threephase effects - diglet48/restim GitHub Wiki
When operating a device with a single channel, current flows directly between two electrodes. The intensity ratio between two electrode can only be controlled by making electrodes smaller or larger. The sensation is usually unbalanced, one of two electrodes on the channel dominates the sensation.
When adding a third electrode, we create what is called a three-phase configuration. Careful signal generation allows for adjusting the intensity ratio between electrode without adjusting their size.
The definition of three-phase has led to much confusion. While all devices with two channels can be wired in three-phase, and thus could be considered three-phase capable. Some devices can only generate a small subset of available sensations. Those can, in my opinion, not be called true three-phase.
Below is a series of signals that demonstrate fading in/out a single electrode. The numbers are best understood as the amplitude of the waveform at the specific electrode. For simplicity, we ignore path-dependencies.
Below are some signals that demonstrate focussing a particular electrode.
Finally, some signals that demonstrate smooth rotation. A reasonable approximation can be made interpolating rows 1 to 6, but it doesn't feel quite the same as 'analogue' rotation.
The coyote is only capable of generating row 1 and 2.
The 312 family generates variations of row 1 and 2 in phase1 mode. Audio 3 mode is capable of generating rows 1, 2, 4 and 7. Audio 3 mode combined with serial commands can generate all rows source.
The NeoDK (4-triac version) in three-phase mode is capable of generating rows up to 5.
Stereostim and FOC devices are capable of generating all 9 rows.
Skin resistance and nerve sensitivity also plays a role. The coyote and NeoDK can only generate their rows fully with very specific combinations of skin resistance and nerve sensitivity. Stereostim, FOC and 312 devices can generate their rows under most conditions.
Four-phase
The concept can be extended to four electrodes, demonstrated by NeoStim family and FOC-Stim V4. This greatly expands the range of sensations. The FOC-Stim V4 is capable of generating all possible current paths between 4 electrodes.
Pulse-based vs sinusoidal
It is a mathematical fact that some of the sensations can only be generated in one shot with sinusoidal signals. A pulse-based device has to combine multiple pulses to approximate the desired nerve activation.
Because current generations of pulse-based device insert significant deadtime between pulses, reproduction on that hardware is only approximately possible. How approximate exactly, is an open question.