MOD Duo background noise level


#1

greetings!

…just received my new MOD Duo, and i’m doing initial testing…

one thing i’ve noticed is that there is a disappointing amount of background noise. i suppose it’s possible that i’m expecting a cleaner signal than is reasonable for such a device, but just wanted to check, in case my unit has a fault that should be addressed…

the nature of the noise is not ground hum – it’s not 60Hz or multiples, and it occurs whether anything is connected or not. also, there is a cyclic component to it – looks like at least some of it is processing noise.

i’ve made a soundfile, so you can see how the level relates to a normal, healthy signal level. the noise is fairly low, but is disturbing because it is still clearly audible at reasonable listening levels for a full-level signal.

plutek MOD Duo noise

at the beginning of the file, my clarinet with pickup is plugged in but i’m not playing.
then there are a couple of bits of playing, with the input level to the MOD adjusted appropriately on my pickup. then, a few seconds after the last playing (at about 52.5 seconds in the soundfile), i unplug my pickup (you can see the spike there). you can then see that the noise after that point is virtually the same as before, so it’s not related to the input device but is produced by the MOD alone. the steady-state noise is at about -62dB, while the cyclic peaks are at about -52dB. …but it’s much more noticeable than those numbers might suggest, because it’s not just broadband noise. also, if you listen more carefully, you can hear some white(ish) noise which comes in and out from time to time.

this is all with no plugins – just a straight connection from input to output, and no level adjustments anywhere in the MOD.

please let me know if this is considered “normal”. if it is, i’d suggest there’s some work to be done, because it makes the device of questionable usefulness in my applications. :frowning:

cheers!
.pltk.


High noise level at all times
#2

Yeah, I can confirm that. It’s very audible when running the MOD Duo in front of my guitar amp.

This has been recorded using my phone in front of the speaker cab. https://collie.ca9.eu/owncloud/s/ugu5wXJhLd98bd9

It’s not that prominent when playing, but I imagine it to become an issue when recording.

Cheers
Bollie


#3

right… yesterday, while i was playing around with some plugins, i did get to the point of thinking, “hey, it seems ok”… until i started doing some quieter things with silence in between. then, it was pretty easy to get bummed out by that “digital whine”. hmmmm…

in my regular computer-based performance rig, i recently switched from an ART usb interface to my trusty RME multiface (with an analog preamp up front) specifically because exactly this sort of noise was getting annoying. the sense of relief is palpable when the audible background is just CLEAN, even in live situations where the noise might be on the edge of being inaudible. in recording, it’s another story.

maybe i’ll try an analog gate between MOD Duo and my amp, but it’s a shame to have to resort to a kludge like that! :frowning:

thanks for your report, @Bollie!

cheers!
.pltk.


#4

Hi @plutek!

What’s a nice sophisticated reedman like you doing here!? :grinning:

I also have noticed this underlying noise level, as Bollie said at a show through a cranked guitar amp it’s just part of the noise wash but for non-guitar work like horns and vocals I can see it becoming prohibitive to direct recording.

On top of that for me sitting in front of a display monitor and having the MOD plugged in to a guitar and running to an amp with USB cable connected to my computer (ie to use the web browser interface to tweak and try out pedalboards) increases the noise tenfold even with humbucking pickups… anyone else notice it’s worse when plugged in to USB?


#5

Reeeeeds!!!
Haven’t noticed any unusual backgroundnoises myself.
Well, obviously your background noise in the room will be recorded… Other than that i would have to do some more recordings (and i already have done some, no compaints from the soundengineer…)
What kind of mic setup are you using?


#6

hey @domlo… i’m using a pickup, which is very well isolated; there’s no room noise. also, the noise is there even when i’m not plugged into the MOD Duo at all – the noise level really doesn’t change when i plug in or disconnect.

hey @GMaq! how’s it going? :slight_smile: reedmen show up just about any place, ya know?!..

yeah, my noise is a little worse when USB is connected, but it’s still there even with the MOD running totally free… certainly not “tenfold” worse with USB host connection. i’ll be checking again to be sure though…

the disturbing thing about this noise, even in my live contexts, is more that it has that really noticeable “digital whine” quality about it so it’s more noticeable even at a low level; analog-type circuit noise is not so objectionable since it’s usually more neutral and broadband, unless there are actual grounding issues.

cheers!
.pltk.


#7

just another anecdotal observation:
I have some background noise when my unit is connected to my desktop PC via USB. I notice this especially when using plugins like the Big Muff.

When I disconnect the USB this noise is gone. When I’m connecting the MOD Duo to the laptop (on battery) I don’t notice the noise either.

@plutek: do you use symmetrical cables from the instrument to the MOD and back to the recording computer ? As much as I know the MOD should be have symmetrical inputs and outputs and this may help eliminating background noises.


#8

@brusch Inputs are unbalanced. Outputs are balanced.
Maybe the noise is amplified by ground noise ( https://en.wikipedia.org/wiki/Ground_noise ).
@plutek ok, i will experiment more. for my noise i held my room setup (not a pickup, computer/street noise) responsible. Will investigate.
I also have ordered some pickups for my bass clarinet, that’ll be perfect for doing so.


#9

cool… @domlo check my comments in the “swag” thread about pickup installation.

@brusch: what do you mean by “symmetrical” cables? also, note that i hear the noise even when neither the computer, nor the instrument are connected - so i’m quite sure the MOD itself is making it. try just running the MOD output directly to an amp, with nothing else connected at all (no instrument, no USB, no recording), then turn up the amp and you should hear a whine that’s primarily a G quarter-tone flat. if i play with the output level on the MOD, that digital whine stays constant but the white-noise-ish analog hiss goes up and down. when i have nothing else connected, connecting or disconnecting the USB makes no difference (whether the laptop is on battery or on AC).

so, like i say… it’s pretty definite that this is processing noise from the MOD Duo.

cheers!
.pltk.


#10

Ok, did some investigating.
DISCLAIMER: I turned the volume way up and don’t have a comparison for the noise LEVEL but i recorded the noise itself. (it’s mostly F# :slight_smile: )


the black line is output 1 and the red one output 2.
Connecting the MOD to the PC with the usb cable doesn’t make a difference, bluetooth also.
Here’s the audio. (i hope left is one and right is two but it’s possible both are one. doesn’t matter, they’re pretty much the same) https://www.dropbox.com/s/tmvu2ih74unq1cs/MOD%20NOISE.mp3?dl=0
What you hear is the low noise when changing pedalboards, i know this noise from my old laptops headphone jack when the harddrive was working.

AGAIN: This doesn’t reflect the noise LEVEL itself but rather the noise QUALITY
ALSO: No Instrument or Microphone was plugged in.
ALSO2: I only noticed the noise because you told me about it, so i think the level is pretty low.


#11

Is it there if you only plug in headphones, so that there is nothing else connected to power than the mod?

@plutek: In German you say symmetrisch instead of balanced.


#12

Hi Guys,

I’m getting lots of noise but only when USB is attached, tried different leads including ones with chokes but no luck.

It’s pretty unusable with USB attached :frowning:

Andy


#13

I assume thats attached to a desktop computer or laptop that is plugged in?

I think we’re all creating ground loops which is causing extra noise. With just a mod plugged in and headphones see what the noise level is, then start plugging in your other equipment. Not sure the best way to address this though other than using a laptop on battery power when you need low noise floor.


#14

Yeah, I guess it’s noise from the computer on the ground of the USB.

I guess the USB and audio components are on the same ground in the Mod Duo?

I have ordered a bluetooth dongle to give that a go, if that’s not fast enough I guess I will snip the ground on a usb cable and see if that helps.


#15

Hi everyone

Sorry for the late reply. Busy times here.

We’re following this issue closely and I’d like to share with you some of our findings and measures being taken.

The issue is related to the audio driver and how it manages the power of the CPU when the audio codec is in use. There is a unwanted current discharge at each buffering cycle that is “contaminating” the input circuits. Because the audio buffering is done at every 128 frames and, as we run at 48kHz sampling rate, the frequency of the current discharges is at 375Hz. Harmonics of this frequency - 187.5Hz, 750Hz, etc - also appear. If you enable “development mode” in the GUI - type “enable_dev_mode()” at Chrome’s Developer Console - and change the buffer to 256 samples you can notice the noise frequency change.

As many interference issues, the overall result is a sum up of multiple factors that range from circuit component deviations to the actual set-up and cables hook up.

The USB connection, although not related the audio, has a strong impact on the noise depending on your computer USB implementation and also on the way that the computer and the Duo are hooked up to the electrical mains. When using some laptops, disconnecting it from the AC power and running on the battery completely changes the noise behavior.

We were working on the improvement of the audio driver in order to improve the power handling but right now, this task is temporarily paused. We are using a forked version of the Linux kernel called sunxi and, because of it’s forked nature, it proved to be unworkable due to the never ending “fix here, break there” cycle.

That said, there is an ongoing effort to move to the mainline Linux kernel (the official version) and, as soon as the migration is done, the audio driver improvement will start moving again.

In the meantime, here are some measures to be taken in order to minimize the issue.

1 - The “contaminated” circuit is in the pre-amplifiers analog section. Changing the Gain stages - Menu/Volumes and Gains - makes a huge difference so the users that require a setting different than Low Gain should try to find the best balance between the Gain Stage, the Fine Gain and also on the internal volumes of the Pedalboards.

2 - Due to the location of the circuits in the PCB, the Input 1 is more vulnerable than Input 2. That said, Input 2 should be preferred for users that require higher gain settings

3 - Try different USB connections and ports between the Duo and the computer

I hope this info helps.

I’d like to remind that we consider audio quality to be one of our top priorities and I’d like to assure all users that we are constantly working towards offering the best sounding device we are able to.

Best regards to all members of the MOD Community


Some more measurements (and possibly a HW bug)
Disapointed
#16

thanks a lot for that, @gianfranco!

it did seem clear to me that this is not a grounding issue, although groundloops can, of course, compound the general noise problem. so, it’s good to hear that you are able to confirm the distinct noise phenomenon related to buffering and periodic current discharge – this is an explanation which corresponds to the worst of what i’ve been hearing.

regarding your comments about input 1 vs 2: is it easy to completely disconnect input 1? i.e. does the physical configuration of input 1 contribute to noise even when nothing is connected to it, and would it help to completely decouple that hardware from the MOD circuit? i usually only use 1 input anyways (which could be just input 2 of course), so wouldn’t mind completely eliminating input 1 most of the time.

cheers!
.pltk.


#17

One thing Gianfranco, I have quite a bit of audio gear here connected via USB and the Mod Duo is the only thing I am getting noise on.

Is the USB isolated inside the Mod Duo?


#18

This is not necessary

As long as you don’t make any internal connection using Input 1 it will not affect the audio in any way, independently of the noise level it has.


#19

…this would imply that it might be possible to go a long way towards solving this in software, right? is this being addressed in the “code room”?

the 1.3 testing version seems the same as regards this noise issue.


#20

Hi Gianfranco,

I don’t use input 1 but do have loads of noise when usb is connected. I have to use bluetooth rather than USB to get noise free operation.