Digital noise from MIDI signal (DIN / UART)

Hi, I’ve got a basic MIDI circuit wired up using an optocoupler (H11L1), following the circuit I’ve attached here. I’m getting signal correctly, but every MIDI message generates a “click / tick” sound through the audio output. That means if I slide the mod wheel up from 0 - 127, I hear 127 clicks coming out the audio.

I thought that the optocoupler would have removed the midi circuit from the audio circuit, but obviously something is happening here. These clicks do not happen when I switch to MIDI over USB, so this is not a software issue.

Thanks for any help!

My guess would be that even though the ‘optocoupler’ (or opto-isolator as I would call it) is separating the circuits electrically (galvanically) the sharp transitions of MIDI are creating harmonics that make it through as Radio Frequency Interference (RFI). Possible solutions include using small coils, or capacitors, or ferrite beads to slow the transitions down (make them less square) or physically repositioning the circuits involved to avoid creating little antennas. I imagine you could find examples of this on the schematics of Daisy Pod, Patch, Petal, Field or even non-Daisy circuits.

Question about power… I just noticed that I’ve got the voltage line for my midi circuit coming from the 3v3 Analogue pin (same as my audio out / potentiometers) and in the daisy patch midi schematic, it shows 3v3_D. Does this sound like something that could be causing the issue?

yes (Post must be at least 20 characters)

I moved from the analogue 3v3 to the digital and the clicking is significantly quieter. It’s still present though, which is a huge bummer. Going to continue to poke around.

If anyone has solved a similar issue I’d love to hear about the solution(s). Thanks!

I’m out of ideas here… I stripped everything down and just have a breadboard with the daisy and the midi IN circuit and I still get the “clicking” every time I send a midi signal in (clicks for noteOn, noteOff, CC messages…). Could it be the type of optocoupler I’m using (H1L11m)? I have a 6N138 coming in the mail, since I’ve seen other circuits with this one in it. Does anyone know which optocoupler the POD uses?

I also tried switching from USB power to a straight 9v battery and I get the same thing, so that rules out USB interference of some sort. I’ve also tried several different MIDI setups - MIDI cable from a controller that’s connected to the computer via USB, midi cable from my Juno 106 with no connection to the computer, and a USB to MIDI cable straight from the computer… same for all of them.

Does anyone have 5 pin midi working on their daisy seed? If so, would you be willing to share your schematic / part list?

I can’t imagine why it would be any different, but I’ve got the parts coming in the mail to switch to TRS/MIDI, just in case there’s something going on that’s specific to the 5 pin.


TRS/MIDI is just a different jack, no?

no - TRS vs 5 pin din is just a different jack, indeed.
What we’re concerned with here is TRS MIDI vs USB MIDI where there are very different protocols that the MIDI commands-data are transported over-in as opposed to TRS where it’s just good old asynchronous ASCII data.

Yes, i know the difference between USB and serial MIDI.
I think you’re missing the point here, which is, how to get serial MIDI working without noise on each incoming message.

First and foremost, thank you for all the time and feedback, it helped point me in the right direction. The issue is fixed now.

Ground issue.

I’m using a breadboard for this circuit, so I have two ground rales (one on either side of the daisy). I’ve got D ground going to the left rail and A ground going to the right rail. The two rails are bridged at the end of the board, thus connecting A / D grounds.

I am now bridging D ground, via jumper, directly to A ground. It is no longer connected to the breadboard rail. This cut down on the majority of the issue. I then moved my midi circuit ground very close to A ground as well and all noise was vanquished.

I presume this has to do with the quality of the breadboard ground plane… or simply the distance between the grounds. I’m a novice in electronics, so if anyone has thoughts on this I’d love to hear.

Bonus: I mirrored this onto my more complex project, done on a soldered prototype board, and not only did it solve the MIDI noise, it solved a bunch of other noise issues I was having - including the high pitch whine noise I experienced at 6kHz (sample rate / audio block size).