In Sporth, the "mtof" unit-generator does a MIDI to frequency conversion for every audio sample, thus making it audio rate.
Perhaps it is better to think of control-rate signals as input signals rather than output signals. The "osc~" object, for instance, will update the frequency at every audio block. This would be the control rate. In Sporth, oscillator frequency values are updated every sample inside the audio block.
> But that's not what "mtof" does. Instead, it computes the frequency for an inputted MIDI value at the time it receives that value. That time could be once every block, once a minute, a single time when I load the program, at random intervals on Tuesday, or even never.
The important distinction here is that the resolution can't be smaller than the audio-block size, which in turn defines the control-rate.