SYSTEM FOR PHASE CORRECTION OF A COLOR TELEVISION SIGNAL

Abstract: A video signal correction circuit of a signal processing system includes means (13) for identifying the chroma line-type, means (12) for inserting line-type information in the video signal, and means (20) for extracting the line-type information from the video signal after it has been passed through a memory (14) to allow selecting the phase of the chroma subcarrier.

Description: This invention relates to a circuit for enabling the correction of the phase of a color signal in a video processing system, and particularly to a circuit and method for inserting chroma phase information in a video data path and for subsequently recovering the chroma phase information downstream. Presently known video signal processing systems generally employ a main memory for storing video signals and a timing correction circuit for correcting the time base or frequency errors that occur during the processing of the video signal. Such errors in timing or frequency degrade the video signal and the resultant television image. Another problem that is experienced with video signal processing systems that incorporate relatively complex circuitry is color instability that adversely affects the color representation of the television image. In the United States, television signal processing systems employ a standard NTSC format wherein each successive horizontal line is characterized by subcarrier signals of opposite phase, so that there are two different line types when using such a format. With the NTSC system, it is necessary that the chroma signal appearing at the output of the video-tape recorder be coincident with the phase of the station reference signal, in order to ensure the correct color of the television picture. The television recording systems employ a time base correction network which includes a chroma processor for correcting color and a dropout compensator to compensate for missing video signals. The time base correction system requires the identification of the incoming subcarrier phase, as well as the output subcarrier phase which then are compared in order to provide proper color correction and the correct color in the resultant television picture. In countries that use a PAL standard format which employs horizontal lines of four different subcarrier phases resulting in four different line types, identification of each of the line types is necessary for proper color correction. In prior art systems, circuits for identifying the chroma line type are generally located in a stage or stages preceding the main memory and the time base corrector (TBC) circuit. However, it has been found that with recently developed TBC systems, which are substantial improvements over prior art systems, color correction effectively is accomplished when processing the video signal utilizing the chroma line type information after time base correction is achieved. It would be desirable to accomplish the chroma phase correction after time base correction and with minimal circuitry. The present invention overcomes the various disadvantages of previous mention in a time base correction system which includes chroma phase correction, wherein color picture stabilization is achieved after the time base correction error has been removed. To this end, the invention provides means for carrying the required line type identifying signals through the main time base correction process of the main memory for use on the reference side of the TBC, utilizing a minimal amount of circuitry while eliminating the need for the overhead of additional data bits. The state of the art indicated in the precharacterising portion of the broadest claim is represented by US-A-4251830, which comprises a vertical interpolator which reduces vertical picture bounce when a video tape recorder is running in a variable play mode. Signals identifying line-type are compared in a color field detector with reference signals and a control signal is used to control a device which provides averaging in order, for example, to generate a synthetic odd field which is not displaced vertically with respect to the evenly produced field on which such averaging is not performed. US-A-4258384 is directed to a similar technique which does not store a signal which represents line type. US-A-4145704 employs a synchronizing signal which does not carry information about the phase relationship of the sync and sub-carrier of individual horizontal lines. The present invention provides a system for processing a reproduced composite color video signal having at least two different line types distinguished by different phases of a chroma signal at a sub-carrier frequency, comprising a main memory in which digital samples constituting the video signal are stored and from which the stored samples are read at a controlled rate in order to correct time-base error in the reproduced video signal, characterised by means for selectively inserting, in each of a succession of lines of the video signal, before the line is stored in the memory, digital data identifying the chroma line type of the respective line of the video signal; means for comparing the said digital data in respect of each line after it has been read out of the memory with a reference line-type identification signal to develop a line-type control signal for correcting disparity between line-types denoted by the digital data and the reference signal; and means responsive to the control signal for shifting the phase of the chroma signal of the respective line so as to match the phase of the chroma signal to that of said reference signal. The reference line-type signal may comprise at least one signal which is at half or a quarter of the horizontal line frequency, depending on the television standard in use. The digital data may comprise for each line a digital word of which one or two bits represent the line type of the line. The inserting means may be constituted by a multiplexer arranged for receiving the video samples and the digital data and for inserting the digital data before the active picture portion of the respective line of the video signal. The multiplexer may be controlled by means which detects signals denoting the line type of each line in the reproduced video signal, and may be arranged to insert in the absence of the said digital data and in the presence of a signal denoting the occurrence of a dropout in the video signal a data sample indicating the dropout. The system may include means for stripping said digital data from the video signal after the video signal has been read from the memory, the comparing means being coupled to the output of said stripping means, which may comprise a clocked latch circuit. The invention is described in greater detail with reference to the drawing in which: FIGURE 1 is a block diagram of the circuit for inserting digital chroma phase information in the video data. FIGURES 2A-D are waveforms showing the relationship between the horizontal sync and burst of the video signal and the horizontal signals derived from the recorded tape. FIGURE 3 is a schematic diagram of an implementation of the multiplexing means of FIGURE 1. FIGURE 4 is a schematic diagram of an implementation of the line type control circuitry integral with the multiplexing means and the comparator of FIGURE 1. In NTSC and PAL television systems, the chroma signal differs in phase successively for each horizontal line. The horizontal lines of the NTSC system appear in a sequence of four different fields consisting of two different types of lines, whereas the PAL system employs an eight field sequence with four different types of lines. Since chroma signal phase correction is required downstream of the main memory, chroma line type references also are needed for identifying the line type in order to be able to process the color signal properly. The line type signals, designated as H/2 and H/4 identification signals (FIGURES 2C, 2D) represent respectively one half and one-quarter of the horizontal synchronizing pulse frequency as used in NTSC and PAL format TV systems. Although the following description primarily is directed to the use of H/2 identification signals as used in an NTSC system, the circuit of this invention is also applicable to the PAL system in which H/4 and H/2 identification signals are used. Further, when a helical scan VTR is operated to create special effects upon the playback of recorded video information, such as slow motion, fast motion, or still frame, or is operated to reproduce recorded video for display as the tape is shuttled at high speed, the speed of transport of the tape is varied during reproduction of the recorded video information and the position of a movable playback head is controlled to maintain it registered with the recorded tracks of video information being reproduced. Thus for special effects and shuttle modes of a VTR, it often is necessary to repeat or skip recorded tracks of television fields. The invention line type insertion technique provides the proper line type identification at the reference side of the memory regardless of the mode of operation. With reference to FIGURE 1, an analog video input signal (FIGURE 2A) is obtained from a recorded tape at terminal 11. The off-tape video data signal is converted in an analog-to-digital converter 10 to digital form and is applied to a timing correction circuit 9 and thence to a multiplexing means 12. The timing correction circuit 9 may comprise a four line memory which basically is controlled by tape 2Fsc and 4Fsc clocks on lines 24 and reference 2Fs and 4Fsc clocks on lines 26. The video data signal is written into the timing corrector circuit 9 via the tape 2Fsc and 4Fsc clocks in blocks of selected number of samples corresponding to a horizontal line. Since the data is written into the circuit 9 at the tape rate, with timing derived from the color synchronizing burst that occurs at the beginning of each horizontal line interval of the composite color television signal, the data is not corrected for color subcarrier phase errors as well as other timing errors. To eliminate the subcarrier phase errors, the data stored in the timing correction circuit 9 are read at a time determined by the fixed or stable frequency and phase reference 2Fsc and 4Fsc clocks. Thus the circuit 9 serves to re-time the video signal data obtained therefrom in accordance with the reference clock signals and to supply the data to the main memory in one-line blocks for subsequent time base correction. The re-timed video signal is supplied to the multiplexing means 12 which also receives chroma line type identification signals MEMCON H/2, H/4 derived as indicated in Figure 4 from the TAPE H/2 and TAPE H/4 signals extracted from the incoming video signal. The multiplexer 12 responds to a SELECT command (further described in FIGURE 3 as INSERT WORD and DROPOUT signals) for accepting the MEMCON H/2 and H/4 identification signals from a line identifier 13, whereby the multiplexer switches from the active picture to the corresponding digital chroma line type word representative of the tape line type. More particularly, FIGURE 3 illustrates a schematic of the multiplexing means 12 used for line type insertion as well as dropout insertion when a dropout occurs. After the re-timed video data samples are supplied from the timing correction circuit 9 in the form of 9-bit words, they are rephased by clocking them through buffers 28, 30 with reference clock signals 2Fsc and 4Fsc. Connected to the video bus in parallel with the outputs of the buffers 28, 30 are buffers 32, 34 comprising a line type insertion circuit. Additionally, connected in parallel with the outputs of the first two circuits are buffers 36, 38 comprising a dropout insertion circuit. These three circuits are connected in parallel to the video data bus at this point so that a choice may be made whether to insert, for a particular sample period of the reference 2Fsc video bus clock, either the sample value output from the line memory in timing correction circuit 9, a sample value indicative of the line type, or a sample value indicative of the presence of a dropout. The selection of which value is inserted in a particular sample location is made by a logic circuit 40 from input signals INSERT WORD and DROPOUT which comprise the SELECT signal of FIGURE 1. The selection is accomplished by enabling one of the circuits to allow passage of the sample onto the video bus while the other two circuit outputs are tristated. The signal INSERT WORD is output from a read address counter (not shown) of the timing correction circuit 9 which indicates that the end of a horizontal line has occurred. The read address counter in essence counts the number of samples per active picture line, which number is preloaded and reflects the NTSC, PAL, color standard, and then generates a carry signal which indicates the beginning of the horizontal blanking interval. A low logic level of the INSERT WORD signal selects or enables the outputs of the buffers 32, 34 to insert on the video bus in that sample period the value of the logic levels on their inputs. The values which are inserted are the type of the horizontal line which will next be read out of the line memory in the circuit 9. This information is initiated by a system-memory control module 15 (Figure 1) and is as supplied as the signals MEM CON H/2, H/4 to the line type insertion circuit by the line identifier 13. The signals are transferred to the video bus at the correct time corresponding to the time of the INSERT WORD. The buffers 36, 38 of the dropout insertion circuit are enabled by the low logic level output of a NAND gate 42. When the buffers 36, 38 are enabled they insert a word of all zeros (low logic level) on the video bus to indicate the sample is a dropout. The output of NAND gate 42 enables this function when the DROPOUT signal is clocked through a bistable 44 at a 2Fsc rate. The dropout interval is previously detected as video samples having a dropout condition and is detected by an upstream detector and transferred through the timing correction circuit 9 coincident with that particular period. Thus, the dropout insertion circuit exchanges a dropout value for the sample value which was previously stored during the dropout interval. The gate 42 is enabled for the insertion of the word at the time of the DROPOUT signal unless the INSERT WORD signal is present. The buffers 28, 30 allow the data samples to be clocked through them when the output of a NAND gate 46 is a low logic level, thereby enabling the inverted OE inputs of the devices. The buffers 28, 30 are enabled if neither the INSERT WORD signal nor the DROPOUT signal is present. Otherwise, the NAND gate 46 disables the buffers 28, 30. Therefore, a priority method for the insertion of the sample values on the video data bus is implemented in this manner. The INSERT WORD signal indicating the end of the active video has first priority and causes line type identification to be inserted in the data stream. The DROPOUT signal has second priority and, in the absence of the INSERT WORD signal, causes dropout identification samples to be inserted in the data stream. Otherwise, which is usually the case, the absence of the INSERT WORD and DROPOUT signals allow the data samples to be inserted in the data stream. The digital video data signal, including the digital word indicative of chroma line type information, is applied to a main memory 14. The memory stores the digital video signal and the digital words of the chroma phase information (and the dropout word if a dropout occurs) in blocks of one line of video data. The memory control circuit 15 controls the read/write main memory, and includes a timing generator that produces read and write sync pulses for controlling the timing and location of storage of each video line, including the chroma phase samples. The system/memory control circuit 15 provides row/column addresses and various row/column strobes and control signals for the write and read processes for passing the video data signals and the line type information (and the dropout word if present) through the memory 14. The write and read processes are timed whereby mis-positioning of the horizontal and vertical phase is corrected at the output of the main memory 14 to provide a time base corrected signal synchronous with reference signals. In accordance with the invention, the resulting video signal which has been time base corrected is strobed into a clocked latch 20 that strips two bits from the stored line type word which is read from the main memory 14. One of the bits is used in the NTSC standard to identify the two different chroma line types, whilst both bits are used in the PAL standard to identify the four chroma line types thereof. The clocked latch 20 receives a reference horizontal sync pulse H to enable loading samples of the chroma phase word for each horizontal line. The clocked latch 20 supplies a MEM OUT H/2 (NTSC) and a MEM OUT H/4 (PAL) identification signal, to a comparator 22 which also receives stable reference H/2, H/4 signals from, for example, a sync generator (not shown) of the TBC. If the logic levels of the off-tape signals differ from those of the reference signals, the difference is detected by the comparator 22 and a control signal DECODE H/2 is supplied to an encoder/decoder 16 of a color processor circuit. In the PAL standard both the DECODE H/2 and a DECODE H/4 control signals are supplied to the encoder/decoder 16. The encoder/decoder responds to the control signal and separates the chroma from the luminance signal and then inverts the phase of the chroma signal via a chroma inverter of generally known configuration to match the phase of the chroma signal to that of the reference signal. The video data signal including the inverted chroma signal are added to the luminance signal and passed to a processing amplifier 18. As an alternative approach, the video data is delayed and shifted by one-half of the color subcarrier cycle or by 180° to match the phase of the station reference signal before being passed to the processing amplifier 18. The comparator 22 also supplies the DECODE H/2 control signal to the dropout compensator 17 to identify the V-axis switch sense in the PAL standard which is required for dropout compensation. In response to a selected logic level, the compensator 17 determines the reference phase of a PAL modifier circuit which modifies the chroma signal of an adjacent line of video to supply the same chroma phase as the line having the dropout. FIGURE 4 is a schematic of line type control circuitry for receiving the line type information indicative of an incoming tape field sequence from signals TAPE H/2 and TAPE H/4, and the line type information indicative of the desired output reference field sequence from reference signals REF H/2 and REF H/4. The circuitry in turn supplies signals to different components in the TBC to control line type and field sequencing operations in accordance with the invention. In general, the line type control circuitry comprises two sections; a control signal generation section and a line type data generation section. The control signal generation section is controlled by a PROM 50, and the data generation section is controlled by a PROM 80. A data latch 52 is connected to the write portion of the memory control data bus and provides means for an integral microprocessor (not shown) to write a data word to the PROM 50 via a select line S19. The data outputs D⊘-D4 of the latch 52 are used by the PROM 50 to select particular sections or address blocks of the memory for decoding inputs A⊘-A3. Further, the data lines D⊘-D4 include a bit which either enables or disables the PROM by applying a logic level to its G2 input. In this way the microprocessor chooses certain decoding sections of the PROM 50 to change the decoding algorithm based on TV standard. The TV standard is set by the operator and decoded by the system/memory control circuit 15. Similarly, the PROM 80 has address lines D5-D7 from the latch 52 connected to its address inputs A5-A7. These data inputs select certain decoding blocks in the PROM 80 for decoding data inputs to its other address lines A⊘-A3. The data generation section of the line type control circuitry includes two D-type bistables 88 and 86 which receive a tape write pulse to produce a clocking pulse which is synchronous with the 2Fsc clock. The pulse from the Q output of the bistable 86 clocks line type signals, TAPE H/2, TAPE H/4 of the incoming data into bistable devices 54 and 84. This line type information is then clocked into a latch 82 by a write enable signal at reference horizontal rate. Outputs Q⊘ and Q1 of the latch 82 are connected to two other D inputs of the latch 82. Therefore Q⊘ and Q1 represent the tape H/2 and H/4 signals at the beginning of the present reference horizontal period, while Q2 and Q3 represent the tape H/2 and tape H/4 as they were one reference horizontal period ago. Both pairs of outputs from latch 82 are supplied as four bits of an address for PROM 80. The PROM 80 produces two sets of signals MEM CON H/2, H/4, which are output to the multiplexer 12 from the Q⊘, Q1 outputs of the PROM 80 after being reclocked in a buffer 78 to be synchronous with the 2Fsc clock pulse. The multiplexing means 12 uses these two signals as the two most significant bits of the read address in the line memory integral therein and also inserts these coded signals into the data stream prior to the data of the horizontal line addressed by these bits. In this manner, every horizontal line is preceded by the information as to its type, as it is clocked through the main memory 14 as previously described. These signals are extracted from the data stream after passage through the main memory to become the MEM OUT H/2, H/4 signals which represent available line type to the A1 and A4 address inputs of the line type comparator PROM 50. In combination with the desired line type signals REF H/2, H/4 which are input to the A⊘ and A2 inputs of the PROM 50, these signals are compared and provide enabling signals to the control portion of the line type control circuitry. The reference signals REF H/2, H/4 indicate the desired output sequence of fields after being decoded by the PROM 50 for the TV standard and the MEM OUT H/2, H/4 signals indicate the actual line type output from the main memory 14 after being decoded by the PROM 50 for the TV standard. Therefore, signals Q⊘-Q5 of the PROM 50 indicate certain control actions which must be taken if the actual line type is to be changed to the desired line type such that appropriate field sequence or color correction can occur. The outputs Q⊘-Q5 of the PROM 50 are distributed to the D inputs of the bistables 45, 56, 60, 70, 72, and 74, respectively. These control signals are clocked at the particular times to become control signals to different parts of the TBC. For example, the Q⊘ output is clocked into bistable 45 by the output of a NAND gate 44 to become the 1H shift signal output from a NAND gate 48. The clock signal from the NAND gate 44 is produced at the system reference vertical REF V, if the color processor encoder/decoder 16 is not present or disabled as indicated by the signal OP PRES. This clocks the line type comparator PROM 50 output Q⊘ into bistable 45 at a vertical rate if the color processor is not present or disabled and will add 1 horizontal line delay to the data path of the video data required for color correction mode by memory control in PAL and PAL-M, when a 90° subcarrier phase is needed, which may be accomplished by that delay. If the color processor encoder/decoder 16 is present and enabled, the vertical timing control for color correction should not be used. This output is the signal generated by the NAND gate 48, 1H shift to the vertical timing control. The NAND gate 48 is enabled by the microprocessor set line type correction on/off signal. The bistable 56 is clocked by the output of a NAND gate 76 to produce the signal 180° SHIFT at the output of the HAND gate 58. The D input of the bistable 56 receives the Q1 output of the PROM 50 which is an indication that color correction requires 180 degrees shift of color subcarrier phase. The 180° SHIFT signal is generated if the line type correction on/off signal is a high logic level enabling the gate 58. The clocking signal to the bistable 56 is timed to the reference horizontal sync signal delayed by three subcarrier cycles. This signal is transmitted through the gate 76 which is enabled by the color processor encoder/decoder 16 being present. Likewise, bistables 60, 70, 72, and 74 are clocked by the output of the NAND gate 76. The outputs of the bistables 60 and 70 produce color processor color correction control signals DECODE H/2, H/4 if NAND gates 62, 66, respectively, are enabled by the line type correction on/off signal. If no line type correction is to be applied, then the DECODE H/2, H/4 signals are taken directly from the MEM OUT H/2, H/4 signals by exclusive OR gates 64 and 68, respectively. If the line type needs to be corrected, the MEM OUT H/2, H/4 are inverted by the gates 64 and 68. The color processor takes these decoding signals and produces color correction from the input data of the main memory 14 before providing the data to the processor amplifier 18. The bistables 72 and 74 produce data shift signals for the processing amplifier 18 data shift 180°, data shift 90° through respective inverters 73 and 75 if the color processor encoder/decoder 16 is present. The Q4 and Q5 outputs, respectively, decode into the type of delay that the color processor will produce in the data going into the video bus path for the line type and color sequencing data from the PROM 50. The processing amplifier 18 receives the video from the encoder/decoder 16 and discards the sync and burst portion of the incoming signal and replaces these signal components with new sync and burst derived from the sync generator. The active picture is added to the new sync and burst with a color corrected chroma phase which matches the new burst. The combined signal is passed through a D/A converter and low pass filtered to form the output composite video signal correctly timed to the station reference.

Claims: A system for processing a reproduced composite color video signal having at least two different line types distinguished by different phases of a chroma signal at a sub-carrier frequency, comprising a main memory (14) in which digital samples constituting the video signal are stored and from which the stored samples are read at a controlled rate in order to correct time-base error in the reproduced video signal; characterised by means (12) for selectively inserting, in each of a succession of lines of the video signal before the line is stored in the memory, digital data identifying the chroma line type of the respective line of the video signal; means (20,22) for comparing the said digital data in respect of each line after it has been read out of the memory with a reference line-type identification signal to develop a line-type control signal for correcting disparity between line-types denoted by the digital data and the reference signal, and means (16) responsive to the control signal for shifting the phase of the chroma signal of the respective line so as to match the phase of the chroma signal to that of the reference signal. A system according to claim 1 in which the reference line-type identification signal comprises at least one signal (H/2,H/4) which is at half or a quarter of the horizontal line frequency. A system according to claim 1 or claim 2, in which said inserting means (12) is constituted by a multiplexer arranged for receiving the digital video samples and said digital data and for inserting said digital data before the active picture portion of the respective line of the video signal. A system according to claim 3 in which the multiplexer is controlled by means (13) which detects signals denoting the line type of each line in the reproduced video signal. A system according to claim 3 or claim 4 in which the multiplexer is arranged to insert in the absence of the said digital data and in the presence of a signal (DROPOUT) denoting the occurrence of a dropout in the video signal a data sample indicating the dropout. A system according to any of claims 1 to 5 and including means (20) for stripping said digital data from the video signal after the video signal has been read from the memory (14), said comparing means (22) being coupled to the output of the stripping means (20). A system according to claim 6 in which the means for stripping comprises a clocked latch circuit (20). A system according to any foregoing claim in which said the digital data comprises for each line a digital word of which one or two bits represent the line type of the line.

Inventors: OLDERSHAW, REGINALD, WAGNER, STEVEN D.
Applicants: AMPEX CORPORATION
Application year: 1986
Main classification: H04N9
IPC codes: H04N9