I added a basic breath sensor to the pastry saxophone. It kinda works! The response of the breath sensor is quite slow - I think I need to use something stretchier than a nitrile glove. It doesn't feel sensitive enough to do staccato notes or anything yet.
I didn't get dynamics working with the USB MIDI yet, but on/off breath control works well
The breath sensor works like this guy's video: https://www.youtube.com/watch?v=es92GqYBge0
The black tape all over it is to block out light - I found it was making a big difference to the readings.
I've started printing the mouthpiece and designed the sensor half of the breath sensor. This uses a TCRT5000 IR reflectivity sensor, which shines at a diaphragm clamped between the two halves.
When you blow into the mouthpiece, the diaphragm expands towards the sensor, increasing the amount of IR that's reflected back into it.
Dynamics now work! I'm using the breath velocity value from the cardboard breath sensor to set the MIDI breath controller value, and mapping that to volume in the synth. I'm sure there's a much more complex and nice-sounding config that can be done, but this is cool!
It needs to be a bit more linear, but that just means making some sort of calibration curve for the sensor. That can wait until I've got the 3D printed one working.
I've put the code up on https://gitlab.com/gbrnt/electro-sax
There's currently a latency problem - I need to check whether it's on the Arduino end or on the computer end. The breath sensor has some extra latency - I think that comes from the current prototype but I'm not entirely sure.
The end-to-end latency of the electro-sax is definitely too high right now. It's recommended to get it below 10ms (if you can) and here it is at 160ms.
That's enough that when you're doing something like a run of short notes you do one too many because the sound hasn't caught up yet.
Let's see how much I can improve this just by changing Jack settings. Not sure how Pipewire comes into this.
So far in my attempts to improve the sensitivity of my breath sensor, I've managed to make it worse.
The first cardboard prototype had a range of 0.38V, or 77 counts in an Arduino's 10-bit ADC. Ideally it'd be more than 127 for a completely step-free MIDI reading.
The 3D printed prototype got me 18 with the nitrile glove membrane, or 31 with a white balloon. Not great. Hopefully the sensor I designed it for will arrive tomorrow and I'll be able to try it out.
Hmm, not doing great with the BMP280 pressure sensor. It works really nicely for detecting a change in pressure, but it doesn't seem to be very good at going back to its original zero point.
So in the screenshot you can see the lows are at different points each time I stop blowing. I assume it's doing something clever, but whatever it is it makes it less useful to me.
The best breath sensor I've built is still the original LDR-based one, with a difference between "not blowing" and "blowing hard" of 77 counts.
I could tell that its response was slow, though. So now I've measured it!
Going from LED off to on, the response time is 30ms. From on to off it's 100ms. Luckily that's the ok way round, and normally it won't be varying by so much.
I'd like it to be faster but it's probably an ok solution for now.
Time for a slightly more scientific test of the TCRT5000 based solution. This is the reflective IR sensor with an IR LED and phototransistor.
So far it looks like I can make this good enough by correctly setting the distance between balloon and sensor, and the LED brightness.
I might also test the effect of changing the (current limiting?) resistor next to the transistor, because I've never really got my head round transistors.
It looks like I found the sweet spot for this sensor. With this setup it's 14-16mm from the membrane. My measurement of the distance is bad so I'll still need some adjustability.
I found it was best to set the current-limiting resistor for the LED so that the "non-blowing" voltage was about 4V - maximising the sensitivity.
Not sure why the distances needed in the datasheet are shorter than mine - they're using a mirror to reflect the beam so I'd expect it to be longer.
The electro-sax now has transposing! You press a key combination to activate transpose mode, and then play the key you want to transpose to. So if I want to play alto sax music in the right key, I play Eb and then press the octave key to lock it in. It seems to work well!
It needs a few octave keys so it can play in the correct octave, but that's fine for now - an octave off is still better than attempting to tranpose in my head.
Look mum, I did a science!
This is using the new sensor distance tester. It made it so much easier to do this experiment properly.
It looks like my previous conclusion was about right - 13-15mm between sensor base and membrane is about the right distance.
The good news is that with that distance, I get a range of ~270 ADC counts between "not blowing" and "blowing as hard as possible". That's great - it can easily be mapped into MIDI's 127 breath velocity values.
I tried switching back to Pulseaudio (from Pipewire) to see if that would reduce the MIDI/audio latency I'm seeing with the electro-sax. Haven't measured it yet but it doesn't seem like it made much of a difference.
Hopefully that's an indication that it's not on the (complicated) audio side and instead on the (slightly simpler) MIDI side. Better not be something fucky with USB.
Just to compare audio latency, I'm trying out a live USB of Ubuntu Studio. It looks like without doing anything, that cuts my latency from 150ms to 93ms. And changing my headphones to my front panel output instead of my USB DAC cuts it further to 24ms.
That's still not amazing, but it's a world of difference from being unable to play properly.
Some more progress on the updated breath sensor. It's still quite bulky, but at least with the longer mouthpiece I won't be bumping my nose into it!
Will try this for a while before deciding whether it needs a fake reed - the positioning of the air inlet might be enough to make it feel close enough to a real sax.
@sj_zero Probably, even in a few minutes I can see it leveling out. The problem is I was hoping to use it to detect whether to play a note or not, and that's hard when the baseline keeps changing.
@gbrnt Seems like it would be usably accurate only if the air pressure changes happen at strictly regular instances.
@AskChip Which isn't likely when using it as an instrument! I'm sure there's some software magic that could be done to handle the changing baseline, but I don't feel like doing it!
@gbrnt It looks very much like the result of a leak somewhere. the waveforms all drift up when low, and down when high for any period of time.
@AskChip There's an intentional leak, the chamber is essentially a tube with a flow restriction at the outlet. So when I stop blowing it should always be the same pressure inside (ignoring atmospheric pressure variations)
perhaps the pressure exceeds the 1.1 bar maximum of the sensor, and that's throwing it off somehow?
@till I *think* the values on the graph are millibars so that's not happening, but I could be wrong there.
@gbrnt Based on the graph, any blowing at all is vastly more pressure than the baseline, so it should be easy to programmatically compensate for the sensor drift.
@yngmar It's easy when you're doing on/off blowing like I was in the graph, but I'm guessing it's harder when you're trying to play a piece of music with dynamics
@sj_zero I didn't! This is a more mechanically complex version that's based on your breath flexing a diaphragm. It uses an IR reflectivity sensor to measure the distance to the diaphragm and thus how hard you're blowing.
@sj_zero Yeah, this is one style that Jeppe from Kontinuum Lab on youtube uses, so it's somewhat tried and true.
I was a bit worried about protecting the sensor from the breath moisture anyway, so the electrical simplicity of this is quite nice! Downside is that it's much bulkier.
@gbrnt you could just change the box you're using based on your mood. today it's pastries, tomorrow it's a pasta box. 😃
@arthurlutzim Looks cool! I only see an image for raspberry pi though - is there an x86 one I'm missing?
I think they focus their efforts of kernel patching on a few platforms, not sure x86 is one of them
@ieure 100ms difference is a lot though! I bet it's something you can configure some things to minimise but it's way over my head
@cinebox Yeah that'd do the trick, custom AREF would probably be enough. I think the phototransistor-based sensor that's arriving soon should improve it enough though
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