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Description
MIDI playback - especially with many simultaneous notes - stresses your coil and with an interrupter like this one it is very easy to to exceed the safe operating area (and the meaningful operating area, too). The individual limits are explained below.
Each line in the table above contains the limits of one coil. You'll only see lines for the number of coils your firmware supports (depending on what firmware you flashed lashing). Each field of the table can be selected by a simple touch, and modified with the keypad. Touching the name or password fields opens a fullsize keyboard.
The maximum ontime simply specifies the longest continuous pulse width that the interrupter will generate on this output. Mostly useful for pulse-skipping coils or coils that won't trip the OCD at a too high ontime.
The risk of a too high ontime would be to damage your IGBTs because they heat up too far without an offtime to dissipate it. If your bus capacitance is too small, an excessive ontime could also trip the overcurrent protection of your breaker.
For each output and all the voices (from all modes) that currently play on it, Syntherrupter calculates the total duty. Calculating means that Syntherrupter sums up the product of the current frequency and ontime of every voice. Every time one of those parameter changes (f.ex. because of envelopes) the sum gets updated. If the total duty exceeds the maximum duty set here, all ontimes get multiplied by the same factor (notably max-duty / total-duty), such that the total duty equals the limit.
The purpose/benefit of a duty cycle limitation is to limit the RMS current - both in your primary circuit and what you draw from the grid. The first one keeps the temperatures safe (given a high enough RMS current, every circuit will overheat), while the second one is important for your fuse box.
This method has one major drawback. If multiple voices play together, their ontimes are sometimes closer together, sometimes further away. Thus multiple ontimes can and will sit together, causing a momentary duty cycle spike (<1ms). More about this below under Min Offtime. Forcibly there must be a duty minimum between those spikes and on average the duty won't exceed the given limit. These grouped ontimes shouldn't be an issue in most cases but their risk is similar to the one of too high ontimes.
Important If you plan on setting a minimum ontime, must know that it will be added after the duty limiter. With many ontimes (high number of voices and/or high frequencies) this can and will rise the actual output signal's duty cycle beyond the value specified here. Details below.
As detailed above, ontimes can and will fall closer together because all the voices will have a totally random phase. This will also lead to ontimes falling one right after the other. Here's the issue: pretty much all drivers turn off at zero crossing but they don't turn on with respect to any (residual) primary current because it's expected to be 0. Some drivers even have startup oscillators that switch a couple times at a fixed frequency without respecting any feedback. Normally these are useful features but in this case it's a danger. If two ontimes are too close together, the second one starts before your primary current was able to ring down. Thus you risk hard switching with significant amounts of primary current and in consequence the destruction of your bridge.
The solution is to introduce a minimum offtime after each ontime. This value has a strong impact on the sound quality (try it out; too big values won't do any other damage), therefore it should be chosen as small as possible. The current doesn't need to ring down completely; after all the bricks were originally manufactured for some amount of hardswitching. If you have really no idea what value you should enter here, take your scope and look how long the primary current needs to get to 0 (I suggest you do this using Simple Mode. That value will for sure be enough. Next step could be to slowly reduce it while montioring bridge current and voltage. This obviously needs to be done in MIDI Live Mode with more than one active voice. I suggest you set up your scope to trigger on offtimes smaller than X such that you'll only see the interesting cases. Stop when you see excessive voltage spikes or you notice a temperature rise of your bricks.
Syntherrupter is able to handle a crazy amount of simultaneous voices (well over 30 - more than you can play in a meaningful way on 6 coils). Since MIDI Live Mode is the only polyphonic mode (= the only mode contributing more than 1 voice), it makes sence to limit how many voices it'll generate. This limit can be adjusted from 1 to 16 for each coil.
MIDI Live Mode itself has a global list of currently active notes - independently of what your coil and channel settings are. This list can take up to 64 notes. Then, based on your channel and stereo settings, some of these notes will be forwarded to each coil's output - starting with the most recent note and up to the limit specified here. This allows you f.ex. to play with a sustain pedal that'll quickly raise the number of simultaneous notes beyond this limit. Since internally the notes are still tracked, you can lift the pedal and still hear the notes that were kept pressed. There are other useful cases, too, if this one is not relatable.
This limit is not directly safety relevant but it still is a very useful limitation. Depending on your coil (high/low impedance, big/smal, pulse-skipping, ...) a different number of voices are acceptable and more voices would rather decrease than increase the sound quality. Once again something to experiment with - if you find a MIDI file with that many voices (<= guess you could call that first-interrupter-world-problems).
This parameter cannot be modified through the UI but only via Sysex Commands. It is however stored in EEPROM just like any other command (either by using the EEPROM Update Sysex command or by hitting Settings->Save.
Most coils - especially those that operate at the lower pole - require a minimum amount of primary current before any output is generated. The relation between ontime and output (volume) is thus not linear but has an offset. This has a very strong impact on the audio quality; low volume notes (fade in / fade out) won't generate any output, thus not be audible. Fadings sound weird, too, because parts of them miss.
All of Syntherrupters effects are based on a linear relationship. To "fix" this, a offset ontime will be added to every ontime of every mode. A fade in of lets say 0us to 20us would generate an ouput signal of 30us to 50us (with min ontime set to 30us).
This minimum/offset ontime is added after applying the duty limitation for technical reasons, but before the ontime limitation. It doesn't affect the minimum offtime either. The obvious consequence is that the minimum ontime causes the output signal to raise beyond the duty limit when (or even though) the duty cycle limiter is active. However, the minimum ontime usually covers only a small part of your primary current ring up. Thus the stress on your parts is lower than with a "normal" or "full" ontime. Another consequence of this design choice is that even if the duty limiter is active, the sound quality is not affected; no effect will suffer from dropping below the minimum ontime.
All of the settings above can be controlled by the Coil Settings group of the sysex commands.