[mark_in_manc]

I think I was concerned as not only the pulse rate, but the mark-space ratio will be changing, since the 'mark' length is fixed and the 'space' will just last until the next trigger event, which depends on the period of the signal coming out of the transducer. That doesn't sound like a recipe for a linear relationship between frequency and average output voltage to me. Well, we'll see when the bits arrive - maybe I'll have to go for really short 'marks' and then amplify the output to make it more linear.