The Modern Systems Approach to SCH and Color Framing in NTSC
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Application Note The Modern Systems Approach to SCH and Color Framing in NTSC Introduction SCH and Color Framing have caused more problems, discussions, and confusion than almost any other technical aspect of NTSC television production. This paper presents a modern, standard and proven approach for dealing with these issues. The principles are discussed and the terminology is defined so the problems can be understood. Aspects of equipment performance and system design that are necessary to solve or avoid SCH problems are explained. Fig 1. Sync coincident with zero crossing of subcarrier. RS-170A In the NTSC television system, the frequency of the color subcarrier (S/C) is 2271¤2 times the horizontal frequency. The one half in that number means that the subcarrier phase relative to H sync reverses (changes 180°) every line. Since there are an odd number of lines (525) in a frame, the phase of subcarrier reverses at the start of every frame. The NTSC specification does not define the phase relationship between subcarrier and H (SCH). RS-170A is the de facto standard which defines the relationship. The EIA (Electronic Industry Association) has officially Fig. 2. Sync on Line 10 of Color Field 1 of a Color Frame. approved the RS-170A Standard. A Color Frame consists of 4 fields. subcarrier is equidistant from Color Field 1 is that monochrome Field sync on line 10 of all frames. The following definitions are based on 1 on which sync on line 10 is closest accepted practice and intent of to being coincident with the 3 - Color Framing cannot be RS-170A. positive-going zero crossing of “correct” or “incorrect” without subcarrier. (See Figure 2.) reference to another video Whenever Sync or H are subsequently Several important observations signal. When signals are said to mentioned in a timing context, we should be made about these be correctly Color Framed, it mean the 50% point of the leading definitions: means that they have both edge of sync. Likewise, references to approximately correct SCH, and Subcarrier means the Subcarrier of 1 - Since SCH is defined with that Color Fields 1 of both signals which Burst is a representative sample reference to either the are coincident in time. (i.e. a subcarrier which is in phase with positive-or-negative-going zero burst). crossing of subcarrier, the 4 - Two signals which are burst maximum possible SCH error is timed, horizontally timed, and Correct (“zero”) SCH exists if the sync 90° vertically timed are correctly on every line is coincident with a zero Color Framed. crossing of subcarrier (See Figure 1.) 2 - Color Field 1 cannot be identified unless the SCH is approximately correct. If the SCH error is 90° the positive zero crossing of Application Note 5 - When someone says that the Several methods exist for eliminating SCH of a signal is “180 out” they the 140nS H shift, provided that the are comparing one signal to tape and reference have the same another (whether they realize it or SCH. Therefore, it is necessary to not). They really mean one of the ensure that all signals have the same, following: standard SCH. If all signals in a system have zero SCH, any piece of a) The two signals are equipment can identify Color Frame 1 horizontally and vertically timed, and take the appropriate action to but their burst phases differ by avoid error. If Color Frame problems 180°. exist in a zero SCH system, either a system H timing error (increments of b) The two signals are burst and Fig. 3. SCH measurement set-up. 140nS) exists, or some equipment is vertically timed, but their H defective or obsolete (ie: it is not phases differ by 140nS (or 140nS came into widespread use, SCH and appropriately dealing with Color plus a multiple of 280nS). Color Framing were of little Framing). Timecode editing systems importance. Indeed they were not can be an exception to the previous c) The two signals are burst and even defined. Let’s discuss these statement. To avoid problems, horizontally timed, but their V issues with regard to editing since the timecode must have a consistent phases differ by an odd number problems are easier to describe. Also, relationship to video. Normally, the of lines. even timecode frame numbers should be aligned with Color Field 1. If this The multiplicity of possibilities relationship exists, requesting an edit shows the danger of confusion that violates a strict odd/even inherent in using this sequence of timecode frame numbers terminology. will create a Color Framing violation. 1 Most editing systems will prevent such 6 - In some versions of the RS-170A, errors. zero SCH is defined by stating that the zero crossings of the first Photo # 1 The foolproof way to ensure a correct cycle of burst greater than 50% of relationship between timecode and burst amplitude shall be 19 Color Frame is to always lock a Color subcarrier cycles after sync. This Framing timecode generator to the does indeed define zero SCH in video being recorded. Failing this, the agreement with the definition relationship should be certified at the previously given. However, this time of recording. definition simultaneously defines burst position. If the first cycle of SCH Measurement burst greater than 50% of burst If a system is zero SCH and properly amplitude fell 181¤2, 19, 191¤2, or timed, Color Frame problems are an any multiple of half cycles of Photo # 2 indication of defective equipment. In subcarrier after the 50% point of editing raises the related issue of order to ensure a zero SCH system, it the leading edge of sync, the SMPTE timecode. The importance of is first necessary to measure SCH. SCH would still be correct. Only SCH and Color Framing can be SCH can be measured on any dual the burst position has changed. understood by examining the channel oscilloscope. One channel is sequence of events when a typical fed video, and the other is fed Within reasonable limits, burst VTR locks to an external reference. phaseable source of subcarrier which position has negligible effect on First, the VTR locks the control track is frequency locked to the video. A the system performance, and the frame pulse to a frame pulse derived sync generator locked to the video is effects of incorrect burst position from the reference. Second, tape H usually the most convenient source of are unrelated to those of sync is aligned with reference H sync. this subcarrier (see Figure 3). incorrect SCH. Furthermore, it Third, the TBC aligns the tape burst to can be very difficult to identify the the external reference burst. Since, the The phase of the subcarrier is adjusted first cycle greater than 50% in TBC cannot separate chrominance to exactly match the phase of burst amplitude, especially in the from luminance, the alignment of tape (see photo #1). Then the time presence of distortion or noise. burst to reference burst may shift tape difference between the 50% point of Using this definition is just asking H sync by up to 140nS (180°). If the the leading edge of sync and the for trouble. tape or reference SCH is random, this nearest zero crossing of the subcarrier H shift can be any amount from 0 to is measured (see photo #2). SCH and Color Framing in a 140nS. If the SCH is the same on both System the tape and the reference, the H shift This time, measured in nS, can be VTR editing and composite digital will either be 0 or 140nS. Which of converted to degrees of subcarrier by signal processing require correct these occurs depends on Color multiplying by 1.29. This procedure is SCH. Editing requires correct Color Framing between tape and reference. simple, but obviously is not suitable as Framing. Before these techniques a normal operating procedure. Many Application Note companies, including Leitch, The ideal solution would be to simply devices to achieve the same level of manufacture equipment designed to replace all non-zero SCH equipment. SCH and Color Frame reliability while measure SCH. In addition, SCH However, not all equipment is using standard video or color black as measurement capability is included in currently available in a zero SCH the genlock signal. It is sometimes many modern vector monitors and version, and economy dictates that argued that these proprietary systems VTR’s. Several issues should be borne existing equipment be used. are necessary due to the in mind when choosing and using Nonetheless, it is essential to start the non-phase-linear characteristics of SCH measurement devices. First, a journey to zero SCH on the right foot: video cable which can introduce an clear understanding of the difference with zero SCH sync generators and SCH error. this error is small between measurement resolution and test signal generators. These devices (approximately 3.5° per 30 meters (100 accuracy is required. For example, if a act as the reference for the timing and ft.) for Belden 8281 or equivalent) and, piece of equipment has a resolution of certification of video signals. If SCH as long as a devices meets criterion 4 1° and a specified accuracy of ±5°, and Color Framing are not maintained above, will not normally cause a then two identical devices may reliably by these devices, there is little problem. In any case, cable disagree by as much as 10°. This is hope of establishing a zero SCH plant. equalization removes the insidious since the resolution is cable-induced SCH error. Therefore, immediately obvious to the person The following criteria are important in in the rare instance where genlock making the measurement; however, selecting these devices: cable runs are longer than the accuracy can only be found by approximately 300 meters (1000 ft.), looking in the equipment manual (if it 1 - The output SCH should be zero the cables should be equalized.