Strange behavior using distortion amp and Ardour

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Strange behavior using distortion amp and Ardour

Postby jakob » Sun Apr 24, 2016 9:15 pm

First of all, thank you very much for guitarix! I discovered it a few weeks ago and I have to say I am more than astonished and grateful for such a great piece of software! It will change the way I record in my home studio!! Thank you very much!

I have observed a very strange (?) behavior: I am using guitarix 0.20 with Ardour 2.8 on my Ubuntu Studio 12.04. I recorded some tracks with ardour (click track, drums) and now I want to record my guitar tracks using guitarix. Strangely, the playback that I am listening to (drum and click track) is also fed in as an input into guitarix!! I cannot explain this behavior, the routing in jack is definitely correct and I can play my guitar in a normal way. But very silently, in the background, I can hear (and also see it in the guitarix meter) that the playback is reinserted (barely audible) into the input of the guitarix amplifier. I completely reconfigured the routing in Jack but cannot get rid of this behavior. I also observed that this only occurs when I use a Hi Gain setting. When using clean tones, I cannot see any playback being "reinserted" into the guitarix head.

Do you have any explanation for this? I can even reproduce this issue without connecting any external hardware, so it won't be an issue of some stupid mixer configuration.

I would be very grateful for your help because guitarix seems to be what I was looking for for years but this way, I sadly cannot use it for distorted guitar recording (which is what I do)...
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Re: Strange behavior using distortion amp and Ardour

Postby boomshop » Mon Apr 25, 2016 12:11 am

Hey Jakob,

Ardour 2.8 was released seven years ago so I would recommend upgrading to 4.7 prior to any further troubleshooting.
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Re: Strange behavior using distortion amp and Ardour

Postby jakob » Sat Jun 04, 2016 11:36 am

Hey, I now upgraded to Ubuntu 16.04 (guitarix 0.34). Still the problem persists and does not seem to be connected to Ardour. I also tried using VLC player simultaneously with guitarix and it seems that the output of VLC is also showing up in the guitarix output meter. I attached a screenshot of my jack routing. Hopefully, that is not the problem. In the other screenshot (meter.png), you can see that while VLC is playing background music, there is activity in the guitarix meter (that I could also record in ardour earlier...).

Do you have an idea where the problem is located?
Thanks so much for your help, I would really like to use guitarix more!
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meter.png
meter.png (14.62 KiB) Viewed 265 times
jack.png
jack.png (49.41 KiB) Viewed 265 times
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Re: Strange behavior using distortion amp and Ardour

Postby brummer10 » Sun Jun 05, 2016 2:59 am

Hi

Looks like you need to check your "Mic" settings.
Check it in alsamixer, ensure that your capture PCM is set to zero.

Hint, in alsamixer press F6 and select your soundcard, then press F4 to select your capture settings.
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Re: Strange behavior using distortion amp and Ardour

Postby jakob » Sun Jun 05, 2016 9:22 am

Thanks for the reply!

So, it's definately not the laptop-mic that's being recorded. I just muted my speakers and the problem persists.

Then I tried to configure PCM in alsamixer. I have 2 soundcards, 1 from the laptop which is not used and a USB card that I am using for all recording and playback. Find attached some screenshots from alsamixer: I decreased everything on the laptop's card to zero (mixer1.png). For the USB interface, I only have one slider (PCM). But that one is only for playback... it tells me that "the audio device does not have recording sliders" (mixer2.png).
Attachments
pcm.png
pcm.png (37.46 KiB) Viewed 258 times
mixer2.png
mixer2.png (21.34 KiB) Viewed 258 times
mixer1.png
mixer1.png (30.87 KiB) Viewed 258 times
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Re: Strange behavior using distortion amp and Ardour

Postby brummer10 » Mon Jun 06, 2016 2:29 am

So then I've no idea what happens. guitarix use the so called "In Place Processing", means that as the first step, the input get copied to the output, and the following processing is done on the output buffer.
So, when your playback from other programmes, which are not connected with the guitarix input, enter the guitarix buffer, it could only be a feedback.
You may check, if you get the same results, when you connect a softsynth to guitarix input, instead the capture port. This way we could check if the issue happen on software or hardware level.
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Re: Strange behavior using distortion amp and Ardour

Postby jakob » Mon Jan 02, 2017 12:33 pm

Hi,
after a while, I tried to replicate the bug and conduct deeper analysis:
- On an up-to-date Ubuntu 16.04, I can still replicate the problem.
- The problem arises as soon as I connect the "System Capture_1" (or 2) to the input of the guitarix head.
- When I use a synthesizer (zynaddsubfx) and connect it to the guitarix head, the problem does not occur.
- The problem increases the more I push the gain/drive controls to the max (but as I high-gain guitarist, I cannot live with medium drive settings :-) )
- A tiny and very silent (-60/-50 db) signal of the jack master output is audible on the input and sent into the guitarix amp again although only the "System Capture_1" is connected to the input.
- I also tested without any external devices connected to the interface, so it's not a wiring issue.
- Direct hardware monitoring of the interface is deactivated.
After considering all these points, the only two reasons for the problem that I can imagine would be:
1. A hardware issue in the input channel of my interface (I should maybe test another interface)
2. A software problem with the guitarix drive simulation. I did not look into the source code so I can only speculate: maybe, for the "overdrive simulation/generation" (or however you call that), there could be a bug that sends a tiny bit of the overall master output into the input. Maybe this only happens with high drive settings or as soon as the signal is louder than 0 db.
I suppose the hardware issue is more likely to be the problem but could you maybe just think about whether the second possibility could be the issue as well? Or do you see a third option that I did not consider yet?
By the way, my interface is a Behringer UCA202. I also have a Behringer Firewire Interface that I could maybe test if I find the time. But until now I never had any issues with the interface (and on Windows and also during recordings without guitarix, the problem did not occur. Also on clean tones with guitarix, I cannot hear the issue (maybe for cleans it's too silent to be audible)).
Thanks for a reply and best regards,
Jakob
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Re: Strange behavior using distortion amp and Ardour

Postby jakob » Mon Jan 02, 2017 9:27 pm

I just tested with another interface to verify my theory concerning the hardware issue. But "sadly", the problem also exists not only with my USB interface but also with my FireWire interface (Behringer FCA202). I installed FFADO and made the interface work with ardour and guitarix but still, as soon as I connect the "System Capture_1" to the input of guitarix, it captures strange white noise and the silent playback output.

So I think it's therefore not a hardware issue but probably connected to how guitarix "creates distortion" or how jack is configured.
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Re: Strange behavior using distortion amp and Ardour

Postby brummer10 » Mon Jan 16, 2017 6:42 am

jakob wrote:- When I use a synthesizer (zynaddsubfx) and connect it to the guitarix head, the problem does not occur.


That means, the issue cant be in guitarix, as it didn't exist with virtual ports, in wont be on the software side. Somewhere the output must be mixed in to the input, when using hardware ports.
Problem is, I can't reproduce the issue here.

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hermann
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