Screeching digital feedback/oscillation

  • Anonymous

    Anonymous - 2013-05-12

    Hi there, I have been trying rakarrack 0.6.1 and am getting some strange issues:

    - Certain effects cause a terrible digital oscillation when turned on (even without input) that destroy my eardrums (Echo. MuTroMoJo)
    - Lots of effects just sound harsh and buzzy, like digital aliasing. The input isn't clipping and the output from rakarrack isn't clipping.
    - Some effects, like Distortion or StompBox, just produce no or very low output that can barely be heard even when Level and Main Volume is maxed out. If I do finally get into a situation where I get a decent amount of volume, there's no distortion! It's just clean and loud… even on max drive.

    In jack I have tried:
    - Sampling rate: 44100, 48000
    - Buffer size: 256, 512, 1024
    - Periods/buffer: 3 (USB interface)

    There are always 0 Xruns and the DSP Load is ~16%. I have tried the presets that come with rakarrack and get the same issues. What could I be doing wrong?

  • Transmogrifox

    Transmogrifox - 2013-05-30

    What distribution are you using? Does your "clean" guitar sound ok (routing directly from jack input to output without Rakarrack)?

    I know that Echo and MutroMojo have the capability to go unstable if something was able to submit an incorrect feedback (resonance) value -- for example, a 64-bit vs 32-bit problem, or compiler optimisations that were selected on a package maintainer's build which do not work with your machine.

    That said,
    What is your hardware (processor, #cores, clock speed, 64 or 32 bit...or other, etc)
    Do other programs work as you expect (Guitarix for example)?
    Does it sound ok if you start with a new preset, and make your own settings using some of these effects like MutroMojo or Echo? In other words, does it only happen by loading a preset, or does it happen if you make your own settings?

    Are you sure the input is not connected to ANYTHING when you hear this noise? Perhaps the input of Rakarrack is connected to its own output, then that would make these kinds of problems.

    A good test is to start Rakarrack with nothing else connected to it, then input some music from a media player (like Audacious) so you see the bars bouncing on the input and output. Then connect the output to Jack's output device so you can hear the music, then change FX presets to see if this noise happens.

    On my USB audio interface, there is a significant amount of crosstalk between inputs and outputs and sometimes with certain FX it will cause these kinds of things to happen.

    Also, the audio interface itself can sometimes create a feedback loopback depending on the interfaces mixer settings (alsamixer can help see how these are set). Sometimes the audio interface can make the output mix back into the input, and on certain lower gain FX, no problem, but add a little bit and it shows up. Something like Stompbox may be unstable at a frequency that is barely audible, so you don't hear the screech, but it makes things sound choppy, or you don't have volume, etc.

    Using a media player to drive Rakarrack can help eliminate the chance that something else has been connected back to the input. I know you already said it makes noise without the input connected, but I just wanted to be sure that is the case.

    I hope that helps make progress in identifying the problem anyway. I don't have this problem, so it may be a compiler optimisation problem, or hardware specific...if it's not a simple jack connection or sound card mixer settings problem.


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