FluidSynth Output is Hopping Around Channels L and R


since yesterday, out of nowhere, I have the strangest problem:

I’m using the setup described in this post:

After booting up the Zynthían and loading the snapshot, E-Piano and Grandpiano are on channels R and L respectively, but for the first 2 minutes or so are jumping around channels like crazy.

I play on the keys and every 4th note or so the sound is hopping to the other channel. Then, 2 minutes later and after opening MOD-GUI it stops doing that and seems to work normal.

Anyone encountered this, also? Possible remedies?

I’ll be blunt, as long as it takes only a couple of minutes to “settle down” - I don’ really care, but it is a strange beaviour nonetheless.


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After using it live, I can say the FOH will not be amused by that behaviour…

Could you upload the sound recording from the zynth of this issue? That eill at least confirm the jackd side of the process. . .

It 's really strange. Could you confirm that your Power Supply is a good one? As you probably know, inadequate power supplies are the first cause of strange problems …

Hi I’m aware of that PSU issue. I’m using a 3A PSU, which should be more than enough, I guess. Will do some recordings/video in the evening.


Turns out I am not able to consistently reproduce this behaviour. Now it works fine most of the time. Only occasionaly it happens.

Does i happen with a particular soundfont? I have observed the salamander piano has samples with different stereo position for some notes based on the velocity which makes it unusable.

The stereo swapping is exactly the first thing I heard when playing a grand piano sound in the rehearsal room. It’s a little subtle coming from a PA but I heard it. The soundfont I used was handcrafted, so the problem lies not within salamander…

Then I made a setup where epiano is on one channel and grand piano on the other. There you could hear the swapping clearly.

But as I said, as of now I can’t reliably reproduce it. I have more trouble with the snapshot not taking on the soundfonts I intended.

I know what you mean. It seems to be such a little thing but playing it live that way is indeed impossible.


I’ve been using Zynthian for 3 rehearsals now, and everything went just fine! Well after one or two reboots.


Today I booted Zynthian and it loaded the stereo piano session from yesterday 1#FS (fluidsynth 2.09 /w soundfont app. 140MB). The hopping has reappeared. Which means, it has nothing todo with fluidsynth 2.10 beeing leaky. (see here):


Here is the soundfile for you @wyleu:

And now the strange part (again): after 10 minutes the hopping is gone.

You know guys, I’ve always been baffled by instruments with stereo outputs. :stuck_out_tongue: I mean sure, recreating the experience of a grand pipe organ in a cathedral might warrant that, but little else does.
I’m only starting to get into pro audio stuff on Linux, so I wonder if Jack could be configured to downmix stereo plugins into mono, and then use a mixer and/or 3d sound modeling app to create a stereo mix by hand…

I hear you. The problem with downmixong is: if you have a stereo sample that was recorded in A/B or OTRF (vs XY)


That is to say with downmixing of any spaced micro technique, you will get phase cancellation. And they can sound weird and different for every other sample. That’s the reason I use only one side if the piano soundfont in my setup. Using 3d sound modeling after a downmixed A/B would result in the sound being uncontrollably all over the place…

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I know it’s an mp3 but it doesn’t look so much like hopping as clicks in the data but they are smoothed so that examining them at almost bit by bit level loos ok…

Was this a recording from the zynth Audio recorder or taken from the Zynth output?

IN this case a short WAV might be better to actually see whats going on…

Try removing and replacing your sound-card, it could be i2s pins need stronger connection.

I’m using the kit v3, so i think neither soundcard nor connection should be a problem?

Will try anyway, thanks!

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Ok thanks, here’s a wav snippet. I can clearly here the swapping. And to confirm: if I use two soundfonts panned hard left and right, they indeed swap channels.

Hope this helps…

Most people these days listen on headphones, having a nice stereo chorus washing the sound around is quite a musically appealing process.

Apart from the swapping, i hear a lot of clicks. Are you getting XRuns? It would be strange with FluidSynth …

Can you reproduce the problem with any sounfont or it only happens with some of them? In this case, could you send a WeTransfer (or any download link) with the soundfont causing the problem?


As far as I experienced it, the soundfont doesn’t matter, and yes I got some Xruns. The problem is, I can’t faithfully reproduce this behaviour. Sometimes it happens, sometimes not. Not much I can do about it… I remember it happening more often after a successful shut down from the men; and not so often after a forced shut down.

Having listened to the samples with stereo headphones, it is not clear if the L/R channels are reversing or the image is moving within the stereo image. It would be clearer if you created a soundfont with a sound that is hard panned left and a different sound hard panned right then play the left only sound first several time and then the right only sound several times. This would more clearly demonstrate the issue.
I have not observed this with libfluidsynth 1.2 used in Fluidbox. (Actually, I haven’t noticed this with Fluidsynth in Zynthian either.)

As I said, It happend two times at rehearsal, where hard left is grand piano and hard right is e-piano. I had to swap jacks two times, because it shifted the pianos aorund and then back again. It only occurs within the first five minutes after boot up. I’m sorry I can’t reproduce this faithfully, so I guess I have to live with it. Thanks for your help anyway!