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WO1996009724A1 - Traitement de signal video - Google Patents

Traitement de signal video Download PDF

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Publication number
WO1996009724A1
WO1996009724A1 PCT/GB1995/002261 GB9502261W WO9609724A1 WO 1996009724 A1 WO1996009724 A1 WO 1996009724A1 GB 9502261 W GB9502261 W GB 9502261W WO 9609724 A1 WO9609724 A1 WO 9609724A1
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WO
WIPO (PCT)
Prior art keywords
signal
signals
linear
luminance signal
luminance
Prior art date
Application number
PCT/GB1995/002261
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English (en)
Inventor
Graham Alexander Thomas
Original Assignee
Snell & Wilcox Limited
British Broadcasting Corporation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Snell & Wilcox Limited, British Broadcasting Corporation filed Critical Snell & Wilcox Limited
Priority to AU35291/95A priority Critical patent/AU3529195A/en
Publication of WO1996009724A1 publication Critical patent/WO1996009724A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N9/00—Details of colour television systems
    • H04N9/64—Circuits for processing colour signals
    • H04N9/68—Circuits for processing colour signals for controlling the amplitude of colour signals, e.g. automatic chroma control circuits

Definitions

  • the so-called luminance component does not precisely represent the true perceived brightness of the image.
  • the red, green and blue colour signals from the camera are "gamma-corrected" before the matrix that forms Y', U' and V (primes indicate gamma-corrected signals).
  • This consists of applying a non-linear transfer function to the signals to pre-correct them for the non-linearity of standard cathode-ray tube (CRT) displays. Since the Y signal is formed from a combination of these non-linear signals, it cannot represent true brightness.
  • the coefficients that define the relative contributions to Y of red, green and blue do not generally have the correct values to make Y represent perceived brightness.
  • the effect of the Y signal not representing true brightness is that some information relating to the true brightness of the image travels through the chrominance signals. If all three signals were received intact this would not matter, as the colour decoder reverses the processes described above in the correct order, so that the RGB signals are perfectly recovered and the perceived luminance is correct. However, if only the luminance signal is received, for example on a monochrome receiver, the perceived luminance will be incorrect; this effect is known as 'failure of constant luminance'. In a colour receiver, if the chrominance signals are band-limited, some information relating to true brightness will still be lost, even if no such bandwidth limitation is applied to the luminance signal. In this case, failure of constant luminance manifests itself as, for example, darkening of regions around chrominance transitions and loss of luminance detail in saturated chrominance areas.
  • the circuit 50 for correcting for the failure of constant luminance receives gamma-corrected source signals R', G ⁇ B' at inputs 52.
  • a normal RGB to YUV matrix 54 forms Y'U'V signals from gamma-corrected R'G'B' signals in the conventional manner.
  • the low-frequency part of the modified Y' signal (having frequencies within the passband of the chrominance channels) is formed from the low-frequency part of the conventional Y' signal by a low-pass filter (LPF) 56.
  • LPF low-pass filter
  • the output of the gamma correction circuit 64 is then applied to two combining circuits 66,68.
  • the circuit 66 is a subtracter with its inverting input connected to the output of the gamma correction circuit 64, its non-inverting input connected to the output of the matrix 54, and its output connected to the input of the low-pass filter 56.
  • the circuit 68 is an adder with one input connected to the output of the gamma correction circuit 64, its other input connected to the output of the low-pass filter 56, and its output forming the modified Y' signal.
  • the filter 56 passes the low frequency Y' signal from matrix 54.
  • the low frequency output of gamma correction circuit 64 is subtracted from the output of matrix 54 in subtracter 66 and added back to the signal in adder 68, and is thus ignored over all.
  • the filter 56 passes nothing, and the output of the gamma corrector circuit 64 is passed to the output by the adder 68.
  • the modified Y' signal is sent to the transmission channel along with the chrominance signals U', V.
  • the arrangement shown is equivalent to connecting the matrix 54 directly to the filter 56, omitting the subtracter 66, and connecting the gamma correction circuit 64 to the adder 68 through a high-pass filter that is exactly complementary to the low-pass filter 56.
  • the above reference explains the operation of the system by considering low-frequency and high-frequency signals separately. For low frequency signals (within the chrominance channel bandwidth), the correct brightness will be displayed, since no transmitted information is lost. For high frequency signals (outside the chrominance channel bandwidth but within that of the luminance channel), the reference assumes that the received chrominance signals must be zero. Therefore the decoder works in the same way as a 'monochrome' television receiver, producing a signal whose brightness is determined by the received luminance signal Y'.
  • the first problem is that if the chrominance signals vary without a net change in true luminance, the system discussed above is blind to the changes (since the U' and V signals are not involved in the derivation of the correction signal). If there is significant energy Fn the chrominance signals outside the chrominance channel passband, these signals will be attenuated and this will cause a change in the true luminance value of the displayed signal. This change will not be reflected in the derived correction signal. Even in the more general case, where chrominance changes are accompanied by some change in true luminance, use of the high-pass filtered true luminance signal will not necessarily yield the true luminance level when the signal is decoded and displayed.
  • the second problem is that if the true luminance of the source image varies in areas of the image containing low chrominance frequencies, the high-frequency part of the gamma-corrected true luminance signal which forms the high-frequency part of the modified Y' signal will not necessarily cause the desired change in true luminance of the decoded and displayed image. This is because of the non-linearity (the 'gamma law') of the CRT; although the high-frequency part of the modified
  • Y' signal is derived from the gamma-corrected true luminance, this gamma correction will only be appropriate for the non-linearity of the CRT if each of the three guns is operating at the same point on its transfer function corresponding to the amplitude of the true luminance signal (ie the image is grey). If the chrominance signals are non-zero, then the R', G' and B' signals will all be at different levels, and therefore the small-signal gains of each will be different.
  • the area on the left is magenta and the area on the right is green.
  • the transmitted conventional Y' signal for the areas shown at (a) is approximately the same.
  • the 'true' luminance of the two areas is greater than the value of conventional Y', since some of the true luminance is being conveyed in the U' and V signals, since the areas are highly saturated.
  • the degree of saturation of the two areas has been chosen to make the true luminance of the two areas approximately the same.
  • Figure 4 shows signal waveforms in similar form to Figure 2 for the system shown in Figure 3;
  • Figure 5 shows a block diagram of the decoder circuitry for use in the system of Figure 3 simulating the transfer function of a decoder consisting of a YUV to RGB matrix and a cathode ray tube display;
  • Figure 6 shows a block diagram of a second system embodying the present invention for correcting for the failure of constant luminance, using a small-signal model of the decoder to provide some correction for the deficiency of the circuit of Figure 3;
  • Fi ⁇ ure 7 shows a block diagram of a third system embodying the present invention for correcting for the failure of constant luminance , using a look-up table to determine the value of luminance signal that needs to be transmitted in order to achieve the desired value of perceived luminance.
  • Figure 9 shows a block diagram illustrating the present invention in a more general sense
  • Figures 10 and 11 show block diagrams of still further systems embodying the present invention.
  • a signal 3 representing the true luminance of the image is generated by first passing the incoming RGB signals through compensating delays 37, converting the incoming RGB signal back to a linear form by a non-linear mapping circuit 1 , and then forming a weighted sum of the linear R, G and B signals in a matrix 2.
  • a matrix which is suitable for use on signals corresponding to System I phosphors and illumi ⁇ ant D ⁇ white point is:
  • YUV matrix 4 to generate the signals Y', U', Y that would be produced in a conventional RGB to YUV coder.
  • the U' and V signals are low-pass filtered by filters 5, which simulate the effect of the transmission channel or coding system.
  • the Y' signal is passed through a delay 6 to compensate for the delay in the chrominance filters 5.
  • the delayed luminance and band-limited chrominance signals are then applied to a YUV to RGB matrix 7, which parallels or imitates the operation of the matrix in a normal decoder.
  • the resulting RGB signals are then processed to derive a signal 10 corresponding to the perceived luminance of the image they represent, using a non-linear mapping circuit 8 which simulates the transfer function of a CRT, and a matrix circuit to form a weighted sum 9, in the same manner as the true luminance of the source signal was measured by circuit blocks 1 and 2.
  • the circuits 7, 8 and 9 mirror the operation of a decoder in a receiver, and thus constitute a local decoder.
  • the signal 3 therefore represents the true luminance of the source image and the signal 10 represents the perceived luminance that would actually be displayed by a normal decoder if it was fed by a normal coder.
  • the difference between these two signal represents the error caused by the 'failure of constant luminance'.
  • Figure 3 shows a simple way of deriving a correction to the transmitted Y' signal from these two signals.
  • the two signals are gamma-corrected, by non-linear transfer function circuits 11 and 12, to convert them into signals pre-distorted for the transfer characteristic of a CRT.
  • the difference between these two gamma-corrected signals is formed by a subtracter 13, the inverting input of which is connected to the circuit 12 and the non-inverting input of which is connected to the circuit 11 , yielding an error signal 14.
  • This signal is then added to the conventionally-formed Y' signal by an adder 15 to form a corrected luminance signal.
  • the conventionally-formed Y' signal, as well as the U' and V signals, are first delayed by delays 16 to compensate for the delay in the generation of the correction signal. This delay will be due primarily to the low-pass filters 5.
  • the circuitry of Figure 3 can be implemented either with analogue circuitry, or preferably with digital circuitry. If digital circuitry is used, the non-linear circuits can be implemented using read-only memories, and the matrix operations can be carried out with commercially-available VLSI circuits designed specifically for performing matrix operations on digital video signals. This arrangement works considerably better than the system shown in Figure 1 , since the effect on the perceived luminance signal of the band-limiting applied to the chrominance signals is measured directly.
  • Figure 4 shows the signal levels for two adjacent coloured areas, coloured magenta and green, as follows:
  • the true luminance generated from the output of the local decoder, shown at (c), is equal to the true luminance of the source signal, shown at (a), in areas away from the chrominance transition.
  • the amplitude of the chrominance signals is reduced by the low-pass filter in the chrominance channel, causing a reduction in the true luminance of the received signal as shown at (c). Therefore the error signal 14, which is the difference between the true luminance of the source and that of the received signal, is zero in areas away from the colour transition.
  • the transmitted luminance signal shown at (e) therefore rises in the region of the transition, becoming approximately equal to the true luminance level at the centre of the transition. This increase in transmitted - Il ⁇ luminance level in the region of the transition will result in the perceived luminance of the decoded signal being closely equal to that of the source signal for all points in the image.
  • the luminance has been pre-corrected for a reduction in chrominance bandwidth which is not suffered in the circuit itself, but in a downstream decoder.
  • the arrangement shown in Figure 3 does not provide perfect correction for the perceived luminance at the remote decoder under all circumstances. This happens when the received chrominance signals are non-zero in areas of the image where the correction signal is being applied. In such circumstances, the gamma correction applied to the two true luminance signals 3,10 by the gamma correctors 11 and 12 will not be exactly correct to compensate for the non-linearity of the CRT. This is because the red, green and blue signals will not all have the same value, and therefore each will be at a different point on the 'gamma' transfer function.
  • the arrangement shown in Figure 3- only provides a perfect solution if the gamma correctors 11 and 12 each reflect the actual non-linearity that the added signal will encounter.
  • FIG. 5 illustrates the detailed operation of the matrix 7, the non-linearity circuit 8 and the weighted sum circuit 9 which determine the true luminance output of the decoder.
  • the requirement is to determine the change in perceived luminance ⁇ Y lru ⁇ of the decoded picture by a change ⁇ Y' to the transmitted signal.
  • a change ⁇ Y' to the Y' signal at the input to the YUV to RGB matrix 7 will cause a change to the received true luminance signal 10 given by the relationship
  • ⁇ Yu ⁇ ⁇ Y' (dr/dR'.W R + dg/dG'.W G + db/dB'.W B )
  • the transfer function of the YUV to RGB matrix is unity for the paths Y' to R', Y' to G' and Y' to B'.
  • the values of the gradient dr/dR' depends on the value of R, and similarly for G and B.
  • the small-signal gain of the decoder is therefore:
  • ⁇ Y lru ⁇ / ⁇ Y' dr/dR'.W R + dg/dG'.W G + db/dB'.W B
  • Figure 6 shows a form of a coder that includes circuitry to evaluate this quantity and scale the luminance correction signal accordingly.
  • Much of Figure 6 is the same as Figure 3 and carries the same references and will not be described again.
  • the gamma correction circuits 11 and 12 are omitted and the subtracter 13 receives directly the signals 3 and 10 from the matrix circuits 2 and 20 respectively.
  • the output of the subtracter 13 is applied to a divider 36 where it is scaled by a scale factor.
  • a non-linear function circuit 18, which stores the slopes of the functions stored in circuit 17, is connected to receive the outputs of the matrix 7.
  • a matrix 19 which is the same as the matrix 20 receives the output of the non-linear circuit 18 and applies an output 21 to the divisor input b of the divider 36, which receives the output 22 of subtracter 13 at its dividend input a.
  • the gradients of each of the three non-linearities in circuit 17 simulating the response of the CRT are determined by non-linearities in circuit 18.
  • the output of these three non-linearities is fed to circuitry 19 to form a weighted sum.
  • This circuitry is identical to the circuit 20 used to form the weighted sum of the R, G and B signals themselves, since the same weighting values appear in the equation for the small-signal gain above.
  • the weighted sum 21 is then used to scale the linear error signal 22, by dividing the error signal 22 by the small-signal gain 21 in the divider circuit 36. Since the error signal will be multiplied by the small-signal gain in the remote decoder, the overall transfer function from linear correction signal to change in true luminance of the display should be unity (assuming the small-signal approximation is valid).
  • a further embodiment of a coder shown in Figure 7 allows an even more accurate account to be taken of the transfer function of the decoder.
  • the principle of this embodiment is to pre-calculate the transmitted Y' level that is required to yield the desired true luminance level, as a function of the U' and V signals at the input to the decoder.
  • the coder contains a large look-up table, which generates the Y' signal to be output from the coder, given the desired true luminance signal and the filtered chrominance signals.
  • Such a look-up table may be derived by modelling the processes 7, 8, 9 in Figures 3 and 4 to obtain a table of perceived luminance levels as a function of the YUV signals at the input of the matrix 7.
  • Another way of calculating the contents of the look-up table is by using a recursive method to calculate successive approximations to the required value of the Y' signal, until the error in the transmitted Y value is less than a chosen value. The calculation is carried out separately for each possible combination of the look-up table inputs.
  • Rhapson method may be used, as shown in the following example.
  • V's t ⁇ wi v signal after simulation of channel or coding impairments (27 in Figure 7).
  • Step 2 Let (the first approximation to required output);
  • Step 3 Calculate R'G'B' values corresponding to Y ⁇ mc ⁇ ti , U' fllt ⁇ r ⁇ d ,
  • Step 7 Calculate the gradient, dtrueY, of the transfer function from linear RGB to true luminance level, from a weighted sum of dR, dG and dB using the same weights as used in calculating the true luminance level in step 5. So for example:
  • Step 8 Calculate a correction to the estimate of the output signal, given by:
  • Step 9 Calculate the new estimate of the output, given by:
  • Step 10 If the magnitude of delta is greater than the desired error (for example, 0.1% of the range of Y), go back to step 3.
  • the desired error for example, 0.1% of the range of Y
  • a gamma-corrected true luminance signal 24 is formed by circuitry identical to circuits 1 , 2 and 11 in Figure 3. This signal 24 is fed through a compensating delay 37 into a look-up table 25, together with the low-pass filtered U' and V signals 26 and 27. These chrominance signals filtered by circuits 5 correspond to the chrominance signals at the input to the YUV to RGB matrix in a remote decoder.
  • the look-up table 25 produces an output Y' on line 28 which has been pre-calculated to yield the true luminance level 23 when decoded and displayed as modelled by the blocks 7, 8, 9 of Rgure 3.
  • the 'conventional' Y' signal formed in the matrix 29 corresponding to the matrix 4 is not required in this embodiment.
  • the non-linear circuit 11 (which corrects for the CRT transfer function) could be incorporated into the look-up table 25; however the implementation of the circuitry may be easier if it is not. Since the circuit is likely to be realised with digital electronics and more bits are required to represent a linear signal 23 compared with a gamma-corrected signal 24, the look-up table 25 will be significantly smaller if the non-linear circuit 11 is implemented separately. Further measures may also be necessary to limit the total number of bits at the input to the look-up table, to ensure that the arrangement is practical. For example, the number of bits used to represent the U' and V signals can be less than that used for luminance.
  • a possible configuration would be to use 10 bits to represent the gamma-corrected true luminance 24, 5 bits to represent V on line 27, and 4 bits to represent U on line 26.
  • This requires a look-up table with a 19-bit address range (512K words).
  • the look-up table 25 receives the same input but is adapted to provide an output which is not the corrected signal YO ⁇ a e DUt tne difference between that desired signal and the gamma corrected true luminance signal available on line 24.
  • This true luminance signal is taken along a bypass path to an adder 81 at which it is added to the output from the look-up table 25 to provide the signal Y'-w- ⁇ c ⁇ d on line 28.
  • the system has R G B signals which are combined to form a true luminance signal, in matrix 2, and gamma-corrected chrominance signals U' and V which are band pass filtered to reflect the band-limiting that takes place in the coder.
  • RGB signals are taken to a true luminance matrix 88 providing a linear true luminance signal which is gamma-corrected at block 90 and passed to a luminance pre-corrector 92.
  • the gamma-corrected chrominance will typically pass through some form of pre-channel chrominance processing before being delivered, with the pre-corrected luminance, to a signal channel shown schematically at 96.
  • This optional pre-channel chrominance processing 94 may take the for of a band pass-filter but this should be regarded as simply one example.
  • the pre-channel chrominance processing can include compression or modulation.
  • the signal channel 96 is assumed to deliver luminance information at 98 and chrominance information at 100, the latter chrominance information passing through post-channel processing at 102. Again, the form of post-channel processing will vary widely with the nature of the signal channel 96.
  • Y,U and V signals are taken through an inverse matrix 104 to provide RGB inputs to a display 106, such as a CRT.
  • the processing upstream of the signal channel 96 includes at 108 a replica of the post-channel chrominance processing.
  • This replica 108 receives the chrominance information in the form in which it is outputted to the signal channel 96 and supplies information to the luminance pre-corrector 92.
  • this replica of post-channel chrominance processing comprises U and V low- pass filters. It can be understood, more generally, that the replica of post- channel chrominance processing 108 enables the luminance pre-corrector 92 to deliver to the signal channel 96 a pre-corrected luminance signal.
  • This ability to pre-correct luminance is a powerful feature of the invention.
  • the invention will find particular application in pre-processing video signals before coding and transmission systems such as CCIR Recommendation 601 , PAL, MAC and MPEG, each of which imposes significant reductions in the bandwidths of the chrominance signals compared to that of the luminance.
  • These systems may include operations such as sub-sampling, quantisation, and bit-rate reduction.
  • the effect of the reduction in chrominance bandwidth is apparent more because it changes the perceived luminance level of the displayed image than because of softening of chrominance information.
  • the present invention can be used to restore the perceived luminance level to its original value.
  • the system has been explained as a means of correcting for the effect on perceived luminance of a low-pass filtering operation on the chrominance signals. However it may be applied to correct the perceived luminance for other distortions that the chrominance signals undergo as a part of a coding, recording or transmission process.
  • a coding system in which the chrominance signals are vertically filtered and subsampled.
  • the system can be used to correct for the effect of the vertical filtering by choosing the filters 5 shown in Figures 3, 6, 7 to be vertical filters, having a response equal to the combined response of the pre-filters in the coder and the post-filters in the decoder.
  • the effect of aliasing caused by the vertical subsampling operation may also be corrected for by replacing each filter with a pre-filter, a sub-sampler and a post-filter, to mimic the chrominance response of the coding system.
  • Figure 10 which incorporates an MPEG2 signal channel.
  • this assumes decimation of the chrominance information, effectively discarding alternate lines of chrominance signals.
  • gamma-corrected RGB signals are taken through a conventional matrix 100 delivering signals to pre-channel chrominance processing in the form of a 2:2 to 2:0 decimator 102. This delivers essentially a single chrominance signal to the signal channel which includes an MPEG2 coder 104 and an MPEG2 decoder 106.
  • post-channel chrominance processing is provided in the form of a 2:0 to 2:2 converter 108 which provides an inverse matrix 110 with U and V signals to accompany the Y signal from the MPEG 2 decoder.
  • RGB outputs from the inverse matrix 110 drive the CRT 112.
  • the gamma- corrected RGB signals are taken through a gamma removal circuit 114 providing linear RGB signals to a matrix 116 which delivers linear true luminance.
  • a gamma-corrected at 118 before passing to the luminance pre-corrector is optionally gamma-corrected at 118 before passing to the luminance pre-corrector.
  • Gamma correction is "optional" in the sense that this function can alternatively be incorporated within the function of the luminance pre-corrector.
  • a 2:0 to 2:2 converter 122 is provided as a replica of unit 108 in the downstream processing.
  • This provides the necessary information to the luminance pre-corrector 120 to enable it to deliver a pre-corrected luminance signal which ensures constant luminance, not necessarily within the signal chain but in the YUV signals provided to the inverse matrix 110.
  • the vertical filtering employed as pre- and post-channel filtering in the MPEG2 example will be present in other arrangements including composite coding schemes such as Weston Clean PAL and PALplus. A similar situation may arise in conversion from high definition standards.
  • a further illustrative example of the application in the present invention will now be described with reference to Figure 11.
  • the signal channel incorporates video tape recording.
  • those components which are shared in common with Rgure 10 retain the same reference numerals and will not be described in detail.
  • chrominance information from the matrix 100 is low-pass filtered at 150 and taken to AM modulator 152.
  • the output of luminance pre-corrector 120 is taken through a low-pass filter 154 to FM modulator 156.
  • the outputs of the FM and AM modulation, respectively, are summed at 158 and form the input for video tape recording.
  • the downstream processing comprises complimentary low- pass and high-pass filters 160 and 162 feeding respective demodulators 164 and 166 which provide the YUV inputs to the inverse matrix 110.
  • the post-channel processing comprises low-pass filter 160 and demodulator 164.
  • the present invention proposes the replication of this processing function in the upstream processor with the output from modulator 152 being taken through a low-pass filter 168 and demodulator 170 to provide chrominance information for the pre-corrector

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  • Engineering & Computer Science (AREA)
  • Multimedia (AREA)
  • Signal Processing (AREA)
  • Processing Of Color Television Signals (AREA)
  • Color Television Systems (AREA)

Abstract

Les distorsions au niveau du codage ou de la transmission des signaux de chrominance, comme celles imputables à la limitation de bande, peuvent provoquer un défaut de luminance constante, en particulier parce que les signaux subissent une opération de pré-traitement de correction de gamma non linéaire. Pour réduire cet inconvénient, le codeur comprend des circuits (7, 8, 9) qui simulent un décodeur pour produire une valeur de luminance perçue (10); cette dernière est comparée à une valeur de luminance vraie (3) calculée à partir des signaux des composantes couleur d'origine. A la suite de cette comparaison, la correction obtenue est ajoutée au signal de luminance transmis.
PCT/GB1995/002261 1994-09-22 1995-09-22 Traitement de signal video WO1996009724A1 (fr)

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KR101681059B1 (ko) * 2009-09-22 2016-12-01 삼성전자주식회사 밝기 신호와 색차 신호간의 크로스토크를 최소화하는 비디오 신호 생성 장치 및 방법
US10958886B2 (en) 2019-07-24 2021-03-23 Grass Valley Limited System and method of reducing distortion during downsampling

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EP0524822A2 (fr) * 1991-07-26 1993-01-27 Tektronix, Inc. Procédé et dispositif de traitement de signaux à composantes séparées pour préserver l'information de haute fréquence
JPH06245229A (ja) * 1993-02-16 1994-09-02 Sony Corp ビデオ信号の送信装置および受信装置
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GB2293514A (en) 1996-03-27

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