OK, so I’ve found out. I built a simple square wave generator using a NE555 timer chip. The frequency was about 8.5khz ( a continuous stream of 0s in DCC terms would be 8.62khz I believe). This drove an H bridge motor driver which in turn powered a rolling road. Using my Mallard locomotive equipped with a HM7000 I observed the following:- 1/ the loco was under full Bluetooth control and the square wave power had no effect on this control. 2/ using the standard 5 diode network to generate an asymmetrical square wave similar to that in the manual, the loco behaved as it should dependant on the relevant cv settings as described in the manual. So it is possible to use ABC braking without a true DCC signal being present on the track. A simple square wave can be used which saves all the complexity of a DCC controller. Perhaps someone else might like to repeat the experiment to confirm my findings, but personally I’m happy with the results.