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

Although the KPA is one of the most awesome and important pieces of guitar gear ever, there is an issue that you might want to know about: I noticed it immediately, particularly on high-gain profiles, when playing high notes on the E string from about high G and above (though I can detect it on a few notes below that as well), there is a low-level, but clearly audible, low-pitched harmonic digital noise that is underneath each note played. I tend to believe that this is aliasing due to the processor not being able to accurately render complex high frequencies. I have heard the same artifact in all the PODS up to and including the X3, but Line 6 eliminated it in their HD series--it can be heard in some other modelers as well, but I never expected to hear it in the KPA.

KPA Aliasing Example

I contacted Kemper about this, and they replied: "We can reproduce this and are investigating. We are aware of an issue regarding stomps, which is planned to be fixed, but this is different".
In a later email to me they replied: "The 'wind howling' noise is something our developers are planning to fix in a future firmware release".
I hope they can fix this, but it seems as though its a bit depth / sample rate / processor limitation type of issue... I hope I'm wrong.
I have included a link to a sound file that I made that demonstrates the noise, and is the same one I sent to Kemper--listen closely, and once you recognize it, you will likely begin to notice it in your machine as well, and it is in all amps, even clean ones to some extent (though you have to listen really closely).

You begin to realize how this compromises purity to all profiles, to some extent--though distorted chords and low to midrange single notes don't reveal it, and fortunately it is inaudible under most normal playing conditions, and would likely get lost in a live or recorded mix--still, for the golden-eared among us, it is troublesome.

Maybe Kemper will pull off a magic update and make it all better--after all, they have already demonstrated their level of genius!

Quote from and44

Thats intersting, and also a shock!.
The main reason I always prefer real to sims is for this very reason.
I was able to detect the "fizz" from ALL HW moddelers, Axe, 11R, L6 etc, and they became unusable in my envorinment, but the KPA did not have that on test, hence my purchace. - Might be that your monitering system is a little better perhaps, or cables are finer. - but I certainly dont hear it, and as your the first to really talk of it, then guess im not alone.

Normally I listen out for this always, and felt confident in the KPA.

Granted, I do listen-in very deep with good monitors and headphones, but I can also detect it when playing casually, at times. You should feel confident with your KPA, I am in love with mine--I'm just sharing an observation, and apparently Kemper has acknowledged what I'm hearing.

The fact is that the noise is not at a high degree, but it is there, and can be heard almost subliminally when playing leads. Though not a glaring noise problem, once they eliminate it, we can have the transparency that we deserve with such an otherwise great device.

Many guitarists may never notice it. It wouldn't be the first time I noticed something like this--I'm kinda geeky that way.

The fizzyness on clean sounds is only noticeable when you push the high-end presence for a crystalline sound, not my main concern as much as the "wind howling" on high-gain leads--I think they are related.

Quote from MadH

It does not sound like digital aliasing to me but something else. Do you get the same noise with the noise gate disabled?

Yes, the noise gate doesn't make any difference.

The reason I'm calling it 'aliasing' is because Kemper told me that there were aliasing issues on the distortion stomps that will be fixed on the next firmware update, however, the nature of the amp noise sounds almost exactly like the stomp noise to me, but Kemper says its "something different". I did a test where I turned off everything but the dist stomp, heard the "aliasing" noise, then turned off the stomp and turned on the amp and heard the same noise--which I am referring to as 'aliasing' due to its similarity. Just a coincidence?
So, if it is true that the stomp "aliasing" will be fixed, then that's great, but, if according to Kemper, this "wind howling" really is different in regards to the amp profile, then it concerns me a bit as to whether it's fixable in firmware--after all, the amp profile is what likely consumes most of the processor's horsepower.
I don't want to overstate or seem alarmist about this, because the KPA is seriously rocking my studio right now, but a precision, pro unit like this deserves the scrutiny.

By the way, does anyone know the bit depth and sample rate of the KPA? (A reasonably high resolution is desired).

Quote from mikeb

I hear this type of thing via SPDIF any my EMU sound card, the SPDIFs are incompatible in my case, I just switch over to the XLR outs and I am fine. No high end digital aliasing.

I too am using the analog outs--I'm curious, if you tried a test similar to mine it would seem that you should experience the same thing. I assume you listened to my sound file and heard the noise? It is subtle.

I have a second KPA coming here in a few days, and will of course be A/B-ing to be sure.

Quote from guitarnet70

Another fellow dropped me a PM regarding this thread: Is this not the same Aliasing, Pulsing, Numerical noise issue that have popped up 1 month ago and has been solved?

There are a couple of threads already, GeneAndEddy was the guy IIRC..... Search for "Numerical Noise" or "Aliasing" and you'll find it

The issue I raise has clearly not been resolved, as I have the latest firmware, and the acknowledgement of Kemper. I guess the answer is no.

Quote from r_u_sirius

this "suffocating alien" noise on high notes is there in almost every hi gain profile....not good!

Great description, "suffocating alien"! If you play the same high notes with a clean sound (preferably with 'phones on and listening very closely), the noise has more of an atonal drone compared to the high gain noise which modulates a little bit--they both are related to the same issue, I imagine. It is probably the only reminder sound-wise that it's a digital device that otherwise sounds nearly analog.

I'm super-curious how and when they can remedy this--best of luck to them!