METHOD OF SHOWING 16:9 PICTURES ON 4:3 DISPLAYS

Abstract: In a method of processing 16:9 pictures for display on a screen with a 4:3 aspect ratio, each line of the television signal is compressed and/or expanded so that the ratio of the length of a segment of unprocessed signal to the length of the resultant signal derived from it, varies along the line. Preferably, each line is subject to "cylindrical" processing such that it is compressed as though the line were extended along an arc of a circle (40) and projected along a direction (42) onto a flat plane (44) joining the ends of the arc. The picture may also be subject to slight vertical stretching to give a final image which is subject to distortion at an acceptable level whilst retaining a good level of vertical detail.

Description: METHOD OF SHOWING 16:9 PICTURES ON 4:3 DISPLAYS The present invention relates to a method of processing a television signal representing a plurality of lines which when displayed sequentially form an image. At the present time, there is considerable interest in the adoption of an aspect ratio of 16:9 for television pictures, instead of the existing ratio of 4:3. One particular problem in adopting this new aspect ratio is that it will probably be necessary initially to transmit the 16:9 pictures in a manner compatible with receivers having a 4:3 display so that the same signal gives acceptable pictures on receivers both with 4:3 displays and with 16:9 displays. One method of overcoming this problem involves presenting the pictures on a 4:3 display in a "full letterbox" format as illustrated in Figure 1. With this format, the number of active scanning lines containing picture information is 432 lines rather than the normal 576 lines for a 625-line scanning standard. The main objection to this full letterbox format is the presence of broad black borders at the top and the bottom of the 4:3 display. There is the further disadvantage that vertical detail is lost when the number of active picture lines is reduced. Another method is to allow the 16:9 picture to fill the whole of the 4:3 display, with no black borders, in which case the picture detail will be stretched vertically by 25%, i.e., people would become tall and thin. This method has the advantage that no vertical detail is lost and it would therefore be sensible to use it in studios where the scan height of 4:3 displays could be adjusted if desired to give the correct geometry. For broadcasting to existing domestic receivers, a 25% vertical stretch would be too noticeable to be acceptable; however, the use of a small amount of vertical stretch could provide a useful compromise between the disadvantages of the full letterbox and full-height formats. The method of the invention is characterised in that each line signal is compressed and/or expanded so that the ratio of the length of a segment of the unprocessed signal to the length of the segment of the resultant signal derived from it varies along each line. In a preferred embodiment, the signal representing each line is compressed more adjacent the ends of each line than it is in the middle of the line, each line signal being compressed as though the line were disposed along an arc of a circle and projected onto the chord joining the ends of that arc along a direction perpendicular to that notional chord, the overall compression ratio of the line being given by sin (α)/α where 2α is the angle in radians subtended by the notional chord at the centre of a notional circle of which said arc is a part. This technique may permit an increase in the active picture height but with less obvious geometric distortion than results from a simple vertical stretch. In its preferred form, the technique involves squeezing the sides of the 16:9 picture while leaving the centre of the picture undistorted using a process which will be referred to as "cylindrical" processing. This squeezing produces a picture with all the original details but with an aspect ratio lower than 16:9, e.g. 14:9 as shown in Figure 2. As a result, the active height of the picture is greater than that of a 432-line full letterbox display assuming that the picture occupies the full width of the 4:3 display in both cases. Although cylindrical processing could be used to squeeze a 16:9 picture into a 4:3 aspect ratio, thus entirely eliminating the black borders on a 4:3 display, the resulting geometric distortion has been found to be unacceptable. Hence, if cylindrical processing is to be employed, a compromise is required which results in a residual amount of border. Following a number of initial tests, it was decided that subjective tests should be carried out to assess the acceptability of 16:9 pictures on a 4:3 display using (a) a 16:9 full letterbox format and (b) a 14:9 format achieved using a combination of cylindrical processing and simple vertical stretch; this combination will be referred to as "stretched cylinder" processing. The resulting form of picture distortion is illustrated in Figure 3; further details for this figure are given below. These tests are described below, preceded by a description of the principles of cylindrical processing. Receivers with 16:9 displays would require circuitry to undo cylindrical processing and to restore the picture to full height; the latter would also apply to the full letterbox format. In connection with this processing, it should be noted that the squares drawn in the middle of the picture area in Figure 2 are bigger than those in Figure 1. As a result, the cylindrical format has the advantage that it gives more horizontal resolution at the centre of a regenerated 16:9 display, but less at the edges, compared to that given by a full letterbox format. Also, the vertical resolution is better for the cylindrical format because more active lines are used. (It has been assumed that no use has been made of the black borders to transmit additional information about high-frequency detail in the original 16:9 picture). A method in accordance with the invention will now be described in detail, by way of example, with reference to the drawings, in which:- Figure 1 shows a 16:9 "full letterbox" presentation of a 16:9 picture on a 4:3 display; Figure 2 shows a 14:9 "cylindrical" presentation of a 16:9 picture on a 4:3 display (drawn to scale); Figure 3 shows a 14:9 "stretched cylindrical" presentation of a 16:9 picture on a 4:3 display (drawn to scale); Figure 4 shows diagrammatically the cylindrical projection of a 16:9 picture onto a flat plane; Figure 5 is a histogram illustrating the results of subjective tests carried out using 14:9 cylindrical presentation of a 16:9 picture; Figure 6 is a histogram illustrating the results of subjective tests carried out using 16:9 letterbox presentation; Figure 7 shows the mean values of the results illustrated in Figures 5 and 6; and Figure 8 shows an overall assessment similar to Figure 7 but after separate comparisons on individual sequences. Principles of cylindrical processing The type of geometrical distortion introduced by cylindrical processing is equivalent to that obtained if the undistorted 16:9 picture is wrapped round part of a vertical cylinder and the resulting curved picture is then projected on to a flat vertical surface passing through the sides of the picture. This process is illustrated in Figure 4 which represents a horizontal cross-section of a display. As shown, the 16:9 picture plane 40 is wrapped around a sector ADS of a cylinder. The picture is projected along lines 42 onto a flat plane 44, the chord joining the ends of the arc ADS. It is to be noted that the length L of the line ACB is 2r.sin(α) whereas that of the arc ADB(A) is 2r.α. It can be seen that maximum squeezing effect occurs near the edges A and B of the picture while there is no change in width of objects near the centre of the picture. If L = width of compressed picture, i.e. length of the straight line ACB. A = width of uncompressed picture, i.e. length of the arc ADB. 2α = angle in radians subtended by AB at the centre 0 of the cy1inder. then, by simple geometry:- Overall compression ratio, L/A = sin(α)/α The maximum compression ratio of individual objects within the picture, occurring at the sides of the picture, is equal to cos(α) . If the size of the cylindrically processed picture is adjusted so that it occupies the full width of a 4:3 display, the resulting height can be calculated as follows: Assume we start with a 16:9 full letterbox picture of height 432 lines occupying the full width of a 4:3 display. We then cylindrically process this picture, thereby reducing its width by a factor of L/A. The vertical and horizontal dimensions of this cylindrically processed picture must now be increased by a factor of A/L if the picture is to be restored, without further distortion, to occupying the full width of the display. Thus:- Height of cylindrical picture = 432*A/L active picture lines The resulting compression ratios and active picture heights for different angles of wrap round the cylinder are given in Table 1. Wra An le 2α Ra io L A Max. Com n.cos α Picture Hei ht Table 1. Compression and picture height for given wrap angle. Initial tests indicated that a compression ratio L/A of about 90% was about the lowest ratio that could be used without the geometric picture distortion becoming too obvious on critical scenes. From Table 1, this L/A ratio gives a picture height of 480 lines with black borders each of height 48 lines. Height adjustment The amount of black border given by a compression ratio of 90% is still somewhat obtrusive and it was therefore decided that it might be reduced still further by means of a slight amount of vertical stretch. Initial tests indicated that it would be worth examining the acceptability of stretched cylindrical processing involving a 51 increase in height in addition to an increase in picture height given by cylindrical processing. Subjective tests Various picture sequences originally recorded as full-screen 16:9 pictures were re-recorded for display on a 4:3 screen in (a) full letterbox format and (b) stretched cylinder format. Brief details of the six picture sequences were as follows: 1. Military Band - Changing of the guards. 2. FA Cup Final - 1989 Liverpool v Everton. 3. Ballet - Prince of the Pagodas. 4. Top of the Pops - Tina Turner. 5. Wildlife programme - Wind in the Willows. 6. Shakespeare - Items from Henry V. Six different picture sequences originally recorded as HDTV 1250-line 16:9 aspect-ratio pictures were replayed through a 1250 to 625-line down converter and recorded as 625 line pictures filling the full width of the display but with reduced height. Height reduction was performed in the down converter which employed a high-quality vertical interpolator retaining the maximum possible vertical definition. The sequences were recorded twice, first with an active picture height of 432 lines to give 16:9 full letterbox pictures and secondly with an active picture height of 500 lines to give 14:9 pictures on a 4:3 display. (More precisely, 500 lines corresponds to an aspect ratio of 13.8:9 which is the geometric mean of 16:9 and 12:9.) The 500-line recording was processed using Questec "Cleo" DVE equipment and the resulting signals were re-recorded on a second VTR. This processing involved two operations. Firstly, cylindrical processing was applied, using a cylindrical angle of 86 degrees, giving a compression ratio L/A = 0.909. Secondly, the overall picture width was adjusted in a linear manner to just fill the available active line-period; the active picture height was left unaltered at 500 lines. The resulting geometric distortion is illustrated in Figure 3 where the original 16:9 picture consisted of a grid as shown in Figure 1. Since, according to Table 1, the active picture height for cylindrical processing with L/A = 0.909 would be 475 lines, the fact that the active picture was actually 500 lines means that an additional linear vertical stretch factor of 500/475 = 1.05 was also applied. Finally, the undistorted 432-line full letterbox and the stretched cylinder 500-line recordings were edited together to produce a tape with the following format. Duration Picture content mins sees 5 Caption:- Version A 2 Picture sequence 1, Version A 5 Caption:- Version B 2 Picture sequence 1, Version B 15 Caption:- Vote Now 5 Caption:- Version A 2 Picture sequence 2, Version A 5 Caption:- Version B 2 Picture sequence 2, Version B 15 Caption:- Vote Now and so on for next four picture sequences. This broadcast-quality edited tape was then transferred to VHS format. Two versions of the VHS tape were made, one with the full letterbox format shown before the stretched cylinder format (Version A = 16:9, Version B = 14:9) and the other with the reverse order of presentation. The results obtained from a comparison of these two versions for individual sequences are shown in the form of histograms in Figures 5 and 6. The mean values of the grades shown in Figures 5 and 6 are shown in Figure 7. Figure 8 is similar to Figure 7 but shows the overall assessment after separate comparisons on individual sequences. It can be seen that both methods of obtaining means grades for all six sequences gave very similar results. The results given in Figures 5, 6, 7 and 8 indicate that in general the 14:9 stretched cylinder pictures were comparable to normal 4:3 pictures, 90% of grades being in the range 2 (a little better) to 3 (a little worse). In contrast, opinions about the 16:9 full letterbox format were more widely spread over the whole of the grading scale with a significant proportion (37%) grading it a lot worse than the usual format; on the other hand, 6% (as opposed to 2% for 14:9) graded it a lot better. In addition, the number of people answering "yes" or "possibly" to the question asking if the change of format would not affect their viewing habits was about 60% compared to about 24% for the 14:9 format. These results indicate that the use of the 16:9 format is more likely to reduce the number of viewers for broadcast programmes than would the use of the 14:9 format. The order of presentation of full letterbox and stretched cylinder formats as used on the two different VHS tapes used in the subjective tests did not have any significant effect on the results obtained. Conclusions Initial tests have indicated that the use of "stretched cylindrical" processing to squeeze 16:9 pictures into a 14:9 format provides a more acceptable means of showing "16:9" pictures on a 4:3 display than a 16:9 undistorted full letterbox format. The advantage of the cylindrical format is that it reduces the height of the black borders at the top and bottom of the 4:3 picture area compared to those given by a full letterbox format, whilst not distorting the middle of the picture. The disadvantage is that it introduces geometrical distortion in the form of a horizontal compression squeezes the edges of the edges of the picture. However, the amount of reduction in black borders which can be achieved without excessive geometrical distortion is not quite adequate and a further reduction by means of a vertical picture stretch is beneficial. Cylindrical processing has the additional advantage that it gives more resolution at the centre, but less at the edges, of a regenerated 16:9 display compared to that given by a full letterbox format.

Claims: CLAIMS 1. A method of processing a television picture signal comprising signals representing a plurality of lines which when displayed sequentially form an image, the method being characterised in that each line signal is compressed and/or expanded so that the ratio of the length of a segment of the unprocessed signal to the length of the segment of the resultant signal derived from it, varies along each line. 2. A method according to claim 1 for processing a television picture signal for display on a display device, the display device having an aspect ratio lower than that of the image formed by the sequential display of the unprocessed signal lines, in which each line signal is compressed so that the ratio of a segment of the compressed signal to the segment of the uncompressed signal from which it is derived varies along each line. 3. A method according to claim 2 in which the signal representing each line is compressed more adjacent the ends of each line than it is in the middle of the line. 4. A method according to claim 3 in which each line signal is compressed as though the line were disposed along an arc of a circle and projected onto the chord joining the ends of that arc along a direction perpendicular to that notional chord, the overall compression ratio of the line being given by sin (α)/α where 2α is the angle in radians subtended by the notional chord at the centre of a notional circle of which said arc is a part. 5. A method according to any preceding claim in which the first aspect ratio is 16:9 and the second aspect ratio at which the signal is to be displayed is 4:3. 6. A method according to any of claims 1 to 5 in which the signal is further modified to change the number of lines which when displayed sequentially form the image. 7. A method according to claim 6 in which the number of lines is increased by interpolation or repetition.

Inventors: DEVEREUX VICTOR GERALD, DREWERY JOHN OLIVER, DEVEREUX, VICTOR, GERALD, DREWERY, JOHN, OLIVER
Applicants: BRITISH BROADCASTING CORP, DEVEREUX VICTOR GERALD, DREWERY JOHN OLIVER, BRITISH BROADCASTING CORPORATION
Application year: 1993
Main classification: H04N5
IPC codes: H04N5, H04N7