GB2135850A - Television circuit for the reduction of flicker on a display - Google Patents

Abstract

Signal sources (1) such as, for example, cine film-television converters and "field skip" signal storage and display devices produce a signal which contains picture information which repeats itself in cycles of two consecutive identical field periods and possibly changes therebetween. The circuit comprises a signal combining circuit (6) coupled to a signal input terminal (2) via a delay device (4) and directly, a switching circuit (11) coupling the input terminal (2) and the output of the combining circuit (6) to a signal output terminal (3). During the first field periods of the cycles the signal combining circuit (6) and during subsequent second field periods the input terminal (2) are respectively coupled to the output terminal (2) so that a combined picture signal (PS2) is obtained with reduced flicker on display. <IMAGE>

Description

SPECIFICATION Television circuit for the reduction of flicker on a display The invention relates to a television circuit for the reduction of flicker on a display, for a picture signal whose picture information repeats itself in cycles of two consecutive field periods, the circuit comprising, arranged between an input terminal and an output terminal, a delay device coupled to the input terminal and producing a time delay equal to one or more field periods, a signal combining circuit coupled to the input terminal and to an output terminal of the delay device, and a switching circuit coupled between an output of the signal combining circuit and the output terminal, for forming together combined picture signals having in field periods picture information originating from different field periods.

Such a television circuit is disclosed in the United States Patent No. 3 886 589, more specifically for use in a so-called "field skip" signal storage and display device. Before signal storage is effected, the combined picture signals are formed with the aid of the delay device and the signal combining circuit. Of these combined picture signals alternate picture signals are stored (field skip) during field periods via the switching circuit. Signal storage may be effected every other field period. On display of the stored signal by the signal storage and display arrangement the stored information is repeated once, in the subsequent field period. Thus, the television picture signal obtained contains picture information which repeats itself in cycles of two consecutive field periods with a possible change of picture content therebetween.Forming the combined picture signals prior to storage according to the said patent has for its object to effect a reduction in flicker phenomena on display in a display device of the content changing picture signal.

In addition, the said patent describes that a changing content picture signal may be the result of a cine film-to-television conversion. Herein, the information associated with one cine film frame is nottaken up in one field period ofthe picture signal, but is repeated in the subsequent field period. If thereafter, optionally, the described intermittently effected signal storage of combined picture signals is used, the storage and display device will produce on signal display an adapted content changing picture signal.

Apart from the fact that in both cases some flicker reduction is obtained by forming the changing picture signal to be displayed on a display screen prior to signal storage, with the picture signals combined over several field periods, the changes following after the repetition of information in the cycle will produce flicker. The said patent does not propose measures to counteract this.

The invention has for its object to provide a television circuit for the reduction of flicker on a display, for a picture signal having picture information which repeats itself in cycles of two consecutive field periods, and possibly, changes step-wise therebetween, which picture signal may, for example, directly original from a cine film-to-television converter or a "field-skip" signal storage and display storage and display device combined or not with the above-described signal combining operation prior to signal storage.

The inventon provides a television circuit of the type described in the opening paragraph which is characterised in that the said picture signal is applied to the input terminal of the television circuit, which input terminal is also coupled to the output terminal of the television circuit via the switching circuit, the switching circuit being arranged such that the output of the signal combining circuit and the output terminal of the television circuit are intercoupled via the switching circuit during the first of the consecutive field periods in the cycles whilst the input and output terminals are intercoupled via the switching circuit during the second of the consecutive field periods subsequent to the said first field periods in the cycles.

Th invention will now be further described by way of example with reference to the accompanying drawing, wherein:- Figure 1 shows a block diagram of a television circuit according to the invention and, Figure 2 shows some time diagrams to illustrate the operation of the circuit shown in Figure 1.

In the television circuit shown in Figure 1 reference numeral 1 denotes a signal source. The signal source 1 produces two signals PSO and PS1. The signal PS1 is a line and field-sequential television picture signal or video signal when picture information is combined with line and field synchronizing information present in blanking periods. Whether synchronizing information is present or absent, it is assumed that the signal PS1 comprises a picture signal with picture information which repeats itself in cycles of two consecutive field periods and (possibly) changes content therebetween. For producing such a signal PS1 the signal source 1 may, for example, comprise a "field skip" signal storage and display device or a cine film-to-television (telecine) converter. Hereinaf ter use with a cine film-to-television converter will be described by way of example.To illustrate the operation of the circuit shown in Figure 1, Figure 2 shows some time diagrams suitable therefor, as a function of the time t. In Figure 2 some items of cine film picture are indicated by My~1, Mn and Mn+1 in a time diagram, denoted by M, of a sequence of items of the film picture information. With the cine filmtelevision conversion which may be effected in known manner with a continuous or intermittent film feed forward, with or without flashed light illumination, the information associated with one cine film frame is not present in one television field period only, but it repeatedly occurs in the subsequent field period, these first and the second following field being denoted by TV1 and TV2 in Figure 2.Related to the time diagram M, Figure 2 shows some of the time diagrams associated with the signals PSO and PS1. Figure 2 shows that signal PSO has a repetition rate equal to two field periods (2To) and is a symmetrical square-wave varying between two signal levels T1 and T2. Figure 2 shows an associated time diagram for the signal PS1. The cine film picture information Mn mentioned as an example, results in a television picture information Pn in a first period TV1 and in a repeated picture information Pn' in a second field period TV2. Similarly, the preceding and subsequent frames of cine film picture information Mn~1 and Mn+1, respectively, result in television picture information Pin~1 and Pun'~1, and Pun'+1, respectively.In the case in which a single, 2:1 interlaced television system is used in the cine filmtelevision conversion, as, for example, laid down in a television standard, two consecutive field periods TV provide one picture period. Herewith, the television line rastersforthe picture information Pin~1, Pn and Pn+1 and P'1, P', and Pun'+1, respectively correspond to each other.

The time diagram of the signal PS1 shows that the items of picture information occur in cycles (TV1, TV2) of two consecutive field periods TV1 andTV2. If interlace is not used, the items of picture information P and subsequent repeated picture information P' are identical in the cycles (TV1, TV2). When interlaced is used, the items of picture information P and P' are not identical, but for the same cine film picture information M, they are shifted one television line in the direction of field scan. Changes between the cine film picture informatin M will occur in steps after the cine film-television conversion between the cycles (TV1, TV2) of the picture information P and P', more specifically between the repeated picture information of P' of the preceding cycle and the picture information P of its own cycle.These step changes in the information will frequently occur in the form of abrupt changes due to editing separately filmed scenes. Without further measures, the display of the (abrupt) step changing signal PS1 will result in a flicker phenomena on a display screen. In orderto obtain a reduction of flicker on display, the signal PS1 is first applied to a signal input terminal 2 of a television circuit shown in Figure 1 which then produces at a signal output terminal 3 a signal PS2 which is more suitable for display. Figure 2 shows a time diagram of the signal PS2, which is derived from that of the signal PS2, which is derived from that of the signal PS1 and which as a combined picture signal shows delayed items of picture information P" in addition to the picture information P and the repeated picture information P'.The delayed items of picture information P" may be derived from the immediately preceding, repeated picture information P' or the picture information P, as is preferred in interlaced television systems.

In the foregoing it was assumed that the signal source 1 comprises a cine film-television converter.

As will be apparent hereinafter, it is alternatively possible for the cine film-television conversion to be effected elsewhere, and a resulting step changing television picture signal is received and applied to an input terminal 11 of the signal source 1 for supplying the signals PS1 and PSO.

In the circuit shown in Figure 1 the inputterminal 2 is connected to an input of a delay device 4. the device 4 produces a time delay which is equal to at least one or more field periods TV. In the event in which the signal PS1 is associated with a 2:1 interlaced television system it is more advantageous, in order to obtain an improvement in the quality of the picture on display of the combined picture signal PS2, to use a time delay of one picture period of 2TV. An output of the delay device 4 is connected to an input 5 of a signal combining circuit 6, a further input 7 of which is connected to the input terminal 2. In the circuit 6 inputs of an adder circuit 8 are connected to the inputs 5 and 7, the adder circuit 8 being followed by a halving circuit 9.The output of the signal combining circuit 6 connected to the output of the halving circuit 9 is connected to an input 10 of a switching circuit 11. An input 12 of the switching circuit 11 is connected to the inputtermin- al 2. In Figure 1 the switching circuit 11 is shown as a mechanical change-over switch having two changeover contacts T1 and T2, which are connected to the signal input 10 and 12,respectively, but in practice it will be in the form of an electronic switch. The switching circuit 11 has a switching control input 13.

The input 13 is connected to an input terminal 14 of the television circuit of Figure 1, to which input 13 the signal PSO is applied as a switching signal. The switching circuit 11 has an output carrying the signal PS2 and is connected to the output terminal 3. In the switching circuit 11 the output terminal 3 is alternately connected to the change-over contacts T1 and T2 under control of the switching signal PSO which has the signal levels which are denoted by the same references T1 and T2. The through-connection of the change-over contact T2 and T1, respectively, is indicated at the switching circuit 11 by means of a solid and a dot-and-dash line, respctively, which lines are associated with the similarly indicated lines T2 and T1 in Figure 2, provided with arrow tips, between the time diagrams PS1 and PS2.The lines shown in Figure 2 between the time diagrams PS1 and PS2 serve to illustrate the transfer of information, the dot-and-dash line T1 being associated with either a dashed line T1 or a dotted line T1. As can be seen from Figure 2 and Figure 1,the dashed line T1 is associated with a time delay equal to a field period TV in the delay device 4, whilst in the event in which the delay is equal to 2TV, the dotted line T1 is associated.

From Figure 2 and Figure 1 it follows that the input terminal 2 and the output terminal 3 are coupled to each other during the field periods TV2. As a result thereof the repeated picture information items, for example Pun'~1, Pn' Pun'+1 are passed-on unchanged.

During the field periods TV1 the output terminal 3 is connected to the output of the signal combining circuit 6 in response to which half the instantaneous picture information items Pin~1, P,, Pun+1 are passedon via the input 7 and half of the delayed picture information items Pn8~2, Pun"~1, Pn" via the input 5.

From Figure 2 it follows that a picture information sequence P,, Pn' Pun+1 which is taken by way of example is converted into a sequence 1/2 (Pn/-1 + Pn), P,', 1/2 (Pn" + Pn+1). Asudden change in picture information between Pn' and Pun+1 is attentuated to Pn' and 1/2 (Pn" + Pn+1), which on display on a display screen results in flicker reduction.In Figure 2 it is stressed at the time diagram PS2 by means of three arrows 15 how each picture information is distributed in the time over three consecutive field periods TV on display on a display screen, in order to provide the flicker reduction.

To reduce the flicker it is essential that during the first field periods TV1 of the cycles of the field periods (TV1, TV2) the signal combining circuit 6 is coupled to the output terminal 3 and that during the second field periods TV2 thereof the input terminal 2 is coupled to the output terminal 3.If the coupling were reversed then instead of the desired sequence (Pn"-1 + Pn), P,', 1/2 (Pn" + Pn+1) the sequence (Pn"'-2 + Pr,'-i), P,, 1/2(Pn"'- + Pn') would be obtained when a delay device 4 having a time delay 2TV (interlaced system) or a sequence Pun'~1, Pn, Pn, in the case of a time delay TV. Herein P"' is the repeated picture information P' delayed by the picture period 2TV.In the first case the abrupt change in information remains present between the first and second picture information in the sequence, in the second case the sequence in the time diagram PS2 remains unchanged, compared to the time diagram PS1. In both cases the desired flicker reduction on display is not obtained.

In the circuit shown in Figure 1 the signal source 1 applies the switching signal PSO in the proper phase to the switching ciruit 11, the signal level T1 and T2 of the time diagram Pro shown in Figure 2 being suitable for the field period cycle (TV1, TV2). The time diagram M, PSO, and PS1 of Figure 2 are maintained in the correct phase via the signal source 1 of Figure 1 in which the cine picture-to-television conversion is effected in the way assumed in the foregoing.

Another possibility is to effect the cine picture-totelevision conversion not in the signal source 1 itself, but elsewhere preceding the signal source 1, a consequently step changing picture signal which is, for example, conveyed without change as the signal PS1 being applied to the input terminal 11 of the signal source 1. In orderto obtain the switching signal PSO in the proper phase, information must now be derived from the received signal (PS1) itself about the field period cycle (TV1, TV2). To realise a circuit for obtaining an indication which item of picture information in the sequence of the field periods (TV1,TV2) are associated with each other reference is made to our co-pending patent application (PHN 10-596) 8403716 filed on the same date. In the presence of such a circuit in the signal source 1 a step changing picture signal received from elsewhere can be applied to the input terminal 11 of the signal source 1 for supplying the signals PSO and PS1.

With the aid of the delay device 4 shown in Figure 1 a combined signal PS2 can be obtained at the output terminal 3 in which, combined over three field periods TV, given picture information, a repetition thereof and delayed picture information occur.

When a delay device is used by which the signal PS1 can be delayed by more field or picture periods a signal combination can be formed over those periods.

Claims (2)

1. A television circuit for the reduction of flicker on a display, for a picture signal whose picture information repeats itself in cycles of two consecutive field periods, said circuit comprising, arranged between an input terminal and an output terminal, a delay device coupled to the input terminal and producing a time delay equal to one or more field periods, a signal combining circuit coupled to the said input terminal and to an output terminal of the delay device, and a switching circuit coupled between an output of the signal combining circuit and the said output terminal, for forming together combined picture signals having in field periods picture information originating from different fields periods, characterised in that the said picture signal is applied to the input terminal of the television circuit, which input terminal is also coupled to the output terminal of the television circuit via the switching circuit, the switching circuit being arranged such that the output of the signal combining circuit and the output terminal of the television circuit are intercoupled via the switching circuit during the first of the consecutive field periods in the said cycles whilst said input and output terminals are intercoupled via said switching circuit during the second of the consecutive field periods subsequent to the said first field periods in the said cycles.

2. Atelevision circuit for the reduction of flicker on a display substantially as herein described with reference to the accompanying drawing.