GB2072457A

GB2072457A - Method and apparatus for one line dropout compensation of color television signals

Info

Publication number: GB2072457A
Application number: GB8109240A
Authority: GB
Original assignee: Ampex Corp
Current assignee: Ampex Corp
Priority date: 1980-03-24
Filing date: 1981-03-24
Publication date: 1981-09-30
Also published as: NL8101437A; DE3111334A1; JPS56149190A; IT1144484B; JPH0262997B2; FR2478924A1; CA1171521A; IT8148079A0; FR2478924B1; GB2072457B; DE3111334C2; HK26686A

Abstract

Dropout compensation of composite color television signals is described, in which both the luminance and chrominance components of a dropout compensation signal are derived from the same portion of the original colour television information signal immediately preceding the dropout. One horizontal line of the television signal is continuously stored in a memory 2,3,4. When a dropout occurs, the last received original television signal line is circulated in the memory. The length of delay of the stored chrominance component is controlled line-to-line in response to the phase of the color burst synchronizing component. The length of delay of the stored luminance component is controlled on consecutive lines in response to the horizontal line syn- chronizing component. The respectively delayed chrominance and luminance components are combined into a composite dropout compensation signal which is applied when a dropout is detected. <IMAGE>

Description

SPECIFICATION Method and apparatus for one line dropout compensation of color television signals The present invention relates to a method and apparatus for dropout compensation of color television signals where both the luminance and chrominance component are delayed by substantially one horizontal line period, suitable for utilization in both analog and digital signal systems. As it is well known in the art, television signal dropout compensators are utilized to replace a missing portion of the information signal which has "dropped out" due to unpredictable instantaneous malfunction of the system. For example, when the television signal is recorded and subsequently played back from a recording medium, a dropout may occur due to diminutive defects of the recording medium. When such dropouts in the television signal occur, they visibly disturb the picture.Consequently, dropout compensators are utilized to eliminate the effect of dropouts to the viewer.

Generally, prior art dropout compensators utilize an RF envelope level detector which monitors the amplitude level of the modulated television signal carrier waveform to detect dropouts in the television signal. A delay line is utilized to continuously delay the incoming television signal. When a dropout in the original signal is detected, the delayed signal is applied as a dropout compensation signal to replace the missing portion of the television signal information. More specifically, a switch in the color television signal path is controlled to apply the incoming color television signal, or the delayed dropout compensation signal, respectively, in response to a control signal from the dropout detector. The color television signal is delayed by one horizontal line period and the chrominance component is adjusted to have its phase inverted on consecutive television lines.For example, phase adjustment of the separated chrominance component of the dropout compensation signal is provided by delaying the chrominance component by an additional television line period, or, alternatively, the separated chrominance component is inverted on consecutive television lines, dependent on the particular color television signal system, as it is well known in the art.

However, there is a significant disadvantage in the above-type of prior art dropout compensation, as it will be discussed below. The original color television signal is separated into a luminance and chrominance component and each component is respectively delayed and processed in a separate signal path.

Then, the separately processed signal components are recombined for use as a dropout compensation signal. When the dropout is longer than one horizontal line period, the previously processed and recombined composite signal is again separated and processed, as above described, repeatedly, for every consecutive dropout compensation line.

Consequently, line-to-line distortion and prog ressive degeneration of the dropout compensation signal results. Thus, when utilizing the above indicated dropout compensation method, the obtained dropout compensation signal may become unacceptable after a few consecutive lines.

Accordingly, it is an object of the present invention to provide improved dropout compensation of composite color television signals.

It is a further object of the present invention to provide at least in a preferred form of the invention dropout compensation in which consecutive lines of the dropout compensation signal are derived from the same portion of the original color television information signal immediately preceding the dropout.

It is another such object of the invention to provide dropout compensation in which the same portion of the original composite color television signal is stored in a memory, from which signal portion consecutive lines of a dropout compensation signal are derived and which stored original signal remains unchanged while the dropout compensation takes place.

It is stiil another such object of the invention to provide a color television signal dropout compensation method and apparatus in which both the luminance and chrominance component of the original signal are delayed by substantially one television line interval.

In general, the present invention is suitable for use in both analog and digital color television signal systems, and the various known television signal systems, such as NTSC, PAL, PAL-M, etc. It is also, in general, suitable for use when the television signal is sampled at a frequency equal to an integral multiple, greater than two, of the color subcarrier signal frequencies.

In a preferred embodiment of the present invention, both the chrominance and luminance components of the composite color television signal are stored for a period of time equal to one horizontal line period, and the stored information is circulated in response to a dropout signal. The actual length of delay of the chrominance component is controlled by the phase of the color burst to provide a delayed chrominance component, which is in phase with the color burst component during consecutive horizuntal line periods. The actual length of delay of the luminance component is controlled to provide a luminance component which is in synchronism with the horizontal line synchronizing component of the composite color television signal during consecutive horizontal line periods.The respectively delayed chrominance and luminance compo nents are combined to form a composite dropout compensation signal, which, in turn, is applied to replace the composite color television signal when a dropout is indicated.

The color television signal dropout compensator of the present invention preferably has a circulating storage means for delaying both the luminance and chrominance components of an incoming composite color television sig nai substantially by one horizontal line period in response to a dropout control signal. A first control means which is responsive to the color burst component, may be utilized to control the actual delay of the chrominance component to provide a delayed chrominance component which is in phase with the colour burst component during consecutive horizontal line periods.A second control means, which is responsive to the horizontal line synchronizing component, may be utilized to control the actual delay of the luminance component to provide a delayed luminance component which is in synchronism wlth the horizontal line synchronizing component. A signal combining means may be utilized to combine the respectively delayed luminance and chrominance components to provide a composite dropout compensation signal. A switching means, responsive to the dropout control signal, may selectively apply the incoming color television signal or the composite dropout compensation signal to an output of the dropout compensator.

Other objects, advantages and features of the present invention will become apparent from the following detailed description taken with reference to the accompanying drawings.

Figures la and ib illustrate a preferred method of providing a dropout compensation signal in accordance with the invention.

Figure 2 is a functional block diagram of a preferred embodiment of the invention.

Figures 3a and 3b illustrate another preferred method in accordance with the invention.

Figure 4 illustrates the operation of the apparatus of Fig. 2.

Figure 5 is a functional block diagram of another preferred embodiment of the invention.

Figure 6 illustrates the operation of the apparatus of Fig. 5.

Figure 7 is a functional block diagram of a still further preferred embodiment of the invention.

Figures 8a to 8h are consecutive parts of a detailed electrical schematic diagram corresponding to the block diagram of Fig. 7.

An example of a preferred dropout compensation method of the present invention will be described now with reference to Figs. 1 a and ib. In Fig. la, a sine wave W1 is shown representing a color subcarrier signal waveform comprised in the chrominance component of a composite color television signal, as it is well known in the art. Waveform Wl comprises consecutive samples 1A, 2A, 3A, etc., provided at equidistant sampling periods, which may be obtained by sampling the original color television signal, as it is well known in the art. In this particular example, the above samples are obtained by sampling an NTSC composite color television signal at a frequency equal to three times the color subcarrier signal frequency, that is, at 3 X 3.58 MHz.The obtained samples may be encoded into a digital form by an analog-to-digital encoder, utilizing. for example, pulse code modulation (PCM), where each individual code, or sample, corresponds to a particular amplitude value of the original analog signal at a particular sampling time, as it is well known in the art. As an example, the subcarrier waveform W1 is indicated as occurring during a particular television line interval Al.

It is a well known feature of the NTSC television signal system, that a subscarrier waveform W2 of a next subsequent television line interval B1. will have an opposite phase with respect to W1, as shown in Fig. la. Therefore, if a dropout occurs, for example, after the original television line Al has been received and delayed by one television line period for subsequent utilization as a dropout compensation signal to replace the "dropped out" line Bi, the delayed line Al would exhibit an undesired 180 phase shift with respect to B1. Consequently. samples 1A, 2A, 3A, etc., of the delayed W1 signal will have to be displaced by one and one-half sampling intervals, to obtain the same phase as that of the corresponding samples 2B, 3B, etc., of waveform W2, as it is shown in Fig. 1 a.

Thus, for example, sample 1A has to be displaced with respect to corresponding sample 2B, and similarly, sample 2A with respect to sample 3B, etc., by one and one-half sampling intervals.

By the preferred method of the present invention the above-indicated phase shift of the color subcarrier signal on consecutive lines is obtained by alternatively increasing or decreasing the one horizontal line delay of the television signal by a delay corresponding to one and one-half sample periods. This latter feature is illustrated in Fig. 1 a, showing sample 3B of line B1 as being obtained by delaying sample 2A of the immediately preceding original television line Al by one horizontal line period, increased by one and onehalf sample periods. A dashed line CH 1 connecting corresponding samples 3B and 2A illustrates this feature. Fig. 1 b shows the subcarrier waveform W2 of line Bi, followed by waveform W3 of a next consecutive line A2. Fig. 1 b illustrates an example where line B1 is the last received original television signal line and line A2 is the first dropout compensation line. As Fig. ib reveals, sample 1 A of the subcarrier signal waveform W3 is obtained by delaying sample 2B of waveform W2 by one horizontal line period decreased by a time interval corresponding to one and one-half sample periods. Dashed line CH2, connecting corresponding samples 1A and 2B, indicates the latter feature.

However, when the above-described dropout compensation method is utilized, both the chrominance and luminance component of the dropout compensation signal are displaced by + one and one-half sample periods with respect to the preceding original color television signal line. As it is known in the art, such phase displacement of the chrominance component is not objectionable, since for obtaining a high quality color signal for dropout compensation it is essential to maintain the line-to-line phase of the chrominance component samples as close to the required phase as possible. On the other hand, the aboveindicated displacement is quite objectionable for the luminance component, for which it is essential to maintain the smallest possible line-to-line horizontal displacement of samples on the television picture screen with respect to the original signal.

Note: For brevity of description, "dropout compensation will be abbreviated as "DOC" and "dropout" as "DO" in the following specification.

To eliminate the above-discussed drawback, the preferred dropout compensation method of the invention provides line-to-line adjustment of the luminance component displacement as follows.

When the one horizontal line delay of the original television signal is increased by one and one-half sample periods, for dropout compensation, as it has been described above with reference to Fig. 1 a, the luminance component delay is decreased by one sample period. Thus, the horizontal displacement of the luminance component sample with respect to a corresponding sample of a preceding original television signal line is reduced from one and one-half sample periods to one-half sample period. The foregoing is illustrated in Fig. 1 a by dashed line L1 connecting corresponding samples 3B and 3A.Analogously, for obtaining the above reduction in the luminance component displacement, in case the one horizontal line delay of the original television signal is decreased by one and one-half sample period, as previously described and illustrated in Fig. 1 b, the luminance component delay is increased by one sample period.

The latter case is illustrated in Fig. Ib by dashed line L2 connecting corresponding samples 1A and 1B. It is seen from Fig. 1a that the luminance component of sample 3B of dropout compensation line B1 is obtained by delaying sample 3A of line Al by one horizontal line period increased by only onehalf sample period, as illustrated by line L1.

Similarly, it is shown in Fig. 1 b by line L2 that the luminance component of sample 1A is obtained by delaying sample 1 B of line B1 by one horizontal line period decreased by only one-half sample period. However, in both cases the delay of the respective chrominance components remains unchanged, that is, displaced by one and one-half sample periods, as desired and as it has been previously described.

More specifically, the above features of the preferred method of the present invention are obtained as follows. One horizontal line of the original composite color television signal is continuously stored in a memory. The length of delay of the stored composite signal is controlled responsive to the line-to-line phase of the color burst component of the original color television signal. The resulting delayed composite signal is separated into its luminance and chrominance component and the length of delay of the luminance component is modulated in opposite direction relative to the delayed chrominance component by an integral number of sample periods.With respect to the above-described particular example illustrated in Figs. 1 a and 1 b, the composite color television signal is delayed by one horizontal line period which delay is modulated by + one and one-half sample periods, corresponding to one-half color subcarrier signal period, on consecutive lines. Besides that, the length of delay of the separated luminance component is modulated line-to-line by + one sample period, in opposite sense with respect to the modulation of the delayed composite color television signal, to reduce the line-to-line luminance component displacement.

To illustrate the foregoing, a preferred embodiment of the apparatus of the present invention will be described with reference to the functional block diagram of Fig. 2 of the attached drawings. A composite color television signal is received at input terminal 1 2 and is appjied to first input 20 of a first twoway switch 1. In this particular example, the television signal is received in digital form obtained, for example, by encoding the color television signal into a well-known NRZ code, as it has been previously mentioned with reference to Figs. 1 a and 1 b. A control signal indicating presence of a dropout is received by a control terminal 23, for example, from a conventional dropout detector (not shown).

A suitable dropout detector may be of a conventional carrier monitor type which provides a control signal when the RF envelope of the modulated television signal dropout is below a predetermined level, such as described, for example, in AVR-2 Videotape Recorder, Theory of Operation, Catalog No.

1809179-01, published November, 1977, by Ampex Corporation, pages 5-31 to 5-33.

The control terminal 23 is coupled to a control input 24 of first switch 1, and, also, to control input 29 of a second switch 2. Output 25 of switch 1 is coupled to an output terminal 1 3 of the dropout compensator cir cuit, as well as to first input 26 of switch 2.

The output 27 of switch 2 is coupled via a controlled delay line 3 and a compensating fixed delay line 4 to a second input 28 of switch 2. Elements 2, 3 and 4 form together a circulating memory circuit 44, as it will be described later in more detail. A control termi nal 30 is coupled to a control input 31 of the controlled delay line 3. The output 32 of delay line 3 is coupled to an input 33 of a filter 5 and via a compensating fixed delay line 6 to a first input 35 of a differencing circuit 7. The compensating delay lines 4 and 6 are respectively utilized to compensate for circuit delays in the color television signal path, as it will follow from the further description.An output 34 of filter 5 is coupled to a second input 36 of differencing circuit 7, as well as to a first input 37 of a third two-way switch 10 and it is also coupled via a fixed delay line 9 to a second input 38 of switch 10. A control terminal 43 receiving a control signal HS/2 is coupled to a control input 44 of third switch 10. An output 39 of switch 10 is coupled to a first input 40 of a signal combining circuit 11. To a second input 41 of circuit 11, an output 45 of signal differencing circuit 7 is coupled via a fixed delay line 8.

An output 42 of signal combining circuit 11 is coupled to a second input 21 of the first two-way switch 1.

It is to be realized that the above-described preferred embodiment of Fig. 2 as well as the respective embodiments of Figs. 5 and 7 which will be described later, ail represent respective digital dropout compensators in accordance with the present invention in which high speed digital data is processed. Consequently, the various elements shown in these respective functional block diagrams may be designed as conventional digital circuits in which the high speed data is precisely clocked at the sampling frequency, equal to three times the color subcarrier signal frequency, that is at 3 X 3.58r 10.74 MHz, while the clock signal is frequency and phase-locked to the color subcarrier component of the sampled television signal. Consequently, in the further description we will refer interchangeably to the sampling frequency and clock frequency.For simplicity of representation, the clock signal path is not shown in the above-indicated Figures, however, it is shown in the detailed electrical schematic diagram of Figs. 8a to 8h. corresponding to the block diagram of Fig. 7, which detailed diagram will be described later.

Now the operation of the preferred embodiment of the invention shown in the block diagram of Fig. 2 will be described. A digital NTSC color television signal in the form of discrete data representing consecutive samples is being continuously received at input terminal 1 2 and fed to first input 20 of switch 1. When the system is in normal operation, that is, no dropout in the incoming signal is detected, switch 1 is in its first position as shown, receiving the input signal at 20 and applying it via output 25 to output terminal 1 3 of the dropout compensator, and consequently, applying it also via input 26 and output 27 of switch 2 to the controlled delay line 3. Delay line 3 has a length of delay equal to one horizontal line period less circuit delays in the color television signal path, as it will be disclosed later.

The delayed composite signal from delay line 3 is fed to filter 5 which may be as described in our co-pending Application No.

8036090 or be a digital comb filter and which separates the luminance component from the composite color television signal.

The separated luminance component obtained at the output 34 of filter 5 is applied to input 36 of differencing circuit 7. To the other input 35 of circuit 7, the composite color television signal from output 32 of delay line 3 is applied via compensating fixed delay line 6.

Differencing circuit 7 provides at its output 45 a difference signal of the two signals received at its respective inputs. The resulting difference signal represents the separated chromi- - nance component of the color television signal. Delay line 6 compensates for the circuit delay provided by filter 5, as it is well knowns in the art, to eliminate undesirable relative phase shifts in the respective signal paths of the signals subsequently processed in differencing circuit 7.

With respect to the well known relationship of the color subcarrier signal frequency fse and horizontal line frequency fh of NTSC signals fisc= = 227.5fah, in this particular embodiment of the invention a non-integral number of samples equal to 3 X 227.5 = 682.5 clock cycles is obtained within one horizontal line period. For clarity and simplification of the disclosure, compensation for circuit delays provided by delay line 3 will not be considered in the following description; however, it will be described later. In this regard, the controlled delay line 3 will be considered as having a fixed length of delay equal to one horizontal line period represented by 682.5 clock cycles. Loading and unloading of the samples into and from delay line 3, respectively, is controlled by the control signal CB received at terminal 30, which signal is synchronous with the color burst component of the color television signal during each horizontal line. The latter control signal is applied to control input 31 of delay line 3; it is derived from the color burst synchronizing component of the incoming television signal, in a manner well known in the art. As mentioned earlier, in this particular embodiment one cycle of the color burst corresponds to three sample peri ods equal to three clock cycles. Consequently, the relative line-to-line phase shift by 180 of the color burst component represents a delay of plus or minus 121 sample periods, that is plus or minus 12 clock cycles.Thus, the control delay line 3 provides alternative delays of the composite color television signal equal to 68221 + 1 a = 684 clock cycles and 6822 - 1 2 = 681 clock cycles on consecutive lines. The latter operation may be described as modulating the fixed length of delay of the composite color television signal stored in delay line 3 during consecutive television line periods by the phase of the color burst component. It will be understood that the average signal delay during several consecutive lines approaches 682 2 clock cycles, which is equal to the original one horizontal line period.

From the foregoing description it follows that delay line 3 of Fig. 2 is controlled by a control signal CB synchronous with the color burst component, which signal is received at terminal 30 to provide a delayed composite color television signal whose color subcarrier component is in phase with the color burst component during each consecutive line, as it has been previously described in detail with respect to Figs. 1 a and 1 b.

As previously disclosed with reference to Figs. 1 a and 1 h, it is desirable that the luminance component samples of the dropout compensation signal of the invention be processed to minimize the horizontal displacement of respective sample positions on consecutive lines of the television screen. Now, the line-toline adjustment of the luminance component delay provided by the apparatus of Fig. 2 to achieve the foregoing will be described. As above described, in this preferred embodiment the luminance component is separated in filter 5 from the composite signal having a modulated delay. The length of delay of the delayed separated luminance component is further modulated by + 1 clock cycle in the opposite direction with respect to the above described modulation of the composite color television signal delay.To this effect, the delayed separated chrominance component at the output 45 of differencing circuit 7 is delayed by additional one clock cycle provided by fixed delay line 8. At the same time, the delayed separated luminance component at the output 34 of filter 5 is delayed by additional two clock cycles provided by fixed delay line 9 during alternative television line periods, as follows. The separated delayed luminance component from filter 5 is directly applied to the first input 37 of two-way switch 10 while it is also applied via additional delay line 9 to the second input 38 of switch 1 0.

Switch 10 is controlled by the control signal HS/2 received at terminal 43, having a frequency equal to one-half horizontal line frequency. Signal HS/2 is derived from the horizontal line synchronizing component of the incoming color television signal received at input terminal 1 2. In the resepctive preferred embodiments of the invention described herein, the respective control signals HS/2 are frequency and phase/locked to the horizontal line synchronizing component of the incoming color television signal and, also, to the phase of the color burst component.

During dropout compensation, when the composite television signal is not received, the respective control signals CB, HS or HS/2 are generated synchronously with the respective synchronizing components of the missing television signal, utilizing a flywheel signal general (not shown) frequency and phase-locked to the color television signal, as it is known in the art. It is understood from the foregoing description that the resulting delay of the luminance component is modulated by i one clock cycle delay relative to the chrominance component delay By this latter feature the length of delay of the luminance component of the dropout compensation signal is adjusted on consecutive horizontal lines in resonse to the horizontal line synchronizing component to minimize line-to-line horizontal displacement of samples during dropout compensation.

The respectively delayed luminance and chrominance components are combined in adder 11 and the combined signals at the output 42 of adder 11 represent the composite dropout compensation signal applied to input 21 of first swith 1. The latter signal is continuously available to be utilized for dropout compensation in response to a dropout compensation control signal DO received at control terminal 23, as it has been described previously.

Upon receiving a control signal DO at 23, indicating that a dropout has occurred in the incoming television signal at 12, switch 1 is controlled to switch from input 20 to 21, and switch 2 to switch from 26 to 28. Consequently, switch 1 will apply to output terminal 13, the dropout compensation signal received at its input 21, which is provided by the circuit of Fig. 2 as above disclosed. On the other hand, switch 2 will close the circulating memory circuit 44, and, thus, effect circulation of the last received line of the television signal information immediately preceding the dropout, which signal has been stored in delay lines 3 and 4. Consequently, the aboveindicated original color television signal line will circulate in circulating memory 44 until the dropout is eliminated and the control signal DO at 23 is discontinued.During the dropout compensation, the output signal at 32 from the circulating memory 44 is processed by the dropout compensator circuit of Fig. 2 in the previously disclosed manner, instead of the incoming color television signal.

As above mentioned, the length of delay of controlled delay line 3 is decreased from the previously considered one horizontal line period to compensate for circuit delays in the processed composite signal path. Thus, if designating. A the delay provided by filter 5 in the separated luminance signal path, and designating 8 the additional one clock cycle delay provided in both the separated luminance and chrominance signal paths, respectively, then, it will become apparent from the block diagram of Fig. 2 that to obtain an overall length of delay equal to one horizontal line period of the composite DOC signal, while not considering the above-described modulation of the delay by the respective control signals, delay line 3 must have its delay decreased by A + b. Similarly. it will become apparent from the above disclosure that to provide a circulating memory 44 which has a length of delay equal to exactly one horizontal line period, it is necessary to re-insert the combined delays A + 6 in form of a fixed delay line 4 as shown between the output 32 and input 27 of delay line 3.

As it is seen from the foregoing description, it is a significant advantage of the invention that consecutive lines of the dropout compensation signal are derived from the same portion of the original color television signal immediately preceding the dropout.

It is a further important advantage, that the composite signal is stored in the circulating memory in its original form, and remains unchanged by the line-to-line processing of the signal components throughout the dropout compensation. Consequently, no deterioration of the resulting dropout compensation signal is effected, as it occurred in the known prior art dropout compensators. At the same time, each sample of the dropout signal chrominance component is processed to have the same phase as that of a corresponding sample of the original television signal, from which the dropout signal is formed, while the dropout signal luminance component is processed to have a minimum horizontal displacement of samples on consecutive lines.Since in the above described example related to the operation of the block diagram of Fig. 2, a sampling rate of three times the color subcarrier signal frequency is considered. there is a lineto-line displacement of the dropout compensation signal sample equal to 12 sample periods for the chrominance component and just 2 sample period for the luminance component.

It is understood from the foregoing description that by the above-indicated respective line-to-line displacement of samples, the previously described objectives relative to a high quality dropout compensation signal are achieved.

The foregoing operation of the apparatus of Fig. 2 is illustrated in Fig. 4 where line-to-line displacement of the luminance and chrominance component samples are shown, respectively, on consecutive lines of the DOC television signal. A last received original television signal line Al, immediately preceding the dropout, comprises consecutive samples 2A, 3A, 4A, 5A, etc. It is seen from Fig. 4 that the samples 2B, 3B, 4B, etc., of the first DOC line B1, as well of all subsequent odd DOC lines B2, B3, etc., are obtained from the original line Al in the manner previously described and shown with respect to Fig. la.

For better comparison, corresponding samples in both Figs. 1 a and 4 are designated by like reference characters. However, it is seen from Fig. 4 that the DOC chrominance component samples on the second DOC line A2 and all subsequent even DOC lines A3, A4, etc., are obtained by delaying the chrominance component samples of the original line Al by exactly one horizontal line period. Consequently, the respective chrominance components of samples 3A 4A, etc., on even DOC lines A2, A3, etc., are in phase with those of original line Al, however, displaced by 12 sample periods with respect to an immediately preceding odd DOC line.On the other hand, the luminance component samples of even DOC lines A2, A3, etc., are displaced by one sample period with respect to the original line Al but they are displaced only by 2- sample period with respect to an immediately preced-, ing odd DOC line. As a result of the abovedescribed dropout compensation method shown in Fig. 4, a line-to-line displacement pattern of sample components is formed, in which lines CH2, L2; CH3, L3; CH4, L4; etc., connecting corresponding chrominance and luminance component samples of adjacent DOC lines cross each other. By this pattern the desired respective line-to-line vertical displacement by + 12 sample periods of the chrominance component and by + 2 sample period of the luminance component is achieved.

The foregoing preferred method of the invention with respect to providing a delayed luminance component of the composite dropout compensation signal on consecutive television lines is illustrated in Figs. 3a and 3b. In Fig. 3a a waveform M1 represents a portion of the luminance component of an original color television signal line Al received at terminal 12 by the dropout compensation circuit of Fig. 2, immediately preceding a dropout. Samples 1 A, 2A, 3A, etc., of waveform M1 are obtained by sampling the color television signal at a frequency equal to three times the color subcarrier signal frequency, as described previously. When considering the above-described method related to providing a dropout compensation signal luminance component by the apparatus of Fig. 2, the luminance component of the first dropout compensation signal line Bl, following the original line Al on the television picture screen, is represented as shown at M2 in Fig. 3A. It is seen that waveform M2 is a replica of the original waveform M1 with the exception of samples 1 B, 2B, 3B, etc., of M2 being displaced by only one-half sample period in one direction, for example, to the right with respect to samples 1A, 2A, 3A, etc., of waveform M1.However, waveform M3 of the next consecutive line A2 which waveform is a replica of the original waveform M1, is displaced from M2 of line B1 by one-half sample period, but it is displaced from the original waveform M1 of line 1A by one sample period, as it is seen from the location of samples 2A, 3A, 4A, etc., on M3 with respect to the samples of the previous lines B1 and Al, respectively. The latter effect is not objectionable to the viewer, since only a one-half sample period displacement in horizontal direction is seen between respective samples on consecutive lines.The above method allows forming a composite dropout compensation signal having its chrominance component samples horizontally displaced in one direction, while its luminance component samples are horizontally displaced in the opposite direction with respect to a previous color television signal line on the screen. This pattern alternates on consecutive dropout compensation lines by changing the above directions to achieve an overall compensating effect to the viewer, as it has been described above with respect to Fig. 4.

Consequently, dropout compensation (DOC) line A2, is followed by DOC line B2, which is formed by displacing the luminance component samples of the original line Al by onehalf sample period in the same direction as for line B1 and thus is in phase therewith. Thus, the above method is repeated for all "odd" DOC lines B1, B2, ... BN which have their samples horizontally displaced with respect to the original line Al in one direction by onehalf sample period and alternatively, for all "even" DOC lines A2, A3, ... AN which have their samples horizontally displaced by one sample period with respect to original line A2 in the same direction, as shown in Fig. 3a. it is understood that the overall effect of line-toline horizontal displacement of the DOC signal luminance component to the viewer is + one-half sample period.Thus, by the above described method of the invention, the effect of horizontal displacement of the DOC signal luminance component is minimized.

Fig. 3b depicts the luminance component of an original color television line B1 immediately preceding a dropout. Waveform N1 comprising samples 1 B, 2B, 3B, etc., is identical to waveform M1 pertaining to line Al of Fig. 3a with the exception of having its samples 1 B, 2B, etc., displaced by + sample period with respect to waveform M1. It will be seen from Fig. 3b, that N2 depicts respective waveforms of all the "odd" DOC lines A2, A3, ... AN following the original line B1.

Similarly, N3 represents all the even DOC lines B2, B3, . . BN following the original line B1. There is a difference between forming DOC luminance components on consecutive television lines, as shown in Figs. 3a and 3b in that the samples of the "odd" lines are displaced with respect to the original line by one-half sample and the samples of the "even" lines are displaced with respect to the original line by one whole sample in respectively opposite directions.

With reference to the above disclosure it follows that the present method of line-to-line modulating both the chrominance and luminance component delay sychronously with the color burst phase, and of line-to-line modulating the luminance component delay synchronously with the horizontal line synchronizing component to decrease the line-to-line delay thereof, is achieved relatively simply by applying a color-burst component-related control signal and a horizontal line synchronizing component-related control signal, respectively, to the respective delay lines.Since both control signals CB and HS/2 are frequency and phase-locked to the incoming television signal as well as to each other, the above method may be accomplished by artitrarily assigning the sample displacement direction, or phase, to each dropout compensation line to the right or left as seen on the television screen with respect to the original signal. Once that direction, or phase, is assigned, it is maintained the same on alternative DOC lines in response to the HS/2 signal which has a frequency equal to one-half of the horizontal line frequency. The HS/2 signal is also utilized to modulate the luminance signal delay as it has been described. As a result of the abovedescribed method, a dropout compensation pattern, such as shown in Fig. 4, is obtained.

Another preferred embodiment of the dropout compensator in accordance with the present invention is shown in Fig. 5 and it will be described now. In Fig. 5, similar circuit elements to those previously described with respect to Fig. 2 are designated by like reference numerals. Description of these elements with respect to Fig. 5 will be deleted to avoid undue repetition. In the dropout compensation circuit of Fig. 5, filter 5, separating the luminance component from the composite color television signal, is coupled to the output 25 of the first two-way switch 1. The separated luminance component at the output 34 of filter 5 is fed to input 36 of differencing circuit 7 to whose other input 35 the color television signal from output 25 of switch 1 is fed, via compensating fixed delay line 6.The resulting separated chrominance component at the output 45 of differencing circuit 7 is coupled to a first input 47 of a second twoway switch 14. The output 50 of switch 14 is coupled via a first controlled chrominance delay line 1 6 whose output is coupled to a second input 49 of switch 14. The delay line 1 6 and switch 14 thus represent a first circulating memory circuit 81 for the separated chrominance component. The delay line 1 6 is controlled by the previously described control signal indicated CB received at control terminal 30.The separated luminance component at the output 34 of filter 5 is further coupled to a first input 46 of a third two-way switch 15, and it is fed from output 51 of switch 15 via a second controlled luminance delay line 1 7 to a second input 48 of switch 1 5. It is seen that delay line 1 7 and switch 1 5 form a second circulating memory circuit 82 for the separated luminance component. The delay line 1 7 is controlled by a control signal indicated HS received at control terminal 43. A dropout control signal DO received at control terminal 23 is coupled to respective control inputs 24, 85 and 86 of switches 1, 1 5 and 14, respectively.

In operation, a dropout control signal DO, which indicates the presence of dropout in the incoming composite color television signal received at 12, activates all three switches 1, 14 and 15. respectively. Consequently, to output 25 of the first switch 1 a dropout compensation signal from its input 21 will be supplied, which signal is provided by the circuit of Fig. 5. The above-indicated dropout compensation signal is formed as follows.

During normal circuit operation, when no dropout is indicated, filter 5 continuously receives the digital color television signal at 1 2 having a frequency equal to three times the color subcarrier signal frequency. The separated luminance component at the output 34 of filter 5 is fed via switch 1 5 to the luminance delay line 1 7. Delay line 1 7 has a length of delay equal to one horizontal line period. Consequently, in delay line 1 7 at any given time of normal circuit operation, one horizontal line of the separated luminance component obtained from an immediately preceding portion of the color television signal received at terminal 12 will be stored.The loading and unloading of the signal stored in delay line 1 7 is controlled by the control signal HS received at terminal 43. Signal HS is derived from the horizontal synchronizing component of the received color television signal and is synchronous therewith. With respect to the above feature, a separated luminance component is obtained which is delayed by one horizontal line period and, at the same time, which is coherent with the horizontal line synchronizing component during consecutive television lines.

Similarly, the chrominance delay line 16, which also has a length equal to one horizontal line period, continuously receives the separated chrominance component from differencing circuit 7, via switch 14. Thus, at any given time of normal circuit operation, one horizontal line of the separated chrominance component will be stored in the chrcrninance delay line 16, which portion of the chrominance component corresponds to that of the separated luminance component stored simultaneously in the luminance delay line 1 7.

Loading and unloading of the chrominance component stored in delay line 1 6 is controlled by the control signal CB received at terminal 30, which signal is derived from the color burst synchronizing component of the color television signal received at terminal 12.

By controlling the chrominance delay line 1 6 by the control signal CB a separated chrominance component is obtained which is delayed by one horizontal line period, and at the same time, which is in phase with the color burst component during consecutive television lines.

As it is shown in Fig. 5, the respectively delayed separated luminance and chrominance components are combined in the signal combining circuit 11 and the combined signal at the output 42 of circuit 11 is fed to the second input 21 of first switch 1, as it has been described above. The signal at 21 represents the dropout compensation signal which is utilized by the apparatus of Fig. 5 when a dropout control signal DO is received.

Whena dropout is indicated, switches 14 and 1 5 controlled by signal DO close the respective circulating memory circuits 81, 82.

Consequently, both the separated luminance and chrominance component circulate in their respective circulating memory circuits, synchronously, controlled by the previously-mentioned clock signal at a frequency equal to three times the color subcarrier signal frequency.

It follows from the above description, that the circuit of the preferred embodiment of Fig.

5 has an advantage with respect to the circuit of Fig. 2 that it does not require additional modulation of the delayed separated luminance component with respect to the delayed separated chrominance component to reduce the line-to-line horizontal displacement of the luminance component samples. Instead, in the embodiment of Fig. 5, two separated criculating delay lines are provided, one delay line for the separated luminance component and another one for the separated chrominance component, respectively. Each delay line 1 6, 1 7 is controlled by a separate control signal CB or HS, respectively, to provide a signal component in synchronization therewith.

For illustration of the foregoing description, the operation of the embodiment of Fig. 5 is shown in Fig. 6, where the respective horizontal displacements between the luminance and chrominance component samples on consecutive dropout compensation lines is shown. To facilitate comparison with a previously described diagram on Fig. 4, related to the circuit of Fig. 2, the last received original color television signal line preceding the dropout, the consecutive dropout compensation (DOC) lines, as well as consecutive samples of these respective lines are designated by like reference characters in both diagrams of Figs.

4 and 6. It is seen in the diagram of Fig. 6 that the luminance component of each DOC sample 2B, 3B, 4B, etc., on odd DOC lines B1, B2, etc., is obtained by delaying the luminance component of a corresponding sample 2A, 3A, 4A, etc., of the original line Al by one horizontal line period increased by one-half sample period. At the same time, the chrominance component of each above-indicated DOC sample 3B, 4B, etc., on odd DOC lines is obtained by delaying the chrominance component of a corresponding sample 2A, 3A of the original line Al by one horizontal line period increased by one-and one-half sample period.On the other hand, the luminance component of each DOC sample 3A, 4A, etc., on even DOC lines A2, A3, etc., is obtained by delaying both the luminance component as well as the chrominance component of the corresponding samples 3A, 4A, etc., of the original color television signal by one horizontal line period. Thus, in the operation of the preferred embodiment of the invention of Fig.

5, the relative line-to-line vertical displacement of the DOC signal samples is + + sample period for the luminance component and + 1 > samples for the chrominance component.

However, it is an advantage of the preferred method of the invention shown in Fig. 6 when comparing to the method of Fig. 4 that every other DOC line is in phase with the original television signal line with respect to both luminance and chrominance components, and no horizontal displacement of samples takes place on these lines with respect to both luminance and chrominance components.

It will be understood from the description of Fig. 5 that in this particular embodiment of the invention it is not necessary to have the color television signal sampled at a frequency equal to an integral multiple of the color subcarrier signal frequency, as it has been the case in the embodiment of Fig. 2. In the embodiment of Fig. 5 a sampling frequency equal to a rational number multiple of the subcarrier frequency may be utilized.

For the purpose of a more complete disclosure of the apparatus and method of the present invention, a further embodiment of the present invention is shown in the block diagram of Fig. 7. A corresponding detailed electrical schematic diagram is shown in consecutive Figs. 8a to 8h. The embodiment of Figs. 7 and 8a to 8h has been designed for a specific DOC apparatus in which a two horizontal line delay of the original color television signal is required. First, the block diagram of Fig. 7 will be described, followed by the description of the corresponding detailed circuit diagram. In the embodiment of Fig. 7, a color television signal is received at an input terminal 90, in the form of consecutive digital samples at a frequency equal to three times the color subcarrier signal frequency, as previously described.The received signal is applied to input 91 of a two-way switch 52 and through its output 93 to a first controlled delay line 54, providing one horizontal line delay less circuit delays designated 9. The delayed color television signal V from output 11 5 of delay line 54 is applied via a second compensating fixed delay line 56 providing a delay equal to 9, to a second input 92 of switch 52. Thus, a circulating memory circuit 94 is formed by elements 52, 54 and 56, similarly as previously described with respect to the embodiments of Figs. 2 and 5, respectively.To a control input 1 25 of delay line 54 a control signal CB is applied, which is received at control terminal 1 24. The latter signal is similar to that described previously with respect to Figs. 2 and 5, respectively.

The color television signal V from delay line 54 is also applied via a third compensating fixed delay line 11 3 having a delay designated T to a first input 101 of a signal differencing circuit 64, as a delayed signal V'.

Signal V from the output of delay line 54 is also applied to input 104 of a digital comb filter 60, which may be as described in our co-pending Application No. 8036090, and which separates the liminance component from the color television signal, in a manner described in the copending application. The separated luminance component L' obtained at the output 105 of filter 60 is applied to a first input 107 of a second two-way switch 62 and it is also applied via a fixed two-clock cycle delay line 67 to a second input 108 of switch 62. An output 109 of switch 62 is connected to a first input 98 of a signal combining circuit 66. Thus, via switch 62 the delayed separated luminance component L is applied from filter 60 to signal combining circuit 66 either directly or delayed by addi tional two clock cycles indicated 2 .Switch 62 is controlled at its control input 11 2 by a control signal designated HS/2 received at a control terminal 111. An inverted separated luminance signal component L' obtained at an inverting output 106 of filter 60 is applied to a second input 102 of the signal differencing circuit 64. Circuit 64 provides at output 103 a difference signal C' representing the separated chrominance component obtained as a difference between the delayed composite color television signal V' and the delayed separated luminance component, respectively applied to its first and second input. In this preferred embodiment the differencing circuit 64 is implemented as a signal adder and the separated chrominance component C' is obtained by adding the delayed composite signal V' and inverted separated luminance component L'.

The separated chrominance component C' from output 103 of differencing circuit 64 is applied via a one-clock-cycle delay line 68 to one input 99 of signal adder 66. As previously mentioned, to the other input 98 of signal adder 66, the separated luminance component L from the second switch 62 is applied. Signal adder 66 combines the received chrominance and luminance components into a composite color television signal at its output 100, which signal represents the color television dropout compensation signal.

The latter signal is applied to a first input 95 of a third two-way switch 58. To a second input 96 of switch 56 the composite color television signal received at input terminal 90 is applied via first switch 52, first controlled dealy line 54 and via second compensating delay line 56. To a control input 97 of switch 58 a dropout compensation control signal DO is applied via a delay line 53 providing a one horizontal line delay. The latter control signal is received at input terminal 110 when a dropout in the incoming color television signal at terminal 90 is detected, for example, by a conventional dropout detector (not shown), as it has been previously described. The dropout control signal at 110 is also applied to a control input 114 of the first switch 52.An output 1 27 of switch 58 is coupled via a further controlled delay line 1 26 to an output terminal 70 of the dropout compensation circuit of Fig. 7. Delay line 1 26 provides a one horizontal line delay and is controlled by signal CB applied to its control input 128, simultaneously with and in the same manner as previously described with respect to controlled delay line 54.

Now the operation of the dropout compensation circuit of Fig 7 will be described.

When a dropout in the composite color television signal is detected, for example, by a conventional dropout detector (not shown), a dropout control signal DO received at terminal 110 is applied to switch 52 and via delay line 53 providing a one horizontal line delay, to switch 58. Switch 52 closes the circulating memory circuit 94 and, consequently, the last received horizontal line of the color television signal immediately preceding the dropout, which has been stored in delay lines 54 and 56 will circulate via input 92 and output 93 of switch 52 at a a clock signal frequency equal to 3 X 3.58 MHz-""10.74 MHz. The clock signal which is utilized for clocking the respective circuit elements of Fig. 7 is not shown in the block diagram for better clarity of representation, but it is shown in the corresponding detailed schematic diagram of Figs.

8a to 8h, which will be described later. Filter 60 receives the composite signal V, delayed by substantially two horizontal line periods, from the circulating memory 94 and separates the luminance component L' therefrom. The separated luminance component is applied via switch 62 to signal combining circuit 66 which is implemented in the preferred embodiment as a signal adder. Switch 62 in response to the control signal HS/2 alternatively applies the separated luminance compo-.

nent L' directly or delayed by two additional clock cycles in delay line 67, via switch 62 to signal combining circuit 66. The control signal HS/2 has a frequency of one-half of the horizontal line frequency. It is derived from the horizontal synchronizing component of the color television signal received at terminal 90.

The phase of signal HS/2 is controlled by signal CB received at terminal 1 24 to assure that the same phase-relationship of these two control signals will be maintained during the entire operation. Thus, switch 62 is controlled by the signal HS/2 to alternatively apply during consecutive horizontal lines signal L' undelayed and delayed by two clock cycles 2 6, respectively, to input 98 of adder 66. On the other hand, as previously described, signal adder 64 receives both the delayed composite signal V' and the inverted luminance component L' and provides a separated chrominance component C' at its output 103.To compensate for the circuit delay T effected by processing the separated luminance comonent in filter 60, signal V' is delayed by T in the third fixed compensating delay line 11 3. The separated chrominance component C' is delayed by one clock cycle 6 in delay line 68 and the delayed chrominance component C is combined with the above-described luminance component L in signal adder 66.

It follows from the foregoing description that when the separated luminance component L' delayed by additional two clock cycles during alternative horizontal line periods, and the separated chrominance component delayed by an additional one clock cycle during consecutive horizontal line periods, are combined together in adder 66, they represent a composite dropout compensation signal, whose luminance component delay is modulated by + 1 clock cycle with respect to the chrominance component delay during consecutive horizontal line periods. Thus, the previously mentioned feature of the invention related to decreasing the luminance component delay with respect to the chrominance component delay of the resulting dropout compensation signal is achieved in the preferred embodiment of Fig. 7 by the above-described combination of circuit elements.

It will become apparent from the abovedescription of the block diagram of Fig. 7, that delay 7 compensates for the delay in the filter circuit 60 and that delay 9 compensates for the combined delays r and delay 6 pro vided by delay line 68; thus, 9 = X + when considering that circuits 64 and 66 have no circuit delays.

It will also become apparent with respect to Fig. 7 that in this particular preferred embodi ment the controlled delay line 54 is coupled in the main composite color television signal path. Therefore, when no dropout compensation takes place, the incoming composite signal received at 1 24 is continuously delayed in delay lines 54 and 56 by one television line period, which delay is controlled synchronously with the color burst component. Thus, the original composite signal at the output 1 27 of switch 58 exhibits an undesired + 180 phase shift on alternative lines. To compensate for this effect, an additional delay line 1 26 providing a one horizontal line delay is coupled in the main color television signal path.The loading and unloading of delay line 1 26 is controlled by the above-described con tr6l signal CB received at control terminal 1 24 and applied to control input 1 28 of delay line 1 26. Since the color television signal received by delay line 1 26 has been previously delayed by one horizontal line period in delay lines 54 and 56, and both delay lines 54 and 126, respectively are controlled by the same signal CB, respective phase shifts of + 1 80" in opposite sense are effected on respective output signals of these controlled delay lines 54 and 126.Consequently, these opposite phase shifts cancel, and no phase shift is present in the output signal at output 70 of the dropout compensation circuit of Fig. 7.

As it follows from the above description, in the block diagram of Fig. 7 the dropout compensation signal at the output terminal 70 is delayed by three horizontal line periods to satisfy the specific requirements of the particular apparatus in which this particular embodiment of the invention is utilized. However, as it is seen from the foregoing disclosure, the actual dropout compensation signal obtained at output 1 27 of switch 58 is still formed from the last received original television signal line at input 124, substantially in the same manner as that previously disclosed with respect to Figs. 2 and 5.

It is noted, however, that the previously described respective embodiments of Figs. 2 and 5 do not have their respective controlled delay lines coupled in the main composite color television path and, therefore, they do not exhibit an undesired phase shift such as provided by the embodiment of Fig. 7 during normal system operation when no dropout compensation is provided.

The detailed circuit diagram shown in consecutive Figs. 8a to 8h essentially corresponds to the previously described simplified block diagram of Fig. 7. For better comparison, individual circuits in the detailed diagram corresponding to elements of the block diagram are indicated by dashed lines and designated by like reference numerals. Similarly, connecting lines between the circuits of the detailed diagram are designated by reference numerals corresponding to input/output designations of corresponding blocks of Fig. 7.

For the purpose of complete disclosure, the integrated circuit components shown in the preferred embodiment of Figs. 8a to 8h are designated by respective part numbers commonly used by manufacturers.

Specifically, the filter circuit 60, which is utilized to separate the luminance component from the composite color television signal and which is shown in consecutive Figs. 8d and 8e is similar to that disclosed in the aboveindicated copending application SN. It is understood, however, that a well known digital comb filter may be utilized instead. As it will be seen from the detailed circuit diagram of Figs. 8a to 8h, it is an advantage of the digital dropout compensation circuit of the present invention that all signal processing is provided in real time utilizing standard TTL (transistor-to-transistor logic) circuitry.The circuit of the above-indicated Figs. is designed for dropout compensation in a color television signal recording and reproducing system where an NTSC color television signal is encoded in digital form by sampling at a frequency equal to three times the color subcarrier frequency of the television signal, that is, fsampi = 3 X 3.58MHz 10.74 MHz. The sampling signal is phase locked to the color burst component of the subcarrier signal as is well known in the art. The sampling frequency is equal to the clock frequency as previously mentioned with respect to the description of Fig. 2.

Generally, for operation of the dropout compensator of the invention, the sampling frequency utilized to encode the composite analog signal, for example, the color television signal, does not have to be the same as the clock signal frequency utilized to synchronize the various elements of the dropout compensation circuit. In the latter case the samples may be received and stored, for example, at the sampling frequency, and subsequently recovered at the clock frequency, while the latter frequency is utilized for synchronization of the circuit.

Now the preferred embodiment of the invention shown in the detailed circuit diagram of Figs. 8a to 8h will be described. In Fig. 8a consecutive samples S1, S2, S3, etc., of the digital color television signal are received at input 90 of the dropout compensator as 8-bit parallel data by a first set of inputs 91 of two data selector/multiplexers A24 and A34 of two-way switch 52. These multiplexers also receive data at second input 92 from output 116 of delay line 56, shown in Fig. 8c. A dropout control signal DO is received at input 110 by the multiplexers at 114 via flip-flop A16 from a conventional RF envelope level dropout detector circuit (not shown), as mentioned before. Flip-flop A16 is utilized to precisely clock the dropout control signal DO, as it is known in the art. In normal operation the multiplexers apply the input data 90 via flip flop A23 to output 93.Generally, flip-flops similar to A23 are utilized throughout the entire circuit to delay the processed data by one clock cycle to assure precise data clocking. When the DO control signal at 110 is received, the multiplexers switch from input 91 to input 92. The data from 93 is fed to output 70 of the dropout compensator circuit via first controlled delay line 54 of Fig. 8b; second compensating delay line 56 of Fig. 8c; switch 58 of Fig. 8g; and a further controlled delay line 1 26 of Fig. 8g, as it will follow from the description below.

Delay line 54 of Fig. 8b comprises eight identical 4 X 256 bit random access memories of which six memories designated B1, B11, 831. B51. B61 and B81 are shown.

Two groups of four memories each are utilized for receiving higher and lower order bits, respectively. The controlled delay line 54 has a length of delay equal to one horizontal line period of the color television signal less the compensating delay a provided by fixed delay line 56 of Fig. 8c coupled in the delayed color television signal path. Delay line 56 is implemented by eight shift registers of which four registers A51, A61, A52 and A82 are shown.

Flip-flop B41 at the output of delay line 54 is utilized to assure proper timing of the output.

data to achieve the foregoing delay. The delayed composite signal at output 11 5 is applied via fixed compensating delay line 56 to input 96 of switch 58 of Fig. 8g.

As shown in Fig. 8d, the data S1, S2, S3, etc., from output 11 5 of delay line 54, representing signal V is also applied to one set of inputs 104 of filter 60. As disclosed in the above-identified copending application, filter 60 continuously provides an average value of three consecutive samples S1, S2, S3. As shown in Fig. 8c, a sample S1, applied via a portion of delay line 56, is obtained at output 11 7 thereof, where it has been delayed by one clock signal period to assure its proper timing for addition with a sample S2 received one clock signal later. The latter delayed sample is applied to a second set of inputs 104 of filter 60.Samples S1 and S2 are added in two 4-bit binary adders A33, A43 of Fig. 8d and their sum S1 + S2 is delayed in flip-flop A53 by one clock signal in preparation for addition with the subsequently received sample S3 from output 11 5 of 54. The latter summation is performed by two 4-bit binary adders A54, A44 and an output signal therefrom represents the sum S = S1 + S2 + S3.

Signal S is fed through flip-flop A55 to assure proper timing for further processing. In this particular embodiment of the invention an average sample value is obtained by dividing signal S by 3. The division by 3 is performed with a 0.13% accuracy by an approximation algorithm: S S S S + - + - 3 4 16 256 For the particular application of averaging the samples in the presently described preferred embodiment, the approximation algorithm is implemented in two steps as follows: S S PS = - + - 4 16 S PS -- 2: PS + 3 16 These two steps are performed in the remaining portions of the circuit diagram of filter 60 shown in Fig. 8e as described below.

Two 4-bit binary adders A64, A65 of Fig.

8e receive the signal S at two sets of inputs, they shift the signal S to become S/4 at one of the sets of inputs as it is known in the art, and they provide a sum of (S + S/4). The output signal from the latter adders is further shifted to obtain an output signal correspond ing to (S + S/4)/2 as known in the art. The latter output signal represents twice the partial sum PS defined above. The signal 2PS is applied to flip-flop A75 which supplies signal 2PS to two sets of inputs of two 4-bit binary adders A66 and A76. The latter adders shift the signal 2PS to obtain 2PS/16 at one set of inputs and they provide an output signal corresponding to (2PS + 2PS/16)/2 as known in the art. This output signal repre sents S/3 of the approximation algoirthm indicated above.The obtained signal S/3 corresponds to the average value output sig nal provided by filter 60 in accordance with the disclosure of the above-referenced copend ing application. The output signal of adders A66, A76 thus repesents the chrominance less color television signal, that is, the separ ated luminance component, as it has been disclosed in the foregoing specification with reference to Fig. 7. Signal S/3 is applied to flip-flops A67 and A77 which provide both an output S/3 indicated L' at output 105 to input 107 of switch 62 in Fig. 8f and an inverted output signal (- S/3) indicated r at output 106 and applied to a first set of inputs 102 of adder 64 of Fig. 8f. Adder 64 is implemented by two 4-bit binary adders, A36, A46.

In this preferred embodiment the compen sating fixed delay line 11 3 shown in the block diagram of Fig. 7 and providing a delay, is implemented by a portion of the fixed com pensating delay line 56 at output 1 30 thereof as it is shown in Fig. 8c. The thusly delayed composite color television signal from output 1 30 of delay line 56, indicated V', is applied to a second set of inputs 101 of two 4-bit binary adders A36, A46 of adder 64 in Fig.

8f. Adder 64 provides at output 103 an output signal indicated C' which represents the separated chrominance component, as it has been described before with reference to Fig. 7. Signal C' is applied via flip-flop A56 and A5 representing the compensating fixed delay line 68 to a first set of inputs 99 of two 4-bit binary adders A84, A85 of adder 66 in Fig. 8g. To the other set of inputs 98 of these adders the separated delayed luminance component L is applied from the outputs 109 of multiplexers A87, A86, representing switch 62 in Fig. 8f. As described above, to one set of inputs 107 of multiplexers A87, A86 the separated luminance component L' from output 105 of filter 60 in Fig. 8e is applied.To the other set of inputs 108 of these multiplexers representing switch 62 in Fig. 8f, the luminance component L' is applied, which has been delayed be delay line 67, implemented by flip-flops A68, A78 shown in Fig. 8f.

Multiplexers A87, A86 of Fig. 8f are controlled at input 11 2 by the control signal HS/2 of 7.8 kHz, received at 111, which signal has been described with reference to Fig. 7. As it has been previously disclosed with respect to block diagram of Fig. 7, switch 62 applies to input 98 of signal adder 66 the separated luminance component L directly from output 105 of filter 60, or via the two-clock-cycle delay line 67, alternatively, on consecutive horizontal lines, in response to control signal HS/2. Adder 66 of Fig. 8g combines the respective luminance and chrominance components received at its respective inputs 98, 99 into a composite color television signal, representing the dropout compensation signal.The latter signal is applied to a first set of inputs 95 of switch 58 in Fig. 8g, which is implemented by multiplexers A73, A74, via flip-flop A89, utilized for proper timing of data. On the other set of inputs 96 of multiplexers A73, A74 an output signal from delay line 56 of Fig. 8c is received. The output signal at 127, from switch 58 is applied to delay line 1 26 which is shown as a block. The latter delay line is utilized in the specific embodiment of Figs. 8a to 8h to eliminate the undesired 180 phase shift as previously explained with reference to Fig. 7. Delay line 1 26 provides a one horizontal line delay and is controlled by the control signal CB, simultaneously and in the same manner with the controlled delay line 54.

Delay line 1 26 may be designed utilizing random access memories similar to those of delay line 54 in Fig. 8b. The output signal of delay line 1 26 thus represents the previously described output signal of the dropout compensation circuit of Figs. 8a to 8h provided at output terminal 70.

Switch 58 of Fig. 8g is controlled by the dropout control signal DO received at input terminal 110 in Fig. 8a and applied via output 112 and output 55 of delay line 53 of Fig. 8a to control input 97 of switch 58.

Fig. 8h shows a memory address generator circuit 1 50 providing output signals at memory address lines Ao to A7, coupled to control the data flow via a memory bus line AB, through the respective delay lines 53, 54 shown in Figs. 8a and 8b, respectively. In Fig. 8h counters A2, Al 2, A22, B2, B12 and B22 are utilized to count the respective clock cycles corresponding to the actual delay provided by these delay lines. The diagram of Fig. 8h reveals the memory address generator 1 50 in sufficient detail, consequently, no further disclosure is necessary.

To provide a complete disclosure of the detailed circuit, respective clock signals indicated 10.7 MHz and inverted clock signals indicated 10.7MHz utilized for driving various circuit elements are shown in the detailed circuit diagram of Figs. 8a to 8h. The sampling signals are derived at this clock signal frequency as it is well known and described, for example, in the above-indicated copending application.

In the foregoing specification, the preferred embodiments of the invention have been described with respect to a digital dropout compensation circuit, in which an NTSC composite color television signal is sampled at a frequency equal to three times the color subcarrier signal frequency, fsampl - 3 X 3.58 MHz = 10.74 MHz. As it will become apparent to those skilled in the art, the respective block diagrams of Figs. 2, 5 and 7 could also be utilized for other digital as well as analog color television signal systems, such as NTSC, PAL, PAL-M, etc. For example, if the dropout compensation apparatus of the invention is utilized in an analog color television signal system, it may be perferable to implement the controlled delay lines, for example, delay line 3 of Fig. 2, by conventional charge-coupled devices (CCD).As it is known in the art, the latter devices could be utilized as delay lines for delaying signals in analog form, while the information to be delayed may be clocked in and out of the delay line in response to a control signal, similarly, as it has been disclosed with respect to the above-described embodiments. At the same time, delay lines, such as 8 and 9 of Fig. 2, may be implemented by conventional analog delay lines. In the later case, delay line 8 would provide a desired amount of the analog signal substantially corresponding to the line-to-line offset of the color subcarrier signal, and advantageously delay line 9 would provide substantially twice that amount of delay to achieve a desired luminance component delay modulation with respect to the chrominance component delay, as above disclosed.

Generally, the invention with respect to Fig.

2 may be utilized in any digital color television signal system where the sampling fre quency is equal to an integral multiple of the color subcarrier signal frequency. For example, if an NTSC color television signal is sampled at a frequency equal to four times the subcarrier signal frequency, then, to achieve a desired dropout compensation signal, having a proper line-to-line phase of the chrominance component, for example, by the embodiment of Fig. 2, the original signal chrominance component samples would have to be displaced on consecutive television lines by plus or minus two sampling periods. Consequently, to reduce the luminance component displacement on consecutive lines, the delay line 9 of Fig. 2 would have to provide a four clock cycle delay 4 S. while the delay line 8 would have to provide a two cycle delay 2 8.

The above-disclosed invention is applicable also the PAL or PAL-M signal systems, since the respective control signals, such as signsl CB, HS and HS/2, respectively, in Figs. 2, 5 or 7 are frequency and phase locked to the incoming color television signal. It will be appreciated by those skilled in the art that the so-called one-quarter cycle offset corresponding to the 90 degree phase shift occurring in the PAL subcarrier component during consecutive television lines may be compensated for in the dropout compensator of the present invention by applying the above control signals to the respective controlled delay lines of the above-indicated embodiments, which will be loaded and unloaded on consecutive lines synchronously with the control signals of the particular signal system utilized.

As it will become apparent to those skilled in the art, alternative embodiments similar to the disclosed detailed circuit diagrams of Figs.

8a to 8g as well as alternative circuit elements in these embodiments, may be utilized to obtain the disclosed operation of the dropout compensator in accordance with the method of the present invention. Thus, the differencing circuit 64 may be implemented as a subtracting circuit to which respective signals L', V' of the same polarity are applied, as it is known in the art. Similarly, known alternative circuit elements in the summing circuit 66 may be utilized to obtain combination of the chrominance and luminance components. As an alternative, different means of obtaining the delay in delay lines 54 may be utilized, such as shift registers, instead of the random access memories. Likewise, to obtain division by 3 of the samples in filter circuit 60, read only memories may be utilized instead of the disclosed circuit elements implementing the approximation algorithm.

The individual circuit elements of all the above disclosed embodiments of the invention may be implemented by standard integrated devices.

Claims

1. A method of compensating color television signals for dropouts wherein a dropout interval occuring in an input color television signal is replaced by a dropout compensation signal derived from a previously occuring corresponding interval of the input color television signal including luminance and chrominance components defining consecutive horizontal lines of color television information; the steps comprising: storing intervals of each horizontal line of the input color television signal without dropouts; storing intervals of an immediately succeeding horizontal line of the input color television signal without dropouts in place of corresponding stored intervals of said each horizontal line:: retaining in storage corresponding stored intervals of said each horizontal line in place of intervals of said immediately succeeding horizontal line of the input color television signal when dropouts occur in said immediately succeeding horizontal line; separating the luminance and chrominance components of the stored color television signal; adjusting the time of storage of the stored chrominance component of successive horizontal line intervals between a first time greater than and to a second time less than an interval corresponding to one horizontal line to phase synchronize the stored chrominance component with the corresponding component included in the input color television signal;; adjusting the time of storage of the stored luminance component of successive horizontal line intervals between a third time greater than and a fourth time less than an interval corresponding to one horizontal line to reduce the phase difference between the stored luminance component and the corresponding component included in the input color television signal; combining the adjusted chrominance and luminance components to form the dropout compensation signal; and inserting the dropout compensation signal in place of the input color television signal when a dropout occurs in said input color television signal.

2. A method according to Claim 1 wherein said input color television signal is a digital signal obtained by sampling the analog form of the color television signal with a sampling signal having a rate equal to an integral multiple of at least three times the frequency of the unmodulated color subcarrier frequency of the color television signal, said first time is greater than and said second time is less than said interval corresponding to one horizontal line by a first amount less than one cycle of said unmodulated color subcarrier frequency, and said third time is greater than and said fourth time is less than said interval corresponding to one horizontal line by a second amount less than one cycle of said unmodulated color subcarrier frequency.

3. A method according to Claim 1 wherein the color television signal is sampled at rate of three times the frequency of the unmodulated color subcarrier frequency; the intervals of a first and alternate subsequent horizontal lines of the input color television signal are stored for a first interval corresponding to one horizontal line plus one half cycle of the sampling signal prior to separating the chrominance and luminance components; the intervals of a second and alternate horizontal lines of the input color television signal are stored for a second interval corresponding to one horizontal line less two and one half cycles of the sampling signal prior to separating the chrominance and luminance components; the separated chrominance component of each horizontal line is stored for a third interval equal to one cycle of the sampling signal whereby the storage of the chrominance components of consecutive horizontal lines are adjusted by said first and second times, respectively; and the separated luminance component of said second and alternative horizontal lines is stored for a fourth interval equal to two cycles of the sampling signal whereby the storage of the luminance components of consecutive horizontal lines are adjusted by said third and fourth times, respectively.

4. A method according to Claim 1 wherein the color television is sampled at a rate of three times the frequency of the unmodulated color subcarrier frequency; the input color television signal is first separated into chrominance and luminance components; the separated chrominance component of a first and alternate subsequent horizontal lines are stored for a first interval corresponding to one horizontal line plus one and one half cycles of the sampling signal; the separated chrominance component of a second and alternate subsequent horizontal lines are stored for a second interval corresponding to one horizontal line less one and one half cycles of the sampling signal whereby the storage of the chrominance components of consecutive horizontal lines are adjusted by said first and second times, respectively; the separated luminance component of the first and alternate subsequent horizontal line intervals are stored for a third interval plus one half cycle of the sampling signal; and the separated luminance component of the second and alternate subsequent horizontal line intervals are stored for a fourth interval corresponding to one horizontal line interval less one half cycle of the sampling signal whereby the storage of the luminance components of consecutive horizontal lines are adjusted by said third and fourth times, respectively.

5. A dropout compensator for color television signals including luminance and chrominance components defining consecutive horizontal lines of color television information in which a dropout detection signal is generated upon the occurrence of a dropout interval in an input color television signal for controlling the replacement of said dropout interval with a corresponding interval that previously occurred in the color television signal; comprising: memory means having an input for receiving said color television signal, said memory means having a storage capacity sufficient to store at least one horizontal line of said color television signal; memory control means operatively associated with the memory means for effecting storage of intervals of each horizontal line received at the input of said memory means in place of corresponding intervals of an immediately preceding horizontal line received at said input of said memory means; first switch means for coupling the input color television signal to the input of the memory means, said first switch means being responsive to said dropout detection signal to decouple said input color television signal from said input of said memory means; means for separating the luminance and chrominance components of the stored color television signal;; means for adjusting the time of storage of the stored chrominance component of successive horizontal line intervals, said time of storage of the chrominance component being adjustable between a first time greater than and a second time less than an interval corresponding to one horizontal line to phase synchronize the stored chrominance with the corresponding component included in the input color television signal; means for adjusting the time of storage of the stored luminance component of successive horizontal line intervals, said time of storage of the luminance component adjustable between a third time greater than and to a fourth time less than an interval corresponding to one horizontal line to reduce the phase difference between the stored luminance component and the corresponding component included in the input color television signal; ; means for combining the adjusting chrominance and luminance components to form a composite color television dropout compensation signal; and a second switch means for selectively coupling the input color television signal and the dropout compensation signal to an output, said second switch means responsive to the dropout detection signal to couple the dropout compensation to said output, and said second switch means coupling the input color television signal to the output in the absence of the dropout detection signal.

6. A dropout compensator according to Claim 5 wherein said input color television signal is a digital signal obtained by sampling the analog form of the color television signal with a sampling signal having a rate equal to three times the frequency of the unmodulated subcarrier frequency of the color television signal; the memory means includes a first memory having said input coupled to said first switch means, a second memory having an input coupled to receive the separated chrominance component and a thrid memory having an input coupled to receive the separated luminance component, said second memory providing a first delay of less than one cycle of the sampling signal and said second memory providing a second delay two cycles of the sampling signal, said first memory being responsive to the memory control means for storing a first and alternate subsequent horizontal lines of the input color television signal prior to coupling to the luminance and chrominance separating means for a first interval corresponding to one horizontal line plus one half cycle of the sampling signal and for storing a second and alternate subsequent horizontal lines of the input color television signal prior to coupling to the luminance and chrominance separating means for a second interval corresponding to one horizontal line less two and one half cycles of the sampling signals; and further comprising a third switch means having one input coupled to the output of the third memory and second input coupled to receive the separated luminance component from the luminance and chrominance separating means, said third switch means being responsive to the memory control means to couple to the combining means the luminance component received from the luminance and chrominance separating means during said first and alternate subsequent horizontal lines and to couple to the combining means the luminance component received from the third memory during said second and alternate subsequent horizontal lines.

7. A dropout compensator according to Claim 5 wherein the input color television signal is first coupled to the luminance and chrominance separating means; and said memory means includes a first memory and a second memory each having a storage capacity of at least one horizontal line, said first memory being coupled to receive the separated chrominance component and said second memory being coupled to receive said separated luminance component, said first memory being responsive to the memory control means to provide said first and second times of storage for the chrominance component, and said second memory being responsive to the memory control means to provide said third and fourth times of storage.