NTSC Studio Timing: Principles and Applications Copyright Grass Valley Group, Inc. 1987 Editorial Staff: Mike Guess & David Colborn The most critical design in every teleproduction facility is the system timing. The final video product will always reflect the quality of the system design. This booklet will review the principles of video, discuss system timing, and offer approaches to system timing design. A definition of subcarrier to horizontal phase and an explanation of how to achieve and maintain SC/H phase is also included. SECTION 1 Video Basics The Camera and Pickup Tube Light from a scene enters the camera through the lens and creates a pattern of electrical charges on the pickup tube's target. An electron beam scans across the target and completes an electrical circuit with the pattern of electrical charges on the target. Electrons representing the scene in lightness or darkness flow from the target and become the video signal. In this way, the pickup tube inside the camera changes the varying brightnesses of light that it "sees" into varying electrical voltages called video. Scanning In order to accurately reproduce a scene, the scanning must be done in an organized way. In both the camera and the television receiver, the scanning of the target or screen is done by an electron beam moving in horizontal lines across the target plate or screen. At the same time, the electron beam gradually moves down the scene. Between horizontal scans the beam returns to the left side of the viewer's screen (called "horizontal retrace" or "line flyback"). When the beam reaches the bottom of the scene, the beam is sent back to the top (called "vertical retrace" or "field flyback"). There are 525 horizontal lines in a complete picture. Fields and Frames Each scan of the scene is called a field and only involves half of the total 525 lines or 262.5 lines. Two complete scans of the scene (525 lines) is called a frame. Because the fields are scanned in rapid sequence (60 per second), the viewer only perceives the completed picture. Field one is scanned as the beam moves from the top to the bottom of the screen. The beam is then quickly returned to the top of the screen and field two is scanned. The lines of the second field are "interleaved" between the lines of the first field. This is called "interlaced scanning". By the way, vertical retrace ( field flyback ) takes a certain amount of time so the actual number of lines displayed on the screen is less than 525. Horizontal and Vertical Sync For accurate reproduction, both the camera and the television receiver must be synchronized to scan the same part of the scene at the same time. At the end of each horizontal line the beam must return to the left side of the scene. This is called "horizontal retrace". Coordination of the horizontal retrace is handled by the horizontal sync pulse. At the bottom of the scene, when 262.5 horizontal lines have been scanned, it is time for the beam to return to the top of the scene. The start of vertical retrace is signaled by the vertical sync pulse which is different in width than horizontal sync pulses. Since the vertical retrace takes much longer than the horizontal retrace, a longer vertical synchronizing interval is employed. Blanking During the time when horizontal and vertical retrace are taking place, the electron beams in the camera and home TV are cut off. This time period is called blanking. Blanking means that nothing will be written on the television receiver screen. During horizontal blanking, sync and "burst" (to be described in more detail later) occur. During vertical blanking, vertical sync, vertical equalizing pulses, and vertical serrations occur. The equalizing pulses are inserted to cause the video fields to begin at the proper points to achieve interlace. The vertical serrations keep the television receiver's horizontal sync circuitry from drifting off frequency during the time when no horizontal picture information is present. Black and White Vs. Color Black and white (monochrome) television was the first system to be successfully transmitted and many television sets had been purchased by the time color was being considered. One of the constraints placed on the new color system was that it had to be compatible with the monochrome system. Everything that we have considered so far is applicable to both monochrome and color television systems. Both systems use interlace scanning, synchronizing, and blanking pulses. In the illustration above you will recognize the sync pulses and the active video that we have already discussed. You will also see a small segment identified as "burst". We will be looking into the color system, along with the concept "burst", in the next few pages of the Video Basics section. Carrier Waves Ever since radio was invented, a "carrier" wave has been used to "carry" electrical information through the atmosphere. A carrier wave is a signal that goes up and down in voltage very rapidly and evenly. One complete up and down is a "cycle". The carrier wave can be sent through the air for considerable distances and it can be easily picked up by a receiver like a radio or TV. The carrier wave is somewhat like a rapid and exaggerated ocean wave Subcarrier In the video signal, a subcarrier wave is included to carry the color information. The subcarrier, with its color information, is combined with the black and white information and together they modulate (impress) the main carrier. The subcarrier has a particular frequency (cycles up and down a certain number of times per second) and that frequency is abbreviated as 3.57MHz (Mega Hertz). Mega means a million times and Hertz means per second. Encoding Color Onto the Subcarriers The color signal is composed of luminance (Y), red (R), green (G), and blue (B). To make the total video signal more compact for broadcasting purposes, the Y, R, G, and B components of the color signal are combined as Y and two color difference signals called R-Y (V) and B-Y (U). Each color difference signal contains the information for hue (color) and saturation (brightness of the color). The amplitude values of the two color difference signals are modulated onto two subcarriers which have the same frequency but are 90 degrees apart in phase (see section on Phase below). These two modulated subcarriers are then further combined to form one chrominance signal that changes in amplitude and phase (see illustration in margin). The final transmitted signal contains both a lumininance (Y) and a chrominance component. The original Y, R, G, & B components of the scene are decoded by the television receiver from the transmitted signals. Phase For our purposes, phase relates to a time comparison between two signals. For example, signal A starts at zero microseconds and signal B begins several microseconds later. Both signals are a sine wave like the carrier wave on the previous page. A sine wave travels through a path that is described as 360 degrees (see sidebar below). Signal A's starting point is 0 degrees and, depending on the time lag for the start of signal B, signal A's phase relationship to signal B is described in terms of degrees. In the sidebar below the phase relationship between the first signal and the second is 90 degrees. It is this phase relationship between two signals that is used to derive the color information in the television system. Burst/Color Burst Burst or color burst is produced from a small section of the 3.57MHz subcarrier wave and is transmitted separately as the reference for the color information (hue). The subcarrier is first subjected to a process called "phase splitting" which splits it into two signals, E'V and E'U, that have a 90 degree phase difference but an equal amplitude. E'U is permanently inverted to -E'U. Components -E'U and E'V are summed to form burst. Burst has a resultant degree position of 135 degrees. Burst then acts as a constant reference. The chrominance signal is compared to burst to determine the exact color that is being transmitted. The color is determined by the number of degrees the hue information is shifted in relation to reference burst. Burst is placed, as we've seen before, in the horizontal blanking interval following sync. When a video signal contains burst and horizontal and vertical sync, it is said to be "composite video". Summary All you have learned so far has stressed the point that all the equipment used in a television system must be synchronized together. This is achieved by feeding each item one or more reference signals from a sync pulse generator (SPG). When used as a master timing reference, the SPG must meet the performance specifications as required by the governing body. (In the case of the NTSC system, this is the National Television Standards Committee.) The SPG must also be extremely reliable. Many studios will use two SPGs combined with an automatic changeover switch that will switch to the backup SPG if the master should fail. The outputs of sync pulse generators are as follows: Mixed Syncs - Also known as Comp Sync or just Sync This signal consists of combined horizontal and vertical synchronizing pulses. Mixed Blanking - Comp Blanking or Blanking Combined horizontal and vertical blanking pulses. Horizontal Drive - Line Drive A signal that can be used to trigger the horizontal flyback in older cameras. Vertical Drive - Field Drive A pulse that can be used to trigger the vertical flyback in older cameras. Burst Gate - Burst Flag Pulse used to gate the burst onto the color black output. Note: All pulses should be selectable for 2 or 4 volt amplitude Subcarrier - SC A 3.57MHz sine wave that is used as a phasing reference for chrominance signals. Subcarrier should be selectable between 1 or 2 volts. Color Black - Black Burst A signal that contains mixed syncs (H and V) and burst. The color black signal should have provision for a white flag that indicates line one of field one. This is explained in more detail in the SC/H Phase section. Nearly all equipment designed today is designed to lock to color black. This equipment will have controls for horizontal timing and subcarrier phasing and also provide the system designer with the opportunity to design flexible and expandable television systems. SECTION 2 System Timing It is imperative that all video signals arrive at the video switcher (the central combining point) in synchronization. This means that the scanning sequence of every source must start and stay in time. Without this, the picture on the television receiver or monitor will roll, jump, tear, and/or have incorrect colors when the source video signals are combined. Careful system design is necessary to assure synchronization at the point of input to a video switcher. The degree of accuracy with which these events must occur requires a precision reference. In all television facilities, this timing reference is provided by a synchronizing pulse generator. Establishing and maintaining precise timing involves a multitude of variables that will be described in detail in this booklet. Advance and Delay Defining advance or delay between two video signals is dependent on which signal is defined as the reference. Advance on Camera 1 means its output occurs earlier in time than Camera 2's output. If viewed from the other perspective, Camera 2 is delayed when referenced to Camera 1. It must be understood that advance is not really possible. Advance or negative time delay does not exist. Video signals take time to move just as you and I do. A marathon runner wins because he had the least delay in his running time. On the other hand, he is the most advanced at the finish line, but only because the other runners had more delay in their running times. Video frame synchronizers make video advance appear possible, but in reality they introduce delay to achieve the apparent advance. This is proven by the fact that the audio associated with the video going through a frame synchronizer must also be delayed to avoid lip-sync errors. Studio Planning Before the actual assembly of a teleproduction facility can begin, a system plan must be completed. This can only be accomplished upon definition of studio timing requirements. It will be necessary to know the timing requirements of the equipment to be installed. This information is usually available from the manufacturer's published specifications. Most newer source equipment locks to color black. This implies the device has its own internal sync generator. Typically this source equipment will have adjustments to allow the video output timing to be adjusted relative to the reference color black. You should verify that the adjustment range is sufficient for your requirements. Planning For Timing Advances The ability to lock to color black has not always existed. In the early years of television, cameras needed separate horizontal and vertical drive pulses from the sync generator to drive their scanning circuits. Sync, blanking, and subcarrier were also needed. System design required that all drive pulses be advanced by the path length of the camera. The delay from pulse input to video output may have been as long as one microsecond (a very long delay). These older cameras would receive pulses directly from the sync generator. Drive pulses to other pieces of source equipment would then have to be delayed to time that equipment. This delay could be several hundred feet of coaxial cable or some equivalent lumped delay. There are still cameras in use today that require sync, blanking and subcarrier (horizontal and vertical drive are now virtually obsolete). These cameras have no internal timing adjustments so it is necessary to adjust the advanced pulse drives to time the camera. One way to resolve this timing requirement is to drive the camera with a source synchronizing generator. New cameras lock to color black and have internal timing adjustments available. Until now, most character generators have required pulse drives and external adjustment of timing. This is often done by dedicating a source synchronizing generator to the character generator. Newer character generator models, like other devices, are beginning to lock to color black. Digital video devices such as digital effects generators, time base correctors, and frame synchronizers work on the basis of storing digital video data. This allows timing to be easily adjusted and as such, digital video devices are inherently able to time internally. Color black locking is very common. Nearly all production switchers require sync, blanking and subcarrier. Some switchers have some limited adjustment of horizontal (H) delay but still require advanced pulse drives. Subcarrier phasing is normally built in and allows for color timing of the switcher. Dedication of a source synchronizing generator to a switcher will simplify system design. Some switcher designs now incorporate color black locking. Planning For Timing Delays Coaxial cable is necessary for the proper distribution of video, pulse and subcarrier signals. Coax has an inherent delay of up to 1.5 nanoseconds per foot. This is cumulative and must be considered in system design. Very long runs can introduce significant delay. Coaxial cable can be used for delay but it should be remembered that coax introduces frequency response loss that increases with frequency and length. Distribution amplifiers (DAs) introduce delay that will need to be planned for. This can vary from 25 to 70 nanoseconds depending on the model. Variable cable equalization adjustment will also affect electrical delay. Equalization should be adjusted prior to final system timing. Special purpose video distribution amplifiers are available to provide delay beyond 1 microsecond. These should be used because they have frequency response compensation that is superior to coax and passive video delay lines. Pulse DAs are available to allow for adjustment of pulse delay of up to 4 microseconds and regenerate the pulse to eliminate distortion. Video processing amplifiers have a fixed electrical path length even though regenerated sync and color burst are adjustable. The propagation delay of the GVG 3240 Video Processing Amplifier is about 225 nanoseconds. Sometimes multiple studio facilities have the output of one switcher feeding a second and both share some common video sources. In this instance, the common video sources to the second switcher will need to be delayed by the path length of the first switcher. This delay may be as little as 50 nanoseconds for a small routing switcher to 700 nanoseconds for a large production switcher. Final Considerations There are products available to aid in system design. One such product is the 3230 Isophasing System. This is an automatic delay distribution amplifier that will correct source timing errors of up to 115nS. The Isophasing System can provide up to 32 channels with 5 outputs each, keeping all outputs within one degree of subcarrier phase. The 3230 simplifies system design and daily system maintenance. Once all the timing requirements of the equipment are known, you can begin to actually lay out a system plan on paper. It is important that a specific piece of equipment be defined as the zero timing point. It will become the timing reference by which all calculations and measurements are made. It is desirable to have this be a source in the plant that is not easily altered, such as the test output from the master reference sync generator. It should be remembered that all equipment signal levels and frequency responses must be correct before the timing process is begun. System Design Using Delay The illustration below shows a small system that will use cumulative delay to achieve system timing. This system consists of a camera, a character generator, two 1/2" and one 3/4" video cassette recorders (VCRs). All of the video cassette recorders have time base correctors that lock to the color black reference. The color black signals come from the master synchronizing generator and is distributed by a DA. These time base correctors provide ample timing adjustment for the VCRs. The sources in this system that are to be mixed, keyed or wiped with the video switcher must be exactly in time at the switcher input. Hence the obvious point of reference for this system is at the switcher input. This point is therefore designated the zero timing point, or time 0. In the illustration below the timing requirements of the equipment have been plotted relative to Time Zero. Camera 1 has 850 nanoseconds delay from its composite sync input to its composite video output and represents the longest signal path of any source device in the system. The character generator, switcher, and color bars will need delay added to make their total delays the same as the camera. Since the camera has the longest path length, the pulse drives will be provided directly from the sync generator so that the camera gets the most advanced pulses. The camera has a subcarrier phase control for color timing adjustment. The Camera 1 output becomes the reference input at the switcher. To make the video switcher internal color black and color background generator synchronize with the camera, both sync and blanking drives must be delayed to the switcher by 400 nanoseconds. This is accomplished with two adjustable pulse delay distribution amplifiers. The switcher has a subcarrier phase control for color timing adjustment. Timing of the character generator can be handled in two ways. Delay can be introduced either in the pulse drives, or in the video and key outputs of the character generator. In this system, video delay distribution amplifiers are added to the character generator video and key outputs. This method provides six timed outputs. The amount of delay necessary is 250 nanoseconds as calculated in the illustration below. The last source to be timed is the color bars from the master sync generator. The color bar output is 30 nanoseconds later than the sync output from the sync generator. With the camera as a reference, we can calculate that 820 nanoseconds delay to the color bar output is required to match the camera's delay. The sync and subcarrier required as external reference inputs for the video processing amplifier should come from the distribution amplifiers feeding the switcher. The video processing amplifier has sufficient timing range for both sync and subcarrier. The sync generator is a known SC/H phased source, and the color bar output will be SC/H phase correct. Fine system timing can now begin by adjusting the color bars and the camera. Measurements are made at the switcher output by selecting between the reference source and the source under adjustment on the switcher. An externally locked waveform monitor and vectorscope should be connected to the switcher output. The following steps, in this order, are necessary to ensure correct timing and SC/H phase of all sources. The first step will be to adjust the color bar delay DA. Adjust so that the timing of the half amplitude 50% point of the color bar horizontal sync leading edges match the timing of the camera sync. A timing match within 10nS is desirable. Camera 1 subcarrier phase needs to be adjusted to match its burst phase to the color bar burst phase. The switcher sync and blanking pulse delay DAs must be adjusted so that the switcher color background sync 50% point and blanking are in time with the sync and blanking of Camera 1's output. Switcher color timing (internal color black and background) is matched to Camera 1 with the switcher subcarrier phase control. The character generator video delay DA should be adjusted to match the character generator and Camera 1 horizontal sync leading edges. Adjust the internal subcarrier phase to color time the character generator. The key delay will be adjusted to center the character generator fill video within the hole produced by the key signal. Finally, adjust the VCR time base corrector H and SC phase controls to match each VCR to Camera 1 at the switcher. The procedure will result in all sources being SC/H phase correct only if the color bar video signal is SC/H phase correct. If an SC/H phase meter is available, the SC/H phase of all sources can be verified. This approach to system design is usually the least expensive but does have serious deficiencies. We are distributing sync and subcarrier to equipment through many different paths. This will make establishing and maintaining SC/H phase very difficult. With the many variables in this system, SC/H phase may drift with time and temperature. Additional source equipment may be difficult to integrate in the future and could require major system design changes. System Design Using Source Synchronizing Generators Most of the difficulties encountered in system design can be avoided with a master/source sync generator system. This system provides maximum flexibility and the best SC/H phase stability. The approach below will be used with the same equipment employed in the previous delay system. This time, rather than using the camera as the reference at the switcher input, the master synchronizing generator's color bars will be used. These color bars are fixed in their time relationship to the other outputs of the master sync generator and thus make a rock solid, SC/H phase-correct reference. All the sources still need to be in exact time at the switcher input. This time SC/H phased pulse drives will be provided to the camera and character generator by their own dedicated source sync generators. The source synchronizing generator has the convenience of a single-line locking signal and output advance or delay relative to the lock reference provided. This results in a much simpler system to design and maintain that uses far less cabling. There is also redundancy in the system since the source sync generators will continue to freerun if the master should fail. Camera 1 still requires drives which are advanced 850 nanoseconds to produce a timed, composite video output, but this advance will now come from the source synchronizing generator. The same is true for the character generator and video switcher, provided they each have a dedicated source synchronizing generator. Final system timing is now a matter of looking at the switcher output and comparing each of the sources to the master sync generator's color bars. Each source sync generator is adjusted to time the source it is driving. If the source device has a subcarrier phase control built in, you should adjust horizontal phase using the source sync generator and subcarrier with the source device's SC phase control. This will establish correct SC/H phase and afterwards only the source sync generator should need adjustment. A SC/H phase meter will allow the source to be SC/H phased prior to adjustment of the source synchronizing generator for final timing. Sync and subcarrier for the 3240 Video Processing Amplifier should come from the switcher source sync generator. The source synchronizing generator on the video switcher could be removed and the video switcher and processor could be driven directly from the master sync generator. This would require that about 430 nanoseconds of delay be placed in the color bar path going to the switcher. This is the amount of delay required to generate switcher color black and background from the applied drives. Master/Source Reference Selection The single line reference signal for this master/source synchronizing generator system can be color black or encoded subcarrier. Grass Valley Group developed encoded subcarrier to improve and simplify the locking of source synchronizing generators. The encoded subcarrier signal consists of a continuous 3.579545MHz sine wave which contains two phase inverted cycles, once per color frame. This brief phase inversion is very precisely positioned on the front porch of blanking preceding line 11 on field 1 of the four field sequence. The phase inversion thus communicates horizontal, vertical, and color frame information to the source synchronizing generators. Encoded subcarrier provides a number of advantages over color black as a locking signal. Subcarrier does not have to be regenerated form the periodic color burst, so jitter becomes much less of a problem. Non-ambiguous color frame lock is guaranteed. Since encoded subcarrier is a single frequency, the group delay problem encountered with color black traveling through coaxial cables does not exist. (Group delay will cause large SC/H phase errors if the coax is not equalized, and some SC/H phase error even when the coax is properly equalized.) A color black reference sync generator must first regenerate subcarrier from the color burst. Jitter can result if this is not done precisely. Second, it must very precisely compare the regenerated subcarrier with the exact 50% point on the leading edge of horizontal and vertical sync to determine color frame. If this process is not done precisely, the result may be SC/H phase instability, jitter, and color frame error. A cheaper, less acceptable approach is to independently lock to sync and subcarrier. An independent locking sync generator cannot provide a color frame output because the color frame was never determined. The output SC/H phase will track reference input SC/H phase error. Sometimes SC/H error indicators are provided to help overcome these deficiencies. Multiple Studio Timing The illustration on the facing page shows a three studio system in which the timing of entire source clusters and studios can be changed. This will allow one studio to feed any other studio which is in time. It will also allow for the priorities to change very easily. This entire system is being driven by a dual master reference synchronizing generator with an automatic changeover switch. This provides additional security since each master sync generator is powered from a different circuit. The master sync generators can have ovenized crystal oscillator options for higher frequency stability against temperature variations. An external frequency reference option allows a rubidium or cesium frequency standard to be used as the frequency standard, with the internal oscillator as a backup. Each of the three studios are similar to the one just designed. The studios have some dedicated source devices, with additional cameras and/or video tape machines that can be assigned. A routing switcher is used to assign these sources to the studios. Every studio output is fed to a routing switcher input for assignment as a timed input to another studio. Every studio is being driven by a reference synchronizing generator which will adjust the timing of that entire studio. Each source cluster is driven by a source synchronizing generator so the source cluster timing will stay together. The reference output from each studio reference synchronizing generator is sent to the routing switcher. The reference input to any source cluster synchronizing generator can be assigned to any studio. This automatically times the source cluster to the studio using it. If the reference synchronizing generator has a phase preset option, the phase setting for every configuration can be stored and recalled. A typical configuration could be Source Cluster 1 timed into Studio 1, the output of Studio 1 and Source Cluster 3 timed into Studio 2, Source Cluster 2 timed into Studio 3, which is also a timed input to Studio 2. These timing assignments can easily be interchanged with the phase preset option and routing switcher once the initial timing is completed and stored in each reference synchronizing generator. This system provides maximum flexibility in tailoring each studio for the production it is to be used for. The cameras would be assigned to a studio doing live production and the video tape machines could be used for post production in another studio. Many more sources can be added when using this design without causing major system design problems. Distributed synchronizing generator systems also provide what may be an important advantage: redundancy. Should a failure occur in the master generator, the reference and source generators will freerun and thus the equipment being driven by them will continue to function. SECTION 3 Definition of SC/H Phase In the late 1940s the Electronic Industries Association (EIA) established monochrome television standard RS-170. In recent years proposed color standard RS-170A has received increasing acceptance. RS-170A fully outlines the phase relationship of the color subcarrier to horizontal sync. A graphic representation of this standard is included on pages 12 - 13. If we look at the equation that relates horizontal sync to subcarrier and consider the number of lines in each frame, several conclusions can be made. H = 2 X 3579545 /455 First, there are 227.5 subcarrier cycles per horizontal line, so subcarrier phase reverses every line. This is desirable to reduce the visibility of color subcarrier on monochrome receivers. Second, with 525 lines per frame, there are 119437.5 subcarrier cycles each frame. This causes subcarrier phase to reverse every frame. Because of the extra half cycle of subcarrier, it takes two frames to complete one full four field color sequence, called a color frame. It is clear from the horizontal frequency equation above that horizontal is frequency locked to subcarrier, but it does not define the phase relationship between them. Proposed color standard RS-170A clearly defines SC/H phase as: the zero crossing of the extrapolated subcarrier of color burst shall align with the 50% point of the leading edge of horizontal sync. For color field one, the extrapolated subcarrier zero crossing will be positive going on even lines. This definition of sync to subcarrier phase (SC/H) is required for the unambiguous identification of the four field color sequence. The operational ramifications of these definitions are not obvious and require further explanation. Operational Importance Of SC/H Phase The importance of SC/H phase lies primarily in the video tape editing environment. If during playback the video signal coming off the tape is not of the same color frame as the house reference, the video at the machine's time base corrector output must be shifted horizontally. The shift can be in either direction and be up to 140 nanoseconds (one half subcarrier cycle). This may result in narrowing of active picture and a widening of blanking since the output processor blanking is referenced to the house. Even if the off-tape video is of the correct color frame, the machine-output video will be shifted horizontally to a smaller degree in an amount equal to any SC/H phase difference between the off-tape and house video. These horizontal shifts are troublesome in a tape editing environment, especially when editing scenes together of similar content. At the edit point the background will appear to jump horizontally. This is unacceptable and thus dictates the need for an entirely SC/H phased facility. To ensure the proper operation of the tape machine color framing circuits (to avoid incorrect color frame operation), the SC/H phase relationship of the video recorded on tape and house video must match. As a matter of uniformity correct SC/H phase is defined by RS-170A. It is important that all recorded video have a constantly correct SC/H phase relationship. The reference input to the tape machine should also be a stable SC/H phase source. Problems Achieving and Maintaining SC/H Phase Subcarrier timing in a studio is a well understood concept in the industry; if it is not correct, there will be color hue shifts between sources. If sync timing is not correct, horizontal shifts will occur at the video switcher. The concept of SC/H phasing in a studio requires a higher level of thought regarding each element within the studio. First, and most obvious, is the house sync generator. If the sync generator cannot generate consistent SC/H phased outputs, maintaining SC/H phase in the plant will never be possible. It is equally important that all the sync generators in a multiple sync generator facility maintain correct SC/H phase and color frame relationships. Once SC/H phase has been defined by the sync generator none of the elements in the system should alter the SC/H phase. Some elements are obvious such as the video processor which regenerates sync and burst. If the phase of the regenerated sync or burst is different from the incoming video, the SC/H phase is altered. Less obvious are sources which derive timing from externally applied sync and subcarrier. If sync and subcarrier are fanned out through DAs, then their phase can be altered independently. This dictates that the output of each source device be SC/H phased prior to or at the input of the switcher. There are many distortions which make the determination of color frame and SC/H phase difficult. The most prominent is sync to subcarrier time base error. This can be generated by many devices, such as sync generators with noise in the horizontal sync circuits, linear and regenerative pulse DAs which suffer from pick-off jitter or low frequency response problems, or any device that has separate sync and subcarrier regeneration circuitry. Smear due to poor low frequency response, noise, hum, and power glitches are distortions that may occur in signal transmissions. If these are not removed prior to sync separation, determination of the exact 50% point of sync will be difficult. Video time base error is different than sync to subcarrier time base error. Sync to subcarrier time base error is seen when triggering a scope on the leading edge of sync and viewing color burst. What should be seen are two overlapping cycles of subcarrier that are not blurred. An example of sync to subcarrier time base error is shown in the accompanying oscilloscope photos. If sync to subcarrier time base error occurs either on the reference pulses to a tape machine, or exists on the recorded video tape, color frame lock will be difficult. In the normal playback mode, excessive sync to subcarrier time base error will cause the tape machine to shift horizontal lines by 279nS (subcarrier cycle) increments. This phenomenon is seen as a tearing of the picture. Building An SC/H Phased Plant The first consideration must be the heart of every system, the synchronizing generator. The requirements for the sync generator should include the following: 1) Less than 1nS sync to subcarrier timebase error 2) Less than 10nS long term SC/H phase stability 3) Consistent SC/H phase regardless of operational mode or initial conditions 4) Compatibility with other equipment Many studios use multiple sync generators to provide advanced drive pulses and subcarrier to various source equipment. Every source synchronizing generator must meet these requirements as well as being able to precisely color frame lock to the master reference synchronizing generator. This need has been met by the 9500 Series Synchronizing Generators. The 9500 Series can address any system requirement, including both ultra precise encoded subcarrier locking and color black locking source synchronizing generators. Every model in the 9500 Series is unconditionally SC/H phased, whether locked or freerunning. Every video locking 9500 Series Synchronizing Generator will lock to a non SC/H phased reference and produce correctly SC/H phased outputs. This is done by identifying color frame of the incoming video and assigning the nearest color frame. Once this has been achieved, the sync generator will tolerate SC/H phase drift beyond 100 degrees for the source synchronizing generator and over 330 degrees for the master. Should the incoming SC/H phase exceed the 100 degree limit, the source synchronizing generator will shift its horizontal phase by one-half subcarrier cycle to maintain color frame match to its reference. The master generator will shift its horizontal timing by a full subcarrier cycle should the 330 degree limit be exceeded and thus not change color framing. A stable reference is ensured under any condition. The 9510 and 9520 Master Synchronizing Generators feature protected video genlock. In this mode the generator achieves color frame lock and then maintains frequency lock to the color burst of the incoming video only. This mode provides immunity to incoming jumps in video sync which would otherwise cause severe disturbances in the generator's output. The transition out of protected video genlock into freerun will occur if the burst abruptly changes phase, disappears, or there is a complete loss of video. The transition to freerun will be smooth and not disturb the plant. Every model in the 9500 Series has a wide retiming range of 2.5 lines advance to 1.5 lines delay. Output SC/H phase is correct at any timing setting. A one line wide color frame pulse which occurs on line 11 of field one of the color frame is available on every model. This color frame pulse provides absolute, positive identification of color framing to all equipment in the plant that will accept it. All pulse outputs are negative going four volt peak-peak and are shaped. The 9505 Source Synchronizing Generator is available for either color black reference or ultra precise encoded subcarrier reference. Both models are unconditionally SC/H phased and have superior performance specifications. Test signals are optional in every 9500 Series Synchronizing Generator. An optional Source Identification submodule can be added to the Test Signal Generator module to place up to a 14 character identification over the test signal output. This identification is also positionable both vertically and horizontally. The 9520 will accept two Test Signal Generator/Source Identification options, a High Stability Ovenized Crystal Oscillator option, and an External Frequency Reference option. The External Frequency Reference option permits the 9520 to frequency lock to an external 3.579545, 5.0, or 10.0MHz atomic frequency source for superior timebase stability. The 9510 has a phase preset option that will store 16 different phase settings in a non-volatile memory. These can be recalled via local or remote control. This option permits retiming of a source device or entire studio with a single binary control. Conclusions To achieve an SC/H phased plant, the timing of sync becomes as important as subcarrier, and each element should be viewed in that light. To aid video tape editing, it is important to record video with proper SC/H phase and also supply SC/H phased reference to the machine in playback. These criteria do not have to be compromised with the system approach offered by the Grass Valley Group. Measuring SC/H Phase The SC/H (subcarrier-to-horizontal) phase is the time relationship between the subcarrier and the leading edge of horizontal sync. A properly adjusted SC/H phase occurs when the 50% points of the leading edge of sync and the subcarrier zero crossings are coincident. The color frame pulse (V1) appears on line 11 of field 1. V1 identifies field 1 of the 4 field color sequence. Test Equipment Required The following test equipment is required to perform the SC/H phase measurement procedure. Equivalent test equipment may be substituted but must be equal to or superior in performance. Dual Trace Oscilloscope Tektronix 465 (with delayed sweep and one channel input inversion) Switchable Delay Line Mathey 511 or Subcarrier Delay DA (360! range) Test Procedure SC/H Phase Measurement 1. Connect a video source requiring SC/H phase measurement to the inverting channel of the oscilloscope. 2. Connect subcarrier (3.58 MHz continuous) to the second channel of oscilloscope. 3. While observing the oscilloscope (triggered at a horizontal rate), adjust subcarrier to match amplitude of burst. 4. At the oscilloscope, invert the video display and set mode to alternate sweep. Figure A shows inverted video (top) and continuous subcarrier (bottom). 5. Adjust the oscilloscope for A plus B mode. 6. Adjust subcarrier phase and fine level at the generator or delay line for a null burst as shown in Figure B. 7. Adjust the oscilloscope for chop mode, noninverted video, and adjust vertical positions to exactly overlay subcarrier and sync. 8. Adjust the oscilloscope delayed sweep for a display showing the leading edge of sync and the subcarrier. A proper phase relationship requires coincidence at the 50% points of the leading edge of sync and the subcarrier zero crossings. See Figure C. An improper phase relationship is shown in Figure D. Color Frame Pulse (V1) Identification 9. Adjust the SC/H phase as described in steps 1 through 8, for proper coincidence. 10. Trigger the oscilloscope on the leading edge of the V1 pulse with video and subcarrier connected to the two input channels. See Figure E. 11. Increase the oscilloscope sweep rate and, using the delayed sweep option, view a display showing the first leading edge of sync following the trigger. 12. If the negative transition of the subcarrier is coincident with the leading edge of sync, the triggering V1 pulse is a color frame identification pulse that occurs on line 11 of field 1. See Figure F. NOTE: The SC/H phase is easiest to observe on a display that is horizontally triggered. Because of the low repetition rate of V1 and the fast sweep rates (50nS/div.) required, only the direction of subcarrier signal can be easily observed by triggering on V1. GLOSSARY OF TIMING TERMINOLOGY APL Abbreviation for average picture level. The average luminance level of the part of a television line between blanking pulses. Active Picture Period That part of the video signal that produces the television picture, as distinguished from that portion of the video signal that occurs during blanking (horizontal and vertical retrace). Amplitude Modulation (am) Modulation in which the amplitude of a wave is made to vary. In television, the color video signal modulates the subcarrier, causing its amplitude to vary. Automatic Changeover Switch Equipment that receives the outputs of two sync generators and automatically switches to the backup sync generator should there be a failure of the sync generator in use. Backporch The blanking signal portion which lies between the trailing edge of a horizontal-sync pulse and the trailing edge of the corresponding blanking pulse. The color burst is located on the back porch. Bandwith The complete range of frequencies over which the television system can function. The information carrying capability of a particular television channel. Blanking The time period when picture information is shut off. Blanking is a voltage level at black picture level and acts as a signal to turn off the scanning beam. synchronizing pulses which control invisible retrace of scanning are active during the blanking period. Breezeway That portion of the "back porch" between the trailing edge of the sync pulse and the start of the color burst. Burst (Color Burst) Nine cycles of 3.57MHz subcarrier, placed near the end of horizontal blanking, which is the color reference for the color signal. Color timing refers to adjustment of the phase of the subcarrier. Carrier Wave A single frequency wave which is transmitted and modulated by another wave which contains the information. Character Generator A device used to generate text or captions for television broadcast. Chrominance That portion of the video signal that contains the color information (saturation and hue). Clamping The process that establishes a fixed reference level for the picture signal, normally keyed off the horizontal synchronizing pulses. A major benefit of a clamp is the removal of low-frequency interference, especially power line hum. Coaxial Cable A cable with a noise shield around a signal-carrying conductor. In television, the cable impedance is 75 ohms. Color Background Generator Circuit that generates a full-field solid color for use as a background in a video picture. Color Bars A video test signal widely used for system and monitor setup. Color Black (Black Burst) A composite video signal that produces a black screen when viewed on a television receiver. Composite video is a video signal that contains horizontal, vertical, and color synchronizing information. Color Frame In NTSC color television it takes four fields to complete one color frame. For a detailed definition, see the SC/H PHASE section. Composite Sync (CS) Horizontal and vertical sync pulses combined. Often referred to simply as "sync". Sync is used by source and monitoring equipment. Composite Video A video signal that contains horizontal, vertical, and color synchronizing information. Color Track Frame Pulse A pulse laid down on video tape by a video tape recorder to enable the machine to lock up correctly when played back. Cut A transition between two video pictures which is nearly instant, without any gradual change. DC Restoration The reestablishment by a sampling process of the DC and the low-frequency components of a video signal which have been suppressed by AC transmission. DC Signal Bounce Overshoot of the proper DC voltage level of the blanking pulse due to multiple AC couplings in a signal path. Causes sudden brightness in picture. Delay Distribution Amplifier An amplifier that can introduce adjustable delay in a video signal path. Distribution Amplifier Device used to multiply (fan out) a video signal. May also include cable equalization and/or delay. Referred to as a DA. Drive Pulse(s) (Pulse Drives) A term commonly used to describe a set of signals needed by source equipment such as a camera. This signal set may be composed of any of the following: sync, blanking, subcarrier, horizontal drive, vertical drive, and burst flag. Equalization Process of altering the frequency response of a video amplifier to compensate for high-frequency losses in coaxial cable. Equalizing Pulses A series of pulses occurring at twice the line frequency before and after the serrated vertical synchronizing pulse. Their purpose is to adjust the scanning sequence for proper interlace. Fade A gradual transition of the video picture (and signal) to black. Field Half of the horizontal lines (262.5 in NTSC system) needed to create a complete picture. Two interlaced fields create a complete frame. Fill The video information that fills the "hole" cut in the video picture by the key signal. Flyback (retrace) The movement of the camera or television monitor electron beam back to the starting point for the next line or field. Frame A complete picture composed of two fields. In the NTSC system, 525 interlaced horizontal lines of picture information. Frame Synchronizer A digital buffer that by storage, comparison of sync information to a reference, and timed release of video signals, can continuously adjust the signal for any timing errors. Frequency The number of cycles of a waveform in a given length of time. Frequency Modulation (fm) Modulation in which the frequency of a carrier wave is made to vary. Front Porch The blanking signal portion which lies between the end of the active video picture information and the leading edge of sync. Frequency Response The maintenance of a uniform video signal level (amplitude) over a range of frequencies. Gate A signal used to trigger the passage of other signals through a circuit. Group Delay A defect in a video signal caused by different frequencies having differing propagation delays (delay at 1MHz is different than delay at 5MHz). In the television picture, group delay will cause an object's color to shift outside the object's outline. Harmonic A wave having a frequency that is an integral multiple of the fundamental frequency. For example, a wave with twice the frequency of the fundamental is called the second harmonic. Hertz Unit of measurement for the number of cycles of a waveform in one second. Horizontal Sync Pulse The synchronizing pulse at the end of each line that determines the start of horizontal retrace. Hue The actual color that appears on the screen. Hue defines color on the basis of its position in the spectrum - i.e., whether red, blue, green, or yellow, etc. One of the three characteristics of television color. See Saturation and Luminance. Impedance The total opposition (resistance and reactance) a circuit offers to the video signal at a given frequency. Key A signal that can electronically "cut a hole" in the video picture to allow for insertion of other elements such as text or a smaller video picture. Linear and Regenerative Pulse DAs Linear pulse DA will handle up to 4V p-p signals (pulses) but is limited to amplifying and fanning out the signal. Regenerative pulse DA reconstructs the signal and allows for adjustment of delay. Luminance (brightness) The brightness of the picture or area of the television screen being considered. See Hue and Saturation. Master Reference Synchronizing Generator A synchronizing pulse generator that is the precision reference for an entire teleproduction facility. Microsecond (uS) One millionth of a second: 1 x 10 to the negative sixth or 0.000001 second. Modulator/Demodulator Modulator is a circuit that modulates or impresses the carrier wave by amplitude and/or frequency. Demodulator is a circuit that demodulates or decodes the amplitude and/or frequency information from the carrier wave. In television, the information typically modulated and demodulated are the hue and saturation components of the color signal. Monochrome (black and white) The video signal which represents the brightness values (luminance) in the picture, but not the color (chrominance) values in the picture. Nanosecond (nS) One-billionth of a second: 1 x 10 to the negative ninth or 0.000000001 second. NTSC National Television System Committee which worked on formulation of standards for present United States television system. Now describes the American system of color telecasting which is used mainly in North America, Japan, and parts of South America. Ovenized Crystal Oscillator A crystal oscillator that is surrounded by a temperature regulated heater (oven) to maintain a stable frequency in spite of external temperature variations. Overshoot Amplitude of the first maximum excursion of a pulse beyond the 100% level. Pulse exceeds its defined level temporarily, before settling to the correct level. Overshoot amplitude is expressed as a percentage of the defined level. PAL Abbreviation for Phase Alternating Line. PAL is the name for the color television system in which the E'V component of burst is inverted in phase from one line to the next in order to minimize hue errors that may occur in color transmission. PAL-B is a European color TV system featuring 625 lines per frame, 50 fields per second, and a 4.43361875 MHz subcarrier. Used mainly in Europe, China, Malaysia, Australia, New Zealand, and parts of Africa. PAL-M is a Brazilian color TV system with phase alternation by line, but using 525 lines per frame, 60 fields per second, and a 3.57561149 MHz subcarrier. Phase The relative timing of a signal in relation to another signal. If the time for one cycle of a signal is represented as 360 degrees along a time axis, the phase position for the second signal is called phase angle, expressed in degrees. Path Length or Propagation Delay The time it takes for a signal to travel through a piece of equipment or a length of cable. Phasing (Timing) Adjusting the delay of a video signal to a reference video signal to ensure they are synchronous. This includes horizontal timing and subcarrier phasing. Pick-Off Jitter Jitter is a random aberration in the time period due to noise or time base instability. Pick-off means sample point. Processing Amplifier/Proc Amp (See Video Processing Amplifier) Pulse Width Measured between the 50% amplitude points of the leading and trailing edges. Reference Video Signal A composite video signal used to compare all other video signals to, for timing purposes. Rise Time Time required for a pulse edge to rise from 10% to 90% of the final value. Return Loss At a connecting point in a video system, the difference between the signal amplitude on the connection and the signal amplitude reflected from the connection. The difference is measured in decibels (dB). Saturation (chroma, chroma gain) The degree of purity of a color. Adding white to a color reduces its degree of saturation. SC/H Phase The phase relationship of the subcarrier to the leading edge of horizontal sync. Alignment of the zero degree crossing of subcarrier with the 50% point of the leading edge of sync. SECAM Abbreviation for sequential couleur a'memorie (sequential with memory). A color-tv system with 625 lines per frame and 50 fields per second developed by France and the U.S.S.R. and used in some countries that do not use either NTSC or PAL systems. Source Video producing equipment such as a camera, tape recorder, or character generator. Source Synchronizing Generator A synchronizing pulse generator used to drive a specific piece of source equipment. It is referenced to a master reference synchronizing generator. Spurious Signals Any portion of the signal that is not part of the fundamental video signal and its harmonics. Spurious signals include transients and noise. Switcher, Production Switcher, Vision Mixer Device that allows transitions between different video pictures. May also contain special effects generators. Subcarrier A continually cycling waveform at 3.57MHz on which color information is added or encoded; subcarrier is added to the monochrome signal to carry color information. Synchronizing Pulse Generator (SPG) Equipment that generates synchronizing pulses needed by source equipment. Also known as a sync generator. Time Base Corrector Device used to stabilize the video picture on replay from a tape machine. U Color difference signal (B-Y) used to modulate "U" component of subcarrier. V Color difference signal (R-Y) used to modulate "V" component of subcarrier. Vertical Serrations A vertical synchronizing pulse contains a number of small notches called vertical serrations. These serrations provide horizontal synchronization during the vertical interval. Vertical Sync Pulse The synchronizing pulse at the end of each field which signals the start of vertical retrace. Video Processing Amplifier A device that stabilizes the composite video signal, regenerates the synchronizing pulses, and can make other adjustments to the video signal. Zero Timing Point The point at which all the video signals must be in synchronization (typically the switcher input). NTSC REFERENCE TIMING DATA Subcarrier Frequency 3.579545MHz Subcarrier Period 279.37nS Horizontal Frequency 15.734264KHz Horizontal Period 63.5565S Vertical Frequency 59.94Hz Vertical Period 16.683mS Vert. Equalizing Pulse Width 2.35S Horizontal Sync Width 4.75S Horizontal Blanking Width 10.95S Vertical Sync Width 27.15S Vertical Blanking Width 21 lines Front Porch Width 1.55S Breezeway 0.65S Burst Width 2.55S Color Back Porch Width 1.65S Color Timing Data: 1! = .776nS 1nS = 1.289! For Cable With 66% Propagation Factor 1! = 6.035" = .503' 1nS = 7.778" = .648' Zero SC/H phase is the coincidence of the zero crossing of a subcarrier the same phase as color burst with the 50% point of the leading edge of horizontal sync. On color frame one, the subcarrier zero crossing will be negative going on odd numbered lines. Your Feedback Is Appreciated