Abstract:
A switched digital video broadcast network provides in-service testing of digitized broadcast video signals subject to analog-to-digital and digital-to-analog conversion. The network includes a plurality of gateways, each gateway coupled to video signal sources and sink. Video frames transmitted on the network are subject to analog-to-digital and digital-to-analog conversion and compression in an MPEG  2  encoder/decoder. Each gateway and includes a test pattern generator and test measurement analyzer for in-service testing of the video signals. The test pattern generator inserts a test signal on pre-selected lines ( 22, 23  or  261,262 ) in a Video Blanking Interval (VBI) and time periods of a video frame. The test signal may be dynamically placed at any location in the frame using concealment techniques The video lines are not seen by television viewers. The test pattern generator and test measurement analyzer are synchronized using vertical integral time code and a trigger packet sent by the transmitting station. The test signals enable both in-service and out-of-service testing to be performed for the entire suite of EIA/TIA 250C standard video tests.

Description:
RELATED APPLICATIONS 
     Copending applications entitled “System and Method of Automated Testing of a Compressed Digital Broadcast Video Network”, Ser. No. 09/221,864, Filed Dec. 29, 1998 (BC9-98-078), and “Apparatus and Method of i-Service of Audio/Video Synchronization Testing”, Ser. No. 09/221,868, Filed Dec. 29, 1998 (BC9-98-103), both assigned to the same assignee as that of the present invention and fully incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to video data transmission systems. More particularly, the invention relates to systems and methods of in-service testing of compressed digital broadcast video. 
     2. Prior Art 
     In video transmission networks the Motion Picture Expert Group (MPEG) 2 compression algorithm and ATM networks can cause perturbations in video transmission signals that are not normally seen in an analog only network. To detect network anomalies and failures real-time, inservice circuit testing is performed so that service may be restored with minimum circuit outage. The technique of EIA/TIA 250 C in-service testing is well-known in the television industry. Broadcasters have historically used the Video Blanking Interval (VBI) lines 10-20 of both fields to insert test signals. Also embedded in the VBI are closed caption text and Society of Motion Picture and Television Engineers (SMPTE) time codes. The VBI is not part of the active video area and, therefore, is not seen by viewers. 
     MPEG-2 video encoders however preclude in-service testing because the encoder filters VBI lines 1 through 21 exclusive of each screen or frame in order to reduce the transmitted bandwidth. The video encoder copies the SMPTE time code into the Group Of Pictures (GOP) header and the closed caption text is passed as user data in the MPEG-2 transport stream. The test signals are ignored. At the receiving end, the VBI lines are regenerated by the MPEG-2 decoder and the SMPTE time codes and closed captions text are reinserted, however, the VBI is devoid of any test signal. Thus, in-service testing is limited to (1) synchronization pulse amplitude test and (2) chrominance burst amplitude test which basically confirm the presence or absence of video. These tests are incapable of measuring or assessing video quality. 
     The MPEG-2 video encoder limitation also encumbers broadcasters which often seek to insert test signals into the VBI at the point of signal origination to test end-to-end video quality. If their video transmission undergoes MPEG-2 to compression at any stage, the VBI test signals are lost. 
     Prior art related to in-service testing of video transmission systems includes: 
     U.S. Pat. No. 5,617,148 issued Apr. 1, 1997, filed Sep. 18, 1992 (Montgomery), discloses a controlled element for spectrum attenuation or a controlled filter used to aid in the insertion of a secondary signal into a video signal without distorting the blanking intervals or closed caption data contained in the blanking interval of the video signal. 
     U.S. Pat. No. 4,969,041 issued Nov. 6, 1990 (O&#39;Grady), discloses data embedded in a video signal by adding a low level waveform to the video signal. The low level waveform has a level below the noise level of the video signal and corresponds to the data. To detect the data embedded in the video signal, the video signal is correlated with low level waveform corresponding to the data to produce a correlation coefficient. A high correlation coefficient indicates a presence of a low level waveform which is converted into data. The low level waveform extends over many video lines so that it does not occur at or near the same location within a video frame for many frames to avoid fixed pattern noise anomalies that may be detected by a viewer. 
     U.S. Pat. No. 5,585,858 issued Dec. 17, 1996, filed Apr. 15, 1994 (Harper), discloses a system for simulcasting a fully interactive program with a normal conventional program in the same standard video signal bandwidth. Unused lines of the video are preferably used for embedding additional interactive response audio channels and graphics and control data. Alternatively, interactive audio segments are provided either serially or one after another in the audio subcarrier or in cable frequency guardbands or pre-stored in memory at the interactive program box. More audio and graphics can be provided through the use of an external storage device or game cartridges. The additional data is entered at designated trigger points in the system through the use of overlaid logic sent down in embedded codes in the signal or resident in the software at the receiver location. 
     U.S. Pat. No. 5,572,247 issued Nov. 5, 1996, filed Jun. 14, 1991 (Montgomery), discloses signal processors for permitting the transparent reception of a data signal in the video bandwidth of a cable television system. The received signal has video and data components that are frequency interleaved in the video bandwidth in the active video interval. The data signal is modulated with a carrier at a non-zero multiple of a horizontal scanning rate of the video signal. The receiver selects a forward channel transmitting the combined signal responsive to a control signal and extracts the data portion of the transmitted combined signal. 
     U.S. Pat. No. 5,557,333 issued Sep. 17, 1996, filed Feb. 24, 1994, and U.S. Pat. No. 5,327,237 filed Jun. 14, 1994 (Jungo), discloses signal processes for permitting the transparent, simultaneous transmission and reception of a secondary data signal with a video signal in the video band. The signal processor in the transmitter rasterizes the data at the horizontal scanning rate and modulates the data with a data carrier at a non-integral multiple of the horizontal scanning rate to obtain frequency interleaving. The data is transmitted during the active video portion of each video line. 
     U.S. Pat. No. 5,663,766 issued Sep. 2, 1997, filed Oct. 31, 1994 (Sizer), discloses a system for communicating digital information in a video signal and comprising an encoder arranged to add a carrier signal modulated by digital information to the video signal. The modulated carrier signal at other than a frequency corresponds to a peak in the video spectrum. A receiver is arranged to optically sense the video signal and to recover the encoded digital information in the video signal. 
     In view of the prior art, a need exists to provide in-service testing of video transmission subject to the MPEG-2 compression algorithm where a video test signal may be inserted within the VBI or overscan or active area of a video frame for measuring or assessing video quality in a manner transparent to a viewer. 
     SUMMARY OF THE INVENTION 
     An object of the invention is a system and method of in-service testing of video transmission subject to MPEG compression. 
     Another object is the method of inserting a test signal in a video transmission for assessing video quality wherein the transmission is subject to MPEG-2 compression. 
     Another object is a system and method of concealing a video test signal in a video transmission in a manner that renders the testing transparent to a viewer. 
     Another object is a system and method to dynamically position a video test signal within the active viewing area of a base video transmission based on the content of the video broadcast. 
     Another object is a system and method for removing a video test signal from the video blanking interval and periodically injecting the test signal into the active video area of a video transmission to circumvent MPEG-2 video encoder filtering. 
     Another object is a system and method for extracting a test signal from the active video area of a transmission and placing the test signal into a newly created video blanking interval. 
     Another object is a system and method to generate a continuous test signal in a restored video blanking interval while receiving an intermittent test signal of varying periodicity. 
     These and other objects, features and advantages are accomplished in a video transmission system including a command and control operations center (CAC) coupled to a wide area TCP/IP network for remotely controlling local video station. Each station is coupled to an Asynchronous Transfer Mode (ATM) network through gateways as Points Of Presence (POP). Each POP interfaces with the network via an ATM switch. At the origin POP, subscriber video feeds are routed to an analog video switch which is coupled to Vertical Interval Test Signal (VITS) generator and a signal analyzer. The switch is connected through MPEG-2 encoders to a multiplexer which multiplexes the output of the MPEG-2 encoders into a single OC-3 video transport stream of pictures occurring in a series of I, B and/or P fiames for delivery to the ATM switch. To test video quality, the VITS generator sends a NTSC color bar test pattern to the remote POP. The test signal is placed into the Video Blanking Interval (VBI) of a frame without affecting the active video. The modified digital stream is passed to a MPEG video encoder for encoding. A test signal injection controller builds a trigger data packet that is sent downstream to a test signal extractor. Each trigger packet is transmitted twice as a precaution in case a CRC error is detected. The trigger packet is stored in a VBI test signal table. The test signal is placed outside the viewing area using concealment techniques. Video logic determines the concealment of the test signal. The over-scanned area of a frame is tested to determine whether there is available space for the test signal. If so, a line is chosen and a concealment mode is calculated. If the over-scanned area is full, all available safe action lines are ranked by motion and content. Test signals already inserted reduce the number of available lines and the line separation parameter removes other lines from consideration to prevent test signals from being juxtaposed. A concealment score is calculated for each line. The safe lines are ranked by concealment score. If the top score is greater than the predetermined quality threshold, the line, mode and score are returned. Else, the available title safe lines are ranked according to motion content score. Scoring is weighted towards lines devoid of content and lines with static motion. Then, synchronously with the presentation of each frame, a test signal from the test signal store is inserted into the video blanking area for each active video blanking interval line entry. The test signals in the active video area are concealed by the insertion of a video line retrieved and built from a picture stored according to the concealment mode contained in the trigger packet. The repaired video line is forwarded for digital-to-analog conversion. The test signal injection controller determines when and where to inject the test signal into the digital video transport stream. At the receiving end, the analog switch switches the output into the test signal analyzer for measurement and analysis. A time code associated with a frame is matched to the current frame. The entry is removed from an extractor Test Signal Trigger Table and a flag is set in a VBI Line Table. During frame reconstruction, the test signal extractor loops through the active VBI table entry and inserts the stored test signal. If a conceal flag is set, the frame currently being processed is moved out of the current picture into a test signal line. The test is then copied from a signal line store into a target VBI line field of the current picture. If the current frame contains a conceal flag, the video line is concealed. A concealed mode in the VBI table is decoded. The line containing the test signal is repaired using the next line above and below the line needing repair. Thus, in-service testing is accomplished in digital compressed video using concealed video test signals inserted into the video blanking area of an I frame where the concealed test signals are identified by an extractor test signal trigger table and an extractor VBI line table during frame re-construction of the I frame. 
    
    
     DESCRIPTION OF THE DRAWING 
     The invention will be further understood from the following description of a preferred embodiment taken in conjunction with the appended drawing, in which: 
     FIG. 1 is a representation of a Command and Control facility (CAC) serving Point-Of-Presence (POP) locations and incorporating the principles of the present invention. 
     FIG. 2 is a representation of a video in-service test signal generator at a first POP and a test signal analyzer in a second POP included in the system of FIG.  1 . 
     FIG. 3 illustrates the components of a CAC center of FIG.  1 . 
     FIG. 4 is a representation of a bandwidth reservation order by a video subscriber for a future video transmission 
     FIG. 5 is a prior art illustration of an MPEG-2 video layer hierarchy. 
     FIGS. 6A and 6B are prior art illustrations of MPEG-2 sampling formulas 4:2:2 and 4:2:0 sampling formats, respectively. 
     FIG. 7 illustrates a few of the well-known television test signals used to test video performance and quality over transmission systems. 
     FIG. 8 illustrates a block diagram of a test signal injector  130  shown in FIG.  1 . 
     FIG. 9A is a representation of a test signal trigger data structure. 
     FIG. 9B illustrates an MPEG 2 multiplexer multiplexing the data structure of FIG. 9A 
     FIG. 10 is a flow diagram for processing of each frame in an MPEG-2 encoder. 
     FIG. 11 is a flow diagram for processing I-frames in a test signal injector of FIG.  8 . 
     FIG. 12 is a flow diagram for building a Trigger Packet. 
     FIG. 13 discloses a television screen showing the areas of test signal insertion. 
     FIG. 14 is a flow diagram for video line concealment in the text injector of FIG.  8 . 
     FIG. 15 is a flow diagram for calculating concealment scores for the injector of FIG.  8 . 
     FIG. 16 illustrates an MPEG 2 demultiplexer. 
     FIG. 17 is a block diagram of the test signal extractor. 
     FIG. 18 is a representation of an extractor test signal trigger table for the extractor of FIG.  17 . 
     FIG. 19 is a flow diagram of processing a trigger packet in the extractor of FIG.  16 . 
     FIG. 20 is a flow diagram for processing I-frames in the extractor of FIG.  17 . 
     FIG. 21 is representation of a VBI line table for the extractor of FIG.  17 . 
     FIG. 22 is a flow diagram for frame reconstruction in the test signal extractor of FIG.  17 . 
     FIG. 23 is a flow diagram for concealing video lines in the extractor of FIG.  17 . 
     FIG. 24 is an alternative embodiment of the invention of FIG.  1 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     FIG. 1 depicts a video transmission system  10  including three points-of-presence (POP) or gateways  100 ,  105  and  110  coupled to an Asynchronous Transfer Mode (ATM) network  115  for transmitting high bandwidth, broadcast video to a gateway(s) by digitizing and compressing the analog digital signals. The digitized signal is converted back to analog at the receiving gateway and passed to subscribers. A Command and Control Center (CAC)  120  remotely controls the POP gateways. 
     The POP gateways are connected to the ATM network through OC-3 (155 Mbps) access lines 118 from an ATM switch  140 . A POP gateway has a set of access lines 128, 129 that carry the analog video signal to/from subscribers. The access lines are connected to an analog switch  132  that allows the lines to be switched into dedicated MPEG-2 encoders  136 ,  137 . The POP gateway interfaces with the ATM network via an ATM switch  140 . On the transmit side, the ATM switch is connected to a multiplexer (mux)  138  which multiplexes the output of the MPEG-2 encoders into a single OC-3 transport stream. To test video quality of each newly established video circuit, a vertical interval test signal (VITS) generator  130  and a signal analyzer are wired into the switch  132 . 
     Network data that is addressed to a POP gateway is routed into a demultiplexer  139  that demultiplexes the OC-3 data into individual MPEG-2 transport streams. The output subscriber lines 129 provide the signal to the subscriber. 
     The CAC establishes and removes video connections by commands issued under program control of software executives executing in computers  172 ,  173 . These computers maintain continuous connection to each POP gateway over a TCP/IP wide area network  174  to control the POP equipment and monitor alarm conditions. 
     FIG. 2 depicts a typical video connection from a POP gateway in Los Angeles  300  to a POP gateway  400  in Washington, D.C. The gateway  300  provides analog video from a video tape recorder  321  on a customer&#39;s private access line 328. The signal  380  is routed by an analog switch  382  into the first available MPEG-2 encoder  336 . The signal continues on a multiplexer  338  where it is given an ATM address that permits it to be properly routed by the ATM switch  340  and ATM network  445 . At the receiving POP  400 , the demultiplexer  452  demultiplexes the aggregate OC-3 signal receiving from the ATM switch  450  and routes the demultiplexed MPEG-2 transport stream  453  into the dedicated MPEG-2 decoder  454 . The analog video output of the MPEG-2 decoder is passed to the analog switch  456  which switches the signals into the customers private access lines of  463  for viewing on a video monitor  470 . 
     Immediately after establishing a connection, but prior to switching the subscriber lines 328,463 into the connection path  380 , 485 , the video test equipment  332  is switched  382  into the connection to send a NTSC color bar test pattern to the distant end  400 . At the receiving end  400 , the analog switch  456  switches the output of the video connection  453  into the test signal analyzer  458  for measurement and analysis. After a test duration of a few seconds, the test equipment is switched out and subscriber lines into an active connection and the circuit is turned over to the subscriber. If the color bar tests had failed, the circuit would have been deactivated and a new video connection would have been established using an entirely different set of network resources. The new connection would then be tested prior to release to the subscriber. 
     FIG. 3 illustrates the components of the CAC center  402 . The CAC maintains a database  418  of each subscriber reservation. The subscriber makes a reservation for video circuits from his/her computer  405  which is configured with a web browser. After connecting to the reservation web server  416 , the subscriber is printed with a web page that solicits reservation data. A reservation request is passed on to system  417  which then queries the network resource manager  410  to ensure that there are adequate resources in the network to establish the circuit at the requested time. The resource manager  410  in turn queries the Network Resource Database  412  to check the availability of access lines, encoders, and decoders, and network bandwidth. If neither resources will be available to honor the future connection, the Network Resource Manager updates resource database and responds affirmatively to the reservation system  414  which then updates its reservation database  418 . The reservation web server  416  informs the subscriber of a confirmed reservation by refreshing the reservation request web page. 
     FIG. 4 illustrates a reservation order web page  600  to which a subscriber reserves bandwidth for a future video transmission. The subscriber enters a start date and time  605  and the end date and time  610  of the connection. Also specified are origins  615  and destination  620  POP locations. Upon submission,  640  of the reservation, the web page is updated with the computed duration  625  of the connection, a reservation status of confirmed  630  and the unique reservation ID  650  with which to reference the reservation in future transactions. 
     Returning to FIG. 3, in addition to accepting new reservations, the Network Resource Manager  410  creates new video connections at the requested time and destroys connections when the reservation expires. This process is done by issuing commands to a set of program executives that control the ATM switches  430 , analog video switches  432 , video test sets  434 , and MPEG-2 equipment  436 . These program executives in turn issue hardware specific commands to slave equipment over a Wide Area Network  445  that is accessed through an IP router  440 . Each executive runs in a separate computer with a user console that permits network operators to take manual control of the POP equipment if operator intervention is deemed necessary. However, control of the entire network is fully animated under the control of the network resource manager  410 . As each executive issues commands, detects POP alarm conditions, network status is updated at the operator&#39;s console  438  and printer  425  which are continuously monitored by the network operators. 
     Each POP is configured with an IP hub  490  that is the gateway to the network  445 . From the hub, Ethernet lines  495  are connected to each set of networking equipment and the terminal server  462 . The terminal server permits the test equipment controller  432  to remotely control and monitor all test signal generators and measurement sets via an RS-322 control interface from a wide area IP network  174 . The present invention is incorporated as an adjunct to MPEG-2 video encoder/decoder system. MPEG-2 is described in Motion Pictures Expert Group (MPEG) Standard, Coding of Moving Pictures and Associated Audio ITU Recommendation H.262, which is incorporated herein by reference. Some pertinent aspects of MPEG-2 compression will be reviewed in order to facilitate an understanding of the invention. 
     FIG. 5 is a prior art illustration of an MPEG-2 video layer hierarchy. In FIG. 5, a MPEG-2 data stream consists of a video stream and an audio stream which are packed together in a system&#39;s streams. There are other layers within the video stream. The highest layer is a video sequence layer  80  which contains information pertaining to the entire sequence. The video sequence  80  is divided into Groups of Pictures (GOP)  82  which comprise one or more video frames. Video frames, the odd and even fields interlaced to make a 525 line frame, make up the third or picture layer  84 . Pictures are further divided into slices of horizontal sections  85 . The slice layer is made up of consecutive macro blocks  87  which are built from 8×8 pixels blocks  89 , the lowest layer. 
     In FIGS. 6A and 6B, MPEG-2 encoding uses either 4:2:0 or 4:2:2 format which referred to the type of sampling used to encode the luminance (Y) and chrominance (CrCb) video components. In FIG. 6A, 4:2:2 or 4:2:0 format sampling provides 2 Cr and 2 Cb samples for every four samples of luminance. The CrCb samples are reduced to 1 each in the 4:2:0 format. The location of the single CrCb with respect to the Y sample is shown in FIG. 6A. A superior result is obtained in using 4:2:2 sampling when compressed at a rate of 15 Mbps and higher as shown in FIG.  6 B. MPEG-2 encoders supporting 4:2:2 sampling format became available in 1998. 
     Returning to FIG. 5, within a GOP there are three types of pictures: I or intra-frame; P or predictively motion compensated frame; and B or bi-directionally motion compensated frames. B-frames are the smallest size picture since they are encoded using motion estimations from both the previous and the next I or P-frame. P-frames are predicted from the previous I- or P-frames and they are larger in size. I-frames are coded independently and they are much larger. 
     A GOP series of frames begins with an I-frame. The I-frame is followed by a series of B and/or P-frames. The more B-frames that are used, the more efficient the compression at a price of reduced video quality. The IBP sequence is a video encoder configuration parameter set by the user according to the requirements of an application. I-frames may be automatically generated for a scene change or when motion compensation cannot be effectively used. 
     FIG. 7 illustrates a few of the well-known television test signals used to test video performance and quality over transmission systems. FCC multiburst  700  provides test signal for frequency response. NTC7 Composite  710  allows amplitude and phase measurements. NTC7 combination  720  provide frequency response and distortion testing. FCC color bars  730  provide amplitude and timing measurements. One or more of these test signals may be placed into a video blanking interval for in-service testing as will be described hereinafter. 
     Test Signal Injector Operation: 
     FIG. 8 illustrates a block diagram of a test signal injector  1102 . A video signal input  1100  is processed to monitor the SMPTE time code and the VBI of each frame and detect the presence of VBI video test signals  1110 . Any non-black signal in VBI lines 10 through 20 of either field is treated as a test signal. As test signals are detected, they are stored in test signal store  1170 . The motion detector  1130  examines the incoming video for motion and scene change while the video concealment  1140  compares the video lines from the current frame with lines from the prior and next frame. The individual video lines are scored by completeness of the repair. In order to accurately score the concealment results that will take place in the downstream extractor to the video that has been compressed and decompressed, it is necessary to compress and decompress the signal in the injector and process the current, forward and backward picture store. A MPEG-2 video encoder  1195  is used as a quasi-pass encoder for the purposes of obtaining the picture stores of predicting I-frame generation. This will be described in more detail hereinafter. 
     The test signal injection controller  1150  determines when and where to inject  1155 , the test signal, into the digital video stream  1100 . The modified serial digital video  1160  is then passed on to the MPEG-2 video encoder for encoding. The controller  1150  also builds a trigger data packet  1180  that is sent downstream to alert a TS extractor (see FIGS.  16 / 17 ) of the incoming I-frame for obtaining a test signal. The VBI test signal table  1197  stores injection controller state data. 
     In FIG. 9A an injector trigger packet is stored in VBI test signal table  1200  which also contains a transmit_count  1204  to ensure the packet is transmitted twice, and an I-frame_count  1202  that tracks the number of times an I-frame was sent without an inserted test signal. During times of high scene changes or when it is deemed that concealment would not be effective, no test signals is inserted into the I-frame. This has no effect on the output of the TS extractor because the TS extractor continues to repeat the test signal on the VBI line. Up to three consecutive I-frames may be skipped without consequence. After the extractor sees four consecutive I-frames without a test signal, it drops the test signal and restores the VBI line to black. The signal sent flag  2206  indicates the TS extractor has processed at least the I-frame with an injected test signal. 
     The injector test signal trigger packet  1200  is diagrammed in table  1208 . A timecode  1210  of the modified I-frame is passed in the first field. The timecode is the same value and format as the timecode found in the GOP header preceding the I-frame. Markers  1215  and  1235  are placed within the data structure to preclude start code emulation per the MPEG-2 specification. The next two fields are the VBI_line_ID/field  1220  from which the test signal was taken and the video_line_ID/field  1230  to which it was moved. A line_count  1240  is passed indicating one or three lines were used to transmit the test signal. Three lines are used in 4:2:0 format. The recommended concealment_mode  1250  is also sent to optimize the repair of the usurped video line. The concealment_mode field stores up to three concealment_mode values (3×16 bits) to facilitate the repair of three lines in 4:2:0 mode. Each concealment_mode value  1255  is a 16-bit structure that contains the conceal mode, a pixel shift count, and the lines used to form the repair. The entire data packet is protected by CRC  1270  for error detection. The packet is sent as PES_private_data which are 128 bits of private data available in the PES header. The stuffing field  1260  used to pad out the data blocks to 128 bits. In the event that there are multiple test signals present in the VBI, a separate trigger packet is generated for each one. 
     Each trigger packet is transmitted twice as a precaution in case a CRC error was detected in the first packet. When the test signal is detected, it is scheduled to be sent on the next I-frame. Given the amount of encoding delay or latency inside the video encoder, the downstream TS extractor will receive the trigger packets several frames before the arrival of the test signal. Using timecode to detect the target I-frame makes injector/extractor interaction insensitive to frame buffering or latency within the system. 
     In FIG. 9B, the test signal trigger data  1280  is packetized as a video Program Elementary Stream (PES) private data embedded into video Private Elementary Stream. The test signal is then multiplexed into the NPEG transport stream in transport stream multiplexer  1284  along with the audio and program system information. Embedding the trigger packet into the video PES tightly couples the test signal trigger packet with the video. There may be several video PES streams being multiplexed together, each one carrying its own test signal and trigger packets. 
     FIG. 10 describes the logic flow of the test signal injection controller shown in FIG.  8 . 
     On each frame a loop is entered  1510 - 1570  to see if there are any trigger packets to transmit  1520 . If so, the packet is sent  1530  and the transmit count is zeroed  1535 . 
     Each VBI line is tested in both fields for the presence of a non-black signal  1540 . If found, the test signal is copied into test signal store  1550 . If the current frame is an I-frame  1560 , additional I-frame logic is invoked  1570 . Processing ends  1580  after the last VBI line has been processed. 
     FIG. 11 depicts injector I-frame processing invoked in operation  1570  shown in FIG.  10 . Each line on the test signal portion of the VBI  1310  is again examined for the presence of a test signal  1320 . If not found, the VBI table entry for that line is cleared  1325 . Otherwise, the motion score is retrieved  1330  and a test is performed to determine if there is currently high motion or if this is an out of sequence I-frame  1340 . During a rapid scene change, the MPEG-2 video encoder may encode an I-frame even though an I-frame was not the expected frame type in the GOP sequence. If either of these conditions test true, the test is made to see if the test signal has been sent at least once to the TS extractor  1350 . If not, the I-frame may be safely skipped. Otherwise, the I-frame count is checked to ensure I-frames haven&#39;t been suppressed for more than 3 frames  1355 . If not, the I-frame count is incremented  1360  and the loop iterates. Otherwise, the test signal is necessarily injected into the video. Another test is performed in operation  1345  to determine if the video encoder is configured for I-frame only mode. If so, the test signal is again suppressed. The test signal is transmitted on every fourth frame when operating in I-frame only mode. I-frame only encoding is rarely used when transmitting video due to the very high bandwidth consumption. The highest ranked line in concealment mode are retrieved  1365  and the score is compared against the user specified quality threshold  1370 . If the score is higher, the test signal is sent. Otherwise, the I-frame count is checked again  1375  and the test signal is sent only if the count is greater than 3. The trigger packet is built and transmitted in  1380 . The test signal is inserted in video input sent to the MPEG-2 video encoder in  1385 . 
     In FIG. 12, a Trigger Packet is built by the injector shown in FIG.  8 . The Trigger Packet includes all information needed by the TS extractor to remove the test signal from the active video area and place the signal into the correct VBI line. The trigger packet is built in the following sequence of steps. The transmit count is set to 1 in block  1420 . The I-frame is cleared in block  1430 . The timecode is stored in block  1435 . The concealment mode is stored in block  1440 . The video line ID field is stored in block  1445 . The VBI line ID field is stored in block  1450 . A test is performed to determine the presence of the 4:2:2 format. A “yes” condition stores the line count of 1 in block  1460 . A “no” condition stores the line count of 3 for a 4:2:0 format in which three consecutive video lines are used to transmit the signal instead of 1 as in the 4:2:2 format. The “no” condition stores the line count of 3 in block  1480  and both blocks  1460  and  1480  advance to block  1465  to calculate and store a CRC for the trigger packet. The trigger packet is transmitted in block  1470  and a signal sent flag is set in block  1475  indicating that the downstream TS extractor now has an active test signal in the VBI after which the process ends in block  1490 . 
     FIG. 13 discloses a television screen including an active video area  1700  of an NTSC broadcast of 525 video lines comprising two interlaced fields of 2625 lines each (not shown). The active video further comprises a title safe area  1705  and an action safe area  1710 . These areas serve as boundaries that guide producers in the framing of a scene or placement of title text. Lines 22 through 33 and lines 250 through 262 of each interlaced field lay in an overscanned area  1715 . The size of the overscan will vary for each television and even within the same television due to fluctuations in the voltage regulation of a power supply. To prevent viewers from seeing the non-picture area of the horizontal and vertical scans, the overscan is typically set for 5% of the active video which, effectively renders unviewable approximately 12 lines of each field at the top of the screen. The overscanned area offers a high degree of confidence that the concealment of test signals will not be noticed by viewers regardless of video content. It should be noted however that the number of test signals that may be passed in the overscanned area is limited in 4:2:0 mode because each test signal requires 3 lines of active video for transmission. To optimize repair of three contiguous video lines, the adjacent lines must not be used which allows only 4 test signals to be passed in the upper or lower overscan area. A Video Blanking Interval (VBI)  1702  is made up of video lines 1 through 21 and completes the frame of a video signal. 
     When the overscanned area is fully utilized, the video lines are ranked or scored by the effectiveness of the concealment. The placement of the test signal within the active video area is then determined by comparing the highest concealment score against a predetermined threshold. The lower the threshold, the less effective the concealment. When scores fall below an acceptable threshold, 1-3 I-frames may be skipped where no test signals are injected. 
     Although the preferred embodiment teaches placement of the test signal outside the viewing area, concealment techniques used in the present invention make possible other embodiments that pass signals or other data in the title safe area  1710  because the current video content in this area facilitates optimum concealment. Concealment is enhanced because the TS injector instructs the TS extractor how to best repair the missing video line. The usurped video line is replaced using spatio-temporarily adjacent video lines in one of several modes using: 
     1. The same line from the prior frame, opposite field; 
     2. The same line from the next frame, opposite field; 
     3. The adjacent line (upper) in the current frame; 
     4. The adjacent line (lower) in the current frame; and 
     5. A composite line built from two of the four (4) above-mentioned lines. 
     These repair techniques, when combined with optimum placement of the test signal within the video area by the TS injector, result in a high degree of concealment. 
     FIG. 14 describes a concealment process. The test signal  1600  is applied to test block  1602  to determine whether there is available space for the signal. A “yes” condition places the signal into an overscanned area in block  1610  and a concealment score is calculated in block  1620 , after which processing ends in block  1690 . A “no” condition indicates the overscanned area is full and the line separation distance is loaded in block  1625 , after which all available safe action lines (33-45 and 238-250) (See FIG. 14) are ranked by motion and content in block  1630 . In test block  1635 , each available line receives a concealment score in block  1640  until all lines have a concealment score, after which test block  1635  is exited in a “no” condition to rank action safe lines by concealment score in block  1645 . In test block  1650 , the concealment score for each line is compared against the threshold. If the top score is greater than the predetermined threshold, a “yes” condition returns the line, mode and score to block  1690  to end the process. Otherwise, a “no” condition ranks the available title safe lines 46-237 according to motion/content score in block  1655 . Scoring is weighted towards lines devoid of content (black, continuous color or pattern) and lines with static motion for three frames. In test block  1660 , the top ranked 25 lines are each calculated for a concealment score in block  1665 . When all concealment scores have been calculated, a “no” condition returns the lines to block  1670  in which the title safe lines are ranked by concealment score and the top candidate is returned to block  1690 . 
     FIG. 15 describes the process for calculating line concealment scores. The line differences are calculated between the target video line and the same line in the prior and next frames in block  1800 . A “yes” condition initiates a calculation to determine the line difference for luma and chroma components in block  1805 . In test block  1810  the temporary adjacent lines are tested for a difference of zero indicating a perfect match. A “yes” condition awards the line a maximum score in block  1865  and the mode is recorded in block  1870 , after which the process returns to test block  1800 . 
     A “no” condition from test block  1810  returns the process to block  1800  from which a “no”condition returns each temporarily adjacent line to test block  1820 . For each spatially adjacent line a “yes” condition initiates block  1825  which computes the difference between target video lines and the two spatially adjacent lines in the other field of the current frame. A test block  1830  compares the difference between the target video line and the two spatially adjacent lines to zero. A “yes” condition indicates a match has occurred and the maximum score is given in block  1865  which is stored in blocks  1870 , after which the process returns to block  1800 . 
     A “no” condition from block  1830  indicating no match, indicating temporarily adjacent lines do not have a difference of zero, the process returns to block  1820  to determine an alternative concealment score. The “no” condition for block  1820  refers the spatially adjacent lines to block  1835  which determines the direction of motion using motion vectors computed by the video encoder and the same line from the prior frame is shifted in that direction in block  1840  until the luma difference is minimal in block  1840 . Next, pixels are taken from the same video line of the next field in block  1845  to fill in the pixels that were shifted out in a luma/chroma difference is calculated in block  1850 . Test block  1855  compares the difference against zero and a “yes” condition returns the output to block  1865  which awards a maximum score followed by storing the mode in block  1870  and returning to block  1800  from block  1875 . Where the difference is not zero in test block  1855 , the highest score of the previous five calculations are stored in block  1860  and the mode is saved in block  1870  followed by return to block  1800 . Test Signal Extractor Operation: 
     FIG. 16 describes a transport data stream flow in the MPEG 2 demultiplexer. The transport stream  2100  from the ATM switch  140  (See FIG. 1) is received by demultiplexer  2110  which parses the stream into a video PES  2160 ; an audio PES  2162 ; and a system control  2164 . The video PES  2160  is forwarded to the test signal extractor to demultiplex the test trigger signal. The video PES is also provided to video decoder  2122  which provides a decoded video. The audio PES  2162  is provided to an audio decoder  2124  which provides a decoded audio output. 
     FIG. 17 is a block diagram of a test signal extractor  2900 . The extractor contains a video demultiplexer  2901  which demultiplexes the video PES stream and extracts the SMPTE timecode embedded in the GOP header used to identify I-frames. A trigger signal demultiplexer  2905  and a depacketizer  2908  extract the test signal triggers and stores them in the test signal trigger table  2205 . Synchronously with the decode of each frame, an inter-frame processor  2910  receives a picture type and GOP timecode and compares the timecode of the decoded I-frame with the trigger table entries in table  2205 . Entries are made in the video blanking line table  2605  for every matched I-frame and trigger packet. The test signal insertion and concealment device  2915  which may be software or hardware, receives the current picture indicator; the forward picture indicator; the backward picture indicator; and picture type, then synchronously with the presentation of I-, B-, and P-frame, generates a test signal from a test signal line store  2930  which is inserted into the video blanking interval  2940  for each active VBI line entry in block  2605 . Additionally, the test signal in the active video area of the I-frame is concealed (using the process of FIG. 14) by insertion of a video line retrieve/built from picture store according to the concealment mode contained in the trigger packet. The repaired video  2945  is forwarded for digital-to-analog conversion and subsequent display in the TV screen. 
     FIG. 18 describes the test signal trigger data structure  2200  for lines 1 through 22 of the video blanking interval. Each test signal data structure is stored in a table  2205  comprising Column 1, the test signal component; Column No. 2, the number of bits in a component; and Column No. 3, a mnemonic for the component. The components of the signal trigger data structure include a timecode in block  2210 ; a marker in block  2215 ; a video blanking interval line ID/field in block  2220 ; a video line and ID/field in block  2230 ; a marker in block  2235 ; a line count in block  2240 ; a concealment mode in block  2250 ; and stuffing or padding digits in block  2260 ; and a cycling redundancy code in block  2270 . 
     FIG. 19 describes processing the trigger packet received at the extractor. In block  2300  the packet is checked for a bad CRC. A “yes” condition discards the trigger packet in block  2320 . A “no” condition transfers the packet to test block  2310  to determine whether an entry already exists for the VBI line ID contained in the packet. A “no” condition initiates test block  2325  to determine if the timecode has expired for the packet. A “yes” condition returns the packet to block  2320  which discards the packet. A “no” condition returns the packet to block  2360  which creates a new entry in the test signal trigger table  2200  after which the process ends. If an entry does exist for a video blanking interval line test block  2310 , a “yes” condition transfers the packet to block  2330 . The video line ID field of the entry and the trigger packet are compared in test block  2330 . If the packet video line equals the entry video line, a “yes” condition forwards the packet to block  2350  which updates the entry for the packet in the trigger table. If the packet video line and the entry video line are not equal, the packet is returned to block  2340  which removes the entry from the test signal trigger table and creates a new entry in the test signal trigger table in block  2360  after which the process ends. A feature of the present invention is to dynamically move the test signal within the active video area. Operations  2340  and  2360  allow the test signal to be relocated on an I-frame making the TS extractor responsive to the injector on a I-frame basis. 
     FIG. 20 describes the processing of an I-frame in the test signal extractor. Upon arrival of each I-frame, a test block  2400  determines whether or not an entry exists in the timing signal trigger table. A “yes” condition initiates a test  2410  to determine if the frame timecode equals the table entry timecode. A “no” condition returns the process to block  2400 . A “yes” condition initiates block  2420  the VBI line active flag is set after which the I-frame count is cleared in the VBI line table in block  2430 . The conceal flag is set in block  2435  and the new concealment mode is moved into the video blanking interval line table in block  2440 . The video line ID field and count are also moved into the video blanking interval table in block  2445  after which the video blanking line ID and field are moved into the video blanking interval table in block  2450  and the I-frame is deleted from the timing signal trigger table in block  2455  and the process returns to block  2400 . If an entry does not exist in the timing signal trigger table, a “no” condition transfers the frame to the video blocking interval line table shown in FIG. 22 which will now be described after such description the process will return to FIG.  20 . 
     In FIG. 21,  22  entries exist in the video blanking interval line table  2600 , one for each video blanking line from line 10 to line 20 exclusive in each field. Each entry in the table  2600  is shown in block  2605 . The video blocking interval line active flag is installed in block  2610  which signals the TS extractor to move the test signal from the video store onto the video blanking interval line. An I-frame count block  2615  is used to timeout the test signal should it appear from the test signal injector input. Test system injector does not generate a separate trigger to signal the removal of its test signal from the video blanking interval. After three consecutive I-frames are received without an embedded test signal, the test signal extractor removes the entry from the video blanking interval line table and ceases to generate the test signal. A timeout of 3 I-frames in a video network using a GOP count of 12. A concealment flag is stored in block  2620  and the concealment mode in block  2630 . The concealment flag and concealment mode are used at I-frame reconstruction to repair the video line used to carry the test signal. The video blanking interval line and ID field is stored in block  2640 . A video line ID field is stored in block  2650  and a video line count is stored in block  2660 . The video line ID field identifies the video line to repair and the video line count indicates the number of lines which were used to transmit the test signal. 
     Returning to FIG. 20, test signals in the video area, the I frame process  2400  loops through each entry in the extractor test signal trigger table. Upon exit of the loop, each entry in the VBI line table is examined in block  2460  to determine if the VBI line is marked “active”. A “no” condition ends the process. A “yes” condition initiates test block  2465  to determine if the VBI line is marked active. A “no” condition returns the process to block  2460 . A “yes” condition increments the I-frame count  2615  in table entry  2605 , and the process transfers to test block  2475  in which the I-frame count is examined to determine if three (3) I-frames are exceeded. A “yes” condition initiates test block  2480  which resets the VBI line “active” flag in table  2605 . The VBI entry is marked “inactive” in test block  2480  and the test signal is cleared from the video store in block  2485 . After all VBI lines have been examined in the loop comprising operations  2460  through  2485 , the process ends in block  2490 . 
     Now turning to FIG. 22, frame reconstruction will be described. During frame reconstruction the TS extractor loops through each “active” VBI line entry in test block  2500 . A “no” condition ends the process. A “yes” condition initiates test block  2510  to determine whether the VBI line is active. A “no” condition returns the process to test block  2500 . A “yes” condition determines if the I-frame and the conceal flag are set in test block  2515 . If the conceal flag is set, the frame currently being processed is an I-frame that carried a test signal so the test signal is moved out of the current picture into test signal line in block  2520 . The test signal is then copied from the signal line stored into the target VBI line field of the current picture in block  2525 . A test block  2530  determines if the I-frame and the conceal flag are set. A “no” condition returns the process to test block  2500 . A “yes” condition conceals the video line in block  2540 . The process of concealing the video line will next be described in conjunction with FIG.  24 . After the video line is concealed, a conceal flag is cleared in block  2550 , after which the process returns to block  2500 . 
     Turning to FIG. 23, the process of concealing a video line implements the concealment mode stored in line  2630  of the VBI line table  2605  in the VBI table  2600  of FIG.  21 . The video line ID field and count in the VBI entry table are loaded into the process in block  2700 . The concealment mode is loaded in block  2705  after which the test block  2710  determines whether the concealment mode is the same line or a prior frame. A “yes” condition moves the video line from the back frame store in block  2750  after which the process ends. A “no” condition from test block  2710  initiates test block  2715  to determine if the same line is in the next frame. A “yes” condition initiates block  2760  which moves the video line from the forward frame store after which the process ends. A “no” condition for the test block  2715  determines whether the concealment mode is in the same line of the next frame. A “yes” condition initiates block  2760  which moves the video line from the forward frame store after which the process ends. A “no” condition initiates test block  2720  in which determination is made as to whether or not it is a line above in the other field. A “yes” condition moves the video line from the current frame store  2770 , after which the process ends. A “no” condition initiates test block  2730  to determine if the concealment is going to be in the line below in the other field. A “yes” condition initiates block  2780  which moves the video line from the current frame store, after which the process ends. A “no” condition initiates test block  2740  to build a composite line. A “yes” condition builds the composite line in block  2785 . The composite line is built by shifting carrier lines and back filling with the next line, after which the process ends. A “no” condition ends the process. 
     Summarizing the operations  2750  and  2760  repair the line using the same lines/field from the previous frame and next frame, respectively. Block  2770  and  2780  use the next line above and below the line needing repair. These lines are taken from the other field of the current frame. Blocks  2730  and  2740  involve averaging the lines separated either temporarily or spatially. In block  2785  the line is concealed by building a composite line by using part (indicated by pixel shift count) of the lines from the previous frames, then using pixels from the next frame to fill in the shifted out pixels. 
     FIG. 24 describes an alternate embodiment of the invention. In FIG. 24, in-service testing is accomplished using video test signals inserted into the VBI. At the origin POP  3500 , the subscriber&#39;s video feed  3528  is routed through the analog video switch  3582  into the Vertical Interval Test Signal (VITS) equipment  3532  which inserts a test signal into the VBI without affecting the active video. The analog switch routes the output of the VITS  3532  into the MPEG-2 encoder  3536  for encoding and transmission. The injector  3534  moves the test signal from the VBI into the active area before the video is encoded. At the receive end  3505 , the extractor  5384  moves the test signal from the active area into the VBI and the decoded signal  3585  is routed into two output ports by the analog switch  3556 . One port is connected to a VITS and the other to the measurement set  3585  that performs analysis and measurement of the video test signals. The VITS  3557  receives the video test signal  3546  and inserts black into the line that contains the in-service test signal, effectively removing the test signal from the broadcast. The video is then routed back into the analog switch  3544  where it is switched into the subscribers&#39;s outbound access line  3562 . In this manner, the network  115  is tested end-to-end affecting neither the viewed broadcast not the subscriber&#39;s VBI signals or data. 
     Summarizing, the present invention provides the detection of a test signal located in the vertical blinking interval; extracting the signal and moving it into the active video; concealing the line used to send this test signal; dynamically positioning this line within the viewing area depending upon the area of the screen that we best conceal it; the dynamic positioning can vary several times a second where we place it; and finally, an extractor removes the signal that then provides a continuous signal to the measurement test set even though the test signal is only being sent downstream 2.5 times a second, approximately. The test signal sees a solid continuous signal from the extractor. 
     While the invention has been described in a specific embodiment, various changes may be made without departing from the spirit and scope of the present invention, as defined in the appended claims, in which: