Why is the gamut monitoring on the WFM700 series different from the other Tektronix digital waveform monitors?
All Tektronix gamut alarms have horizontal filters to prevent gamut alarms on brief violations of the gamut limits. This is necessary since almost all signals have small overshoots in RBG space due to the unequal rise-time of the Y and PbPr signals.
The WFM700 also has an optional vertical filter for the gamut error detection displays, the rest of Tektronix's instruments including the WFM601 series,WVR600 series, WVR6100, and WVR7100 do not.
The effect of the WFM700 vertical filter is to make the gamut alarm less sensitive to gamut errors that only occur on a few lines. For example, if only line is out of gamut, then it will have to be significantly past the threshold before it will trip an alarm. Without the vertical filter then any excursion past the limit will generate an alarm.
This filter was implemented in order to reduce the incidence of gamut alarms which users consider unnecessary. In practice there seem to be relatively few signals that have gamut alarms on only one line so this feature has not been carried forward into newer instruments.
However it is possible that the newer boxes will give different results on the gamut alarm detection than the WFM700, if the WFM700 has the vertical filter enabled.
The WFM700 and newer boxes also have the ability to only alarm if a significant percentage of the screen is out of gamut. For example if the area threshold is set to 1% then the gamut alarm will not trigger until 1% or more of the active area is out of gamut.
Using this area threshold at even 1% will have much the same effect as the vertical filter, in that if only a few lines are out of gamut then the instrument will not generate an alarm.
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