What waveforms to start with when making stereo content for the audio input?

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wetotter
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What waveforms to start with when making stereo content for the audio input?

Post by wetotter »

I'm interested in making some "audio" files to use with the 2B, and I'm curious about where to start. What kind of wave shapes should I be starting with? Sawtooth, sine, square, something else? What frequency range is effective? Should I be using a continuous wave or short pulses? I'd like to make sure I'm putting energy in the right frequency bands to get good sensations, and not just hitting the current limit with useless current that the user can't feel. It's quite hard to find good solid information about this so I thought I'd tap the experts here.


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Stimaddict
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Re: What waveforms to start with when making stereo content for the audio input?

Post by Stimaddict »

Not an 'expert' by any means but as a rule of thumb sine waves tend to feel smooth while sawtooth can feel like it sounds, sharp and cutting.
I'd suggest you experiment with a Tone Generator like this one: https://www.nch.com.au/tonegen/index.html
Play around with it to get a feel of what's what.

The 2B is a digital device with an internal waveform generator that processes audio input to produce an intelligently interpreted output generated as a Pulse Width Modulation (PWM). The advantages are a generally smoother, safe output with less sudden shocks and surprises. Disadavantage is that some very high or low frequencies in certain stimfiles will not be felt. However, uncomfortable spikes usually are high frequency and most people do not like very low frequencies. The 2B's frequency range is generally between 50Hz and 5-6Khz, although it tails off towards the higher frequencies. In reality very high frequencies cannot be felt even using an analogue StereoStim device, but they can still potentially cause damage which is why the 2B ignores them.

It's not as simple as 'liking' this or that frequency but I'm probably happiest in the low to mid-ranges. Continuous stimulation doesn't really do it for me. I prefer a moving range of frequencies in a stimfile to produce a mixture of waveforms and their consequent sensations. For inspiration you should take a look at other stimfiles out there to see which ones float your boat, and how the author has created them.




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wetotter
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Re: What waveforms to start with when making stereo content for the audio input?

Post by wetotter »

OK, so a quick update. What I've actually been doing is using DSP to turn a normal audio signal into something more suitable for the 2B. What I've got so far is two signal paths, one for each channel. The first path runs the audio into a high-pass filter at about 600 Hz to cut the bass, then an envelope follower to find its amplitude. The 20 s moving average of the envelope is subtracted from the instantaneous value, to give me a signal that's on when the audio gets louder but sags if the audio stays the same level or gets quieter. This then controls the level of a simple FM waveform with the carrier about 90 Hz and the modulator less than half that. The idea of this channel is to give a mild, continual buzz that vaguely follows the audio.

The other channel is intended to give bigger zaps for boomy audio like kicks. I run the same source audio through a high-Q band-pass filter around 120 Hz, and then it gets split. It goes into a note onset detector (which should have a similar effect to the envelope follower, but more responsive to sharp peaks), the output of which triggers an AD envelope with a short attack and a decay of a few seconds. There's no synthesis in this one: the envelope just controls how much of that band-passed signal gets sent to the 2B directly.

I'm very happy with that second signal path but the first needs more work: I've tried a few electrode placements and across the board it's too sharp, when I wanted it to be more warm and comfortable.

I've just remembered that last year I fed the 2B's output into an oscilloscope to look at its waveforms. I should refer to the notes I took then, but I'll have to find them first...
hurtmeplease
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Re: What waveforms to start with when making stereo content for the audio input?

Post by hurtmeplease »

Resurrecting an old thread, I know...

I think the idea to run some DSP filtering is really clever. Can I ask what SW you were using to do this?

If the generic audio files use high frequency for pain, I wonder if you have thought about a path that merges in a high-pass > pitch-shift -> volume boost filter. My thinking I that it would pull the high frequencies down to the point where the 2B could detect them, although maybe adding in an echo or decay or something would be needed to make the signal long enough to be felt
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