Micro:bit limits of tone generation

I played a sliding tone, but found because of the clock divider logic that as the frequency went up the resolution of the output frequencies turns into steps.

Here is the output of an iPad spectrum analyzer on a program that just increments frequency by 10 hz up to 30khz to see if I could use the microbit for ultrasonic communications.

The multiple lines come from the fact that its outputting square waves that have odd harmonics.