Here is a case where an RPM math channel was applied to a one degree optical CMP a few years ago.
http://www.autonerdz.com/cgi/yabb2/YaBB ... 08802930/7
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This technique, of course, works best with higher resolution signals.
In this case, it was very helpful to easily see if signal degradation was the cause or the effect of a stalling situation as it illustrated where the RPM fell.