We will discuss:

• Traditional signal generation.
• Pulsed signals.
• Traditional modulation by mixing multiple carrier signals.
• Various signal generation ideas.

Traditional signal generation

Traditionally, e-stim signals are just sine waves, the frequency is called the carrier frequency.

These signals are not efficient, they stimulate the nerves are much higher rates than the nerves can physically generate action-potentials. As a result, energy efficiency is low.

Pulsed signals

One improvement is to modulate the signal, this reduces the signal energy but keeps the nerve activation almost the same. In some cases this may even increase the nerve activation because a constant signal can cause neuronal inhibition and/or rapid energy depletion.

(Signal generated by Restim, 500hz carrier, 50hz amplitude modulation, 0.50 low bias)

Most commercial devices use square pulses.

How fast should we fire these bursts? When firing slow bursts of 1-10hz most nerves will have recovered every time we send a burst, this results in spiky nerve activation and will feel painful. If we fire a bit faster at 50hz, not all nerves will have recovered but this makes the firing pattern less spiky, the signal will feel smoother. Image below gives experimental data.

https://www.frontiersin.org/articles/10.3389/fnins.2018.00783/full

If we fire too fast, the electricity starts to mess with the recovery process of the neurons and this results in a decrease in neuron firing rate. We believe the firing rate that results in the maximum nerve activation is slightly below 100hz. Therefore, frequencies below 100hz are preferred.

In my testing, pulse frequencies around 10hz feel really rough and not pleasurable. 50hz feels very smooth, 100hz feels a bit smoother, but not much. Some individuals prefer a slightly rough signal in the 30-40hz range.

Restim offers various ways to control the modulation shape. This needs additional research.

It works different for muscles, also needs research.

Additional reading: https://gitlab.com/mantrid0/mpgenerator/-/blob/master/Readme.md#the-theory

Multiple carrier signals

Some software uses multiple carrier signals, such as 420Hz + 520Hz + 620Hz. This is mathematically the same as amplitude modulation, just more confusing. See product-to-sum formula on wikipedia.

Various signal generation ideas

Varying pulse rate

Rather than stimulating with a constant 100hz pulse rate, some academics are running experiments where they vary the inter-pulse-interval (IPI). One approach is to randomize the IPI between 5-10ms. This seems to have the effect of slowing numbing significantly. In my testing, such signal causes a slightly tingly sensation.
https://www.frontiersin.org/articles/10.3389/fnins.2020.00397/full
https://www.mdpi.com/2076-3425/11/4/509

A different approach would be to stimulate with a constant 100hz pulse rate, but randomly omit a pulse, or insert one extra pulse.
https://iopscience.iop.org/article/10.1088/1741-2552/acb013/meta

More testing is needed, but the following effects of random IPI have been mentioned:

• Slightly more tingly sensation
• Less painful when suddenly increasing the volume or with fast movements
• Feels slightly more natural in long sessions
• Slows down numbing

Varying intensity

Instead of randomly varying the IPI, we can also randomly vary the amplitude of individual pulses. Needs research.