The Aliasing Problem

  • I know it has been discussed here before. But there's something that I don't quite understand, let me explain my case.
    I own a Kemper since barely a few days. Love it, BTW.
    However there's the Aliasing problem. I know this is an inherent problem with converters, but I don't totally understand where it happens.
    Is it my guitar that introduces higher frequencies in the AD conversion? I don't think so, I don't hear anything on clean sounds, but it could just be not apparent.
    Is it the processing that - in the case of very distorted tone particularly - generates harmonics higher than the Nyquist frequency?


    The thing is: I am experiencing this with all heavy profiles. Most of them show it only on really the highest notes (say, 20th fret E string) or some harmonics, others are apparent even starting from 14th fret on the G string.
    On the other hand, I profiled a preamp - which did generate aliasing when recorded on my PC, so we're really comparing apples with apples - and very surprisingly the resulting profile shows no sign of aliasing!
    Why do you think that happens?

  • So, I read up on the aliasing discussions that took place and on TGP in the past.
    I understand that the problem was addressed, some say solved (in 1.6.1), and yet it's still very, very audible. I might be doing something wrong myself, in which case any tip would be welcome, cause I find really hard that big shot producers would live with that. Andy Sneap for instance doesn't really go easy on the distortions!
    In a post (Be aware of the digital sonic artifacts (aliasing) in your KPA!) Christoph Kemper wrote:


    "We have a large headroom of calculation power left, that I was planning to keep for future features and improvements for upcoming firmwares.
    However, it takes only three seconds for me to speed up our algorithms for the next firmware"


    Well, personally I would speed the algorithms in a blink of an eye. Somebody even suggested to make it optional, which clearly would be the best solution. Maybe because I care way more about the cleanest possible tone (i.e., without artifacts) than running all 8 effects at a time. Sure, if a Kemper 2 was somehow in the works, with relevantly more processing power... =P (cross my fingers)

  • I have no aliasing problems with my Kemper. What you seem to describe is how real tube amps oscillate and do crazy stuff when at high volume and high gain. Maybe people are not used to hearing this in the context of low overall volume and can sound like aliasing.

  • Laimon, no aliasing problems here at all.
    Please demonstrate and post sound clips of this if possible. Since you have so short time experience with the kemper maybe you have one of the noise gates up too much?
    I saw you just posted the claims about it also on TGP, post 112. http://www.thegearpage.net/boa…read.php?t=1443961&page=8
    It's always good to use a scientific approach and a good rule is that extraordinary claims require demonstrable evidence.


    Remember, just like any real amp the Kemper can recreate frequencies and harmonics "fighting" eachother which can cause some strange sounds, but that is a real phenomenon with guitars and amps, not a digital one.

  • Since you have so short time experience with the kemper maybe you have one of the noise gates up too much?


    This!


    When I first got my PowerRack, the only disappointment for me was the weird "aliasing" I was getting using high gain profiles. I realised that it was worst when picking very lightly or when letting notes die, which of course led me to the culprit : an over zealous noise gate!


    To the OP, try disabling any gates in the stomps section and then turn the input gate all the way down, then see if you can recreate the effect. I'll bet you can't!


    Cheers,
    Sam

  • here's the clip:


    https://soundcloud.com/simone-frau-2/alias


    On record it's not as audible as through my guitar cab, but still one can hear this anomalous, descending frequency right after picking each note.
    Noise gate is complete off.


    PS: I would share the rig for you to see what's going on, but it is a commercial one and I don't think I am supposed to.

    Laimon, no aliasing problems here at all.
    Please demonstrate and post sound clips of this if possible. Since you have so short time experience with the kemper maybe you have one of the noise gates up too much?
    I saw you just posted the claims about it also on TGP, post 112. http://www.thegearpage.net/boa…read.php?t=1443961&page=8
    It's always good to use a scientific approach and a good rule is that extraordinary claims require demonstrable evidence.


    Remember, just like any real amp the Kemper can recreate frequencies and harmonics "fighting" eachother which can cause some strange sounds, but that is a real phenomenon with guitars and amps, not a digital one.

  • The only thing I can see (hear) from that clip is that you might consider to work a little bit harder on your bending .
    Good Luck ! :thumbup:

    Tom Andersons Drop Top 2010 >Gibson ES 335 2011 Custom Shop Cherry of course >Charvel GG sig Caramelised Ash>Steve Vai vwh with SS frets and Sustainac 2006 > Fender YJM 2008>Double Trouble, Honey Bee pedal >TS 2002 > etc... >Kemper>QSC's and/or JP Le Roux amp and Mesa Cab

  • Have you heard the sound clips posted on the Kemper forums back when the aliasing issue was relevant?
    I'm not sure what you're hearing is aliasing, and the difference between your 'aliasing' preamp and your Kemper profile of it might just be due to the Profiler's tendency to roll off a little low end, depending on your refining process.

    "But dignity is difficult to maintain
    stamina requires constant upkeep
    repetition is boring
    and you pay for grace."

  • The only thing I can see (hear) from that clip is that you might consider to work a little bit harder on your bending .
    Good Luck ! :thumbup:

    How cute. I suffer from folic acid deficit which some times leads literally the tip of my nails to detach from my skin, and that hurts like a bitch. You should try that, it'd give you a little perspective.
    I did the clip despite the fact that my fingers were hurting, thinking that listeners would focus on the alias.
    BTW: I've been playing for 20 years, and when I don't have this problem I can bend just fine, thanks.



    Have you heard the sound clips posted on the Kemper forums back when the aliasing issue was relevant?
    I'm not sure what you're hearing is aliasing, and the difference between your 'aliasing' preamp and your Kemper profile of it might just be due to the Profiler's tendency to roll off a little low end, depending on your refining process.

    Yes I did, it was indeed more prominent, but this is clearly audible on my full setup.

  • Yes I did, it was indeed more prominent, but this is clearly audible on my full setup.

    I don't know what your 'full setup' is, but i've got two pairs of studio monitors in a treated room here and i'm hearing nothing wrong.


    I'm pretty sure that's not convincing, however. Could you post a link to the .wav file? If there's aliasing it should be visible on an FFT.

    "But dignity is difficult to maintain
    stamina requires constant upkeep
    repetition is boring
    and you pay for grace."



  • Can't hear anything wrong here - did you check with another speaker, headphones?


    How do you use your KPA?
    e.g.
    guitarBrand - KPA - main out - mixerBrand - speakerBrand


    Did you use the digital out or any digital amp / active speaker?

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  • Ok, let's try again from the start. This time I picked a profile from Keith Merrow, KM-ANDROMEDA, because freely available (Kemper download page) and because there the problem is more prominent.
    The signal path is:



    Yamaha Pacifica 821 with DiMarzio Evolution bridge =>
    Kemper (rig KM-ANDROMEDA, no effect on) =>
    Engl Ironball poweramp =>
    Mesa 1x12 Mini Rectifier cab


    Here are clips: https://soundcloud.com/simone-frau-2/sets/alias, with a version without and one with cab sim.
    The one without cab sim shows evident signs of aliasing, because there all higher harmonics are not cut by the cab sim (which acts as a filter).
    The alias in the one with cab sim on is of course way more subtle, but listen with headphones at enough high volume and you'll hear (especially after picking the 3rd, 4th and 6th note) this wobbling sound in the background. (the overall background noise does not make it easy to perceive, and in fact I can not claim I would hear it myself after some of the notes).


    I also uploaded the wav versions of this files, in my google drive account: https://drive.google.com/folde…94cS1KUFNCNEU&usp=sharing
    Feel free to do any analysis.


    Guys, you might think I am splitting hairs.
    When in use with the cab sims on, it is very subtle to hear it, and if I didn't know there was I would not look for it.
    When feeding the signal with cab sim off to a power amp and then to guitar cabinet, it is very very clear.


    I guess I could live with the first (as I don't do any professional recording at home), but not with the second, because I would be playing most often in that setup, where the problem is more apparent.
    The solution I would propose - if anyone knows how to make one, maybe Christoph Kemper himself - is to provide a fake cab IR, something that does not quite behave like a cab but rather as a low-pass filter to cut out the higher frequencies that in the DA conversion would produce aliasing. I think that would be an excellent compromise.

  • Yes everything is there as you mentioned it and it is very disturbing for my ears.
    It is like a basic noise covered by your picking (maybe they are right , it is not an aliasing
    problem).I dont know how to help but I remember once I had the same base noise with a
    line 6 dt 50 .It drive me crazy!After a while I found the cause ,my guitar cable!Check every component!

  • Yes everything is there as you mentioned it and it is very disturbing for my ears.
    It is like a basic noise covered by your picking (maybe they are right , it is not an aliasing
    problem).I dont know how to help but I remember once I had the same base noise with a
    line 6 dt 50 .It drive me crazy!After a while I found the cause ,my guitar cable!Check every component!

    Hi Redspecial, I think you're rather talking about this background noise (that I have to deal with this afternoon if I find the time), but that's hum, not the wobbling sounds that I am talking about.
    Listen closely right after picking a note (if you can, try sending the signal without cab sim to a poweramp and cabinet, there the effect is far from subtle, sounds like bumblebees on LSD), you will hear strange artifacts that have are not related to background noise, but they clearly come from erroneous interpolation of samples.