Author Topic: compact midi interrupter  (Read 1548 times)

Offline TMaxElectronics

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Re: compact midi interrupter
« Reply #20 on: June 20, 2020, 09:52:14 PM »
Quote
Ok I see I should have spent more time with your code before commenting it.
No problem! I do admit that my coding style is very confusing ;D , especially when I program c and java more or less at the same time :D

Quote
For my interrupter its no problem to use an uncommon connector since I don't sell them.
I don't really plan to make a product out of them either (unless of course somebody wants lots of them :P ), but since i have some more boards and uControllers laying around and some people were interested in trying one out, I was just wondering if they would even be able to use it with their connectors. Should be fine though, since the range of transmitters has most common connectors available (except the no connector one).

And i have now got four voices working from just the timers :D
I'll now need to limit how close together the pulses are, i think i will use a holdoff time. should i make this variable or not? I guess it kind of depends on the coil, but maybe not enough to require adjustment...
« Last Edit: June 20, 2020, 11:36:08 PM by TmaxElectronics »

Offline Max

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Re: compact midi interrupter
« Reply #21 on: June 21, 2020, 09:14:32 AM »
Quote from: TmaxElectronics
And i have now got four voices working from just the timers :D
I'll now need to limit how close together the pulses are, i think i will use a holdoff time. should i make this variable or not? I guess it kind of depends on the coil, but maybe not enough to require adjustment...
Congrats! 
It does affect the sound quality so you want to keep it as low as possible. I guess the values range from ~200us for a low frequency high impedance coil to only ~30us for a high frequency low impedance coil. Having 200us fixed instead of 30us will be audible (at least i heard a difference...).
Long story short: I‘d make it adjustable :)

Kind regards,
Max

Offline TMaxElectronics

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Re: compact midi interrupter
« Reply #22 on: June 21, 2020, 01:10:52 PM »
Quote
Long story short: I‘d make it adjustable
Will do! ;)

The other thing I'm not sure about is how to limit the dutycycle with multiple notes playing. I was thinking of a counter to check for how long the notes were on in a given time period, that then either disables the output if it was too much or scales it back until, at the limit, it is off all the time. But the graceful one sounds really bad, as you can hear the base frequency of the background limit counter resetting :(.
The hard limit also sounds quite bad (though is not as noticeable), but only if the limit is reached (which happens quickly with four voices :P).
So is this actually a problem? Or should I instead limit by not playing a note if it pushes the on time over the limit, as that would make the music not sound bad, but instead have a part missing from it.
It is of course fairly simple to "fix" a song that triggers the limiter, by just adding a note offset to the programm (in the control software).
« Last Edit: June 21, 2020, 01:12:23 PM by TmaxElectronics »

Offline Max

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Re: compact midi interrupter
« Reply #23 on: June 21, 2020, 02:42:48 PM »
Although it's actually pretty easy netzpfuscher had to give me a push, too. Here is my code (line 512 and 566-579): https://github.com/MMMZZZZ/Syntherrupter/blob/9d442716ccf55b9ead32166d53c178af5c0f897b/Tiva_Interrupter_Source/MIDI.cpp#L566

Basically what you do is calculate the sum of the duties of all notes (current ontime multiplied by current frequency) and check if this exceeds your limit. If this is the case you reduce all ontimes by the same factor so that the sum does no more exceed your limit.

Now this does not prevent the very short "duty spikes" when the ontimes of all notes fall closely together (all notes "in phase"). Those spikes should be shorter than 1ms in 99% of the cases (estimated 4x 200us ontime + 50us minimum offtime. And 200us ontime is a lot for 4 voices). For the fuses and the thermal limits of the setup this works just fine.
If a coil cannot handle a few ontimes quickly after each other, I think the best way to go is to simply increase the minimum offtime or reduce the primary current to increase the margins.

This duty cycle limiting is noticeable by a drop in the volume. Listen to the following video of my interrupter and compare 0:40 with 1:36. If I'm not mistaken, the latter one has a higher pitch, causing the duty cycle limit to reduce all ontimes. At least it sounds exactly like that.

Edit: It is actually not that easy at all to know in advance if a MIDI file is going to exceed the duty limit or not. Lets just say I tested it pretty hard and I'm glad I have it. ;D


Kind regards,
Max
« Last Edit: June 21, 2020, 02:48:09 PM by Max »

Offline TMaxElectronics

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Re: compact midi interrupter
« Reply #24 on: June 21, 2020, 04:32:17 PM »
That sounds cool, i can't wait to get my coil working and test with it ;D

Quote
Basically what you do is calculate the sum of the duties of all notes (current ontime multiplied by current frequency) and check if this exceeds your limit.
Good idea. But i think i might have come up with a better one:
Have the controller keep track of how long the output was on and how long it was off, then use those two values to count a value up and down, which then scales the on time. This would also be able to limit multiple pulses very close to each other.
Luckily the pic32 architecture has a core timer, so no need to remove a voice :P
It does work (i have just tested it), so i will continue with this for now and only if testing shows that it does not work reliably, switch to the other method.

EDIT: i think i should explain myself a bit better (i just read my own text and it was hard to understand :o )
Basically what i want the controller to do is to integrate the output waveform, and then scale back the volume depending in the area underneath it. From this I can limit two parameters:
    the "duty cycle", by scaling the rate at which the accumulated on time is reduced when the note is off and
    the maximum cumulative on time, by reducing the on-time the closer we get to the maximum (this would be yet another setting in the software)

So the question is, would this system actually work well enough for a tesla coil...
« Last Edit: June 21, 2020, 05:21:11 PM by TmaxElectronics »

Offline Max

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Re: compact midi interrupter
« Reply #25 on: June 21, 2020, 10:34:04 PM »
Honestly, I can't really see the benefit...

First of all, how long would you choose your integration time? 1ms? 10ms? 100ms? If you chose it too small, you will effectively filter out low notes (example below). If you chose it too high, you don't have real benefits over the method Netzpfuscher and I use. Especially if you chose it low enough to filter burst ontimes you're effectively not limiting your overall duty, but introducing a ontime limit and a minimum offtime.
Additionally it costs quite a lot of CPU time. It has to continually track all timers and calculate the average instead of doing it once when a MIDI command comes in.

Here's an example from "Gimme Gimme Gimme" (which I posted as video in my topic, too): Maximum allowed duty is 10%. MIDI note 45 and 88 play together. Ontime is lets say 50us for both. Gives the following values:
MIDI Note | Frequency [Hz] | Period [us] | Ontime [us] | Duty [%]
451109090500.6
881319758506.6

Total duty is 7.2% which is well below the given limit of 10%. How would you chose your averaging times in this case?
  • Lets say 500us. This means each time a note fires you have reached your duty limit for these 500us, effectively introducing a minimum offtime of 450us. This will likely ruin your sound quality for the given example - especially for the lower note.
  • Lets say 750us. Now you have the problem that one ontime does not exceed your limit. Thus a second ontime can follow as fast as the minimum offtime allows. Bursts are possible again.
  • The further you increase the averaging period the more your solution becomes in its behavior similar to Netzpfuschers and my solution. For some values you might get a lower frequency oscillation in volume because of aliasing.
Kind regards,
Max
« Last Edit: June 21, 2020, 10:35:58 PM by Max »

Offline TMaxElectronics

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Re: compact midi interrupter
« Reply #26 on: June 21, 2020, 11:08:07 PM »
Quote
First of all, how long would you choose your integration time?
its not a time frame based integration, instead it is constantly running.
The way I implemented it is also not very cpu intensive; Basically the timer interrupt that turns off the output also saves the value of the CP0 timer (running at 24MHz, not adjustable) to a variable. Then when the output gets turned on again, it subtracts the time since switch off from the accumulator:
Code: [Select]
accumulatedOnTime -= ((_CP0_GET_COUNT() - noteOffTime) * Midi_currCoil->maxDuty) / 3200;    //the 3200 is the factor by which the CP0 counts faster that TMR1 multiplied with the % division from the max duty cycle variableThe maximum duty cycle impacts how fast the counter counts down.
afterwards it adds the on-time for the next note, which is scaled by the accumulated on time (if it is larger than half of the continuous on time limit).

Quote
...you don't have real benefits over the method Netzpfuscher and I use.
That is one thing I was wondering about too. The only benefit i see is that it will limit the duty cycle, even if an effect changes it, and it has the ability to scale pulses coming very close to each other. If this is necessary or even worth it is another question.
Lets say one note has a sustain power level of 30%, if you use the 100% level to calculate your duty cycle, you potentially scale back further than needed. (but i have no idea if that is actually an issue)

I think i will leave it in, as it does work and does not cause any audible effects, except when it limits, which only makes the sound quieter.
And sorry if my explanation of this is still bad, I don't really know how to explain it any better :(

Offline Netzpfuscher

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Re: compact midi interrupter
« Reply #27 on: June 21, 2020, 11:27:35 PM »
In my implementation the sustain power level goes into the calculation.

https://github.com/Netzpfuscher/UD3/blob/master/common/ud3core/tasks/tsk_midi.c#L766

And the pitchbend results also in a recalculation. The other ADSR phases are not calculated.

On the UD3 the Volume doesn't modulate the pulsewidth. The pulsewidth is fixed until the duty cycle limiter kicks in. The volume modulation is done with the primary current (pulse skipping) every single pulse which goes to the inverter contains a pulsewidth and a volume. If two or more notes are played the OCD changed for every pulse between the min and max current range.

I have seen a few float calculations in your code. Does your micro has a FPU? I haven't seen that by a quick look. If it has no FPU you should change your calculations to fixed Point.


Offline TMaxElectronics

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Re: compact midi interrupter
« Reply #28 on: June 21, 2020, 11:41:37 PM »
Quote
Does your micro has a FPU?
No it does not, but really the only drawback to using FP is the speed isn't it? So far I have had no issues with that, but i will change it once some show up :)

Quote
On the UD3 the Volume doesn't modulate the pulsewidth.
I'm afraid my little drsstc controller is waaaay behind yours in terms of features, so duty cycle control will have to do :P

Offline Max

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Re: compact midi interrupter
« Reply #29 on: June 22, 2020, 01:49:42 PM »
Quote
First of all, how long would you choose your integration time?
its not a time frame based integration, instead it is constantly running.
The way I implemented it is also not very cpu intensive; Basically the timer interrupt that turns off the output also saves the value of the CP0 timer (running at 24MHz, not adjustable) to a variable. Then when the output gets turned on again, it subtracts the time since switch off from the accumulator:
Code: [Select]
accumulatedOnTime -= ((_CP0_GET_COUNT() - noteOffTime) * Midi_currCoil->maxDuty) / 3200;    //the 3200 is the factor by which the CP0 counts faster that TMR1 multiplied with the % division from the max duty cycle variableThe maximum duty cycle impacts how fast the counter counts down.
afterwards it adds the on-time for the next note, which is scaled by the accumulated on time (if it is larger than half of the continuous on time limit).

Quote
...you don't have real benefits over the method Netzpfuscher and I use.
That is one thing I was wondering about too. The only benefit i see is that it will limit the duty cycle, even if an effect changes it, and it has the ability to scale pulses coming very close to each other. If this is necessary or even worth it is another question.
Lets say one note has a sustain power level of 30%, if you use the 100% level to calculate your duty cycle, you potentially scale back further than needed. (but i have no idea if that is actually an issue)

I think i will leave it in, as it does work and does not cause any audible effects, except when it limits, which only makes the sound quieter.
And sorry if my explanation of this is still bad, I don't really know how to explain it any better :(
Okay it seems to cost less CPU time than I thought; probably not more than what I did. I didn't mean that you divide your time in discrete chunks; more the idea of a moving average. And that's how it still looks to me.

What I do in my code is that after all effects (pitch bend, modulation, volume, ADSR, etc) have been applied - at the moment where I know exactly which ontimes and periods will appear on the outputs - only then I calculate the duty of the active notes and adjust the ontimes if needed. This is the final step before outputting the signals.

To take your example: If 8 notes attack at the same time, causing a duty spike, the limiter will only be active during the spike - or to be more precise: during the part of the spike which exceeds the limit. Instead of having a triangular Attack/Decay shape, it will be a trapezoid. The sustain and release shape won't be modified at all (assuming that the total duty falls below the threshold during decay. Otherwise the limiter wil of course stay active).


Kind regards,
Max

Offline TMaxElectronics

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Re: compact midi interrupter
« Reply #30 on: June 24, 2020, 05:48:05 PM »
I finally got my coil to make some sparks, and tested the Interrupter:
Videos are potato quality, because I only had my phone in my workshop at the time.

There are still some problems with the duty cycle limiting though. If too many notes are on at one time and the limiter kicks in, the pulses tend get so short, that they don't make sparks anymore. At that point it would have to decide between too short or missing Pulses...
That could be because of the limited primary voltage I was running at though (120V instead of 330V).

Offline Max

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Re: compact midi interrupter
« Reply #31 on: June 24, 2020, 07:09:28 PM »
Wohoo congrats!

Love the comments ;) iPhones have the possibility to lock exposure and focus with a long press to whatever part of the image you want. It will adjust for that part and keep those settings no matter what happens. I'd guess that Androids have similar functions?

The Ievan Polka sounds deeper than what I remembered? Is that a bug or a feature?  ;D
In the heart of courage video some notes seem to have quite .. expressive attacks? Interestingly only if they are not legate (played immediately after the previous one without audible pause inbetween). Couldn't hear something similar in the other video, which makes me wonder if it is part of the MIDI file.

Btw I'd love to try the same MIDI file on your and on my interrupter for comparison.


Kind regards,
Max

Offline TMaxElectronics

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Re: compact midi interrupter
« Reply #32 on: June 24, 2020, 07:59:15 PM »
Quote
iPhones have the possibility to lock exposure and focus with a long press to whatever part of the image you want.
Mine can do that too, but the touchscreen went a bit crazy and decided to unlock it again.
I have just made some more Videos with a better camera app, they are on my youtube channel.

Quote
The Ievan Polka sounds deeper than what I remembered? Is that a bug or a feature?  ;D
I might have been afraid of high notes and their power draw initially ::)  Its one octave lower than usual.
That's why I added the adjustable note offset to the Synth :P

Quote
In the heart of courage video some notes seem to have quite .. expressive attacks?
Idk, I did not have any effects on for that. Maybe an effect of the limited coil power?
Here's a Video with some ADSR enabled:

Maybe I should have used my minimum on time feature. Especially the fadeout in the end sounds weird...

« Last Edit: June 24, 2020, 08:50:41 PM by TMaxElectronics »

Offline Max

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Re: compact midi interrupter
« Reply #33 on: June 25, 2020, 12:07:03 AM »
A full octave - oh boy my ears are out of practice. Should have heard that.

Limited power is likely the cause. You powered the coil from a variac, right? I bet the bus voltage dropped by a significant amount! You could check that with a multimeter.

The fade out is tricky (except if you can, like Netzpfuscher, control the primary current). Here‘s why: by reducing the ontime, you increase the high frequency content of the note while decreasing the fundamental tone. Or in other words: your signal to noise ratio gets worse. And I don‘t think there‘s much you can do about it if you have a normal DRSSTC driver like the UD1.x or UD2.x.
If you set a minimum offtime you won‘t get as noisy sounds, but you won‘t get a complete fade out either. There might be a good compromise though. Has to be tested.
I have to say, after having heard lots of music on a piezo and some metal plates, the issue is not that bad in your video. Maybe it‘s because I‘m already used to it...?


Kind regards,
Max

Offline TMaxElectronics

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Re: compact midi interrupter
« Reply #34 on: June 25, 2020, 01:16:14 AM »
Quote
I bet the bus voltage dropped by a significant amount!
Very likely. especially since I am pushing the output from my variac slightly past 230V, so I can get a little more voltage out of my Isolation transformer.  Which is really just an old toroidal one from a power amp.

Quote
The fade out is tricky
certainly, but i think at some point it is just part of how a tesla coil sounds. If we just wanted to listen to music we would be using speakers after all :P

I was actually thinking about some ways to recreate instrument sounds (or for that matter any audio signal) with a tesla coil, either by using a fairly high base frequency and doing PWM modulation to get an "analog" output. But that would be damn near impossible at normal resonant frequencies.
For example using 20KHz carrier (is that actually the right word?) with a 100KHz coil frequency would only allow for 5 cycles of the primary in one interrupter cycle, so at least very poor resolution, if the coil even managed to output anything at all.
In addition there is the extremely non-linear relationship between interrupter pulse width and output audio amplitude. I did kind of get this to work with my first coil, but that had a Fres of almost 400KHz, and even that sounded bad.
So the other thing I was thinking of, would be to use an FFT on the audio signal and then output some of the peaks in the spectrum from the voices of the synth. No idea if that would work though, and it would probably also require an FPGA - definitely a project for after my exams  :o.
I have seen a post here some time ago about somebody using a saxophone to control a tesla coil, but i believe they used the zero crossing of the signal as the interrupter input.

Offline TMaxElectronics

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Re: compact midi interrupter
« Reply #35 on: July 12, 2020, 05:06:10 AM »
Ok so its been a while since the last update, but not because I stopped working on it :D
I didn't give any updates because nobody has the hardware to test potential updates at the moment anyway...

But much has happened since:
  • Multi channel support (before it only played a single one), with different parameters for ADSR for each one (kind of obvious honestly)
  • support for custom bendranges if the midi file supports it
  • the ability to switch the aux pin to an enable switch (also usable as an E-Stop)
  • LOTS of bug fixing, so the firmware is stable now (it wasn't 100% before :P)
  • added support for the sustain and damper pedal
  • added the option to select what the LEDs indicate (kind of a gimmick but why not :P)

So now all that is missing is a bootloader, and the ability to do a bulk write of all changed program presets (right now only one write is supported and its inefficient).
I'll also be soldering together another one of the boards soon so I can test with more than one simultaneously (maybe even including support for the pan command, as the competition already can :P)

I also have confidence in the Hardware now, and am ready to manufacture some, so if you wan't one just send me a PM, and I will order some extra parts with my next digikey order.
Unfortunately i won't be able to do it for free though, and it would cost 15€ WITHOUT the optical transmitter, which costs 20€ in addition to that (direct digikey price :/)). If you have some with a compatible pinout you could use yours: https://docs.broadcom.com/doc/AV02-0176EN
I really hoped for it to be cheaper but the bom cost is already at ~7.50€ without the PCB, so sorry to disappoint :(... (all with 19% VAT though).

And I have committed to continue with this project, despite my new controller being UD3 based and not supporting an external interrupter (at the moment at least), so updates will continue :D
I'll post some test footage with a speaker tomorrow (its 5am right now, so not really the time to play loud music :P)
« Last Edit: July 12, 2020, 05:07:50 AM by TMaxElectronics »

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Re: compact midi interrupter
« Reply #35 on: July 12, 2020, 05:06:10 AM »

 


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July 08, 2020, 09:00:04 PM
post Re: Configuration of a half bridge.
[Electronic Circuits]
Mads Barnkob
July 08, 2020, 04:25:53 PM
post Re: Configuration of a half bridge.
[Electronic Circuits]
ElectroXa
July 08, 2020, 10:03:25 AM
post Re: Configuration of a half bridge.
[Electronic Circuits]
hammertone
July 08, 2020, 08:39:10 AM
post Re: Measuring LED efficiency
[Light, Lasers and Optics]
Twospoons
July 08, 2020, 08:34:02 AM
post Re: Measuring LED efficiency
[Light, Lasers and Optics]
davekni
July 08, 2020, 04:39:10 AM
post Measuring LED efficiency
[Light, Lasers and Optics]
TMaxElectronics
July 08, 2020, 12:46:23 AM
post Re: Configuration of a half bridge.
[Electronic Circuits]
hammertone
July 07, 2020, 10:28:37 PM
post Re: Configuration of a half bridge.
[Electronic Circuits]
ElectroXa
July 07, 2020, 10:09:22 PM
post Re: Configuration of a half bridge.
[Electronic Circuits]
hammertone
July 07, 2020, 09:36:53 PM

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