Help, strange digital artifacting sound when using amp sims

...and how to do so...
Laserpulse
KVRer
4 posts since 29 Sep, 2019

Post Sun Sep 29, 2019 8:25 am

Sorry if this isn't the right place to post this, it's my first time posting here and I couldn't find a general support area on the forum. I've posted this elsewhere on the internet but I turned to here because I didn't get any response on the issue. Here it goes:

I connect my guitar, a PRS SE Custom 24 with stock pickups, to a Behringer UMC204HD which is connected to my PC which has an i7 4790k cpu, where I usually have the audio interface set to 41000Hz and a latency buffer of 128. The computer can more than handle this. I've tested this with multiple different kinds of headphones and monitors connected to the audio interface and this problem happens on every set.

On the majority of amp sims that I've used, when playing high notes around the range of the B and e strings above the 12th fret, if I listen close, there's a strange digital sounding warble that's lower in frequency than the note I'm playing. The audio interface is not clipping and the gain is set rather low. The two biggest amp sims I've had this problem with have been BIAS and Amplitube.

The only amp sims I didn't have this problem with is the free Ignite amp you get when downloading NADIR, and the PRS Supermodels amps if you turn the amp's bias setting to 0, which is unfortunate since I like the sound of that setting turned up a bit. This also does not happen when increasing the harmonics knob in NADIR.

Normally I'd just assume something with my hardware is messing up somewhere, but the fact that the free amp sim doesn't have this issue while the ones people pay hundreds of dollars for does is a little insulting, and I would hope there's something that can be done to reduce or eliminate the artifacting in the other sims. I *could* use the amp sims that don't have the problem and not worry about it, but I'd like to get my moneys' worth for the others that I've paid for. Any suggestions for this?

User avatar
Rockatansky
KVRist
324 posts since 3 Jun, 2017

Re: Help, strange digital artifacting sound when using amp sims

Post Sun Sep 29, 2019 9:09 am

Laserpulse, meet Aliasing, a typical symptom of digital distortion. Distortion creates new harmonic signals at multiple frequencies of the original signal. If the frequencies of those new harmonics are higher than a certain threshold frequency (1/2 samplerate = Nyquist = 22.050 at 44.100) then they will fold back below. Simple calculation: folded back frequency = Nyquist frequency - (harmonic frequency - Nyquist frequency). So e.g. a harmonic at 23.000 will fold back to 22.050 - (23.000 - 22.050) = 21.100 Hertz, and a harmonic at 44.000 will fold back to 22.050 - (44.000 - 22.050) = 100 Hertz. The game continues in the same way as soon as the folded back harmonic hits 0 Hertz, then it ping-pongs back upwards. The higher the distortion amount is, the more harmonics will be created, and the higher up past the Nyquist frequency they will reach, so the more they will pollute the audible spectrum by bouncing back and forth. The common way to battle this is oversampling, which in simple terms 'extends' the sample rate and moves the Nyquist cutoff frequency higher up. This means the harmonics are calculated at their actual frequencies, and are then filtered out when reducing the artificially extended sample rate again. If the distortion amount is high, oversampling doesn't get rid of all aliasing, but it helps. The effectivity of oversampling for distortion units largely depends on the used filters, and it's always a bit of a trade-off between CPU efficiency (=speed) and filtering efficiency (=precision). If your amp sims have an oversampling option, enable it and see what happens. You can test all of this by running a simple sine wave into the distortion and checking with a spectrum analyzer behind it.
Confucamus.

Laserpulse
KVRer
4 posts since 29 Sep, 2019

Re: Help, strange digital artifacting sound when using amp sims

Post Sun Sep 29, 2019 12:13 pm

Rockatansky, thank you for your time and the in-depth explanation of what I'm hearing. I started to look and play around with the amp sims again with this new knowledge and found:

1) BIAS does not have an oversampling option, but raising the sampling rate to 96KHz and the latency buffer to 256 eliminates the aliasing. Passed!
2) On all 3 of the PRS amps the aliasing is very noticeable even at 96KHz when the bias setting is set to anything other than 0. There is no oversampling option. Failed.
3) There is an oversampling option in Amplitube, but it's just a checkbox and doesn't tell you by how much it oversamples what you're doing. Even with the boxes checked there is some faint aliasing when the amps are cranked, which isn't something I'd be doing with Amplitube anyway, and other instruments in a recording would most likely cover it up. Passed!
4) The Ignite Emissary amp has a clearly labeled oversampling option right under the amp interface and hasn't been a problem.

A bit disappointing how the amp that's free is better designed and is easier to just plug-and-play, but I blame advertising and having to make a program look attractive in a screenshot to sell it. As an outsider who doesn't know much about this it's easy to write the developers of the paid sims as lazy, but to be fair, the Ignite amp is just one amplifier with no effects, while something like BIAS and Amplitube could have thousands of unique permutations of amps and effects, I can't imagine it being easy to optimize every permutation for an optimal sound. Feel free to prove me wrong, though!

I was able to find a spectrum analyzer in Reaper, but I couldn't find an option for it to run a sine wave. I'm not too sure what I was looking for in it, but I did find spikes in lower frequencies than the note I was playing, which is what I assume you were mentioning with the Nyquist and harmonic frequencies with the calculations. Still, your post has helped a lot. Thank you again.

imrae
KVRian
830 posts since 2 Jul, 2010

Re: Help, strange digital artifacting sound when using amp sims

Post Sun Sep 29, 2019 12:36 pm

It's probably worth mentioning that the Ignite Amps sims have been widely praised as being of outstanding quality. They're not necessarily representative of freeware in general.

Interesting to hear that the Waves PRS has such noticeable aliasing. I wonder if its supporters have been working with lower gain levels?

Laserpulse
KVRer
4 posts since 29 Sep, 2019

Re: Help, strange digital artifacting sound when using amp sims

Post Sun Sep 29, 2019 1:07 pm

Noted.

I was sold on the PRS amps from watching videos and all, and even after using the free trial I didn't notice it. I only started noticing it after having spent a lot of time in it, and I'd already recommended them to others before noticing. I bought them on a big discount for only $30, so it's not such a big loss for me, but others might not have been so lucky. At first listen the amps sound great, way more natural sounding than other amp sims, especially since the standalone non-DAW version lets you load third party IRs. I still might use them but have the amp itself turned off just for the air knob, since I really like the sound of that.

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