Those of you who have been with us from the early days may remember my aliasing tests. I have repeated these tests on some of the latest products available, including the Axe-Fx II. For those who don't remember, aliasing is the bane of modelers. It creates the dreaded fizz and can even create noticeable tones if bad enough. To prevent aliasing requires oversampling and well written interpolation/decimation algorithms.
So I gathered up the Axe-Fx II, a modeler we'll call "Brand D" and another modeler we'll call "Brand K". I wanted to included a modeler from "Brand L" but for the life of me I couldn't figure out how to shut off the cabinet modeling in it (which distorts the measurements) so I was unable [edit: I have the lower tier version of the product which I have found out does not allow turning off the cabinet modeling]. All of the modelers are running the latest firmware from the manufacturer (as of today's date). The Axe-Fx II is running Version 7.00 but that has no bearing on these tests. Much care was taken to ensure that the input/output range of the devices was not exceeded. The sine-wave source was the synth block in the Axe-Fx II as it is a very high-quality signal generator and actually produces a purer tone than my SRS synthesized signal generator.
On each modeler I used an SLO 100 model with the drive roughly at 7.0 and the cabinet modeling turned off. I chose the SLO100 because it is fairly high gain. The more gain the worse the aliasing but the SLO100 models were fairly close in tone between the three modelers so that seemed to be a good choice even though not the highest gain models available. I matched the gains using an intermodulation measurement and then verifying be ear. In my original tests I input a 10 kHz sine wave into each device and measured the output spectrum. An ideal device would only create harmonics of 10 kHz, i.e. 20, 30, 40, etc. kHz. Any tones that are not harmonically related are aliases. 10 kHz is a nice value because the aliases are then mostly below the fundamental and easy to recognize.
Since my original tests I have devised a new test which is more illustrative. I now sweep the input from 9 kHz to 11 kHz. This excites all the various aliases and prevents cheating since one could adjust the input frequency slightly to improve the results. I set the spectrum analyzer to peak-hold and recorded the results. The graph below consists of the results of all three devices. The blue trace is the Axe-Fx II, the magenta trace is Brand D and the green trace is Brand K:
The ideal result would be a rectangular pulse sitting above the noise floor. Anything outside the pulse from 9 to 11 kHz is aliasing noise. 60 dB is considered the threshold of audibility so if the aliasing noise is above 60 dB, then theoretically it is audible.
The Axe-Fx II scores best in this test. Brand D does well and, actually, better than the original Axe-Fx. The results of Brand K are surprising. I have repeated the tests at least a dozen times fearing that I have made an error but, alas, the results are correct.
I can also post sample clips of tones into the three devices that demonstrate the audibility of the aliasing, if desired.
[edit: I have been informed that you can indeed turn off the cabinets in Brand L now. I still can't figure out how but once I do I will try to repeat this test on that modeler and update the results.]
