@Jim; fantastic. Thanks. The code is a copy of some other code posted in this forum and - frankly - we didn't quite understand it all, but it's much clearer now (relatively old hands at Arduino, but new to the Due)...
So to make sure we understand it correctly, the frequency corresponds with the calling frequency of the routine (uint32_t frequency) delivering a 20Khz timed interrupt, but the digitalWrite(13, I = !I) flips the pin 13 logic value causing a 50us high, followed by a 50us low - creating a 10Khz square wave. That makes perfect sense.