Be aware of the digital sonic artifacts (aliasing) in your KPA!

  • thanks for the update!


    so does the aliasing happen at an intermediate stage where a sample rate conversion is done to save computation cycles in subsequent code? if this is the case, wouldn't it be cheaper to use a more aggressive anti-aliasing filter before down-sampling than to stay at the higher rate for functions that don't require it?


    if not, i like the idea of making this optional. there are too many who have told us we aren't hearing it, or it's our poor technique, or it doesn't matter, who will likely explode if they don't get their prized new feature because of this. otoh, this will take additional coding effort as well.


    i would rather see a fix for this than an attempt to get every feature under the sun in one box. if you need one box to rule them all and in the darkness bind them, there is already an option...

  • Yes, I think in case the DSP has power left it should be very easy to get rid of the artifacts.


    Please give us a switch 'normal resolution' - 'high resolution'.


    For most profiles (clean and soft distortion) do I not get any artifacts at all - so I could use 'normal resolution' and may have additional features - for some higher gain profiles I would love to use 'high resolution' and don't need so many effects.

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    Edited once, last by Armin ().


  • I like this idea.


    Further, could it be an option that is enabled on a rig-by-rig basis. So, for example, clean or "dirty" rigs could have that parameter switched off. High-gain rigs, on the other hand, could have it enabled. :thumbup:

  • In a scenario where it's best aliasing performance or more features?


    I bought the KPA for the best tonal representation of amps, regardless of gain. I'll go with the best amp representations possible and forego the special feature doodads... Had I been willing to accept less than the best tone in trade for additional feature? I'd have gone with the AXE 2. If it's as simple a thing as can be done in just 3 seconds??? Please do so...


    Maybe a FW update that is for folks who could care less about "feature headroom" ? An alternate upgrade path, as it were? A choice on the part of the owner... If catering to "purists" is a 3 second affair??... with all due respect... do both. I would suspect you have a very focused understanding of what an anti- aliasing patch would address and who might want it. Also you know who has no need in the world for it, in actuality.




    Consider that some folks went with, and will continue to choose, the KPA primarily for the best possible amp tones and not any additional FX, signal chain options, etc., etc. I am also quite aware that some of the aliasing examples are able to be gotten out of certain amps in the analog world...

    Edited 2 times, last by 1fastdog ().

  • In case anyone missed these sound files from my previous posts that further reveal artifacts:


    [1] This shows that aliasing is audible in clean sounds--the first six notes are a Grace DI, and the next six notes are a profile of that Grace DI.


    Grace-KPA Clean Test 1


    [2] This reveals the furry (unwanted) distortion that the KPA adds to pristine clean sounds--the first 3 strums are the Grace DI, and the next 4 strums are the profile of the Grace DI.


    Grace-KPA Clean Test 2


    [3] This reveals the clocking noise ticks from the KPA's S/PDIF digital output--changing cables and converters 3 times now, the KPA still emits random digital clicks and crackles (I only use the analog outs now for recording):


    Click Test 1

  • 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.
    But this would eat up some of the headroom for the future.
    Still we don't know how much headroom we will use in the future.


    What do you think?


    CK



    I would err on the side of saying I'm fine loosing a bit of headroom, but also that you could release 2 separate firmwares; an "anti-aliasing" one, and an "aliasing" one, so to speak. I bought the KPA to have the best representation of amps available; if that means sacrificing some random effects features down the line then so be it.


  • 1. I can hear the same sound you describe as aliasing in the first clip in this thread on the portion of the clip which you say is the Grace DI (no Kemper). Subtle, but its there. Harshest on the attack. And IMHO, the second part of clip one sounds...well...pretty much exactly like the first. So unless this is some sort of trap/trick I'm not sure what your trying to prove. Is the Grace a convertor? What convertor are you using?


    2. Did you have the Kemper set to clean when you profiled? Also, how is it being recorded? Did you watch the input meters? I've recorded some fastastic 100% prestine clean tones with the Kemper, not a challenge at all. The Kemper sends an extremely loud signal when its profiling, hot enough to overdrive the input of even pretty high headroom amps. Resultant profiles can have more distortion than the source for this reason. Many users turn down the gain a notch or 2 as a result.


    3. Clicks on SPDIF pretty much always have to to do with clocks not syncing correctly. When using SPDIF, you have to set which item is the master. If your card doesn't allow you to do this, you will likely have clicks when the Kemper (or any other unit which is set to be a master clock by default) is hooked up. The Kemper unfortunately does not allow an external clock sync, as such click city.

  • 1. I can hear the same sound you describe as aliasing in the first clip in this thread on the portion of the clip which you say is the Grace DI (no Kemper). Subtle, but its there. Harshest on the attack. And IMHO, the second part of clip one sounds...well...pretty much exactly like the first. So unless this is some sort of trap/trick I'm not sure what your trying to prove. Is the Grace a convertor? What convertor are you using?


    2. Did you have the Kemper set to clean when you profiled? Also, how is it being recorded? Did you watch the input meters? I've recorded some fastastic 100% prestine clean tones with the Kemper, not a challenge at all. The Kemper sends an extremely loud signal when its profiling, hot enough to overdrive the input of even pretty high headroom amps. Resultant profiles can have more distortion than the source for this reason. Many users turn down the gain a notch or 2 as a result.


    3. Clicks on SPDIF pretty much always have to to do with clocks not syncing correctly. When using SPDIF, you have to set which item is the master. If your card doesn't allow you to do this, you will likely have clicks when the Kemper (or any other unit which is set to be a master clock by default) is hooked up. The Kemper unfortunately does not allow an external clock sync, as such click city.


    With all due respect, I think you are wrong in your first assertion--the first clip is 6 notes of a Grace M101 Preamp/DI straight into an Avid 192 interface then into a Pro Tools HD Accel system, followed by 6 more notes of the KPA profile direct into the same system--after 30 years in the music business, I don't play the "trap/trick" game! Turn up your monitors and listen to the low alien sounds underneath the high notes I'm playing.


    As for your next two questions, suffice it to say I am fairly meticulous in my testing--I spent some time trying to get that furry distortion out of clean profiles, and nothing would get rid of it. The Grace M101 was hooked directly into the KPA for profiling and all input and output gains were optimized, and there were no stomps.


    Finally, I have several pieces of digitally interfaced gear, and the KPA is the only one that doesn't sync perfectly with my other gear.


    Other than those issues, I love my KPA and have no other complaints.

  • About the clip 1 and 2 I can definitely hear the artifacts in your clips, but I never noticed that on my KPA with my profiles... I don't know if it is something only related to that profile or you have similar problems with other profiles.



    Finally, I have several pieces of digitally interfaced gear, and the KPA is the only one that doesn't sync perfectly with my other gear.


    3) I don't understand. Are you setting every other gear's digital clock to slave? If not it's very regular that you'll have those clicks...


  • As for your next two questions, suffice it to say I am fairly meticulous in my testing--I spent some time trying to get that furry distortion out of clean profiles, and nothing would get rid of it.


    I think what he meant was: did you choose 'Clean' on page 2 out of 3 in the profiling process in this case.
    Just making sure.

  • miles,
    I'm hearing the issue your having, most audible in the last note, the Ab.


    here's a quick recording I made of the Vab Clean profile, first your Grace DI, then your profile with the issue, then the Vab Clean profile - where what I can hear in your profile isn't happening:
    (the Vab Clean profile ist just the Stack section of course, Noise Gate, Stomps and Effects are disabled. Gain shows @ 0.0; bridge HB, SD JB, Clean Sense @ 5)


    http://soundcloud.com/donpetersen/vab-clean-01


    could you please contact me via pm, so I can check out the profile you made?


    thanks.


    p.s.
    here's a picture of the waveforms. first the Grace DI then Grace DI profile, then the Vab Clean profile.
    you can see the limiting going on in the attack of your profile (not in the Vab profile, tho)


    [Blocked Image: http://i40.photobucket.com/albums/e229/donelectro/Fullscreencapture1025201210208AM.jpg]


    p.p.s.
    I repeated´this with the bass set to max and the Clean Sense maxed (clipping the input and output) - still issue-free.
    I think I need your profile.
    thx