only voice signals and thus isolate the voice 06 and data signals, and having a loss frequency 07 characteristic effective to minimize the effects of 08 low frequency interference signals affecting the data 09 transmission.
Preferably the means for modulating the 11 carrier for transmission uses frequency shift keying 12 techniques and comprises an oscillator, an active 13 bandpass filter, and an electronic switch which 14 controls the filter characteristics to produce the two 15 FSK frequencies.
16 According to yet another aspect of the 17 present invention one of the terminals has a loopback 18 switch which has a first state in which normal 19 transmission and reception takes place and a second state in which a data signal received and recovered is 21 looped back into the modulator and retransmitted from 22 that terminal to facilitate a test procedure to test 23 the system.

lZ30~2 01 _ 3 _ 02 In a preferred embodiment the carrier detection circuit 03 comprises an amplifier for amplifying the incoming signal, a 04 balanced rectifier for rectifying the amplified signal, and a 05 Schmitt trigger circuit for receiving the rectified signal and 06 effective to clamp the output in the event of the level of the 07 carrier signal falling below a predetermined minimum value.
08 According to yet another aspect of the present invention 09 the terminal has a synchronous oscillator circuit for providing a synchronising clock signal from the terminal, the oscillator 11 circuit comprising an input for receiving decoded data, an edge 12 detector for producing a short pulse for each data 13 transmission, and a binary counter arranged to count a local 14 oscillator signal which is reset by the short data transmission pulses to provide the synchronising clock signal.
16 In order that the invention can be clearly understood 17 reference will now be made to the accompanying drawings, 18 wherein:
19 Fig. 1 is a block circuit diagram of a data and voice transmission system according to an embodiment of the present 21 invention;
22 Fig. 2 is a detailed circuit diagram of the FSK decoder 23 and carrier detection circuit of Fig. 1, 24 Fig. 3 is a detailed circuit diagram of the synchronising oscillator output circuit of Fig. l;
26 Fig. 4 is a detailed circuit diagram of the transmit 27 oscillator and data switch of Fig. l;
28 Fig. 4A is a detailed circuit diagram of the transmit 29 bandpass filter circuit of Fig. 1:
Fig. 5 is a block diagram of a data and voice 31 transmission system according to another embodiment of the 32 invention in which one of the subscribers is a dedicated data 33 terminal, and 34 Fig. 6 shows circuit details of a loopback switch for test procedures, together with interface circuits ~or the 36 subscriber terminal A shown in Fig. 1.
37 Referring to Fig. 1 there i~ shown a subscriber terminal 38 on the left hand side and an exchange terminal on the right 39 hand side connected b~ a nominal 140 Ohm ~, line which would normally be a twisted wire pair. The two terminals are similar and comprise similar components except that the transmit frequency for the subscriber terminal is 40kHz and the receive frequency is 64kHz 5 whereas the exchange terminal transmits at 64kHz and receives at 40k~z. This could be the other way round.
The system provides a data link over an existing telephone network, in such a way that the transmission of data over a cable pair does not affect the simultaneous lO transmission of speech. The speech and data use the same cable pair as far as the local exchange, but thereafter they may be routed to the same destination or to different destinations, as desired.
Carrier-frequency techniques are used to provide t a full duplex data channel, together with a completely independent normal telephone service, over a single unloaded cable pair. A subscriber unit interfaces directly with the data terminal equipment, which may be a visual display unit, a word processor, facsimile equipment, or telemetry and security equipment. The telephone may continue to be used in the normal manner to originate and receive speech calls by using audio frequencies. At the exchange, the exchange unit separates the derived data channel from the physical voice channel.
Alternatively it is possible to patch the exchange with a passive bridging filter so that only two terminal units are required, one at each subscriber terminal. This would be applicable to a "dedicated"
telephone line and is described later with reference to Fig. 5.
Yet another alternative is for example a PABX
which can handle carrier frequencies, would be once again to have no terminal unit i.e. in the exchange, simply a terminal unit at each subscriber i.e. a call-connect system. We have found that the equipment to be d~sc,ibed can cope with a line loss of up to 40dB. Therefore even with a 20dB loss in the exchange, caused mainly by the ~Z3V~62 fairly massive loading coils, a very significant telephone line length can still be accommodated for example at 64kHz the attenuation of a 0.5mm LDN cable is about 6dB/km giving a workable line length of over 3km for a 20dB loss.
We have found that the main guard against impulsive noise components in the speech band resides in the passive low pass LC filters 2 and 2A and the design of this is critical. Passband loss must be low and stop band loss high with good impedance presentation to the telephone handset with d.c. passage up to lOOmA. The loss over the frequency range 200-4000kHz is less than 1.5dB where the send and terminating impedance is 600 ohms. At 150 ohms termination and send impedance the loss is not greater than:-7.5 dB at 4 kHz 1.0 " " 6 "
18.0 " " 10 "
35.0 " " 16 "
40.0 " " 20 "
50.0 " " 30 "
70.0 " " 50 "
80.0 " " 70 "
80.0 " " 100 "
Each terminal has a transmit and receive circuit and an audio path 1 via a 3.4kHz low pass filter 2 which isolates the audio signals from the data-signals. In the transmit circuit a data terminal input 3 is connected via a V24 interface 4 to a data switch 5 which switches an oscillator circuit 6 to modulate the data by frequency shift keying onto the transmit carrier at 40kHz. Switch 7 locks on to the ~ata Terminal Ready interface 8. The bandpass filter 9 feeds the signal to the telephone line.
In the receive circuit a received data signal passes through a receive bandpass filter 10 centered at 64kHz, a receive amplifier 11 and a receive signal detector 12. A carrier detect circuit 13 detects the presence of a carrier having at least a predetermined 1~3(~162 minimum level, below which the equipment is clamped in a non-operate state. A synchronising circuit 14 provides a data clock signal for synchronising the data terminal equipmen. for synchronous operation although a synchronous operation is normal since this data transmission system is designed to be transparent to the transmission of data. A V24 output interface 15 couples the signal detector output to the data terminal equipment.
The above description relates to the left hand side of the block diagram 1, and on the right hand side similar components have been given similar reference numerals with the suffix A. I
The system operates as follows. One of the terminals connected to a data terminal unit (DTU) receives a Data Terminal Ready (DTR) signal from the DTU.
This switches the data-bearing carrier frequency to line.
At the same time the terminal A (for example) returns a Data Set Ready tDSR) signal to the DTU. Alternatively the same interface protocol can be achieved using a Request to Send (RTS) signal and returning a Clear to Send (CTS) signal: this procedure also switches the carrier frequency to line. Data can now be received by the terminal A from the DTU and sent to line. On receipt of the carrier frequency at the far end, terminal B, it is converted to data and fed to the Received Data interface at the far end DTU. The Carrier Detection Circuit 13A detects the incoming carrier and produces a 12volt d.c. output. It passes the signal to the DTU
thereby signifying that valid carrier signals are being received from the originating DTU.
Referring now to Fig. 2 the receive FSK decoder 12, carrier detector circuit 13, receive amplifier 11, and receive interface circuit 15, of Fig. 1, are all shown in detail. The input to the decoder is buffered by a pre-amplifier Al with resistors Rl and R2 performing the gain function. The output of Al is a.c. coupled by capacitor Cl to the input pin of the FSK integrated - 7 - j circuit decoder DEC. This is a phase-locked loop decoder whose frequency is set by resistors R3, R4 and capacitor C2. The centre frequency is derived from the relationship:-fo (fl + f2)/2 where fl and f2 are the mark and space frequencies.
The system bandwidth is set by resistor R5.
Capacitor C3 sets the loop filter time constant and the loop damping factor. Capacitor C4 and resistor R6 form a post-detection filter. Resistor R7 provides positi~e feedback of the FSK decoder chip, which is infact a comparator, to facilitate rapid transactions between logic output states.
The presence of an incoming carrier of at least a predetermined minimum level is detected by the carrier ' detection circuit 13 of Fig. 1. The function of the carrier detect circuit is to determine whether the incoming data signal is a valid signal or not. Thus if the loss of the telephone line ~or telephone line plus PABX loading coils in an embodiment where the PABX switch can transmit data and the transmission terminals A and B
are both located at subscriber locations) exceeds 4ndB 'i then the carrier detect circuit will lock out. In detail in Fig. 2 this circuit is enclosed in bro~en line as indicated. The incoming amplified signal (referring to Fig. 2) is further amplified by amplifier A2, the resistors R8 and R9 performing the gain function. The signal is then rec.ified by a full wave rectifier arrangement FWR comprising an amplifier A3, diodes D1 and D2, capacitor C5, and resistor R10.
The rectified signal is fed to a Schmitt trigger circuit STC whose bias is set by resistors Rll, ~12, R13 and R14. The hysteresis of the circuit is set by resistor R15 for ap~roximately 3dB, and the switching 3S point is set by resistor R16 for a minimum level of -40dB
ref., the voltaye corresponding to QdBm~140 Ohms.
The output of the Schmitt trigger STC on the one lZ~ 2 hand drives 2 light emitting diode LED which indicates, when illuminated, the validity of the incoming carrier signal and is extinguished when invalid, on the other hand the output is fed to the V24 interface corlnection terminal OPl, and via line Ll to the negative input of logic circuit LCl.`
The decoded data output signal from the FSK
decoder DEC is also fed to the negative terminal of the logic circuit LCl whose positive terminal is set by voltage divider resistors R31, R32 to either a positive or negative 6 volt DC potential (M.P.Q.) depending on the desired logic state of the output of circuit LCl when no incoming signal is detected. The output of circuit LCl is fed to a V24 interface connection, i.e. part of block lj 15 in F'ig. 1.
Referring now to Fig. 3 for synchronous versions of the data transmission system receive and transmit synchronous oscil]ators (synchronising circuit 14, Fig.
1) are provided. These form a basic part of the equipment and are either connected in circuit for synchronous versions or are left unconnected for asynchronous versions of the equipment.
The transmit clock source comprises a crystal oscillator CO (in broken line) which consists of an inverting operational amplifier A4, a crystal CRl having a frequency of 2.4576MHz, start-up capacitors C6 and C7, and resistor R17. The oscillator output is fed by an integrated circuit buffer inverter A5 to the clock input of an integrated circuit binary counter BC whose reset is held low.
The counter outputs BCl to BC5 are at twice the data frequency and they are fed to an inte~rated switching circuit SC. Only one switch is in the operate state at any one time. The lowest switch number corresponds to the highest baud rate as shown below:-Switch position: BCl BC2 BC3 BC4 BC5Baud ra~e: 3600 480~ 2400 1200 600 The receive clock circuit is synchronised as ~2~V~;2 follows. The data output from the decoder DEC is fed via line L2 to a CMOS edge detector circuit ED (enclosed in broken line) which produces a short pulse for each data transition. The pulses vary in sympathy with the received data and are used to reset an integrated circuit binary counter BC'. The clock signal to this device is derived from the crystal oscillator CO via ~ine L3. The outputs of the binary counter ~C' are derived by dividing the crystal oscillator frequency by 2 where n = 10, 11, 12, 13, 14 and 15.
These outputs are fed to a second integrated switch circuit SCI ~hose switch positions 1 to 5 correspond to the baud rates as shown below.-Switch position: 1 2 3 4 5 1~ Baud rate: 9600 4800 2400 1200 600 The wave form is then inverted by an integratedbuffer inverter A7 to ~ive a negative edge at the centre of each da-ta bit (i.e. the clock runs at twice the data fre~uency). The resulting waveform is fed via a level shift circuit A8 to the V24 interface connection.
Referring to Fig. 4 the transmit oscillator 6 and data switch 5 will now be described. The oscillator is a frequency keying oscillator which produces a clipped sine wave which is filtered to produce a pure sine wave fundamental waveform. An amplifier A9 forms part of a unity gain inverting amplifier. Its output is set by diodes D4 and D5 and resistor R18 controls the current through the diodes. Amplifier A10, resistors Rl9, R20, R21 and R22, potentiometers R22, R24 and capacitors C8 and C9 form an active 40kHz (or 64kHz) bandpass filter.
In the input data switch the transistor TR1 acts as a switch to shunt potentiometer R24 and resistor R22 across potentiometer R23 and resistor R21, thus changing the filter characteristics to produce the two FSK
fre~uencies, which are 43.86kHz (level 0~ and 36.14kHz (level 1) or 67.88kHz (level 0) and 60.14kHz (level 1).
For oscillation to occur the phase shift around the oscillator must be 0 or multiples of 360. Therefore ~L~3~

the fre~uency which is phase shifted by 180 through the filter is the oscillating frequency. The gain of the oscillator is controlled by resistor R25 and the output frequency appears at terminal Z.
If fc is the carrier frequency, and ~O is the FSK frequency, then fc ~ fO corresponds to a data "1" and fc + fo corresponds to a data "0".
The transmit bandpass filter g of Fig. 1 shown in detail in Fig. 4a consists of a capacitor C10, inductors Ll and L2, capacitors Cll and C12. It is a passive two-section filter which operates between source and load im2edance of 1 kilohm.
In one embodiment of the invention the two transmission terminals shown in Fig. 1 can be located each at a subscriber station where the data equipment is arranged locally "in-house". Fig. 5 shows this. In addition to the terminals A and B there is provided a passive bridging filter PBF where carrier transmission through the local exchange switeh is not possible (e.g. a

2~ digital exchange). This bridging filter P3F includes data patch connected via a high pass filter HPF with low pass filters LPF allowing the voice or speech path to feed into the exehange for switching as required.
Referring to Fig. 6 there is shown a loopbaek switeh L~S eontrolled by a manual eontrol button on the subscriber terminal A of Fig. 1. This switeh on the subseriber unit provides a loopbaek test faeility when in the operate position (i.e. depressed). ~s shown in the drawing it is in its normal (unoperated) state. When operated it eonneets together the transmit and reeeive paths via terminals 10, 12, 6 and 4; and it holds the Data Terminal Ready DTR interfaee 9 (Fig. 1~ to l12 volts via terminals 3 and 5, thus allowing the data-bearing carrier frequeney to be siwtched by switch 7 (Fig. 1)

3 eontinuously to line via the bandpass transmit filter 9 (Fig. 1). Thus the reeeived data is looped baek to the source.
A V24/V28 interface between the Data Terminal ~23~

Equipmen-t and the Unit A is provided by amplifier A11 which operates as a comparator. When data is to be transmitted from the DTE the Unit A receives either a Data Terminal Ready (~TR) signal or a Request to Send (RTS) signal which is generated by the DTE. The unit responds by returning a Data Set Ready (DSR) or Clear to Send (CTS) signal, respectively, to the DTE. This is a level 1 interface which signifies that data can now be transmitted by the DTE.
On receipt of a DTR or RTS signal the input of amplifier All will go to a high state, which will cause the logic level at terminal AB to be low. This in turn switches transistor TR2 off, which enables amplifier A12, thus allowing the carrier frequency to be transmitted to line via the bandpass transmit filter.
Similarly, data is presented to the Unit A in the V24/V28 format, i.e. 12 volts to + 3 volts working into a load between 3 and 7 kilohms. The ouput of amplifier A13 of FIg. 4 is the inverse of the input data;
23 this switches transistor TRl on/off in sympathy with the incoming data, as previously explained, in the data switch 5.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A transmission system in which data and voice signals can be transmitted independently yet simultaneously over a telephone line, the system comprising first and second terminals, the terminals being connectible between a data terminal unit or a telephone exchange and the telephone line, each terminal unit comprising; first and second bandpass filters used for selecting respective first and second different carrier signals in respective transmit and receive circuits of the terminal, the receive passband of the first terminal being centered on the same carrier frequency as the transmit passband of the second terminal and the transmit passband of the first terminal being centered on the same carrier frequency as the receive passband of the second terminal; said transmit circuit including a modulator for modulating the first carrier signal with data for transmission from the transmit circuit to the line; said receive circuit including a demodulator used for recovering data from the second carrier signal received from the line and an amplifier used for amplifying the recovered data; and a low pass LC passive filter connected to receive both voice and carrier signals from the telephone line and having a passband effective to pass only voice signals and thus isolate the voice and data signals and having a loss frequency characteristic effective to minimize the effects of low frequency interference signals affecting the data transmission.

2. A transmission system as claimed in claim 1, wherein the modulator for modulating the transmit carrier signal uses a frequency shift keying technique and comprises a frequency keying oscillator, an active bandpass filter connected to and determining the oscillator frequency, and an electronic switch which is so connected as to change the filter characteristics when it is operated to thereby produce the two FSK frequencies.

3. A transmission system as claimed in claim 1, wherein one of said terminals has a transmit and a receive path for connection to said data terminal unit or exchange, and a manually operable loopback switch connected to the transmit and receive paths and having a first state in which normal transmission and reception takes place between the unit or exchange and said terminal, and a second state in which data from the transmit path is looped back into the receive path to facilitate a test procedure to test the system.

4. A transmission system as claimed in claim 1, further including a carrier detection circuit comprised of an amplifier for amplifying the incoming signal, a balanced rectifier for rectifying the amplified signal and having a switching point set to clamp the output in the event of the level of the carrier signal falling below a predetermined minimum value corresponding to the switching point.

5. A transmission system as claimed in claim 1, in which each terminal is arranged for connection at a subscriber location on a PABX or equivalent, said system comprising also an exchange bridging filter including a low pass filter coupled to each telephone line associated with the respective terminal and effective to isolate from the voice path in the exchange the data carrier signals carried by said telephone lines and a high pass filter coupled to each said telephone line to carry only the data carrier signals directly between said telephone lines.