Audio bleed with USB

M

mcfarljm

Member
When recording direct over USB to my DAW with a backing track, I notice that I get some audio from the backing track bleeding onto the guitar track, if I'm using a high-gain patch. These artifacts are definitely noticeable at normal listening volumes if the guitar is silent during that passage and the backing track is near full scale. The strange thing is that it happens even when my studio monitors are powered off during the recording (so there is no sound being produced). Been trying to figure this out for a while now... Anyone have any ideas what might be causing this?

Some more details:
- MFC disconnected
- Axe FX main outs -> mixer -> studio monitors (tested with monitors powered off)
- DAW: Reaper
- No fancy settings or unusual preferences in Reaper. Just created empty project, imported media file for backing track, and added one track for recording guitar. Set to "IN 1". No buses or other routing.
- Mostly been testing with preset 024 (Friedman HBE), with noise gate turned off. The audio bleed is most apparent with high-gain patches with noise gate off.

Some things I've tried:
- Turned off "Send realtime sysex"
- Looked through this page: Help and answers - Axe-Fx II Wiki
- Contacted FAS support. Initially got suggestions that there could be crosstalk between the monitors and guitar pickups, but haven't heard anything else back from them after pointing out that I can reproduce the problem with monitors powered off (no sound being produced whatsoever during recording).
- Also tried unplugging cable from the guitar (so pickups are not in the equation), and I still get the bleed. It does go away though if I unplug the cable from the Axe FX instrument input.
- Can also recreate when plugging into the rear input, but the bleed is not as loud in this case.
 
mcfarljm said:
It does go away though if I unplug the cable from the Axe FX instrument input.
Click to expand...

There's your answer. Something is coupling into your guitar cable. If you are using high gain even the slightest amount of coupling into the cable will get amplified. The better the cable shielding the less coupling there will be.
 
When you have your monitors powered off, in your setup the output signal is still going to the mixer and could still be the source of the interference. Unplug the output cables from the Axe to rule that part of the equation out. What happens when you use headphones from the Axe and have nothing plugged into the Axe output?
 
So your setup is guitar > instrument cable > Axe > USB cable > PC and nothing else is connected? Odd. I can't imagine where the coupling is coming from. Doesn't seem like there's any source left for the bleed. Do you have a patch panel connected to the Axe?
 
Yes sir (and I can also get the same results without guitar in the signal chain). Nothing else connected to the Axe (don't know what a patch panel is). That is exactly why it's had me so confused. The source of the coupling must be analog, and I've tried to eliminate any presence of the analog signal (the one containing the backing track audio) external to the Axe FX (hence turning off the monitors and unplugging Axe FX outputs). I think Cliff is probably right, but wouldn't that indicate that the source of the coupling is internal to the Axe FX (I'm thinking it must be the output signal after D->A conversion)? I'm going to try to do some more testing with different brands of guitar cables, but I feel like I did that already and didn't see any difference.
 
Keep you cable plugged in, move it around, and see whether som positions are worse than others. That could give you a clue as to th source of the interference.

if the interference were inside the Axe, you'd still hear it with the cable unplugged. There are analog parts inside that can't be shielded, and they're on the noisy side of the case, which acts as a shield itself.
 
It could be internal. There's only so much isolation that can be achieved. If you are using very high gain then the tiniest bit of coupling will be amplified. Look at it this way: if you've dialed in, say, 80 dB of gain and there is -80 dB of isolation (which is a very good figure, most products only get about -60 dB), then you'll get 0 dB net crosstalk.

If you want a product with perfect isolation then you need to be willing to pay a lot more, probably on the order of $10K or more.
 
Added a dry track to record and got some numbers on the peaks. The peak for the backing track itself is around -1db.

Dry guitar (humbucking guitar, volume up, strings muted):
- Guitar noise level: ~ -100 db
- Bleed from backing track: ~ -94 db

Processed guitar (preset 024, noise gate off):
- Guitar noise level: ~ -45 db
- Bleed from backing track: ~ -31 db

With this patch, the difference in levels between dry and processed for regular chording and single note playing seems to be around 20db to 30db.
 
I should say that for practical purposes -94 db on the dry guitar signal is probably a non-issue, I can just gate it. Really I just opened this thread out of curiosity and for my own edification. Thanks for the comments though, I've already learned a few things...
 
Turn the noise gate on then. I don't know what you are expecting. You've got 55 dB of gain which means the crosstalk is around -80 dB. That is a phenomenally good number and you'd be hard-pressed to find a product that is better than that.
 
Yes I will turn the gate on for actual recordings. I'm coming from a GT-100 and 11R and had never noticed this before, so that is why I wanted to try and understand it (I figured maybe there were some adjustments I could make to my set up somehow to mitigate it). I still have the GT-100 and did some testing and cannot reproduce this at all on the GT-100 over USB. Recorded a dry guitar track with the same backing track being sent over USB, and no sign of any bleed. Normalized the dry guitar track and the backing signal is not there at all (and of course it doesn't show up in a processed recording either). That's the main reason I was surprised to find this with the Axe FX... Again, I'm not trying to knock it or say anything's wrong, I'm just trying to learn.
 
Hi guys!
I have similar problem, this is my signal path: guitar -> axe XL+ -> rme babyface -> monitors. I am using axe as interface (as I want to have clean di for reamping) so rme is just kind of passthrough to monitors. First I thought that my pickups are picking up signal from monitors, so I've turned them off and started recording with headphones but problem still existed. Then in daw I switched audio interface from axe to rme and recorded "analog way" - axe into L & R inputs of rme. Result? Zero bleed at all. I don't want to use a gate cause this bleed is audible on long chord decays etc so gate will not help here.

EDIT:
I've just compared XL+ with MarkII, the same preset, identical behavior. Recording through usb is unusable for me :(
 
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Don't know if this addresses the issue, but I found that when I asked Reaper to mute or solo a track during playback, it did not complete zero out the other tracks but allowed some bleed through. This can be controlled through software settings to completely eliminate bleed through from unwanted tracks or to allow some of the other tracks to bleed through.
 
This is not a solution at all, to eliminate all bleed I would need to record without hearing click, drums etc...
 
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