GB1604721A

GB1604721A - Data terminals

Info

Publication number: GB1604721A
Application number: GB706378A
Authority: GB
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1978-02-22
Filing date: 1978-02-22
Publication date: 1981-12-16

Description

(54) IMPROVEMENTS IN OR RELATING TO DATA TERMINALS (71) We, THE GENERAL ELECTRIC COMPANY LIMITED, of 1 Stanhope Gate, London W1A lEH, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to data terminals and in particular to data terminals for installation remote from an associated computer and arranged to communicate with the computer through a telephone network.

According to the present invention a data terminal for installation remote from an associated computer incorporating a receiver unit capable of receiving, over a conventional telephone line, incoming data information carried in frequency modulated form on carrier signals whose frequency lies within a predetermined range and whose power levels are maintained within set maximum and minimum levels, and a transmitter unit capable of transmitting, over said conventional telephone line, data to said associated computer, wherein the receiver unit incorporates signal compression means to which any signal arriving at the data terminal is fed via signal input means, with the signal compression means being arranged to reduce an incoming signal whose power level lies above said maximum level to substantially that maximum power level such that all components of the signal are reduced in substantially equal ratios, thereby to enable the receiver unit to discriminate between incoming data information carrying signals lying within said frequency excursion and between said maximum and minimum power level, and dial tone or other signals, which are of a level higher than said maximum level and which have components falling within said excursion and between said maximum and minimum power levels, and the terminal is capable, after initiation of a data call by a user of the terminal, of automatic reception from, and/or transmission to, said computer of data information on carrier signals.

Discrimination between data information carrying signals and other signals such as, e.g., dial tone is desirable if the terminal also incorporates a transmitter unit for transmitting data information from the terminal to the computer, and is provided with means, including an automatic dialling facility, for setting up data calls automaticaly, since reception of data information on an incoming data call will, in general, start immediately after a telephone line loop has been established, while for an outgoing data call dial tone will be received as soon as the telephone loop line is connected, and an automatic dialling sequence will have to be initiated to establish connection between the terminal and the computer, before transmission of data information can commence.By reducing the signal level of the dial tone, or any other such signals, which are of a level in excess of the level at which incoming data information carrying signals arrive at the terminal, and reducing all the components thereof in substantially equal ratios, it is possible to reduce components lying within the frequency excursion of the carrier signals to below the minimum level of carrier signals, provided that the highest intensity components lie outside the frequency excursion and are of a level appreciably above the level of components lying within the frequency excursion.

Preferably, the data terminal is suitable for installation remote from the computer and incorporates a transmitter unit and a reciver unit adapted to facilitate the transmission to and reception from the computer, over a conventional telephone line, of data information carried on frequency modulated carrier signals, wherein the transmitter unit incorporates attenuator means and selectively operable attenuator by-pass means interposed between a modulation stage of the transmitter unit and an output thereof, and wherein coupling means provide a weak coupling between said output and an input of the receiver unit for injecting a carrier signal, originating in said means for generating the carrier signals and emerging from said modulation stage, into said input of the receiver unit, with the signal passing through the attenuator means during periods when no transmission of data information takes place, and with the signal passing through the attenuator by-pass means during transmission of data information.

The provision of a masking signal is of special importance in cases where the receiver unit of the data terminal incorporates an audio output so as to enable a user to monitor the progress of a data call by means of an electroacoustic device, e.g. a loudspeaker or the earpiece of a telephone handset, connected to the audio output. Particularly if the audio output is connected to a loudspeaker having a volume control, it may be possible to overhear low level intelligible background signals caused by cross-coupling between pairs of telephone lines, usually known as cross talk. Such signals, if present, will generally appear on a telephone line at a level of around -60 dBm, well below the levels for valid speech or other signals on the telephone line, and will usually be masked by such valid signals.However, in the absence of such valid signals, the cross talk could be overheard with a high setting of the volume control. By injecting a signal into the receiver unit at a level appreciably higher than the level of the background signals, the cross talk will be masked and will usually no longer be intelligible. For instance, injection of a masking signal at a level of 5 dB above that of the background signals will normally suffice for masking purposes, i.e. in the case of cross talk in telephone lines this would mean injection of a masking signal at an equivalent of -55 dBm on the telephone line.

The invention will now be described further by way of example only, and with reference to the accompanying drawings, of which: Figure 1 is a schematic diagram of a transmitter unit and a receiver unit in accordance with the present invention; Figure 2 shows a graph illustrating input level vs output level characteristics of the signal compressor shown in Figure l; and Figure 3 shows a graph illustrating a typical spectral content of a dial tone.

Referring now also to the drawings, transmitter unit 1 and receiver unit 2 shown in Figure 1, are adapted respectively to transmit to, and receive from, an associated computer (not shown) frequency shift keyed carrier signals carrying data information in binary code. Furthermore, whilst the frequency shift keyed carrier signal originating from the computer and received by the receiver unit 2, change in time from 1300 Hz to 2100 Hz, corresponding to a data "1" and a data "0" respectively, the data information carrying signals transmitted to the computer by the transmitter unit 1 vary between 390 Hz indicating data "1" and 450 Hz, indicating data "0".Data carrying signals are transmitted by the transmitter unit 1 at a level of around - 15 dBm, and received data carrying signals are arranged to be of a power level lying subsantially between -15 dBm and -40 dBm.

The transmitter unit 1 comprises a modulator stage 13 having a data input 10, a low pass filter 12, an attenuator stage 7, and an attenuator by-pass loop 8 incorporating switching means 9, whose functions will be explained below.

The receiver unit 2 comprises a signal compressor 11, a band pass filter 15, a demodulator stage 14 having a data output 18, and an audio output 16 to enable data calls to be monitored through a loudspeaker (not shown).

Both the transmitter unit 1 and the receiver unit 2, are coupled to line terminals 5 and 6 by means of a line transformer, or hybrid coil 3, with the connections between the line side coil of the transformer 3 and the line terminal 5 incorporating a line loop switch 4. The line terminals 5 and 6 are in use connected to a line pair forming part of a switched telephone network, and switch 4 when in its dosed position completes a line loop to enable data calls to be made, or received.

The signal compressor circuit 11 of the receiver 2 is arranged to reduce any signal which is in excess of -20 dBm to a level of approximately20 dBm, while leaving signals which are at or below -20 dBm substantially unaltered. The relationship between signal input level and signal output level of the compressor circuit is illustrated in Figure 2. To achieve the required reduction of signals in excess of -20 dBm, the compressor circuit is provided with gain control means (not shown) in known manner. Thus, if a signal has a power level in excess of -20 dBm, a reduction of the signal to -20 dBm will result in a substantially proportional reduction of all the components of the signal, including any components which are already below the level of -20 dBm.

Incoming data carrying signals, which, as aforesaid, are of a level not higher than -15 dBm, and not lower than -40 dBm, will thus suffer only a slight reduction, if any, on passing through the compressor circuit 11. The bandpass filter 15 ensures that only signals lying within a frequency excursion of 1300 Hz to 2100 Hz are able to pass from the compressor circuit to the demodulation stage 14, where the data are recovered from the carrier signal. After demodulation, the data are transferred via data output 18 to data processing circuits (not shown).

Figure 3 illustrates a typical spectral content of a dial tone on Post Office lines in the United Kingdom, and it can be seen that the maximum component, lying below 100 Hz, is well above the level of -20 dBm, reaching nearly 0 dBm. Furthermore, the signal levels in the frequency range between 1300 Hz and 2100 Hz are also well above the minimum level of -40 dBm for data carrying signals, which may, in the absence of the compressor circuit 11, cause misoperation of the data processing circuits referred to above.However, by reducing the signal level of dial tone to -20 dBm, and proportionally reducing all the spectral components of the dial tone, a reduction of those components which lie in the range 1300 Hz to 2100 Hz to below the minimum level of -40 dB of data carrying signals is achieved, and hence misoperation of the data processing circuits on account of these components of dial tone is avoided. The compressor circuit thus in effect provides the receiver unit, and thereby the data terminal, with means for discriminating between data carrying signals, lying in a well defined frequency range, and signals such as dial tone having a wide range of spectral components.

The ability of the data terminal to discriminate between data carrying signals and signals, such as dial tone, is of particular importance where the data terminal is to incorporate automatic call connection arrangements (not shown). Thus, on incoming data calls data reception by the receiver unit 2 will commence as soon as the telephone 'ine loop is completed upon closure of switch 4. For an outgoing data call, however, it will be dial tone, which is returned from the telephone exchange as soon as the line loop is connected, and a dial sequence will have to be initiated before connection between the terminal and the computer is established.To enable a suitably arranged control logic incorporated in an automatic call connection arrangement to make the appropriate decisions, i.e. whether the call is outgoing and a dialling sequence has to be initiated, or whether the call is in incoming call, in which case the dialling sequence is to be by-passed, it is necessary therefore, to provide said means for discriminating between data carrying signals and dial tone.

Transmission of data carrying signals is arranged to commence only when connection to the computer has been verified by an incoming carrier signal arriving at the receiver unit 2, again requiring that the receiver unit 2, and the appending control logic are capable of distinguishing between carrier signals and other signals. During transmission the switch means 9 of the attenuator by pass lcop 8 are closed and the data signals, generated, and modulated, in the modulation stage 13 pass through the low-pass filter 12, the by pass loop 8 and the line transformer 3 to the telephone line connected to line terminals 5 and 6, the signals generated in the modulation stage being at a level of approximately - 15 dBm.

During times, when the line loop switch 4 is closed, but no transmission of data takes place, no data are fed into the input of the modulation stage 13, and switch means 9 are open. The modulation stage 13 stays connected to its power supply, and continues to generate signals of a frequency of around 400 Hz, and at a level of approximately - 15 dBm. These signals are attenuated by the attenuator stage 7 by about 30 dB, i.e. they are fed into the line transformer 3, and the telephone line, at a level of around45 dBm.

By arranging the matching between the line transformer 3 and the telephone line to be such as to cause reflection into the input of the receiver stage 2 of a signal in excess of -55 dBm, a masking signal is provided which prevents a terminal user from receiving background signals, caused by cross talk, in intelligible form via the audio output 16, since cross-talk background signals are usually at a level of -60 dBm. As the masking signal is of a level below the minimum level of incoming data carrying signals, and also, as the frequency of the masking signal lies well outside the frequency range of the incoming data carrying signals and the pass band of the filter 15, interference between these and the masking signal is avoided.

A data terminal incorporating the features of the present invention is especially, but not exclusively suited for use in a telephone linked computer system such as, e.g. the proposed Viewdata Service by the Post Office.

WHAT WE CLAIM IS: - 1. A data terminal for installation remote from an associated computer incorporating a receiver unit capable of receiving, over a conventional telephone line, incoming data information carried in frequency modulated form on carrier signals whose frequency lies within a predetermined range and whose power levels are maintained within set maximum and minimum levels, and a transmitter unit capable of transmitting, over said conventional telephone line, data to said associated computer, wherein the receiver unit incorporates signal compression means to which any signal arriving at the data terminal is fed via signal input means, with the signal

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. signal. After demodulation, the data are transferred via data output 18 to data processing circuits (not shown). Figure 3 illustrates a typical spectral content of a dial tone on Post Office lines in the United Kingdom, and it can be seen that the maximum component, lying below 100 Hz, is well above the level of -20 dBm, reaching nearly 0 dBm. Furthermore, the signal levels in the frequency range between 1300 Hz and 2100 Hz are also well above the minimum level of -40 dBm for data carrying signals, which may, in the absence of the compressor circuit 11, cause misoperation of the data processing circuits referred to above.However, by reducing the signal level of dial tone to -20 dBm, and proportionally reducing all the spectral components of the dial tone, a reduction of those components which lie in the range 1300 Hz to 2100 Hz to below the minimum level of -40 dB of data carrying signals is achieved, and hence misoperation of the data processing circuits on account of these components of dial tone is avoided. The compressor circuit thus in effect provides the receiver unit, and thereby the data terminal, with means for discriminating between data carrying signals, lying in a well defined frequency range, and signals such as dial tone having a wide range of spectral components. The ability of the data terminal to discriminate between data carrying signals and signals, such as dial tone, is of particular importance where the data terminal is to incorporate automatic call connection arrangements (not shown). Thus, on incoming data calls data reception by the receiver unit 2 will commence as soon as the telephone 'ine loop is completed upon closure of switch 4. For an outgoing data call, however, it will be dial tone, which is returned from the telephone exchange as soon as the line loop is connected, and a dial sequence will have to be initiated before connection between the terminal and the computer is established.To enable a suitably arranged control logic incorporated in an automatic call connection arrangement to make the appropriate decisions, i.e. whether the call is outgoing and a dialling sequence has to be initiated, or whether the call is in incoming call, in which case the dialling sequence is to be by-passed, it is necessary therefore, to provide said means for discriminating between data carrying signals and dial tone. Transmission of data carrying signals is arranged to commence only when connection to the computer has been verified by an incoming carrier signal arriving at the receiver unit 2, again requiring that the receiver unit 2, and the appending control logic are capable of distinguishing between carrier signals and other signals. During transmission the switch means 9 of the attenuator by pass lcop 8 are closed and the data signals, generated, and modulated, in the modulation stage 13 pass through the low-pass filter 12, the by pass loop 8 and the line transformer 3 to the telephone line connected to line terminals 5 and 6, the signals generated in the modulation stage being at a level of approximately - 15 dBm. During times, when the line loop switch 4 is closed, but no transmission of data takes place, no data are fed into the input of the modulation stage 13, and switch means 9 are open. The modulation stage 13 stays connected to its power supply, and continues to generate signals of a frequency of around 400 Hz, and at a level of approximately - 15 dBm. These signals are attenuated by the attenuator stage 7 by about 30 dB, i.e. they are fed into the line transformer 3, and the telephone line, at a level of around45 dBm. By arranging the matching between the line transformer 3 and the telephone line to be such as to cause reflection into the input of the receiver stage 2 of a signal in excess of -55 dBm, a masking signal is provided which prevents a terminal user from receiving background signals, caused by cross talk, in intelligible form via the audio output 16, since cross-talk background signals are usually at a level of -60 dBm. As the masking signal is of a level below the minimum level of incoming data carrying signals, and also, as the frequency of the masking signal lies well outside the frequency range of the incoming data carrying signals and the pass band of the filter 15, interference between these and the masking signal is avoided. A data terminal incorporating the features of the present invention is especially, but not exclusively suited for use in a telephone linked computer system such as, e.g. the proposed Viewdata Service by the Post Office. WHAT WE CLAIM IS: -

1. A data terminal for installation remote from an associated computer incorporating a receiver unit capable of receiving, over a conventional telephone line, incoming data information carried in frequency modulated form on carrier signals whose frequency lies within a predetermined range and whose power levels are maintained within set maximum and minimum levels, and a transmitter unit capable of transmitting, over said conventional telephone line, data to said associated computer, wherein the receiver unit incorporates signal compression means to which any signal arriving at the data terminal is fed via signal input means, with the signal

compression means being arranged to reduce an incoming signal whose power level lies above said maximum level to substantially that maximum power level such that all components of the signal are reduced in substantially equal ratios, thereby to enable receiver unit to discriminate between incoming data information carrying signals lying within said frequency excursion and between said maximum and minimum power levels, and dial tone or other signals, which are of a level higher than said maximum level and which have components falling within said excursion and between said maximum and minimum power levels, and the terminal is capable, after initiation of a data call by a user of the terminal, of automatic reception from, and/or transmission to, said computer of data information on carrier signals.

2. A data terminal as claimed in claim 1, adapted to receive from said computer data signals whose frequency excursion lies substantially between 1300 Hz and 2100 Hz.

3. A data terminal as claimed in claim 1 or claim 2, adapted to be used with carrier signals originating in said computer which have a maximum power level of subsantially -15 dBm and a minimum power level of substantially -40 dBm.

4. A data terminal as claimed in any preceding claim, adapted to be used with data information in binary serial code, with the carrier signals being modulated in frequency shift mode.

5. A data terminal as claimed in any preceding claim, wherein an audio output is provided within the receiver unit, thereby enabling a user of the data terminal to monitor the progress of a data call by means of an electro-acoustic device connected to said audio output.

6. A data terminal as claimed in any preceding claim, for use with an associated computer, incorporating a transmitter unit and a receiver unit to facilitate the transmission to, and reception from, the computer of data information carried on modulated carrier signals, wherein the transmitter unit incorporates a modulation stage for modulating the carrier signals, and means for generating said carrier signals, and wherein means are provided to feed the generated carrier signals to an input of the receiver unit at a power level below that of incoming data information carrying carrier signals, but above the level of background signals produced in communication lines between the computer and the receiver unit by cross-coupling between these communication lines and other communication lines, thereby to provide a masking signal to mask the background signals during periods when no data information carrying carrier signals arrive at the receiver unit.

7. A data terminal as claimed in claim 6, for installation remote from the computer, and incorporating a transmitter unit and a receiver unit adapted to facilitate the transmission to and reception from the computer, over a conventional telephone line, of data information carried on frequency modulated carrier signals, wherein the transmitter unit incorporates attenuator means and selectively operable attenuator by-pass means interposed between a modulation stage of the transmitter unit and an output thereof, and wherein coupling means provide a weak coupling between said output and an input of the receiver unit for injecting a carrier signal originating in said means for generating the carrier signals and emerging from said modulation stage into said input, with the signal passing through the attenuator means during periods where no transmission of data information takes place, and with the signal passing through the attenuator by-pass means during transmission of data information.

8. A data terminal as clamed in claim 7, wherein the transmitter unit and the receiver unit are coupled to the telephone line by means of a hybrid coil connected to said output of the transmitter unit and said input of the receiver unit respectively, with the matching between the hybrid coil and the telephone line being arranged such as to provide a weak coupling between the output and the input.

9. A data terminal as claimed in claims 7 or 8, wherein the carrier signals generated in the transmission unit have a frequency excursion of substantially 390 Hz to 450 Hz

10. A data terminal as claimed in claims 7 to 9, wherein the attenuation obtained in the attenuation means is substantially 30 dB.

11. A data terminal as claimed in claim 8, wherein the hybrid coil is arranged to provide a weak coupling between the said output and the said input such that a signal arriving at said input from said output has a power level of substantially55 dBm.

12. A data terminal substantially as hereinbefore shown in, and described with reference to, the accompanying drawings.