W
WhiteNite007
New member
Being that there are so many submission on metal guitar wars that utilize impulses, can someone explain what they do and what they are?
kcearl
I see deaf people
What is an impulse?
Speaker cabinet simulation seems to be the final frontier of guitar amp modelling. While the preamp, tonestack, and power amp sections have been accurately modelled by numerous companies for some years now, the speaker cabinet component of amplifier modellers have not reached the same level of accuracy as the other parts of the signal chain. This problem, however, can be easily solved using what are called impulse responses, or IR’s.
An impulse response is defined as the reaction of a dynamic system (a speaker cabinet in our case) to an external change (a sine sweep or test tone in our case). This reaction is expressed in the difference between the original sine sweep and the recorded signal that has been processed by the power amp, speaker, microphone, and if used, mic preamp. This difference between the sine sweep and the recording is the character of the recorded speaker cabinet. While this explanation may seem a little confusing at this point, however, it will make more sense once you’ve reached the end.
An impulse response is ideal for capturing the so called linear characteristics of a system, such as the frequency response and reverb, but poor for capturing the non-linear characteristics of a system, such as compression or distortion.
Its excellence in capturing the linear characteristics of a system makes it near perfect for capturing the character of a recorded speaker cabinet, which is mostly linear. At time of publication, a cabinet impulse is the most accurate method of software-based cabinet simulation, and has been for some time.
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Where To Get Impulse Responses
Impulse responses come in the form of .wav files. You can get them in a number of places, such as the Andy Sneap forum on the Ultimate Metal site, the "Impulses" subforum on the Guitar Amp Modeling site, and, of course, here. You can also search the internet for them, but the majority of impulses in existence can be found on the aforementioned sites.
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How To Use An Impulse
In order to use a cabinet impulse, you first need to disable the cabinet in the amplifier simulator that you’re using. How to do this varies by program. Some, such as DIG, Amplitube, the Aradaz amps, and BTE Audio’s excellent Juicy 77 (a Soldano SP77 simulation) have cabinet bypass buttons, while others, such as the SimulAnalog amps and Nick Crow’s “Combo”, don’t have the ability to bypass the cabinet. Still others, like LePou’s SoloC, Nick Crow’s Wagner Sharp, 7170, and 8505 don’t even have a built in cabinet, which makes impulses a requirement for any use.
There are a number of programs that load impulse responses, ranging from free products such as KeFIR or Boogex (which can also double as an amplifier simulator) to paid programs such as Peavey’s Revalver or Voxengo’s Pristine Space. Different people have different preferences regarding what program they use, so feel free to make your own decision. I will explain how to properly use each of the free ones, as you probably already know how to use the commercial programs if you own them.
Neither KeFIR nor Boogex’s default settings were designed with cabinet impulses in mind, however, the necessary knob tweaking is minimal.
For Boogex, set the “Pre filter” knobs (Lows, Mids, Highs) to 0db and the “Amplifier” knobs (Tone, Drive, Dynamics, Phase) to 0. Out can be set as needed, as it's the volume control. The “Dry Pre Cab” and “Dry Post Cab” controls should be set all the way down to "-inf dB”, and the "Speaker Cabinet Impulse Response" button should be on.
Boogex also features a low pass filter (LPF) and high pass filter (HPF). While it is not possible to bypass them completely, it is possible to set them so that they have a minimal effect. By clicking on the white point in the left side of the graphic display and moving it all the way to the left, and doing the same for the green point on the other side, only moving it to the right, the filters will be set fully open, and thus only effect extreme frequencies.
In order to load an impulse file, you have the choice of using a number of internal impulses, as well as the ability to use external impulses, such as those of your own cabinet (more on that in the next installment).
For loading an external impulse, click the “File” button, and browse your way to your impulse collection, select the desired impulse, and click “Open”. Once you have loaded an impulse, you can switch to any other impulse in the folder by clicking on the name of the impulse, which brings a list of the other impulses in the folder. Alternatively, you can scroll through the impulses by using the pointer buttons to the right side of the name of the currently loaded impulse.
The biggest downside with using Boogex as an impulse loader, aside from its complexity, is the fact that it will add 96 samples of latency to your signal. While that’s only one 500th of a second at a 44.1kHz sample rate, it still is some latency, which is undesirable when using an amplifier simulator in realtime.
KeFIR, on the other hand, is a lot simpler. It only has three knobs, labeled “Mix”, “Gain”, and “Length”, as well as a button labeled “Load” for loading impulses, and two others, “L” and “R”, for bypassing the impulse on that channel.
The “L” and “R” buttons don’t need to be tweaked, however the “Mix” knob should be set to 100%, as otherwise you will get some dry signal mixed in with the signal that the cabinet processed.
The “Length” knob typically shouldn’t need to be tweaked, as it adjusts itself automatically for the length of each impulse. However, some people forget to cut the tails off their impulses, and as a result you should cut it down to about .05 seconds, or a little after where the green part of the line in the graph goes flat.
The “Gain” knob can be set to taste, and also auto adjusts for each impulse.
http://www.acmebargig.com/forums/viewtopic.php?f=31&t=44
