Amp Modeling in the Digital Age

Having spent the last couple of weeks delving into some guitar effects research, I have come across a lot of mis-understandings on the analog vs digital guitar effects debate. Of course people can decide what they like to hear for themselves, though I’d still like to see some double blind sound tests. Those that claim they can tell a real tube amp from a modeling amp should be put to the test.

The number one myth I keep seeing as I read articles on internet is that digital signal processors cannot truly represent an analog signal because they are taking a finite number of samples per second. Now I don’t want to go into sampling theory for band limited systems here since there are many well written discourses on this subject. I’ll simply state, and leave it as an exercise for the reader to verify, that this belief is completely false. Analog systems also have finite resolution in amplitude because of electronic noise and finite resolution in time because of inherent system bandwidth. The belief that analog signal representation provides for infinite resolution is completely false. If one samples an analog signal at or greater than the highest frequency component in that signal, the reconstruction of that signal is complete and there is no loss of information. This is fundamental to digital signal processing and is why it works.

So what about stomp boxes? which is better analog or digital? Well, it depends on the application. If you are driving a real tube amplifier then an analog pedal is better. Why? because of simplicity. A pedal like the Tube Screamer is a simple OP-AMP circuit that provides for some clipping and equalization. Its purpose is to drive a tube amplifier to get saturated tone without having to crank up the amp all the way. However it is only functioning as a preamp. You cannot take the output from the Tube Screamer and go direct to a recording device. Well, you can, but you won’t get a good sound. There is no point in doing a digital implementation of this trivial analog circuit. So, its not because a digital implementation would sound inferior, its because it doesn’t make since to implement a preamp in DSP.

Now, how about amp emulation for direct recording. This is an area that was pioneered by Tom Sholze of Boston fame. Tom Sholze was obsessed with direct recording of saturated guitar amp tone. That is, recording guitar distortion sounds without using microphones set up around a real overdriven amplifier. An early approach he employed was to build a dummy load for a guitar amp. A speaker presents usually around an 8 ohm impedance to the output stage of a guitar amplifier. This impedance represents the conversion of the electrical energy from the output stage of the amplifier to sound energy transmitted to the air by the speaker cone. However, one can build a ‘dummy’ load that will convert most of the amplifier output to heat instead of sound. And then recover the signal as it appears across the dummy load to create a low volume version of the amplifiers output wave form. This way the output stage of the amplifier is saturated as it would be driving a speaker and you can direct record this waveform without even using a speaker. Now these devices are called ‘power soaks’. The second generation of direct saturated guitar tone consisted of an analog circuit that would later appear in the ‘Rockman’ product developed by Tom Sholze. As it turns out, simple solid state clipping circuits, like OP-AMPS with diodes, can be made to sound remarkably like an overdriven tube amp if the proper frequency equalization is employed. This is what Tom Sholze next accomplished. He found the right combination of clipping and frequency equalization to get a very realistic sounding signal that could be recorded directly without using speakers and microphones. For those of you that have played with the Rockman, I think you will agree that it sounds pretty good direct to headphones. As you can see the purpose of an amplifier emulator is quite different than that of a distortion stomp box. The stomp box is a preamp that will augment a tube amplifier into producing saturated guitar tone. The amplifier emulator is a complete representation of a tube amp and speaker cabinet.
So where does digital come in here? The Rockman is pure analog and can emulate an amplifier. Well, actually this area is where the digital approach really shines. The rockman is cool, but the problem is that the range of equalization that can be created is very limited. The Rockman uses real analog active filters based on OP-AMPS and as such cannot implement complex low pass responses that are easily done on a digital signal processor. In addition, the type of nonlinearity used to produce clipping is completely programmable in a DSP system. The companies that build amp modeling units use a combination of white box analysis ( studying the circuits in the amplifiers) and measurement characterization to encode very detailed models of the amplifier they are emulating. These units , when used for direct recording or listening, sound amazingly realistic. There is simply no way for simple analog circuits to accomplish that degree of system modeling, without actually building the real system ( amplifier + speaker cabs).

Musicians tend to be retro oriented and still favor the stomp box to multi-effect units. However that says more about the desired user interface than the guts of the pedals. Digitech, for example, offers it’s effects in stomp box form as well as integrated even though the stomp box’s are still using DSP. The gigging musician prefers the separate pedals perhaps for ease of actuation, but they are , in many cases, still using DSP.

Just as audiophiles have rigid beliefs about vintage gear sounding better, so do guitar players have strong opinions on how to get killer tone. In the end, each musician uses what gets them that tone. But sometimes we have to have an open mind about the new technology and ask ourselves, Are we using some retro piece of equipment because it really sounds better, or because it’s cool to be retro? Recording without microphones is a great thing and digital guitar processors make this possible without giving up the tone.

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