GB2262679A

GB2262679A - Limiting the range of a colour component video signal

Info

Publication number: GB2262679A
Application number: GB9126878A
Authority: GB
Inventors: Barry Donald Ruberry Miles
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-12-18
Filing date: 1991-12-18
Publication date: 1993-06-23
Anticipated expiration: 2011-12-18
Also published as: GB2262679B; GB9126878D0

Abstract

An apparatus for range limiting a component video signal having a luminance component and colour difference components, so that RGB colour-component signals or composite video (PAL, NTSC) derived from the video signal are within a predetermined range, includes means 16 for detecting whether R, G, B colour-component signals would be within the allowable range and if they are within the allowable range bypassing the input luminance and colour difference components direct to output 21. However, if the R, G, B colour component signals are detected as being outside the allowable range, range dipping circuit means 17 including a lookup table processes the component signals to ensure that colour-component signals derived from the video signal are within the range. The high frequency luminance components are arranged 19 to bypass the circuit 17 to retain image crispening effects. <IMAGE>

Description

LIMITING THE RANGE OF A COMPONENT VIDEO SIGNAL The present invention relates to an apparatus for range limiting a component video signal having a luminance component and colour difference components, so that colour-component signals (RGB) derived from the video signal are within a predetermined range.

According to the present invention, there is provided apparatus of the aforesaid type, characterised by detection means for detecting whether colour-component signals would be within said range; bypassing means for bypassing said video signal to an output if component signals are detected as being within said range and processing means for processing said video signal if the component signals are detected as being outside said range so as to ensure that colour-component signals derived from said video signal are within said range.

The invention will now be described by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a system for limiting the range of component video signals having a range detector and a range clipping circuit; Figures 2A, 2B and 2C illustrate typical video signals; Figure 3 shows a range detector; and, Figure 4 shows a range clipping circuit A system for limiting the range of component video signals is shown in Figure 1, in which the component video signals in the form of a 601, serial digital multiplex are supplied to a demutiplexing separator 15. The separator 15 supplies a separate luminance signal Y and two colour difference signal Cr and Cb to a range detector 16. The range detector detects whether the luminance and colour difference signals have values outside an allowable range and, if so, range limiting is provided by the circuit.

Colour television cameras and devices for converting photographic film into video signals are configured such that the output video signals lie substantially within an allowable range. Such signal are said to be legal and, similarly, signals lying outside the allowable range are said to be illegal.

Thus, the present invention may be considered as "legalising" video signals which contain illegal values. Illegal video signal are commonly generated by devices such as colour correctors and by equipment arranged to synthesize video signals electronically.

In accordance with the 601 standard, a digital video component is sampled and quantized at 256 levels so that it may be represented by 8 bits. Thus, a component sample may have any value between zero and 255, although only values between 16 and 235 are considered to be legal. A video signal varying with respect to time is shown in Figure 2A and is made up of samples, all of which lie within the allowable range, that is to say, all of the samples are no smaller than level 16 and no larger than level 235.

This may be compared with a video waveform shown in Figure 2B, in which the amplitude increases to a level above 235, is held at this level and then decreases to a level below 16. These maintained high and low levels will probably result in illegal colours being defined once the video signal is converted into RGB colour signals for display on a monitor. These signals will also result in illegal signals being generated during the formation of a composite video waveform in accordance with broadcast standards, such as PAL or NTSC.

A complication is shown in Figure 2C, in which a video signal rises from a low level to a high level.

However, before rising to the high level, the video signal initially undershoots below level 16 and then rises to a level which causes it to overshoot above level 235, before settling down to a level which is within the allowable range. Such a characteristic is commonly created by, for example, television cameras so as to improve the perceived crispness of the image.

Thus, although the periods of undershoot and overshoot are notionally outside the allowable range, they should be allowed to pass through the range limiting circuit unmodified, so as to retain the crispening effect.

Crispening is perceived through changes in the luminance of the image and variations in colour are not so critical. Thus, crispening is maintained if a characteristic of the type shown in Figure 2C is retained in the luminance signal, although characteristics of this type present in the colour difference signals Cr and Cb may be lost, without any perceived difference in quality. As shown in Figure 1, when the range detector 16 detects that illegal signals are present, the colour difference signals Cr and Cb are supplied directly to a range clipping circuit 17. However, luminance signal Y is supplied to said range clipping circuit 17 via a low pass filter 18, and a high pass filter 19 provides a path which bypasses the range clipping circuit and goes directly to a summation circuit 20.Thus, low frequency components of the luminance signal may be clipped by the clipping circuit 17 while high frequency components, of the type shown in Figure 2C, are bypassed and then combined with the output luminants signal from the range clipping circuit 17.

The range clipping circuit 17 is arranged to receive Y, Cr and Cb signals as an input and to provide similarly coded signals as an output, each of which are supplied to a multiplexing combiner 21 so as to provide a serial component output in accordance with the 601 standard.

The range detector 16 is detailed in Figure 3 and includes a switch 31 for supplying the 3 digital signals to the range clipping circuit 17, when illegal levels are detected, or for supplying said signals directly to the multiplexing combiner 21 when the levels are legal. The 3 signals supplied to the switch 31 are also supplied to a colour-space converting matrix 32, arranged to convert the 3 signals, representing luminance and two colour difference signals, to three colour signals representing the amount of red, green and blue (RGB) present for each sampled pixel. Each of the red, green and blue signals is supplied to a repsective comparator 33R, 33G, 33B which detects whether their respective R, G or B signal is within the allowed range.Thus, each comparator shown in Figure 3 actually consists of a pair of comparators which compare their respective input signals with a minimum level and again with a maximum level. The outputs from the comparators 33R, 33G and 33B are supplied to an OR gate 34, which in turn supplies a operating signal to the switch 31. Thus, if any comparator detects that that a red, green or blue signal is outside its allowable range, a logical "high" signal is produced, which is relayed by the OR 34 to operate switch 31, which in turn is switched so that the luminance and colour difference signals are supplied to the range clipping circuit 17.

The range detector 16 ensures that legal signals are bypassed around the range clipping circuit, for two important reasons. Firstly, the-range clipping circuit itself converts the luminance and colour diffence signals into red, green and blue signals, clips the red, the green, the blue or any combination of these signals and then reconverts them back into luminance and colour difference signals. Even if no clipping is performed, as will be the case if legal signals were supplied to the clipping circuit 17, the process of convering from luminance plus colour difference signals to RGB and then back to luminance and colour difference signals again is not transparent, therefore undesirable signal degredation may be introduced. Secondly and perhaps not immediately apparant, the fact that not all signals need to be supplied to the range clipping circuit 17 may be used to simplify the design of the range clipping circuit 17 itself, given that only illegal signals need to be processed because all of the possible legal signals are bypassed.

The range clipping circuit 17 is detailed in Figure 4 and consists of a first matrix 41, a lookup table 42 and a second matrix 43. The first matrix 41 performs arthimetic calculations in a similar way to the matrix 32 shown in Figure 3, so as to convert the luminance and colour difference signals into red, green and blue colour component signals. One or all of these colour component signals are invalid and must be clipped in some way so as to produce valid signals.

Clipping is achieved using an addressable lookup table, in which the RGB input addresses is the lookup table 43 which, for each illegal input, produces a legal RGB output, which is in turn supplied to the second matrix 43. The second matrix 43 is arranged to re-convert the RGB signals back into luminance plus colour difference signals which are in turn recombined into the serial 601 signal as previously described.

It should be noted that, because the range clipping circuit 17 only receives illegal colours, the size of the lookup table 32 may be reduced significantly, enabling it to be implemented at reasonable cost in a practical system.

The system described above provides a means for legalising component video signals in the form of a luminance signal plus two colour difference signals, while being transparent when only legal signals are received. In addition, short legal transiants are preserved in the luminance signal by means of the high pass filter 19 and the summation circuit 20. The lookup table 42 allows the clipping function to be programmable and, by only clipping illegal values, the lookup-table is realizable.

Claims

1. Apparatus for range limiting a component video signal having a luminance component and colour difference components, so that colour-component signals (RGB) derived from the video signal are within a predetermined range, characterised by detection means for detecting whether colour-component signals would be within said range; bypassing means for bypassing said video signals to an output if component signals are detected as being within said range and processing means for processing said video signals if the component signals are detected as being outside said range so as to ensure that colour-component signals derived from said colour component video signal are within said range.

2. Apparatus according to claim 1, wherein said video signal is processed by converting the luminance and colour difference components to colour-component signals (RGB), modifying said colour component signals so as to place them within the predetermined range and converting the modified colour components back into luminants and colour difference signals.

3. Apparatus according to claim 1, including a high pass filter for bypassing high frequency components of the luminance component around the processing means.