It's a digital representation of a given speaker cabinet ... or an environment, such as a room or instrument body. Just as the raw guitar sound is shaped by the speaker it is amplified through, instruments played through impulse response curves are shaped as well.
To create an impulse response curve of a cabinet -- in this case my own Hard Truckers 2X12 loaded with JBL E-120s, the cabinet is mic'd with a microphone with as flat as response as possible, and a loud sine wave sweep covering the entire audible frequency range is recorded. The sweep goes from low to high then high back down to low.
The resulting WAV file is then convolved (measured) and the resulting impulse response curve is created. When any sound is played through that curve, it's as if the sound is being played through the cabinet itself and recorded with a flat microphone.
(Note that you could use a Sennheiser 421 to mic the E-120s when you're capturing the WAV file and get a very accurate representation of Garcia's sound; I didn't do that because there are several rolloff settings on the 421 and I have no idea how it was set up when Garcia's cabinet was mic'd.)
It's impressive technology ... when I made the curve of my E-120s (and also of my 2X10 with K-110 speakers), I loaded the curve into the AxeFX and took a stereo output off of there, feeding one side, panned with the curve, into my QSC K-12 full range powered monitors, and the other side into a power amp and the original 2X12 cabinet, without the curve applied. I played through both sides, muting one then the other, and it was pretty much indistinguishable from the original. Same with the K-110s. Made a believer out of me.
Any Playing Dead recording you listen to from mid 2009 or so on is my guitar through an impulse response block in the AxeFX instead of a cabinet. On my Garcia preset, the cabinet IR block is last in the signal chain before it goes direct to the soundboard and my powered monitors.
You might be able to hear the difference in a room with just the guitar, because the inherent characteristics of a 2X12 open back cabinet and the resulting room reflections are going to be different from a sealed full-range wedge sitting on the floor aimed at your face, but in the mix and the context of the band it's extremely difficult to discern any difference.
It would be very interesting to obtain a multitrack recording of 94-95 Grateful Dead concert and then run Jerry's recorded guitar signal into the AxeFX through just the E-120 curve and see how it sounds. If anyone is aware of an extended Jerry "noodle" between songs with just his clean guitar off one of those soundboards, I can give it a shot.
yes i know he is referring that but playingdead how did you program it into your axe fx? and there is a crazy fingers 94 on youtube where jerry noodles around in the begining if thats what you were looking for Dan JR
The Stag: The AxeFX is a digital modeling preamp. You can re-create any guitar tone on it if you have the knowledge and the patience to do so. So, yes, you could design a preset for any era of Garcia's tone that you wanted to, and switch between them at a gig, sending the signal direct to the soundboard. But no, you can't just download it somewhere, you have to build it from scratch, or ask someone to do it for you. For example, you could start with an amp block that models the Twin Reverb, adjust the various parameters until it approaches what you hear from Garcia's -- presence, tone stacks, power amp distortion or lack thereof, preamp tube overdrive, etc. -- then add the correct speakers (JBL E-120), spring reverb, various effects, and so on. Or start with a Mesa Boogie amp block, etc.
Phil Lesh101: Yes, That is Exactally what I is refering.
I'd say that with a WHOLE LOT of effort Vic was able to make the Axe-FX achieve a very good and convincing Jerry tone. It was a process, and may even still be a process of refining to some degree. Vic had to really know his stuff to program that thing to do it. It's pretty impressive.
... and it's just like any other day that's ever been...
I'm looking forward to giving my fellow gearheads a little tour of the AxeFX at the Gear Expo next month.
Great News!!! How is it that I can never find the time to really sit down and start working with this thing...I've got summers off and still can't find the time to do what I really want. I'm really looking forward to hearing more about what Vic has figured out about the AxeFX...he's a wealth of information on a complex piece of gear. Exactly what I was hoping the Expo would offer. I going to bring along a flash drive...just in case you've got more patches that I can cop from you.
Vic, I would LOVE a shot at your rig! I won't be making iot to the expo unfortunatly... Also, if I was going to model a late Garcia GD tone it would the Jack Straw jam from this show... When Bobby's rig goes out Jerry is up for the challenge. I am pretty sure this is the Bolt guitars unveiling.
one of those axe fx things sounds like the way to go because i just gigged for the 4rth and carying 4 jbl e120s a fender twin amp matching box for effects is getting old. plus all the other band members equipment.