IRs - I don't get it

Discussion in 'Digital & Modeling Gear' started by gregrjones, Dec 6, 2014.

  1. gregrjones

    gregrjones Member

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    I have a Pod HD500 that is now collecting dust since I bought a Tech 21 Fly Rig 5. The latter has a more open sound for getting Marshall & Fenderish tones.

    But I'm hearing so much about IRs. One person was suggesting that perhaps a Pod HD500 with an IR like the Torpedo Live or Epsi can keep up with a Fractal or Kemper.

    I don't understand IRs. Can someone explain? Seems to me that we're talking about some sort of technology that is supposed to better emulate speakers and speaker cabs? Why would THIS be the shortfall of something like the Pod? And what do IRs do that Pods don't do natively for speaker and speaker cab emulation?

    And how is that is that the IRs can be used with just overdrive pedals? Do they also emulate power amps?

    I've read the tech engineer explanations, but looking for something that makes sense to the average guitarist why something like the Torpedo C.A.B. would be the way to go. Thinking about adding one to my HD500 for a tone improvement but man that's a big investment....
     
  2. slugworth

    slugworth Member

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    It's simple, really. Many of the amp sims included in the HD500 are very good, but the included cab/mic sims are not very good. With a third party IR, you can bypass the POD's cab/mic sims and use much better ones.

    If done properly, the end result gives a much greater quantity of :-D
     
  3. VCuomo

    VCuomo Member

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    In addition to what slugworth said, I suspect that the POD's internal architecture (CPU bandwidth, memory bandwidth, data transfer rates, etc.) can't handle really good amp sims, cab/mic sims, and effects simultaneously (shoot, it's a $500 product) so compromises were made with the cab/mic sims and effects. After all, if you at least start with good amp sims those other things can be done externally to the POD if the user determines that they need better than what the POD itself offers. And the POD really isn't bad at all in those areas. I don't know this for sure, and an L6 guy may shoot this theory straight to hell, but that's my guess.

    Currently, my main rig is a KPA through a powered CLR. Since I kept my HD500 as a backup to the KPA, I decided one day that I should try the HD500 through the CLR (my HD500 rig includes a DT-25 for amplification). I was quite pleasantly surprised at how good that sounded, even with minimal tweaking. I attribute that to how good the CLR really is, and I bet the cause of most HD owners not liking what they hear has more to do with what they play the HD through (and maybe not taking the time, or knowing how, to tweak the HD), not the HD itself. Again, just my $.02...
     
    Last edited: Dec 6, 2014
  4. Imerkat

    Imerkat Supporting Member

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    i'm not keen on how IR's work (i think it's algorithms and not just a filter) but if you look at the EPSi you have a whole DSP chip dedicated to just cab sim. This is something the HD500 can't afford. The EPSi cost less than a the fly too. I do have to make note that a lot of pros using AxeFX still mic and bypass the IR's it has
     
  5. Scott Peterson

    Scott Peterson Administrator Staff Member

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    An Impulse Response (IR) is a collection of data representing sound measurements taken from a speaker cabinet or system. A test signal is played through the actual speaker, recorded, and used to generate an IR of the speaker/cab.

    Source: Axe-FX Wiki
     
  6. Mr God

    Mr God Member

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    An Impulse Response is an audiofile that contains the difference between a signal being played back through something and the recorded result.

    For example: Playback a sinus sweep through a speaker in a church, record it with a microphone, remove the sinus sweep, and you're left with the impulse response. That can be used to capture the characteristics of a room or a device and then used to apply that response to other signals, for example to make your recorded guitar signal sound like it was played in a church.
     
  7. Sadhaka

    Sadhaka Member

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    Mr God is correct, but it's a sine sweep or a full frequency spectrum sweep that is used.

    I think a sinus sweep is the technical term for wiping one's nose! ;-)

    Here's a link to a video of the guys at Altiverb showing you how to make an impulse response:

    http://www.audioease.com/Pages/Altiverb/sampling.php
     
  8. MaxTwang

    MaxTwang Member

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    You can consider an IR to be the 'sound of the speaker or room' or the contribution a specific speaker or room makes to your tone.

    Regarding an Impulse Response of a speaker: If you make a recording of an impulse (sine wave, etc.) though a specific speaker, then subtract the impulse from that recording - you are left with a recording of the speaker's response to the impulse (an Impulse Response). Removing the impulse is done with a process called 'de-convolution'. As you use a microphone and mic preamp to capture the Impulse played through the speaker the Impulse Response will be colored by the mic & preamp (the IR is the sound of the specific speaker, mic and mic preamp).,

    If you then apply the Impulse Response to your POD through a process called 'convolution', you get an approximation of how your POD would sound through the actual speaker/mic/preamp.

    The EPSI is a convolution processor, with EQ, which can apply an IR to the output of your POD (the ESPi can use cab or reverb IRs). The Torpedo is an convolution processor with power amp simulation, eq and a couple other effects depending on the model. When using a convolution processor you will want to disable the speaker simulation in your POD.

    IRs are generally considered 'state of the art' for speaker simulation and require a lot of processing power to convolve your signal with the IR. Whether you find a specific IR to sound better than your POD's cabinet simulation will be up to you the IR, but having a large collection of IRs can help you get closer to the sound you are gong for (or consume the rest of your days on this planet as you get lost in a sea of endless options).

    IRs can also be used to capture the sound/eq of a room, a long Impulse Response can capture the reverb & eq of a room, or hall, or cathedral. Some people have had success obtaining IRs of an acoustic guitar then applying that IR to an electric guitar to simulate an acoustic guitar.

    As processing power gets cheaper I would not be surprised to see IRs added to mid and lower priced devices. Giving those devices access to the large libraries of available IRs. Then the question is: what new and wonderful technology will the big processors introduce that 'blows away' IRs?
     
  9. Scott Peterson

    Scott Peterson Administrator Staff Member

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    For reverb and room response, yes.

    For speaker and cabinet response, most often a pop is used. You do not need (or necessarily want) longer responses with room information in speaker and cabinet responses - which is what I understood the OP is asking about in use.
     
  10. Jay Mitchell

    Jay Mitchell Member

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    That is incorrect. You're confusing the "impulse" in "impulse response" with the test signal that is used to derive that response. A sine sweep is by far the superior of choice of excitation signals for acquiring IRs, regardless of whether you're testing a speaker or an acoustic space.

    Again, you're confusing the impulse response you acquire - which you can (in fact, must) shorten as needed after the fact - with the test signal used to acquire the response. I can - and often do - acquire IRs of 20ms usiing a 10-second or longer test sweep.
     
  11. barhrecords

    barhrecords Member

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    IR's are typically used in guitar rigs to emulate speaker cabinets and or for ambience effects like room / hall reverb simulation.

    For modern devices like the Torpedo or Epsi, you can load different IR's into the pedal.

    This capability gives you more choices in that there are
    iR's available for sale from several commercial IR providers.

    So with something like the Line6, you have the built in choices or the Line6 add on packs.

    With something like the Epsi, you can buy IR's from several different places.

    The Torpedo pedal does have power amp emulation + IR loading. The Epsi pedal is only IR's; no power amp.

    Running a dirt pedal -> Epsi would be like running your pedal board into a flat power amp and then into a Cab and micing it. With the IR loaded into the Epsi being the Cab -> Mic part of the signal chain.
     
  12. nicolasrivera

    nicolasrivera Gold Supporting Member

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    FWIW there are IR out there that will fit any musical style or mixing situation any one could ever encounter, there is a kind of ongoing development on to who can achieve the best IR, but i have encounter situation where a simple IR that didnt had so much stuff in the signal chain work better then one done with the best mics, pres, or most transparent poweramps.

    And an IR will not save a bad pre amp emulator, for example putting the HD500 thru an great IR will not make it sound like an Axe Fx,not even close.

    I advice you to do a search and find as much free irs as you can, there are some trully amazing ones that rival and ofthen beat up those in expensive collections.
     
  13. Scott Peterson

    Scott Peterson Administrator Staff Member

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    True, but I've done my own for a long time with just a pop (or snap or gunshot or bang or whatever you want to call it) and they sound fine with my gear and for my purposes. That's all I'm claiming. I'm not selling IR's (and neither are you) so there's no battle to be held. If you want the room or something more complex and longer to use, a sweep will probably - just as you say - be the better choice. I thought, wrongly?, the OP was discussing guitar speaker IR's only.

    We can (and often do it seems) debate the science - though I don't really debate it, but express it as I understand and use it only (I am not a scientist, engineer or expert in any way; nor do I claim to be), but electric guitars, amps and cabs are lo-fi and frankly the results matter more to me than the science.

    So I don't disagree, but I also rely on my own experience and for speakers and cabs a gunshot, clap or other 'impulse' works just fine. Your experience and results will vary. That's okay. I don't see it as such a black and white/right and wrong thing and I base my opinion on first hand experience. It's okay with me if you disagree.

    The last few years I have relied entirely on commercially available IR's and been exceptionally happy with the results. IMHO, OwnHammer, Fractal and TAF are all extremely good however they capture them (and no, I don't know how they do it).
     
  14. Mr God

    Mr God Member

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    Some IR-measuring uses other signals than sine sweeps, that's why I chose to use 'signal'. A sine sweep is usually the best choice but sometimes they measure spatial reverbs by using starting guns as the original signal and I wanted my statement to be as generalized as possible :)

    Yes. You're completely right. Some methods use signals other than sine sweeps, but a sine sweep is almost always the best option. It is a better option for speakers and cabinets as well.
     
  15. svl

    svl Member

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    Bottom line- a Torpedo (or Epsi, but I'd shell out the extra for the C.A.B. myself) will bring that HD500 to life. It won't equal a Kemper or an Axe, but it'll get you closer to the mark at half the price.
     
  16. Mr God

    Mr God Member

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    or you can connect the Pod to your computer and run IR's there.
     
  17. Jay Mitchell

    Jay Mitchell Member

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    You're certainly free to do as you wish. However, you said "most often a pop is used." That is the incorrect statement I corrected. Knowledgeable practitioners never use a "pop" to measure a loudspeaker. Sometimes starting pistols or balloon pops are used to collect room IRs, but that is usually done because it is not practical to set up a loudspeaker (needed for a sine sweep) in the space.

    A sweep is a vastly superior choice for a cab IR. I provided a detailed explanation of why that is so on the original Axe-Fx forum, but it (the forum) now appears to have vanished.

    See above. A sweep is better, for several reasons.

    I'm talking about practical results as opposed to "science." A sweep will produce superior practical results.
     
  18. GuitaRasmus

    GuitaRasmus Member

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    Now, I know that you use every opportunity you get at bashing the Pod HD, and saying the Axe is the only thing worth having, but have you actually tried this?

    I recently aquired this setup, after having tested it with a Kemper, and Axe II and Eleven Rack. I actually like the Pod HD+CAB better than the Kemper, and it comes VERY close to rivalling the Axe II. This test was carried out with 3 other guys, who own the other devices, and have used them for a long time, so they were very much tuned in, and we were in agreement - it is VERY close to the Axe.

    Saying the Pod HDs preamps are bad is just wrong, plain and simple. The preamp is very good actually - the cabs and power amps are it's downfall... Which the CAB takes care of, luckily.
     
  19. cliffc8488

    cliffc8488 Member

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    An IR stands for "Impulse Response". In mathematical terms it is the time response of a system to a Dirac delta function (also known as an impulse).

    An IR can be used directly as the coefficients for an FIR (Finite Impulse Response) filter.

    In the modeling world IRs are obtained from real speakers and when processed using an FIR filter produce extremely accurate results. In essence an IR is a "sample" of the speaker and microphone and uses very similar principles.

    However the quality of any IR is subject to the talents of the individual(s) capturing the IR. Mic placement, preamp choice, etc., etc. are important as you are essentially recording the speaker. The HD500 uses IRs as do most modern modelers. In the old days modelers used EQ to emulate speaker response but I don't think there are many left that still use that technique.

    So the quality of the IR is really the issue here. The HD500 only allows you to use its internal IRs while many products now allow you to import IRs. The original Axe-Fx pioneered this technology which has since become almost ubiquitous.

    There are products that just process IRs (the Epsi and Torpedo stuff) and can be used in conjunction with modelers that don't offer IR import. You would shut off the cabinet emulation in your HD500 to use it with these products. If you are just playing at home you can use your computer to do the IR processing. There are both free and paid products that will run on a PC/Mac that can import .wav files and you can then connect your HD500 to your computer and use your computer for the cabinet processing.
     
  20. gregrjones

    gregrjones Member

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