>One thing I think might have been touched upon is that for various reasons,
>maybe your particular encoder simply can't handle going beyond a certain
>speed.  I had an encoder that could drive LEDs in that manner.  One thing I
>found when measuring the maximum speed my PIC can count is that it maxed
>out closely to the specified maximum output frequency of my encoder's data
>sheet, so the either the pickup or signalling drive electronics might just
>be hitting the wall, or so to speak.

It is most likely the rise and fall times of the phototransistor itself that
sets the maximum speed. These are very sensitive to the way the transistor
or diode is biased, and to the type of transistor or diode used.

Get an ordinary 4n26 or similar opto and try various circuit arrangements
around the transistor and you will see what I mean.

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