Before I start coding I'll need to come up with some audio samples so I can create a set of tones to use as data symbols. I'd like to make it sound robotic and synthesizer tools are a good way to test what works.

I've been looking at https://onlinesequencer.net/ (no affiliation) mainly because it's online and I can save the output. 

So far I've recognised that high-ish frequencies sounds good, and that using a musical key makes any combinations sound too... musical.

Example audio so far http://onlinesequencer.net/763546