GB1568184A

GB1568184A - Television receiver arrangements

Info

Publication number: GB1568184A
Application number: GB3753377A
Authority: GB
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1977-09-08
Filing date: 1977-09-08
Publication date: 1980-05-29

Description

(54) TELEVISION RECEIVER ARRANGEMENTS (71) We PHILIPS ELECTRONIC AND ASSOCIATED INDUSTRIES LIMITED of Abacus House, 33 Gutter Lane, London, EC2V 8AH 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 television receiver arrangements of a kind (hereinafter referred to as being "of the kind specified") suitable for use in a television transmission system of a character in which coded data pulses representing alphanumeric text or other message information are transmitted in a video signal in at least one television line in field-blanking intervals where no picture signals representing normal picture information are present.

A television transmission system of the above character is described in United Kingdom patent specification No. 1,370,535.

A television receiver arrangement of the kind specified includes data selection and acquisition means for extracting selectively coded data pulses from a received video signal, a data store for storing the extracted coded data pulses, and decoding means for producing from the stored coded data pulses a video signal which can be used to cause the display on a television screen of the particular message information represented by the stored coded data pulses. The television receiver arrangement may include suitable television display means, or it may be adapted to feed the video signal into a separate television receiver.

In a practical realisation of a television receiver arrangement of the kind specified, it has been found to be desirable to further include a signal quality arrangement which is arranged to become operative tQ inhibit the acceptance for storage of coded data pulses which are contained in a received video signal which has a noise content higher than a predetermined threshold.

Such a signal quality arrangement aims at minimising the acceptance for storage of "noise-corrupted" coded data pulses which are likely to result in an unintelligible display. It also prevents coded data pulses which are already stored and are suitable for the intelligible display of the message information which they represent from being adversely affected by such "noisecorrupted" coded data pulses. Mullard Technical Information Article 34, dated September 1976, refers to the inclusion of a signal quality arrangement in a television receiver arrangement of the kind specified.

It is an object of the present invention to provide a particular form of such a signal quality arrangement which determines the signal quality of a received video signal in a simple and advantageous manner.

According to the present invention there is provided a television receiver arrangement of the kind specified including a signal quality arrangement which is arranged to become operative to inhibit the acceptance for storage of coded data pulses which are contained in a received video signal which has a noise content higher than a predetermined threshold, wherein said signal quality arrangement comprises separastor means responsive to a received video signal to produce a first pulse train comprising line synchronising pulses separated from the received video signal, pulse generating means phase-locked and responsive to said first pulse train to produce a second pulse train comprising pulses at line synchronising pulse frequency, gating means for combining the amplitudes of a pair of pulses, one from each pulse train, integrating means responsive to the output from said gating means to produce a voltage which is a function of the magnitude of the gating means output for the pulse width period of the pair of pulses, and threshold detector means responsive to cause the production of an inhibiting signal for inhibiting coded pulse data reception by the television receiver arrangement when said voltage attains a predetermined threshold.

When the pulses of the first pulse train are relatively uncorrupted by noise in the received video signal, the mean amplitude of these pulses will be sufficient for the gating means output not to result in response of the threshold detector means.

However, if the signal-to-noise ratio in the video signal becomes too high, the contribution to the gating means output of the mean amplitude of the pulses of the first pulse train will fall sufficiently for this gating means output to result in response of the threshold detector means.

The present invention thus enables selective inhibiting of codded data pulse reception simply on the basis of the effect of the noise content in a received video signal on the quality of the line synchronising pulses therein. This has the advantage that the need for analysing the quality of the actual coded data pulses to determine whether or not they are suitable to give an intelligable display is avoided.

In carrying the invention into effect the separator means of the signal quality arrangement may be comprised by a sync.

separator which is already provided in the television receiver arrangement for separating synchronising signals from a video signal for normal scanning control. This gives the advantage of a saving in cost in the realisation of the signal quality arrangement.

The invention also extends to a television receiver arrangement as set forth above, embodied in a television transmission system of the character referred to.

Also, a television receiver arrangement as set forth above can be adapted to cause the display of alpha-numeric text or other message information concurrently with, or as a selectable alternative to, normal picture information.

The invention also provides a method of selectively inhibiting in a television receiver the reception of coded data pulses which are contained in a television video signal in field-blanking intervals where no picture signals representing normal picture information are present, which method consists in, combining the amplitudes of a pair of pulses, one from each of two pulse trains of which one pulse train comprises line synchronising pulses separated from the video signal and the other pulse train comprises line synchronising pulses generated by pulse generating means phase-locked and responsive to the separated line synchronising pulses, and inhibiting coded data pulse reception unless the average combined amplitude of said pulse pair over a pulse width period has a predetermined value.

In order that the invention may be more fully understood reference will now be made by way of example to the accompanying drawings, of which: - Figure 1 is a block diagram of a television transmission system of the character referred to; Figure 2 shows diagrammatically a signal quality and comparison circuit for use in the television receiver arrangement of the system of Figure 1; and Figure 3 shows explanatory waveform diagrams relating to the operation of the circuit of Figure 2.

Referring to Figure 1 of the drawings, which shows a television transmission system of the character referred to having a television receiver arrangement for displaying selectively either a television picture which is produced from picture information in a normal broadcast or cable television video signal, or alpha-numeric text or other message information which is produced from coded data pulses which are transmitted in the video signal in verticalor field-blanking intervals thereof. The possibility can also exist for displaying such message information concurrently with a television picture, for instance as sub-titles or captions which are superimposed on the television picture.

The incoming television video signal VS appears at an input lead 1 of the television receiver arrangement via its front end 2 which comprises the usual amplifying, tuning, i.f. and detector circuits. The front end 2 is assumed to be coupled to a television transmitter 3 via a conventional over-air broadcast or cable transmission link 4. The transmitter 3 includes in known manner means for producing television picture information, means for producing alpha-numeric text or other message information, and further means for generating the appropriate composite television video signal containing picture signals representative of the picture information, and coded data pulses representative of the message information, together with the usual synchronising, equalizing and blanking signals which are necessary for the operation of the television receiver arrangement.

For normal picture display in the television receiver arrangement, the received video signal VS is applied to a selector circuit 5 which includes a selector switch 6. When the switch 6 is closed, the video signal VS is applied to a colour decoder 7 which produces the R, G and B component signals for the picture display, these component signals being applied via a video interface circuit 8 to the red, green and blue guns of a colour television picture tube 9. Time base circuits 10 for the tube 9 receive the usual line and field synchronising pulses from a sync separator circuit 11 which extracts these synchronising pulses from the incoming video signal VS.

Coded data pulses representing message information in the video signal VS do not affect the picture display because they occur in one or more lines in the field-blanking interval when there is no picture display. Of the lines occurring in the fieldblanking interval, most could be used to transmit coded data pulses representing message information. However, in the experimental BBC/IBA Teletext System for which the present invention was originally conceived, it is proposed at present to restrict such use to lines 17/18 of even fields and lines 330/331 of odd fields of the 625 line broadcast television system used in the United Kingdom. (See "Broadcast Teletext Specification", September 1976, published jointly by the British Broadcasting Corporation, Independent Broadcasting Authority and British Radio Equipment Manufacturers' Association).

The video signal VS on the input lead 1 is also applied to data selection and acquisition means comprising a data acquisition circuit 12 and a code selector circuit 13. It is assumed that the message information represented by the coded data pulses contained in the video signal VS is divided into different pages of information, and that each page is for display as a whole on the screen of the picture tube 10 with the coded data pulses representing each page of information being repeated periodically with or without updating of the information. It is further assumed that each page of message information is identified by means of a unique page address code which is included in the coded data pulses and defines the page number. In the television receiver arrangement, it is the code selector circuit 13 which controls the particular coded data pulses that are acquired by the data acquisition circuit 12 at any time.

(This control is indicated by a broadarrow connection representing the presence of n parallel channels which form an n bit channel link for carrying n bits of information required for data selection other groups of parallel channels forming multi-bit channel links in the television receiver arrangement are represented similarly as m, p, q, and r numbers of channels and bits of information).

The acquired coded data pulses are fed from the data acquisition circuit 12 to a data store 14 over an m-bit channel link, m bit bytes being required for each character (or other item of information) contained in the message information, where m = 7, for example.

The data store 14 can store a complete page of message information. In a typical Teletext transmission, each page of message information would contain up to 24 rows of characters, with each row containing up to 40 characters. Thus, in order to identify the different characters of a page, it is furthermore assumed that the coded data pulses also include an address code for each character, this address code employing r bits and being fed to the data store 14 from the data acquisition circuit 12 over an r-bit channel link.

In view of the restricted transmission time which is available for transmitting the coded data pulses representing message information, for instance, there is sufficient time to transmit the coded data pulses for only one character row during a television line in the field blanking interval, and character data for a page of message information has to be stored row-by-row in the data store 14 over a relatively large number of television fields. This storing of character data row-by-row in the data store 14 is under the control of the address codes received from the data acquisition circuit 12 over the r-bit channel link.

The television receiver arrangement further includes decoding means comprised by a character generator 15 and a parallel to-serial converter 16. The character generator 15 is responsive to the character data stored in the data store 14 to produce character generating data which can be used to derive what is effectively a new picture signal for displaying the characters represented by the stored character data.

As mentioned previously, different characters can be represented by respective m-bit bytes. The bits of each byte are fed in parallel from the data store 14 to the character generator 15 as p-bit bytes. A character format for characters to be displayed can be a co-ordinate matrix composed of discrete elements arranged in rows and columns, this format being derived from a "read-only" memory which serves as the character generator 15 and which provides bits of character generating data in rows and columns, one row at a time.

Since the character generating data is required as a modulation of a video signal in order to produce selective bright-up of the screen of the picture tube 9 to achieve character display the character generating data is produced serially (as l's and 0's) by using the converter 16 to convert each row of bits of data read out from the character generator 15 (e.g. q = 5) into serial form.

In order to effect character display on the screen of the picture tube 9 using standard line and frame scans, the logic of the television receiver arrangement in respect of character display is so organised that for each row of characters to be displayed, all the characters of the row are built up television line-by-television line as a whole, and the rows of characters are built up in succession. It takes a number of television lines to build up one row of characters. In the first television line, character data from the data store 14 to the character generator 15 would cause the latter to produce character generating data in respect of the first row of discrete elements for the first character of the row, then in respect of the first row of discrete elements for the second character, and so on for the successive characters of the row. In the second television line, character generating data in respect of the second row of discrete elements for each character of the row would be produced in turn, and so on for the remaining television lines concerned.

The logic of the data reception and display aspect of the television receiver arrangement is organised by means of a clock pulse and timing pulse chain circuit 17 which provides appropriate clock and timing pulses to the data store 14, to the character generator 15, and to the data acquisition circuit 12. The circuit 17 is synchronised in operation with the scanning of the picture tube 9 by the line and frame synchronising pulses which are extracted from the incoming video signal VS by the sync.

separator circuit 11.

The output from the convertor 16 is applied to a colour coder 18 which produces R', G' and B' component signals for character display, these component signals being also applied to the video interface circuits 8. The colour coder 18 can be controlled (in a manner not shown) by selected items of the character data in the data store 14 to provide a controlled colour display. Of course, black-and-white picture and character display is also possible, in which event the colour decoder 7 and colour coder 18 would be omitted.

The television receiver arrangement further includes as part of the data selection and acquisition means a data acceptor gate 19 via which the incoming video signal VS on the input lead 1 is applied to the data acquisition circuit 12. This acceptor gate 19 is controlled by a signal quality arrangement which is comprised by a signal quality and comparison circuit 20 and the part of the sync separator 11 which pro ducts the line synchronising pulses. The line synchronising pulses from the sync.

separator 11 are applied to the circuit 20.

As will be described, the circuit 20 also produces locally generated line synchronising pulses, and is responsive to apply an inhibiting signal IS to the data acceptor gate 19 when a predetermined signal-tonoise ratio in the incoming video signal is exceeded. In response to this inhibiting sig nal JS, the gate 19 prevents the incoming video signal VS, and thus coded data pulses therein, from being applied to the data acquisition circuit 12.

Referring now to Figure 2, the signal quality and comparison circuit there shown comprises a pulse generator 21, an ANDgate 22, an integrator 23, a threshold detector 24 and a Schmitt trigger circuit 25.

In combination with the sync separator circuit 11 of Figure 1, which circuit is shown in dotted lines in Figures 2, the signal quality and comparison circuit forms a signal quality arrangement in accordance with an embodiment of the invention.

The incoming video signal VS is applied to an input 26 of the sync. separator circuit 11 which is responsive to produce separated line synchronising pulses SLS at an output 27. These pulses SLS are applied to a first input 28 of the AND-gate 22 and to an input 29 of the pulse generator 21. The pulse generator 21 is formed by an oscillator which is responsive and phase-locked to the pulses SLS applied to it to produce derived line synchronising pulses DLS at an output 30. These pulses DLS are applied to a second input 31 of the AND-gate 22.

In response to the coincidence of a pair of pulses (i.e. a pulse SLS and a pulse DLS) at its two inputs 28 and 31 the AND-gate 22 is responsive to produce at an output 32 a signal output which is applied to an input 33 of the integrator 23. Depending on the amplitude and duration of the signal output from the AND-gate 22, and on the time constant afforded by the resistive and capacitive components of the integrator 23, the latter produces a signal voltage of a certain magnitude at an output 34.

This signal voltage is applied to an input 35 of the threshold detector 24 which is responsive to produce a control signal at an output 36 whenever the signal voltage output of the integrator 23 attains a predetermined threshold. The control signal is applied to an input 37 of the trigger circuit 25 which is responsive whilst this control signal is applied to it to produce at an output 38 the inhibiting signal IS which serves (as already described with reference to Figure 1) to inhibit the data acceptor gate 19 to prevent coded data pulse reception.

The waveform diagrams shown in Figure 3 are idealised and illustrate the operation of the signal qualify arrangement.

Figure 3a illustrates the case when the incoming video signal is relatively noise-free, and Figure 36 illustrates the case when the signal-to-noise ratio of the incoming video signal is sufficiently high for coded data pulse reception to be inhibited. In Figure 3a both the separated line synchronising pulses SLS and the derived line synchronising pulses DLS are "clean" in the sense that each is of constant magnitude over the entire pulse width period w. As a result, the signal output (SLS . DLS) from the ANDgate 22 is a maximum, so that the signal voltage Vp from the integrator 22 has a magnitude greater than a threshold volt age Vth. While the signal, voltage Vp is greater than the threshold voltage Vth, the threshold detector 24 remains unresponsive to the signal voltage Vp. In Figure 3b only the derived line synchronising pulses DLS are "clean". The separated line synchronising pulses SLS have noise on them which produces spikes in the pulse magnitude, so that the pulses SLS do not have constant magnitude over the entire pulse width period w. Therefore, in this instance, the signal output (SLS . DLS) from the AND-gate 22 is not at a maximum, resulting in the signal voltage Vp attaining only a value which is less than the threshold voltage Vth. The threshold detector 24 is now responsive to cause the trigger circuit 25 to produce the inhibiting signal IS. The situation is reversed, to terminate the inhibiting signal IS, when the signal voltage Vp exceeds the threshold voltage Vth again. The time constant of the integrator 23 is chosen to be long compared with the period T between successive line synchronising pulses. Because the pulse generator 21 is phase-locked to the pulse SLS, the pulses DLS which it produces are assumed, for practical purposes, to be always in phase with the pulses SLS, even in the presence of noise which is sufficient to cause coded data pulse reception to be inhibited by the signal quality arrangement.

WHAT WE CLAIM IS: - 1. A television receiver arrangement of a kind suitable for use in a television transmission system of a character in which coded data pulses representing alphanumeric text or other message information are transmitted in a video signal in at least one television line in field-blanking intervals where no picture signals representing normal picture information are present, said television receiver arrangement including data selection and acquisition means for extracting selectively coded data pulses from a received video signal, a data store for storing the extracted coded data pulses, and decoding means for producing from the stored coded data pulses a video signal which can be used to cause the display on a television screen of the particular message information represented by the stored coded data pulses, the television receiver arrangement further including a signal quality arrangement which is arranged to become operative to inhibit the acceptance for storage of coded data pulses which are contained in a received video signal which has a noise content higher than a predetermined threshold, which signal quality arrangement comprises separator means responsive to a received video signal to produce a first pulse train comprising line synchronising pulses separated from the received video signal, pulse generating means phrase-locked and responsive to said first pulse train to produce a second pulse train comprising pulses at line synchronising pulse frequency, gating means for combining the amplitudes of a pair of pulses, one from each pulse train, integrating means responsive to the output from said gating means to produce a voltage which is a function of the magnitude of the gating means output for the pulse width period of the pair of pulses, and threshold detector means responsive to cause the production of an inhibiting signal for inhibiting coded pulse data reception by the television receiver arrangement when said voltage attains a predetermined threshold.

2. A television receiver arrangement as claimed in Claim 1, wherein the separator means of the signal quality arrangement is comprised by a sync separator which is already provided in the television receiver arrangement for separating synchronising signals from a video signal for normal scanning control.

3. A television receiver arrangement as claimed in Claim 1 or Claim 2, adapted to cause the display of said alpha-numeric text or other message information concurrently with, or as a selectable alternative to, normal picture information.

4. A television receiver arrangement as claimed in any preceding Claim, embodied in a television transmission system of the character referred to.

5. A television receiver arrangement comprising a signal quality arrangement substantially as hereinbefore described with reference to the accompanying drawings.

6. A television receiver arrangement as claimed in Claim 5, embodied in a television transmission system substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

7. A method of selectively inhibiting in a television receiver the reception of coded data pulses which are contained in a television video signal in field-blanking intervals where no picture signals representing normal picture information are present, which method consists in, combining the amplitudes of a pair of pulses, one from each of two pulse trains of which one pulse train comprises line synchronising pulses separated from the video signal and the

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. pulse reception to be inhibited. In Figure 3a both the separated line synchronising pulses SLS and the derived line synchronising pulses DLS are "clean" in the sense that each is of constant magnitude over the entire pulse width period w. As a result, the signal output (SLS . DLS) from the ANDgate 22 is a maximum, so that the signal voltage Vp from the integrator 22 has a magnitude greater than a threshold volt age Vth. While the signal, voltage Vp is greater than the threshold voltage Vth, the threshold detector 24 remains unresponsive to the signal voltage Vp. In Figure 3b only the derived line synchronising pulses DLS are "clean". The separated line synchronising pulses SLS have noise on them which produces spikes in the pulse magnitude, so that the pulses SLS do not have constant magnitude over the entire pulse width period w. Therefore, in this instance, the signal output (SLS . DLS) from the AND-gate 22 is not at a maximum, resulting in the signal voltage Vp attaining only a value which is less than the threshold voltage Vth. The threshold detector 24 is now responsive to cause the trigger circuit 25 to produce the inhibiting signal IS. The situation is reversed, to terminate the inhibiting signal IS, when the signal voltage Vp exceeds the threshold voltage Vth again. The time constant of the integrator 23 is chosen to be long compared with the period T between successive line synchronising pulses. Because the pulse generator 21 is phase-locked to the pulse SLS, the pulses DLS which it produces are assumed, for practical purposes, to be always in phase with the pulses SLS, even in the presence of noise which is sufficient to cause coded data pulse reception to be inhibited by the signal quality arrangement. WHAT WE CLAIM IS: -

1. A television receiver arrangement of a kind suitable for use in a television transmission system of a character in which coded data pulses representing alphanumeric text or other message information are transmitted in a video signal in at least one television line in field-blanking intervals where no picture signals representing normal picture information are present, said television receiver arrangement including data selection and acquisition means for extracting selectively coded data pulses from a received video signal, a data store for storing the extracted coded data pulses, and decoding means for producing from the stored coded data pulses a video signal which can be used to cause the display on a television screen of the particular message information represented by the stored coded data pulses, the television receiver arrangement further including a signal quality arrangement which is arranged to become operative to inhibit the acceptance for storage of coded data pulses which are contained in a received video signal which has a noise content higher than a predetermined threshold, which signal quality arrangement comprises separator means responsive to a received video signal to produce a first pulse train comprising line synchronising pulses separated from the received video signal, pulse generating means phrase-locked and responsive to said first pulse train to produce a second pulse train comprising pulses at line synchronising pulse frequency, gating means for combining the amplitudes of a pair of pulses, one from each pulse train, integrating means responsive to the output from said gating means to produce a voltage which is a function of the magnitude of the gating means output for the pulse width period of the pair of pulses, and threshold detector means responsive to cause the production of an inhibiting signal for inhibiting coded pulse data reception by the television receiver arrangement when said voltage attains a predetermined threshold.

other pulse train comprises line synchronising pulses generated by pulse generating means phase-locked and responsive to the separated line synchronising pulses, and inhibiting coded data pulse reception unless the average combined amplitude of said pulse pair over a pulse width period has a predetermined value.