CA1090916A - Line sequential colour television system - Google Patents
Line sequential colour television systemInfo
- Publication number
- CA1090916A CA1090916A CA259,665A CA259665A CA1090916A CA 1090916 A CA1090916 A CA 1090916A CA 259665 A CA259665 A CA 259665A CA 1090916 A CA1090916 A CA 1090916A
- Authority
- CA
- Canada
- Prior art keywords
- line
- signal
- chrominance
- luminance signal
- lines
- Prior art date
- Legal status (The legal status 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 status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
- H04N9/00—Details of colour television systems
- H04N9/79—Processing of colour television signals in connection with recording
- H04N9/80—Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback
- H04N9/86—Transformation of the television signal for recording, e.g. modulation, frequency changing; Inverse transformation for playback the individual colour picture signal components being recorded sequentially and simultaneously, e.g. corresponding to SECAM-system
Landscapes
- Engineering & Computer Science (AREA)
- Multimedia (AREA)
- Signal Processing (AREA)
- Processing Of Color Television Signals (AREA)
- Color Television Systems (AREA)
Abstract
Abstract of the disclosure An encoder which combines the luminance signal of a television picture signal with a chrominance signal which comprises a cyclic line sequence of chrominance components which occupy part of the bandwidth of the luminance signal, in which encoder at least a fraction of each line of the luminance signal is combined with a fraction of at least one of the earlier lines which are spaced from the said line by integral cycles of the line sequence of the chrominance components: A cooperative decoder reconstitutes line-to-line detail by comparing each line of the luminance signal with a line which is the duration of one cycle of chrominance earlier.
Description
l~O~i~
The present invention relates to a line sequential colour television system in which, for each line of a television signal, there is transmitted or recorded a luminance signal together with a chrominance signal which occupies only a part of the bandwidth of the luminance signal and which represents, in line sequence, a succession of different chrominance components. Each chrominance component may comprise a signal which represents the difference between a respective primary colour component, usually red, green and blue, and the luminance signal and which is limited in its bandwidth to, for example, the first five hundred kilohertz of the bandwidth of the luminance signal. The succession of the chrominance components is normally cyclic and normally embraces three television lines.
In order that a decoder, namely a television receiver or a player of a recorded signal, may make all the chrominance components ayailable for every line of the transmitted or played back television signal, at least that part of the television signal which contains the chromatic information is passed through a chain of delay lines so that for example the chromatic content of a line which is tr~nsmitted with a red component is supplemented by signals defining the green and blue content obtained from the previous line and the line before that. Such a decoder averages, over a cycle of chrominance, the information content of signals in the said part of the bandwidth and loses line-to-line luminance detail which is defined by luminance components occupying this part of the bandwidth.
As will be more fully explaimed hereinafter, the aforementioned detail can be recovered during decoding by means of a comparison of each line of the television signal, or at least the low frequency part of it, with the line that
The present invention relates to a line sequential colour television system in which, for each line of a television signal, there is transmitted or recorded a luminance signal together with a chrominance signal which occupies only a part of the bandwidth of the luminance signal and which represents, in line sequence, a succession of different chrominance components. Each chrominance component may comprise a signal which represents the difference between a respective primary colour component, usually red, green and blue, and the luminance signal and which is limited in its bandwidth to, for example, the first five hundred kilohertz of the bandwidth of the luminance signal. The succession of the chrominance components is normally cyclic and normally embraces three television lines.
In order that a decoder, namely a television receiver or a player of a recorded signal, may make all the chrominance components ayailable for every line of the transmitted or played back television signal, at least that part of the television signal which contains the chromatic information is passed through a chain of delay lines so that for example the chromatic content of a line which is tr~nsmitted with a red component is supplemented by signals defining the green and blue content obtained from the previous line and the line before that. Such a decoder averages, over a cycle of chrominance, the information content of signals in the said part of the bandwidth and loses line-to-line luminance detail which is defined by luminance components occupying this part of the bandwidth.
As will be more fully explaimed hereinafter, the aforementioned detail can be recovered during decoding by means of a comparison of each line of the television signal, or at least the low frequency part of it, with the line that
2.
lU90~
precedes it by a cycle of chrominance, normally three lines.
The comparison detects line-to-line changes in luminance.
A signal denoting these changes can be combined with a signal which represents the average of three lines and these two signals can be used to reconstitute the luminance signal in that part of its bandwidth which it shares with the chrominance components. However, the recovery of detail in this manner produces various spurious signals which visibly affect the television picture. It is therefore the purpose of the invention to provide an encoder which modifies the original television signal in a manner which facilitates its decoding and provides at least some reduction in the production of those spurious signals or the visibility of the effects thereof.
According to the invention there is provided an encoder which combines the luminance signal of the television picture signal with a chrominance signal which comprises a cyclic line sequence of chrominance components which occupy part of the bandwidth of the luminance signal, in which encoder at least a fraction of each line of the luminance signal is combined with a fraction of at least one of the earlier lines which are spaced from the said line by integral cycles of the line sequence of the chrominance components.
Thus each line which contains, for example, a red component, or the colour difference signal formed by comparing luminance and the red component, is combined with fractions of earlier lines associated with a red component or red colour difference component as the case may be. The combination of each line with earlier lines may be confined to the low frequency part of the video spectrum, that is to say normally the first five hundred kilohertz of the normal two to two-and-a-half megahertz bandwidth. The part of the bandwidth of most
lU90~
precedes it by a cycle of chrominance, normally three lines.
The comparison detects line-to-line changes in luminance.
A signal denoting these changes can be combined with a signal which represents the average of three lines and these two signals can be used to reconstitute the luminance signal in that part of its bandwidth which it shares with the chrominance components. However, the recovery of detail in this manner produces various spurious signals which visibly affect the television picture. It is therefore the purpose of the invention to provide an encoder which modifies the original television signal in a manner which facilitates its decoding and provides at least some reduction in the production of those spurious signals or the visibility of the effects thereof.
According to the invention there is provided an encoder which combines the luminance signal of the television picture signal with a chrominance signal which comprises a cyclic line sequence of chrominance components which occupy part of the bandwidth of the luminance signal, in which encoder at least a fraction of each line of the luminance signal is combined with a fraction of at least one of the earlier lines which are spaced from the said line by integral cycles of the line sequence of the chrominance components.
Thus each line which contains, for example, a red component, or the colour difference signal formed by comparing luminance and the red component, is combined with fractions of earlier lines associated with a red component or red colour difference component as the case may be. The combination of each line with earlier lines may be confined to the low frequency part of the video spectrum, that is to say normally the first five hundred kilohertz of the normal two to two-and-a-half megahertz bandwidth. The part of the bandwidth of most
3.
1090~3i~
concern is that shared by the normally bandwidth limited chrominance components and the low frequency part of the luminance signal. The combination of each line with fractions of earlier lines may be applied to the composite signal, that is to say to the chrominance component as well as to the luminance component in the frequency range mentioned but may be confined to merely the luminance component in that range. When a signal which has been encoded as described is presented to a comparator which makes a comparison between two points a cycle of chrominance apart, the arrival of a line-to-line transition in luminance causes the production of an appropriate transient; when however the line containing that transition is compared, inevitably, with the line a cycle of chrominance later, the transient of opposite sense which would be produced is somewhat attenuated because the later line contains a fraction of the earlier. Accordingly the effects of a larg~e transient which denotes a sharp line-to-line change in luminance can be confined to the region of the change. They cannot be entirely eliminated but can be reduced to small transients, of the nature of a ringing, occurring at intervals spaced by integral cycles of chrominance after the transition which initiates them.
In order to combine each line with fractions of earlier lines, the encoder may include a recursive filter, which receives at least the said fraction of each line and comprises a delay corresponding to at least one complete cycle of the line sequence of chrominance and a feedback loop by means of which the said fraction of each line is combined with fractions of the said earlier lines, which fractions are made evanescent by their traversals of the loop. The choice of the fractions and the attenuation produced by the loop is somewhat subjective, depending on a compromise of the
1090~3i~
concern is that shared by the normally bandwidth limited chrominance components and the low frequency part of the luminance signal. The combination of each line with fractions of earlier lines may be applied to the composite signal, that is to say to the chrominance component as well as to the luminance component in the frequency range mentioned but may be confined to merely the luminance component in that range. When a signal which has been encoded as described is presented to a comparator which makes a comparison between two points a cycle of chrominance apart, the arrival of a line-to-line transition in luminance causes the production of an appropriate transient; when however the line containing that transition is compared, inevitably, with the line a cycle of chrominance later, the transient of opposite sense which would be produced is somewhat attenuated because the later line contains a fraction of the earlier. Accordingly the effects of a larg~e transient which denotes a sharp line-to-line change in luminance can be confined to the region of the change. They cannot be entirely eliminated but can be reduced to small transients, of the nature of a ringing, occurring at intervals spaced by integral cycles of chrominance after the transition which initiates them.
In order to combine each line with fractions of earlier lines, the encoder may include a recursive filter, which receives at least the said fraction of each line and comprises a delay corresponding to at least one complete cycle of the line sequence of chrominance and a feedback loop by means of which the said fraction of each line is combined with fractions of the said earlier lines, which fractions are made evanescent by their traversals of the loop. The choice of the fractions and the attenuation produced by the loop is somewhat subjective, depending on a compromise of the
4-desired sharpness and the extent to which the ringing effect, which cannot be completely eliminated, can be tolerated.
The aforementioned ringing may be compensated by the introduction of a pre-ring by means of which each television line is combined with various, normally small, proportions of later lines which follow the line which is being treated by integral cycles of chrominance. Preferably each line of the luminance signal is combined with a signal which represents an unbalanced comparison between a later line of the luminance signal, which later line follows the said line by a complete cycle of chrominance, and the line, or the average of the lines, immediately preceding the said later line and embracing, together with the said later line, a complete cycle of chrominance. More than one cycle of compensation may be provided, the proportions of lines in a later cycle being smaller than the lines in a relatively earlier cycle, each cycle here'referring to a group of lines constituting a cycle of chrominance and following the line under treatment.
Another feature of the invention is the sharpening of signals by the encoder. The sharpness of detail recovered by a decoder is in practice a measure of the relative gains of the two signals which are derived firstly by comparing signals a cycle of chrominance apart and secondly by averaging a succession of lines. Increasing the gain applied to the first signal sharpens the recovered line-to-line detail but also emphasises any spurious signals.
Reducing the relative gain of the first signal reduces the amplification of the spurious signals but reduces the sharpness by which a transition is represented. In order to enable the relative gain of the first signal to be reduced in the decoder, the encoder preferably includes means by which each line of the luminance signal is combined with a fraction of 113~0~
an immediately preceding line in order to sharpen line-to-line detail in the luminance signal. For this purpose, the encoder may comprise a recursive filter which includes a one-line delay and by means of which each line is combined with increasingly diminishing fractions of earlier lines and the resultant combination is compared to form an output with a fraction of a similar combination constituted one line earlier.
The various components of each line which is combined as aforesaid may occupy all the bandwidth of the luminance signal; however, if so, then in order to confine the effects of the processing performed by the encoder to that part of the bandwidth which the luminance and chrominance signal components share, each line, after it has been thus combined may be compared with the original luminance signal and the resultant of this comparison may be restricted to the afore-mentioned part of th~ said bandwidth and recombined with the original luminance signal.
It is convenient for each chrominance component to comprise a signal representing the difference between a respective primary colour component, for example red, green and blue, and a modified luminance signal M, which is the simple average of the primary colour components, but in general the term "chrominance component" is intended to mean one of a set of chrominance components which together represent the low frequency chromatic content of the picture which the transmitted or recorded signal represents.
There follows a description by way of example of one embodiment of an encoder according to the invention and a decoder which may be used to decode signals encoded by the encoder. The encoder may form part of a recorder which prepares television signals for recordal in the form of a 6.
1(~90.~1~
video disc and the decoder may form part of a player of such a disc.
In the accompanying drawings:
Figure l is a schematic diagram of an encoder;
Figure 2 is a diagram of a chroma softener which may form part of the encoder shown in Figure l; and Figure 3 is a schematic diagram of a decoder.
The encoder which is illustrated in Figure l includes a compensator l, a luminance softener 2, a sharpener 3, and a corrector 4. The stages shown in Figure l process only the luminance signal and combine it with line sequential chrominance components which are processed as described hereinafter with reference to Figure 2. In this encoder the entire luminance signal passes through part of the compensator 1, but the high frequency part, above five hundred kilohertz, is, in effect, not processed. This part of the luminance signal is merely delayed by the compensator ....
whereas those components of the luminance signal which are processed by the compensator, softener and sharpener are limited in bandwidth in order to remove spurious detail which would otherwise be introduced into the high frequency part of the output signal by the action of the softener 2 and the sharpener 3.
The luminance signal which is received at the input
The aforementioned ringing may be compensated by the introduction of a pre-ring by means of which each television line is combined with various, normally small, proportions of later lines which follow the line which is being treated by integral cycles of chrominance. Preferably each line of the luminance signal is combined with a signal which represents an unbalanced comparison between a later line of the luminance signal, which later line follows the said line by a complete cycle of chrominance, and the line, or the average of the lines, immediately preceding the said later line and embracing, together with the said later line, a complete cycle of chrominance. More than one cycle of compensation may be provided, the proportions of lines in a later cycle being smaller than the lines in a relatively earlier cycle, each cycle here'referring to a group of lines constituting a cycle of chrominance and following the line under treatment.
Another feature of the invention is the sharpening of signals by the encoder. The sharpness of detail recovered by a decoder is in practice a measure of the relative gains of the two signals which are derived firstly by comparing signals a cycle of chrominance apart and secondly by averaging a succession of lines. Increasing the gain applied to the first signal sharpens the recovered line-to-line detail but also emphasises any spurious signals.
Reducing the relative gain of the first signal reduces the amplification of the spurious signals but reduces the sharpness by which a transition is represented. In order to enable the relative gain of the first signal to be reduced in the decoder, the encoder preferably includes means by which each line of the luminance signal is combined with a fraction of 113~0~
an immediately preceding line in order to sharpen line-to-line detail in the luminance signal. For this purpose, the encoder may comprise a recursive filter which includes a one-line delay and by means of which each line is combined with increasingly diminishing fractions of earlier lines and the resultant combination is compared to form an output with a fraction of a similar combination constituted one line earlier.
The various components of each line which is combined as aforesaid may occupy all the bandwidth of the luminance signal; however, if so, then in order to confine the effects of the processing performed by the encoder to that part of the bandwidth which the luminance and chrominance signal components share, each line, after it has been thus combined may be compared with the original luminance signal and the resultant of this comparison may be restricted to the afore-mentioned part of th~ said bandwidth and recombined with the original luminance signal.
It is convenient for each chrominance component to comprise a signal representing the difference between a respective primary colour component, for example red, green and blue, and a modified luminance signal M, which is the simple average of the primary colour components, but in general the term "chrominance component" is intended to mean one of a set of chrominance components which together represent the low frequency chromatic content of the picture which the transmitted or recorded signal represents.
There follows a description by way of example of one embodiment of an encoder according to the invention and a decoder which may be used to decode signals encoded by the encoder. The encoder may form part of a recorder which prepares television signals for recordal in the form of a 6.
1(~90.~1~
video disc and the decoder may form part of a player of such a disc.
In the accompanying drawings:
Figure l is a schematic diagram of an encoder;
Figure 2 is a diagram of a chroma softener which may form part of the encoder shown in Figure l; and Figure 3 is a schematic diagram of a decoder.
The encoder which is illustrated in Figure l includes a compensator l, a luminance softener 2, a sharpener 3, and a corrector 4. The stages shown in Figure l process only the luminance signal and combine it with line sequential chrominance components which are processed as described hereinafter with reference to Figure 2. In this encoder the entire luminance signal passes through part of the compensator 1, but the high frequency part, above five hundred kilohertz, is, in effect, not processed. This part of the luminance signal is merely delayed by the compensator ....
whereas those components of the luminance signal which are processed by the compensator, softener and sharpener are limited in bandwidth in order to remove spurious detail which would otherwise be introduced into the high frequency part of the output signal by the action of the softener 2 and the sharpener 3.
The luminance signal which is received at the input
5 of the encoder is fed through a chain of six one-line delays 6 to 11. The output of the delay line 11 and the outputs of the varlous delays are combined in selected proportions by an adder 12. The purpose of the compensator constituted by the delays 6 to 11 and the adder 12 is to add to each line, that is to say the instant output from the delay 11, at least one signal, and in this embodiment two such signals, which represent, in small proportion, line-to-~090~
line changes in luminance in a respective set of three lines of the 1S~lminance signal. This signal affords compensation for a ringing effect which is produced in the decoder and which arises from a comparison by the decoder of signals which are combinations of fractions of different lines. In this particular embodiment the adder 12 adds to the output signal of the delay 11 signals which each correspond to the difference in detail between a sign sl which occurs three, or a multiple of three, lines later than the line appearing at the output of the delay 11 and the average of the signals one line and two lines earlier respectively than the later signal. The compensatory signal is developed in respect of the three lines immediately following the line appearing at the output of the delay 11 and a further signal, of lesser proportion, is developed for the three lines which are the fourth to sixth lines after the line at the output of the de laY 11 .
Accordingly, the adder 12 combines the output of the delays 6 and 7 and forms the difference between the average of those two outputs and a signal proportional to the signal at the input 5. In like manner, a corresponding average is formed of the outputs of the two delays 9 and 10; likewise a difference between this average and the output of the delay 8 is formed by the adder 12. The formation of the aforementioned averages and differences is achieved by selection of the gains which the adder 12 applied to each of its input signals. In this particular embodiment the relative gains of the signals are -3, +2 and +2 for the second cycle of compensation and -6, +4 and +4 for the first cycle of compensation and +20 for the line under treatment.
me sum of the gains of the two signals which are averaged for comparison with the respective following line is 8.
1090'3i~i different from the gain applied to that following line; the comparison is thereby unbalanced and accordingly a compensatory signal will be produced even if there is no line-to-line difference in luminance of three consecutive lines.
The output of the adder is reduced by an attenuator 13 according to the reciprocal of the algebraic sum of the gains applied to the signals combined by the adder 12 so that the gain in the compensator between the input 5 and the output of the attenuator 13 is unity.
The input luminance signal as it appears at the output of the delay 11 does not contain any proportion of earlier lines, although it has been delayed by the duration of six scanning lines, and is fed to a corrector 4 which will be described later.
The output of the adder is, after attenuation by the attenuator 13, fed to-the softener 2. This softener comprises a recursive filter by means of which each line is converted into a form consisting of a signal of which the amplitude is a fraction of the amplitude of the instant line and other signals of which the amplitudes are successively smaller fractions of earlier lines occurring at intervals three lines earlier than the instant line. The recursive filter comprises an adder 14, three one-line delays 15 to 17 and a feedback loop connecting the output of the last delay 17 in the chain of three delays with an input of the adder 14. The feedback loop introduces a gain of 0.5, that is to say a 6dB
attenuation, and the adder 14 applies a gain of 0.5 to the signal received from the attenuator 13.
Thus each input line repeatedly traverses the circuit of delay lines and feedback loop, and is rendered evanescent by those repeated traversals and constitutes, in progressively 9.
1()~)~3i~
smaller proportion, a component of following lines at three-line intervals. The delay introduced by the recursive filter should be as exactly three lines as possible else the various fractions which are accumulated to form each output line will become progressively misregistered.
Satisfactory performance can be achieved if the delay is sufficiently precise but precision in the encoder averts corresponding precision in decoders, which will normally be more numerous and less susceptible of adjustment than encoders.
The output of the recursive filter is taken from the output terminal of the adder 14 but could be obtained from the output terminals of any of the delays 15, 16 and 17.
The output of the recursive filter is fed to the sharpener 3. This sharpener comprises another recursive filter constituted by an adder 19, a one-line delay 20 and a feedb~ck loop 2~ which adds a fraction of the output of the delay line 20 to the input signal of the adder 19. The action of the recursive filter is to emphasise any line-to-line change in luminance. The augmenting of each line constitutes positive feedback which is counteracted by negative feedback in order that the emphasis is rendered transistory. Accordingly the output of the adder 19 is combined, in an adder 23, with a negative fraction of the output of the delay 20. For this purpose the output of the delay 20 is fed through an attenuator 22, which introduces a gain of 0.8, and an inverter 24.
The effect of the sharpener on, for example, a rise of one line duration in luminance is to preserve the leading edge and the magnitude of that temporary rise but to emphasise the trailing edge, which is followed by an overshoot and then a gradual return to the original level of luminance.
10.
~090~i~
In order to limit the effects of the compensator, softener and sharpener to the lower part of the bandwidth of the video signal, the output of the sharpener 3 is fed to a comparator 25 wherein it is compared with the full bandwidth luminance signal constituted by the output of the delay 11 of the compensator 1. This luminance signal is merely delayed and not otherwise processed by the compensator 1. The difference signal formed by the comparator 25 contains only those components which have been introduced by the compensator 1, the softener 2 and the sharpener 3;
the bandwidth of these components is limited by a low pass filter 26, which the cut-off frequency is five hundred kilo-hertz, and the remaining components are added to the original luminance signal by an adder 27.
The encoder includes an adder 28 which combines the luminance signal, after it has been duly processed as described in the fQregoing, with a line sequential chrominance signal which has been processed by a chroma processor 29 and passed through a low-pass filter 30. The purpose of the chroma processor (Figure 2) is to soften the chrominance components and thereby attenuate the components which might interfere with a luminance component in the higher part of the bandwidth of the chrominance signal. The particular processor utilises the fact that the sum of two modified colour difference signals, for example (G-M) + (B-M), is the negative of the third modified colour difference signal (R-M). It adds to each line of chrominance a fraction of a following line of like chrominance and a fraction of the sum of intervening lines of unlike chrominance so that in effect each line of chrominance is softened by the addition of fractions of chrominance components derived from a succession of lines of the sequential chrominance signal.
11 .
iosos~
The chroma softener comprises six one-line delays 31 to 36. The outputs of these delays are combined in various fixed proportions by an adder 37. The action of the chroma softener is conveniently described with reference to a specific example. If the line under treatment, that is to say the output of the delay 36, is constituted by the colour difference signal (R-M), then the signals at the input terminal of the delay 31 and at the output terminal of the delay 33 will also represent the signal (R-M) for the lines which are six and three lines later. These three like components are fed to the adder through input resistors 38, 39 and 40 respectively in selective proportions. In accordance with the ordinary sequence of chrominance, that is to say (R-M), (B-M), (G-M), (R-M) etc, the output of the delay 32 will be the modified colour difference signal (B-M) for the line four lines after the output of the delay 36 and the output of the.,delay 34 will be the modified colour difference signal (G-M) for the line two lines after the output of the delay 36. The outputs of the delays 32 and 34 are fed through input resistors 41 and 42 respectively to an adder 43 which forms a negative "red" modified colour difference signal which is inverted by an inverter 44 and fed to the adder 37. In like manner the outputs of the delays 31 and 35 are fed through input resistors 45 and 46 to an adder 47, and inverted by an inverter 48 and fed to the adder 37.
The sequential input to the chroma softener would normally be obtained by a sampling switch coupled to the output of a matrix which provides the three modified colour difference signals simultaneously for each line.
Figure 3 illustrates a decoder which is suitable for decoding television signals which have been encoded by the 12.
` ~, ~(~90.'31~
encoder illustrated in Figures 1 and 2. The decoder has an input terminal 50. An input signal which is received by the terminal 50 is fed simultaneously down several parallel branches. The first branch includes a short delay 51 which couples the input signal to an adder 52 and thence to a matrix 53. A second branch comprises three one-line delays 54, 55 and 56. One purpose of the first two delays 54 and 55 is to render the chrominance components of three consecutive lines simultaneously available so that the chrominance signal of each line can be reconstituted from the chrominance component actually transmitted for that line and the chrominance component transmitted for adjacent lines. me reconstitution of the chrominance signal is performed by a commutating switch 57 which feeds three simultaneous different chrominance components to a matrix 58 which forms the two commonplace colour difference signals (R-Y) and (B-Y) to the matrix 53~wherein the luminance signal and the two chrominance components may be formed into an ordinary composite colour television signal.
The input signal and the successive outputs from the delays 54, 55 and 56 may conveniently be termed Dol Dl, D2 and D3 respectively. The reconstitution of chrominance requires only three of these signals, for example Do, Dl and D2, but in order to reduce the interference phenomena known as cross-colour all four signals may be used. In this particular embodiment the signals Dl and D2 are fed to the commutating switch 57 directly whereas the signals Do and D3, which include like chrominance components, are fed to an adder 65 which forms the average (Do+D3)/2 and feeds that to the commutating switch 57.
me passage of the input signal through the chain of delays and its combination for each line of the output signal 13.
~090~3iti averages both the chrominance components and the luminance components which occupy the same part of the bandwidth and which define line-to-line luminance detail. Separate processing of the chrominance signal and the luminance signal is not feasible because the two signals share part of the same bandwidth.
The decoder reconstitutes line-to-line detail represented by components of similar frequency to the chrominance components by deriving two signals, one of which is obtained by comparing the signal levels at points spaced by a cycle of chrominance and the other of which is obtained by averaging the signal values at three corresponding points, one in each of three consecutive lines of the television signal. Although a separate chain of delay lines could be used to derive these signals, the present decoder uses the same delay lines which are used for the extraction of the chromin~nce components. The first signal can be represented , as a (Do-D3) when a is positive and greater than unity and the second signal may be Do+Dl+D2 or Dl+D2+D3; the use of the former is more convenient. The difference signal (Do-D3) denotes the difference in luminance between D3 and Do. The changes in luminance are represented by (Do-D3) as soon as they occur. However, when the line containing them appears at the output of the delay 56, the difference signal which is obtained by comparing Do and D 3, that is to say the line three lines after Do, will not represent those changes to the same extent because the encoder forms D 3 from a fraction of D 3 and a fraction of Do.
The difference signal (Do-D3) represents short term changes in luminance, for example the position of a horizontal division between a white band and a black band in a television picture. A longer term change, for example 14.
lU9~431~
from white to black, is represented by the average signal, for example (Do + Dl +D2). An adder 59 combines the input signal and the outputs of the delays 54 and 55; the proportion of the input signal (Do) is greater than the similar proportions of the accompanying outputs. The output (D3) of the delay line 56 is inverted by an inverter 60 and combined with the output of the adder 59 in an adder 61, which forms a signal which is a superposition of the so-called difference signal and average signal and is in the form (a + 1) Do + Dl + D2 ~aD3 wherein a is positive and greater than unity and is preferably approximately 1.5. This signal is essentially a luminance signal because the difference signal is formed by comparing two lines of like chrominance and the average of three consecutive lines produces, in accordance with the use of modified colour difference signals, purely a luminance signal.
The output of the adder 61 is compared with the original input signa~. This latter signal comprises a luminance signal and a respective line sequential chrominance component. The comparison yields the chrominance component free from averaging but accompanied by some spurious signals such as the ringing aforementioned. The reconstituted chrominance component is fed through a low pass filter 64, of which the delay is matched by the delay 51, and is subtracted from the original input signal by the adder 52.
In effect this adder subtracts the reconstituted chrominance component from the original input signal and yields the luminance component of the original input signal free from averaging.
The decoder will delay the chrominance signal finally obtained by, on average, one and a half times the duration of a line. This delay may be compensated by appropriate mis-registration of the chrominance signal and the luminance 15.
~090~1~
signal during encoding. The matching of different delays in parallel processing channels is commonplace and requires no detailed description.
16.
line changes in luminance in a respective set of three lines of the 1S~lminance signal. This signal affords compensation for a ringing effect which is produced in the decoder and which arises from a comparison by the decoder of signals which are combinations of fractions of different lines. In this particular embodiment the adder 12 adds to the output signal of the delay 11 signals which each correspond to the difference in detail between a sign sl which occurs three, or a multiple of three, lines later than the line appearing at the output of the delay 11 and the average of the signals one line and two lines earlier respectively than the later signal. The compensatory signal is developed in respect of the three lines immediately following the line appearing at the output of the delay 11 and a further signal, of lesser proportion, is developed for the three lines which are the fourth to sixth lines after the line at the output of the de laY 11 .
Accordingly, the adder 12 combines the output of the delays 6 and 7 and forms the difference between the average of those two outputs and a signal proportional to the signal at the input 5. In like manner, a corresponding average is formed of the outputs of the two delays 9 and 10; likewise a difference between this average and the output of the delay 8 is formed by the adder 12. The formation of the aforementioned averages and differences is achieved by selection of the gains which the adder 12 applied to each of its input signals. In this particular embodiment the relative gains of the signals are -3, +2 and +2 for the second cycle of compensation and -6, +4 and +4 for the first cycle of compensation and +20 for the line under treatment.
me sum of the gains of the two signals which are averaged for comparison with the respective following line is 8.
1090'3i~i different from the gain applied to that following line; the comparison is thereby unbalanced and accordingly a compensatory signal will be produced even if there is no line-to-line difference in luminance of three consecutive lines.
The output of the adder is reduced by an attenuator 13 according to the reciprocal of the algebraic sum of the gains applied to the signals combined by the adder 12 so that the gain in the compensator between the input 5 and the output of the attenuator 13 is unity.
The input luminance signal as it appears at the output of the delay 11 does not contain any proportion of earlier lines, although it has been delayed by the duration of six scanning lines, and is fed to a corrector 4 which will be described later.
The output of the adder is, after attenuation by the attenuator 13, fed to-the softener 2. This softener comprises a recursive filter by means of which each line is converted into a form consisting of a signal of which the amplitude is a fraction of the amplitude of the instant line and other signals of which the amplitudes are successively smaller fractions of earlier lines occurring at intervals three lines earlier than the instant line. The recursive filter comprises an adder 14, three one-line delays 15 to 17 and a feedback loop connecting the output of the last delay 17 in the chain of three delays with an input of the adder 14. The feedback loop introduces a gain of 0.5, that is to say a 6dB
attenuation, and the adder 14 applies a gain of 0.5 to the signal received from the attenuator 13.
Thus each input line repeatedly traverses the circuit of delay lines and feedback loop, and is rendered evanescent by those repeated traversals and constitutes, in progressively 9.
1()~)~3i~
smaller proportion, a component of following lines at three-line intervals. The delay introduced by the recursive filter should be as exactly three lines as possible else the various fractions which are accumulated to form each output line will become progressively misregistered.
Satisfactory performance can be achieved if the delay is sufficiently precise but precision in the encoder averts corresponding precision in decoders, which will normally be more numerous and less susceptible of adjustment than encoders.
The output of the recursive filter is taken from the output terminal of the adder 14 but could be obtained from the output terminals of any of the delays 15, 16 and 17.
The output of the recursive filter is fed to the sharpener 3. This sharpener comprises another recursive filter constituted by an adder 19, a one-line delay 20 and a feedb~ck loop 2~ which adds a fraction of the output of the delay line 20 to the input signal of the adder 19. The action of the recursive filter is to emphasise any line-to-line change in luminance. The augmenting of each line constitutes positive feedback which is counteracted by negative feedback in order that the emphasis is rendered transistory. Accordingly the output of the adder 19 is combined, in an adder 23, with a negative fraction of the output of the delay 20. For this purpose the output of the delay 20 is fed through an attenuator 22, which introduces a gain of 0.8, and an inverter 24.
The effect of the sharpener on, for example, a rise of one line duration in luminance is to preserve the leading edge and the magnitude of that temporary rise but to emphasise the trailing edge, which is followed by an overshoot and then a gradual return to the original level of luminance.
10.
~090~i~
In order to limit the effects of the compensator, softener and sharpener to the lower part of the bandwidth of the video signal, the output of the sharpener 3 is fed to a comparator 25 wherein it is compared with the full bandwidth luminance signal constituted by the output of the delay 11 of the compensator 1. This luminance signal is merely delayed and not otherwise processed by the compensator 1. The difference signal formed by the comparator 25 contains only those components which have been introduced by the compensator 1, the softener 2 and the sharpener 3;
the bandwidth of these components is limited by a low pass filter 26, which the cut-off frequency is five hundred kilo-hertz, and the remaining components are added to the original luminance signal by an adder 27.
The encoder includes an adder 28 which combines the luminance signal, after it has been duly processed as described in the fQregoing, with a line sequential chrominance signal which has been processed by a chroma processor 29 and passed through a low-pass filter 30. The purpose of the chroma processor (Figure 2) is to soften the chrominance components and thereby attenuate the components which might interfere with a luminance component in the higher part of the bandwidth of the chrominance signal. The particular processor utilises the fact that the sum of two modified colour difference signals, for example (G-M) + (B-M), is the negative of the third modified colour difference signal (R-M). It adds to each line of chrominance a fraction of a following line of like chrominance and a fraction of the sum of intervening lines of unlike chrominance so that in effect each line of chrominance is softened by the addition of fractions of chrominance components derived from a succession of lines of the sequential chrominance signal.
11 .
iosos~
The chroma softener comprises six one-line delays 31 to 36. The outputs of these delays are combined in various fixed proportions by an adder 37. The action of the chroma softener is conveniently described with reference to a specific example. If the line under treatment, that is to say the output of the delay 36, is constituted by the colour difference signal (R-M), then the signals at the input terminal of the delay 31 and at the output terminal of the delay 33 will also represent the signal (R-M) for the lines which are six and three lines later. These three like components are fed to the adder through input resistors 38, 39 and 40 respectively in selective proportions. In accordance with the ordinary sequence of chrominance, that is to say (R-M), (B-M), (G-M), (R-M) etc, the output of the delay 32 will be the modified colour difference signal (B-M) for the line four lines after the output of the delay 36 and the output of the.,delay 34 will be the modified colour difference signal (G-M) for the line two lines after the output of the delay 36. The outputs of the delays 32 and 34 are fed through input resistors 41 and 42 respectively to an adder 43 which forms a negative "red" modified colour difference signal which is inverted by an inverter 44 and fed to the adder 37. In like manner the outputs of the delays 31 and 35 are fed through input resistors 45 and 46 to an adder 47, and inverted by an inverter 48 and fed to the adder 37.
The sequential input to the chroma softener would normally be obtained by a sampling switch coupled to the output of a matrix which provides the three modified colour difference signals simultaneously for each line.
Figure 3 illustrates a decoder which is suitable for decoding television signals which have been encoded by the 12.
` ~, ~(~90.'31~
encoder illustrated in Figures 1 and 2. The decoder has an input terminal 50. An input signal which is received by the terminal 50 is fed simultaneously down several parallel branches. The first branch includes a short delay 51 which couples the input signal to an adder 52 and thence to a matrix 53. A second branch comprises three one-line delays 54, 55 and 56. One purpose of the first two delays 54 and 55 is to render the chrominance components of three consecutive lines simultaneously available so that the chrominance signal of each line can be reconstituted from the chrominance component actually transmitted for that line and the chrominance component transmitted for adjacent lines. me reconstitution of the chrominance signal is performed by a commutating switch 57 which feeds three simultaneous different chrominance components to a matrix 58 which forms the two commonplace colour difference signals (R-Y) and (B-Y) to the matrix 53~wherein the luminance signal and the two chrominance components may be formed into an ordinary composite colour television signal.
The input signal and the successive outputs from the delays 54, 55 and 56 may conveniently be termed Dol Dl, D2 and D3 respectively. The reconstitution of chrominance requires only three of these signals, for example Do, Dl and D2, but in order to reduce the interference phenomena known as cross-colour all four signals may be used. In this particular embodiment the signals Dl and D2 are fed to the commutating switch 57 directly whereas the signals Do and D3, which include like chrominance components, are fed to an adder 65 which forms the average (Do+D3)/2 and feeds that to the commutating switch 57.
me passage of the input signal through the chain of delays and its combination for each line of the output signal 13.
~090~3iti averages both the chrominance components and the luminance components which occupy the same part of the bandwidth and which define line-to-line luminance detail. Separate processing of the chrominance signal and the luminance signal is not feasible because the two signals share part of the same bandwidth.
The decoder reconstitutes line-to-line detail represented by components of similar frequency to the chrominance components by deriving two signals, one of which is obtained by comparing the signal levels at points spaced by a cycle of chrominance and the other of which is obtained by averaging the signal values at three corresponding points, one in each of three consecutive lines of the television signal. Although a separate chain of delay lines could be used to derive these signals, the present decoder uses the same delay lines which are used for the extraction of the chromin~nce components. The first signal can be represented , as a (Do-D3) when a is positive and greater than unity and the second signal may be Do+Dl+D2 or Dl+D2+D3; the use of the former is more convenient. The difference signal (Do-D3) denotes the difference in luminance between D3 and Do. The changes in luminance are represented by (Do-D3) as soon as they occur. However, when the line containing them appears at the output of the delay 56, the difference signal which is obtained by comparing Do and D 3, that is to say the line three lines after Do, will not represent those changes to the same extent because the encoder forms D 3 from a fraction of D 3 and a fraction of Do.
The difference signal (Do-D3) represents short term changes in luminance, for example the position of a horizontal division between a white band and a black band in a television picture. A longer term change, for example 14.
lU9~431~
from white to black, is represented by the average signal, for example (Do + Dl +D2). An adder 59 combines the input signal and the outputs of the delays 54 and 55; the proportion of the input signal (Do) is greater than the similar proportions of the accompanying outputs. The output (D3) of the delay line 56 is inverted by an inverter 60 and combined with the output of the adder 59 in an adder 61, which forms a signal which is a superposition of the so-called difference signal and average signal and is in the form (a + 1) Do + Dl + D2 ~aD3 wherein a is positive and greater than unity and is preferably approximately 1.5. This signal is essentially a luminance signal because the difference signal is formed by comparing two lines of like chrominance and the average of three consecutive lines produces, in accordance with the use of modified colour difference signals, purely a luminance signal.
The output of the adder 61 is compared with the original input signa~. This latter signal comprises a luminance signal and a respective line sequential chrominance component. The comparison yields the chrominance component free from averaging but accompanied by some spurious signals such as the ringing aforementioned. The reconstituted chrominance component is fed through a low pass filter 64, of which the delay is matched by the delay 51, and is subtracted from the original input signal by the adder 52.
In effect this adder subtracts the reconstituted chrominance component from the original input signal and yields the luminance component of the original input signal free from averaging.
The decoder will delay the chrominance signal finally obtained by, on average, one and a half times the duration of a line. This delay may be compensated by appropriate mis-registration of the chrominance signal and the luminance 15.
~090~1~
signal during encoding. The matching of different delays in parallel processing channels is commonplace and requires no detailed description.
16.
Claims (9)
1. An encoder which combines the luminance signal of a television picture signal with a chrominance signal which comprises a cyclic line sequence of chrominance components which occupy part of the bandwidth of the luminance signal, in which encoder at least a fraction of each line of the luminance signal is combined with a fraction of at least one of the earlier lines which are spaced from the said line by integral cycles of the line sequence of the chrominance components.
2. An encoder according to claim 1, in which there is a recursive filter, which receives at least the said fraction of each line and comprises a delay corresponding to a complete cycle of the line sequence of chrominance and a feedback loop by means of which a fraction of each line is combined with fractions of the said earlier lines, which fractions are made evanescent by their traversals of the loop.
3. An encoder according to claim 1 in which each line of the luminance signal is combined with a fraction of an immediately preceding line in order to sharpen line-to-line detail in the luminance signal.
4. An encoder according to claim 2, in which the encoder comprises a second recursive filter which has a one-line delay and by means of which each line is combined with increasingly diminishing fractions of earlier lines and the resultant combination is compared, to form an output, 17.
with a fraction of a similar combination constituted one line earlier.
with a fraction of a similar combination constituted one line earlier.
5. An encoder according to claim 1, in which each line of the luminance signal is combined with a signal which represents an unbalanced comparison between a later line of the luminance signal, which later line follows the said line by a complete cycle of chrominance, and the line, or the average of the lines, immediately preceding the said later line and embracing, together with the said later line, a complete cycle of chrominance.
6. An encoder according to claim 5, in which each line is combined with a signal which represents an unbalanced comparison between an even later line, which follows the said line by two complete cycles of chrominance and the line, or the average of the lines, immediately preceding the even later line and, together with it, embracing a complete cycle of chrominance.
7. An encoder according to claim 3 or claim 4 or claim 5, in which the various components of each line which is combined as aforesaid occupy all the bandwidth of the luminance signal and each line, after it has been thus combined, is compared with the original luminance signal and the resultant of this comparison is restricted to the aforementioned part of the said bandwidth and recombined with the original luminance signal.
8. An encoder according to claim 1 or claim 4 or claim 5, in which each line of the chrominance signal is softened by the combination therewith of fractions of 18.
chrominance components derived from the chrominance components of other lines of the television signal.
chrominance components derived from the chrominance components of other lines of the television signal.
9. A television system comprising an encoder according to claim 1 or claim 3 or claim 5 and a decoder which includes a chain of one-line delays for making the chrominance components of a succession of lines of the television signal simultaneously available and which re-constitutes line-to-line detail in the luminance signal by comparing each line of the luminance signal with an earlier line which precedes it by a complete cycle of the line sequence of chrominance.
19.
19.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB46202/75 | 1975-11-07 | ||
| GB46202/75A GB1510978A (en) | 1975-11-07 | 1975-11-07 | Line sequential colour television system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1090916A true CA1090916A (en) | 1980-12-02 |
Family
ID=10440269
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA259,665A Expired CA1090916A (en) | 1975-11-07 | 1976-08-23 | Line sequential colour television system |
Country Status (15)
| Country | Link |
|---|---|
| JP (1) | JPS5260022A (en) |
| AT (1) | AT356192B (en) |
| AU (1) | AU497216B2 (en) |
| BE (1) | BE845895A (en) |
| CA (1) | CA1090916A (en) |
| CH (1) | CH609818A5 (en) |
| DE (1) | DE2641517C3 (en) |
| DK (1) | DK416976A (en) |
| FR (1) | FR2331225A1 (en) |
| GB (1) | GB1510978A (en) |
| IT (1) | IT1071398B (en) |
| NL (1) | NL7609673A (en) |
| NO (1) | NO763142L (en) |
| NZ (1) | NZ181839A (en) |
| SE (1) | SE415820B (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5826500Y2 (en) * | 1977-07-07 | 1983-06-08 | 三菱電機株式会社 | liquid cooled wound core |
| US4215414A (en) * | 1978-03-07 | 1980-07-29 | Hughes Aircraft Company | Pseudogaussian video output processing for digital display |
| US4677461A (en) * | 1984-07-26 | 1987-06-30 | Mitsubishi Denki Kabushiki Kaisha | Contour detecting filter |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB1453588A (en) * | 1973-09-28 | 1976-10-27 | Decca Ltd | Colour television system |
-
1975
- 1975-11-07 GB GB46202/75A patent/GB1510978A/en not_active Expired
-
1976
- 1976-08-23 NZ NZ181839A patent/NZ181839A/en unknown
- 1976-08-23 CA CA259,665A patent/CA1090916A/en not_active Expired
- 1976-08-31 NL NL7609673A patent/NL7609673A/en not_active Application Discontinuation
- 1976-09-06 BE BE170386A patent/BE845895A/en unknown
- 1976-09-08 CH CH11400/76A patent/CH609818A5/en not_active IP Right Cessation
- 1976-09-09 FR FR7627079A patent/FR2331225A1/en active Granted
- 1976-09-14 AU AU17727/76A patent/AU497216B2/en not_active Expired
- 1976-09-14 JP JP51110648A patent/JPS5260022A/en active Pending
- 1976-09-14 NO NO763142A patent/NO763142L/no unknown
- 1976-09-15 DK DK416976A patent/DK416976A/en unknown
- 1976-09-15 DE DE2641517A patent/DE2641517C3/en not_active Expired
- 1976-09-15 AT AT685776A patent/AT356192B/en not_active IP Right Cessation
- 1976-09-28 IT IT69311/76A patent/IT1071398B/en active
- 1976-10-25 SE SE7611823A patent/SE415820B/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| AU1772776A (en) | 1978-03-23 |
| GB1510978A (en) | 1978-05-17 |
| IT1071398B (en) | 1985-04-02 |
| NZ181839A (en) | 1980-10-24 |
| DE2641517C3 (en) | 1979-07-26 |
| NO763142L (en) | 1977-05-10 |
| SE7611823L (en) | 1977-05-08 |
| AU497216B2 (en) | 1978-12-07 |
| BE845895A (en) | 1976-12-31 |
| NL7609673A (en) | 1977-05-10 |
| FR2331225A1 (en) | 1977-06-03 |
| SE415820B (en) | 1980-10-27 |
| DE2641517B2 (en) | 1978-11-30 |
| DK416976A (en) | 1977-05-08 |
| JPS5260022A (en) | 1977-05-18 |
| CH609818A5 (en) | 1979-03-15 |
| ATA685776A (en) | 1979-09-15 |
| FR2331225B1 (en) | 1980-09-19 |
| DE2641517A1 (en) | 1977-05-12 |
| AT356192B (en) | 1980-04-10 |
Similar Documents
| Publication | Publication Date | Title |
|---|---|---|
| US4050084A (en) | Comb filter for television signals having adaptive features | |
| US4223340A (en) | Image detail improvement in a vertical detail enhancement system | |
| CA1062361A (en) | Chrominance-luninance separator | |
| US4731660A (en) | Processing methods using adaptive threshold for removal of chroma/luminance cross-talk in quadrature-modulated subcarrier color television systems | |
| US4232330A (en) | Method and system for separating the chrominance and luminance signals in color television signals having quadrature-modulated chrominance subcarriers | |
| GB1515551A (en) | Noise reduction in electrical signals | |
| US4051516A (en) | Digital processing of PAL color television signals in the form of a Y signal and a composite U+V/U-V signal | |
| US5663771A (en) | Adaptive video comb filter with legalized output signals | |
| CA1154536A (en) | Decoding and recoding composite digital television signals | |
| CA1189957A (en) | Television signal filtering system | |
| US4597007A (en) | Circuitry for correcting frame combed luminance signal for motion induced distortion | |
| GB2040640A (en) | Digitising an ntsc television signal | |
| CA1191246A (en) | Digital chrominance filter for digital component television system | |
| US4766484A (en) | NTSC/PAL switchable video color decoder using a digital comb filter and method | |
| JPS5946476B2 (en) | PAL color television signal processing circuit | |
| CA1090916A (en) | Line sequential colour television system | |
| GB1426721A (en) | Line sequential colour television recording system | |
| CA1225144A (en) | Piecewise linear digital video signal processing apparatus | |
| JPH05115072A (en) | Correlation adapting type luminance/color-signal separating circuit | |
| KR0133515B1 (en) | The vertical horizontal contour compensation system | |
| US4829367A (en) | Apparatus and method for encoding and decoding a NTSC color video signal | |
| GB2072991A (en) | Coding and decoding of NTSC colour television signals | |
| US4052736A (en) | Line-sequential color television encoding and decoding system | |
| US4553158A (en) | Circuitry for correcting motion induced errors in frame comb filtered video signals | |
| US4849808A (en) | System for filtering color television signals utilizing comb filtering of liminance with variable combing level at low frequencies and filtering of chrominance by comb filtering time multiplexed color difference signals |
Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |