Nice one. Looking forward for the bass side discussion. Thanks.
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[youtube]http://www.youtube.com/watch?v=CjeleesHTm8[/youtube]

Tonight, I finally watched a video which I had put in my "Watch Later" list on YouTube more than two years ago. Sadly, Mike Scaccia died in the meantime (apparently due to a seizure induced by strobe lights).

What is the point of adding to this thread? Well, the video happens to have a few great illustrations of what I mentioned in FHF Episode #40 about the way digital modeling interprets signals based on algorithms. At several points in the video, listed below, Mike bumps the strings of the guitar he is holding; clearly, the ridiculous response of the modeling amp (an Eleven Rack) does not represent the dynamics of the interaction. The bumps can be seen (or heard) at:

• 2:42
• 8:39
• 8:47
• 8:50
• 8:57
• 8:59
• 9:31
• 9:37
• 10:25
• 12:15
• 12:20
• 12:34
• 12:36

Those might seem subtle, but I am sure a replay or two will be all you need to realize that it does not sound like bumping a guitar while connected to a tube amp (with delay in the loop). In the terms I used in the episode, it is not "an honest reaction." The bumped guitar is the ever iconic Gibson Les Paul Custom: without a doubt, the Eleven Rack was tested with such a guitar prior to release. The presumptions of the algorithms were tweaked to sound as good as possible with such a guitar, but the Stick gets no such favoritism. This is why I do not recommend digital amp modeling.

I don't quite follow what one would expect to hear as far as the difference between a real and modeled amp for those incidental bumps on the guitar?

There seems to be quite a range of approaches to amp (or more generally analog) modeling in the industry, some very simple and some very complex. When we developed Tube Saturator at Wave Arts we realized there is a lot of marketing and not so much concrete detail about how different companies develop analog modeling - a tube amp could be modeled with something as basic as a lookup table that converts input to output samples based on a curve depicting the input to output voltage behavior of a simple triode amp. And many products probably use something like this to generate harmonic content similar to what happens when overdriving a tube, but something this simple won't be very accurate for larger systems with multiple triode stages and complex interactions of many analog components. So Bill Gardner (Wave Arts founder) came up with a sort of real time SPICE simulator (a program used to design analog circuits) that is solving a set of non-linear differential equations every sample period to accurately capture the grid current in a system of analog components, and this leads to very accurate audio output compared to the original circuit it's modeling. The downside is that it's exceptionally CPU intensive and is only practical for small circuits at this time (Tube Saturator is a simple Baxandall 3-band equalizer and a two stage triode preamp) - we didn't even attempt to deal with cabinets, power amps, and other things that contribute to the sound of a instrument rig.

We made a point of not shrouding this product in a bunch of marketing BS, Bill lays this all out in the TS manual: http://wavearts.com/uploads/docs/TubeSa ... Manual.pdf

The point is that I'd be wary of dismissing digital amp modeling outright, when it's apparent there is a huge range of approaches to the problem and we may not be quite at the point where it's practical to model massive circuits down to the last intricate details, but we're getting closer. I'd be very curious to know what kind of approach Cliff at Fractal Audio is using for this amp modeling as it seems to be quite highly regarded. While I've used NI Guitar Rig a lot, I get the feeling that the DSP algorithms aren't exceptionally complex as they need to run many modules at once within a reasonable CPU limit.

Thanks very much for that look behind the curtain, Rob! I do not mean to discount modeling for all time, only modeling as it stands (which, to clarify, divides DSP resources among dynamics, harmonics, distortion, equalization, and compression simulations—and also effects).

I look forward to a day when Bill's SPICE-based approach can be used to model complex circuits (such as my EVH 5150 III guitar amp), in real time, in a cool and quiet rackmount chassis. I also look forward to affording such a beast.

Rob, you handsome devil...

K