cattledog:
I don't confirm what you are seeing. I changed the millis() timer to micros() and set the measurement period = 1000. With tone(11,50000) I am seeing counts of 49 or 50 with a period from 1000 to 1012 us.For more precise high speed counting, you want to have the period defined by a hardware timer an not a software one using millis() or micros(). See Nick Gammon's Frequency Timer sketch at Gammon Forum : Electronics : Microprocessors : Timers and counters
Thanks very much for your help and time CD! And for letting me know that you got counts of 49 or 50!! That sounds promising for me. I'll checking everything again over here, and will let you know what happened on my side. Also - thanks for passing on that link! I'll definitely be learning a lot from it, and also from what you mentioned. Really appreciated. I'll let you know what I did incorrectly later --- about getting the higher counts instead of around 50! Thanks again!