 IEC-835 PhaseIconnew measure to picture quality  SCH Phase Measurement The IEC-835 PhaseIcon measures the SC/H condition of any NTSC video signal and displays the results on a color or monochrome monitor. The waveform displayed in the video monitor is horizontal timing and subcarrier phase. The visual display of the PhaseIcon is keyed over the source video "Compare Input". The waveform shows the "Reference" or "Compare" input, depending on the mode of operation. The "Shadow" mode of the PhaseIcon shows a darkened area that represents an amplitude equivalent to 40 IRE units of burst. Although the primary function of the PhaseIcon is SC/H certification, it does system timing and subcarrier phase as well. Timing of two video sources are accomplished by adjusting the source horizontal timing until the two vertical lines on the right side of the display are superimposed. SC/H is accomplished by adjusting the source subcarrier for a match of the two sine waves. Each source is routed into the compare input of the PhaseIcon until all sources in the system are timed and phased. The RS-170A specification states that the relationship between horizontal sync and subcarrier of field one, line 10 shall have a zero crossing in the up cycle and be coincident with the leading edge of horizontal sync, and have a tolerance of ±40 degrees. The PhaseIcon is capable of displaying a much tighter tolerance, in the order of ±2 degrees of phase and ±2ns for horizontal timing.
IEC-835 SPECIFICATIONS INPUT: (For Reference and Compare) OUTPUT: ACCURACY: FRONT PANEL CONTROLS:
"The PhaseIcon is a precise method for certifying and timing, but in practice other attributes such as ease of operation and readability are also of prime importance." The PhaseIcon is an accurate instrument for Certifying, Timing, and Analysis of the RS-170A video signal. The Picture Line Amplifier Output specifications of EIA Standard RS-170 was modified to include a requirement for a sync-to-burst phase relationship, taking into account the use of videotape recorders in programming. Once video has been recorded, the sync-to-burst phase relationship can no longer be altered. This makes it very important that it be recorded with the same relationship it will need when played back. The only way that relationship can be predicted is to maintain a constant relationship during both recording and playback. This is the object of EIA Industrial Electronics Tentative Standard No. 1, part of RS-170A. Its specifications are deceptively simple. But, although the sync-to-burst phase relationship is easy to describe and relatively easy to achieve, it is not easy to measure or certify. It gives only a very dim trace on even a high quality oscilloscope. It further complicates things by requiring that only one cycle of subcarrier be resolved on the oscilloscope trace, which makes nearly every means of direct view impossible. The RS-170A specification meets only two of the three requirements of a workable specification: establishment of a well-defined standardization objective and the requirement of possible implementation. However, it falls short in the third requirement in that it is very difficult to maintain and certify. Three problems arise. First, just knowing when correct subcarrier-to-horizontal (SC/H) phase is achieved is not enough. One must also know how incorrect the relationship may be. The second problem is related to how well or how badly any two video sources are correlated to each other. The third problem encountered in implementing an RS-170A system is caused by the physical layout of video systems. Normally, the point in the system where the phase relation is to be determined is far removed from that point in the system where the phase relationship is to be controlled. The PhaseIcon was designed to overcome these three problem areas. This device processes the phasing and timing information into a video format that can be viewed on a standard video monitor. The RS-170A specifies that a sine wave (subcarrier) is to be compared to the leading edge of sync Line 10, Field 1, where an up-going zero cross is required. The PhaseIcon displays the literal description of this condition on a picture monitor screen, providing a very accurate means of indicating the SCH phase relationship. If another source is added to the display so that the second source can be compared to the first video source, a truly useful system timing and analysis tool is presented. The RS-170A specifications refer to a specific subcarrier to horizontal phase relationship. In that subcarrier repeats itself once every four fields there must be four color fields in a color frame. This means that there are 1050 lines in a color frame as opposed to 525 lines in a monochrome frame.
INTRODUCTION The RS-170 line output specification has been changed to include a requirement for a sync-to-burst phase relationship. Prior to this, burst phase was acceptable as long as it was in phase with picture chroma. The overwhelming drive behind this change is the ever increasing use of video tape recorders in programming. Once video has been recorded, the burst-to-sync phase relationship can no longer be altered. This makes it very important that it be recorded with the same relationship as it will need when it is played back. It is obvious that the only way the relationship can be predicted is to maintain a known relationship during both recording and playback.
RS-170A OBJECTIVES Two years of development was required to insure that an instrument meet the requirements of timing and phase of the video sources in the system and do it accurately and with speed. The IEC-835 PhaseIcon was a result of this development. This instrument (PhaseIcon) is to precisely measure a reference burst extended 19 cycles toward the leading edge of sync and zero cross at the 50% point. (Figure 2 ) And further, if Line 10 in each of the four color fields is observed, the extended burst will zero cross in the positive going direction in color field one, negative going in color field two, negative going in field three, and positive going in four. A tolerance of ±40° is specified at each zero cross.
IMPLEMENTATION PROBLEMS Although it is easy to describe and relatively easy to achieve, it is not easy to measure or certify. First of all, the burst does not extend toward sync, nor can it be extended without regenerating a subcarrier that exactly phase matches burst. This is not too difficult, but does require a high degree of precision. The second difficulty lies in the fact that any one color field is hard to identify and display. The 15Hz Repetition Rate gives a very dim trace on even a high quality Oscilloscope. To further complicate things, it is required that one cycle of subcarrier be resolved within that field. This makes almost every means of direct view impossible. Therefore, it can be seen that the RS-170A specification meets two of the three requirements of a workable requirement. It meets the standardization objective by requiring that all videos have a specified burst to sync phase relationship. It meets the second requirement in that it can be implemented with a minimum of equipment changes and additions. However, it falls short in the third requirement in that it is very difficult to maintain and certify. There is no test equipment that can directly measure subcarrier to sync phase relationship, commonly referred to as "SCH PHASE". Two out of three are not bad and RS-170A is no doubt the accepted standard for maintaining SC/H timing. Some of the difficulties are related to the extra care and attention required by the added parameter. But to a large extent, the difficulties revolve around the lack of equipment available to do the job. It is to this end that Link's PhaseIcon has been developed. It was the result of two years of development. We feel that it will remove a majority of difficulties in maintaining an RS170A system.
PhaseIcon OBJECTIVES As the problem was approached, it became apparent that there were three main areas of difficulties. First of all, just knowing if the SCH Phase is right or wrong is not enough. It is more important to be able to see the overall phase picture. At this time, SCH Phase meters indicate SCH Phase over a narrow phase angle near the leading edge of sync. This means that as the video signal to be SCH Timed to RS-170A specifications, the subcarrier phase must be changed in a random manner until the sinewave is nulled with the reference burst. The second deficiency is related to how any two video sources are related to each other. Merely establishing a SCH Phased condition on each video source in a system is not enough. This does not guarantee that each video source in a system is in time or phased with every other source in that system. The third problem encountered in implementing an RS-170A system is caused by the equipment location in most video systems. Normally, the point in the system where the phase relation is to be determined is far removed from that point in the system where the timing relationship is to be controlled. This usually requires that at least two people be involved, one relating readings from the measuring point and the other making the actual adjustments. It is in these three areas where Link's PhaseIcon was designed to allow a more efficient means of implementing SCH Phasing. First the indicating range, second the ability to correlate multiple video sources into a system, and third to present the phasing information in a manner that can be easily distributed within a system.
DISPLAY FORMAT Addressing the last area first, it was felt that if the phasing information could be processed into a video format for viewing on a standard video monitor the user could become adept at routing the signal to the point of control very quickly, since handling a video signal would present very little difficulty to the operator. Having decided upon a video format, it followed that an oscilloscope type presentation was desirable. Referring to RS-170A specifications, it states that a sine wave, or subcarrier, is to be compared to the leading edge of sync, Line 10, Field 1 and an up going zero cross is required. If one takes the literal description of this condition and displays it on the screen of a picture monitor, it makes a very graphic means of indicating SCH Phase. A picture showing a single vertical line representing the leading edge of sync in Line 10, Field 1 and a single sine wave that represents the subcarrier phase with respect to sync. If an entire cycle of subcarrier is displayed, it becomes obvious to the user to what degree he is correct. Add a single horizontal line to indicate the 50% point and the picture is complete. (Figure 3) This picture provides all the information required to establish correct SCH Phase. Even if the scope of this device were limited to just this one display, it can be seen that it is a very valuable tool in certifying the SCH Phase condition of a video source. But, if a second vertical line and sine wave is added to the display so that a second video can be phase compared to the first, a truly usable system oriented tool is presented.
CIRCUIT DESCRIPTION Referring to the block diagram, the PhaseIcon is made up of many functions that process, regenerate, compare, measure, rasterize, and finally, line drive the resultant video display signal into the video system. (Figure 5) Of course, the unit is more complex than this drawing might indicate, but a general idea of the sequence of processing can be seen. The two identical video processors precisely strip out the sync and burst information from both the reference and compare video. They also buffer the video for the active bypass mode of operation. The processors are followed by a burst lock subcarrier regenerator for each video. However, the reference video also drives a gen-locked pulse regenerator. This is used in constructing the composite video output. In addition, the regenerated horizontal information is used in the sync comparator block. The real heart of the PhaseIcon is the "A Subcarrier Lock" Circuit. In this circuit a subcarrier is generated that is directly related to the leading edge of sync without regard to burst. Its phase is made to absolutely zero cross with the leading edge of incoming sync. The three subcarriers are compared and two rasterized sine waves are generated that reflect the exact phase relationship between the leading edge of sync and burst. The sine wave derived from reference video is used to certify the "SC/H" condition of that video. The second sine wave is used to match the burst phase of the compare video to that of reference video. In the sync phase comparator circuit, a moving vertical line is generated to indicate the sync timing relation between the compare and reference video. The two sine waves, the sync timing line, a horizontal and vertical cross hair, and regenerated sync and blanking are mixed to form a composite video signal that allows a standard video monitor to display all of this information in a simple and useful format.
OPERATION In operating the PhaseIcon, the initial step is to select one of the video sources in the system as the system reference. The PhaseIcon is then used to certify that this source meets the RS-170A specifications. This is displayed as a vertical and horizontal line with a sine wave showing the relationship between sync and burst. If it is not correct, that source is adjusted either at the encoder or its sync source until it does meet RS-170A specifications. (Figure 7) This source then becomes the "A" reference that all other sources are compared to. The second step then is to verify the timing of the system. Normally the reference video is taken from the "preview out" of the switcher, while the compare video is taken from the "program out". In practice, the compare operation presents two sine waves that null into a single waveform when correctly phased. The horizontal information is also presented in two vertical lines, representing reference H and compare H. A properly timed source video signal is correct when the two vertical lines are nulled into a single vertical line. When the source timing is out by a large amount the H timing lines will be on the left side of the picture monitor. See Figure 8. This type of signal is familiar to most operators and is a very good indicator of match even when viewed from moderate distances. The sources are then presented one by one to the compare input of the PhaseIcon and the timing of each is checked and corrected as required. The horizontal timing is then adjusted at the source for a match of the vertical lines and the burst phase is adjusted for a match of the sine waves. The operational controls are simple and straight forward. (Figure 9) In one mode only the "SC/H" condition of the reference video is displayed. This is the certification mode. In the second mode, a second video source is displayed so that its timing can be compared to the first or reference video. This mode is used to verify the timing of all the video sources in a system by comparison to a certified source. In addition there are no calibration controls. Within five minutes of power on, the PhaseIcon is ready for operation. The PhaseIcon will maintain a high degree of accuracy in its day-to-day operation. See Figure 10 that shows the Reference waveform as a properly adjusted master house black burst signal for 0 E subcarrier phase.In the compare mode, the two vertical lines representing H timing will be nulled when properly timed. The PhaseIcon will display the single vertical on the right side of the monitor when the source video is accurately timed. If the compare video is out of timing by a great amount there will be two vertical lines on the left side of the display. The PhaseIcon recognizes when the source video is adjusted and approaches 200 nS, the two vertical lines will appear on the right side of the display when accurately timed. The PhaseIcon provides two video outputs. The outputs are switched between the Reference black burst and the Compare input signals. This "bypass" mode allows the picture monitor to be used for both its normal service and the added function of "SC/H" certifying or system timing. Although the primary use of the PhaseIcon is directed toward certifying and timing of RS-170A systems, it is very important to remember that it can give equally accurate results in timing non-A systems. When used in the non-A system, the certifying mode is of marginal value since the sync to burst phase relation is not maintained. However, the source to source timing relation can be monitored and accurately set in the compare mode, figure (Figure 12). The reference and compare burst amplitude can be adjusted to proper level using the "Shadow" mode of the PhaseIcon.
CONCLUSION The PhaseIcon is a precise method for certifying and timing, but in practice other attributes such as ease of operation and readability are also of prime importance. But perhaps the most important attribute that any device designed to cope with RS-170A is: understandability. We feel that a visual display that looks like a cycle of subcarrier zero-crossing a line that looks like the leading edge of sync will tend to dispel some of the mystic surrounding RS-170A and perhaps move the industry toward its acceptance quicker than flashing lights or moving meters. We feel that the least experienced operator can look at the PhaseIcon presentation and relate it to what is actually happening in his system. We know that the PhaseIcon provides an accurate method of certifying and timing, but in the long run, we strongly suggest that perhaps, just perhaps, if the operator has a better understanding of what he is doing, he will do a better and more accurate job. |