And modern alternatives (for example c2t, which is for Apple II) can achieve even much higher data rates. I prototyped an encoding that packs 4 bits per pulse, achieving 23kbps but not reliably. One difference, though, is that this is designed for output from a modern digital sound interface, not designed to withstand degradation from analog tape recorders.

https://groups.google.com/forum/#!topic/comp.sys.apple2/qXZy...