Custom IR: muddy side-effects

[saemola]

I'm dipping my toes into creating my own IRs for the Helix. It's mostly experiments with EQ curves, I'm not really interested in cabs right now.

When loading my own IR, I'm finding an odd side-effect: my signal gets muddy.

There is a noticeable boost in low end and I feel like something odd is happening with the phase (though I'm not 100% sure on that one).

This is true even if I'm just sampling a flat eq with a notch at 6kHz.

 

Is there something in the process of creating an IR that might create this side effect?

 

I'm using Logic's Impulse Response Utility, sending the sweep out to Soundflower into a channel in Logic with an instance of FabFilter Pro-Q2. No linear phase is activated.

I then convert the files to Wav, edit them in Pro Tools and bring them into the Helix.

Another thing I'm noticing is that unless I use a hi-pass filter, I get some DC-offset.

Anyone had similar experience?

[saemola]

After some experimenting, I realise that the shorter I trim the IR the better less mud I get.

But there is a tipping point after which if I cut it too short I start losing the effect of the sampled IQ.

That threshold seems to be 3 wave cycles.

Is there a way to calculate how short I can go with the IR?

Should I consider just keeping everything above a specific amplitude and trim the rest?

Also, it sounds to me like fading in/out doesn't matter as long as I'm at zero crossing.

 

I'd love to read something about this, so if anyone has references please send them over. I can't find anything online.

[Ilya-V]

You're doing something wrong in your IR creation process.

 

I created EQ IRs for the Helix in 48kHz 24bit and they perfectly match the EQ curve in my DAW I used to create this IR.

I even created a flat IR (transparent) to see if the Helix IR loader colors the sound in any way... nope, it's absolutely transparent with white noise and spectrum analyzer when the IR is 48kHz 24bit.

I'm using Voxengo Deconvolver to create IRs, there are plenty of tutorials online for this software.

 

Here is the EQ IR, and actual EQ in my DAW:

[Ilya-V]

Note that if you are using 44.1kHz IRs the Helix will convert them to 48kHz during import, this process is not completely transparent and changes the curve slightly.

It appears like a 6db boost above 10kHz when the Helix converts 44.1kHz IRs to 48kHz.

For guitar cab IR's it's completely non issue, even more so if you are using a a lowpass on top of the IR.

[saemola]

Thanks for the reply Ilya.

I did a test sampling the same signal chain, but with none of the EQ bands engaged. The result was a flat sounding IR.

This is a screenshot of the IR giving me troubles. The cursor is at about 0.8 into the sample.

 

If I can ask a few questions, in your experience:

1- Is there a correlation between IR length and frequency resolution?

2- Is everything before the transient "supposed" to be there? I sampled with a sine sweep.

3- Where would you trim the sample?

4- Is a minimal DC offset to be expected?

 

Cheers!

[Ilya-V]

1. Yes, 2ms is enough to capture the entire frequency response.  Helix allows for 2048 samples IR maximum, that's 2048/48000 = 42ms,, Itwill trim everything after that when importing.

2. Proper IR should have nothing before the transient (see my image), that's probably the source of your problem.

3. No need to trim anything, the Helix will do it for you.

4. I don't know, probably not.

 

Whatever your tools for creating IRs, consider changing your method. There are plenty of tutorials online.

 

[saemola]

Thanks Ilya.

The issue is I haven't found anything for Mac other than the Impulse Response Utility. For example, Voxengo is PC only.

I'll keep searching.
Thanks for the tip about the pre-transient bit!

[gunpointmetal]

Is it necessary to send the audio out via Soundflower? Can't you just send the output of the track with the sweep through the input of a second track with the EQ you want and record the output of that track? I've done it in Reaper that way a few times with no issues.

[saemola]

Just now, gunpointmetal said:

Is it necessary to send the audio out via Soundflower? Can't you just send the output of the track with the sweep through the input of a second track with the EQ you want and record the output of that track? I've done it in Reaper that way a few times with no issues.

How do you mean exactly?
The sweep happens in Impulse Response Utility, and needs to be recorded back into IRU.
However, I can't load plugins in IRU, so I have to send the signal from IRU, to Logic to process it with the EQ, back into IRU to record and deconvolve it.

 

Are you suggesting something else and I'm not getting it?

[gunpointmetal]

 

6 minutes ago, saemola said:

How do you mean exactly?
The sweep happens in Impulse Response Utility, and needs to be recorded back into IRU.
However, I can't load plugins in IRU, so I have to send the signal from IRU, to Logic to process it with the EQ, back into IRU to record and deconvolve it.

 

Are you suggesting something else and I'm not getting it?

Right, I get what you're doing. I'm just not sure how the routing in Logic works, or the function of the IRU, but for my IRs I could send the sweep from one instance of ReaVerb (reaper stock plugin with IR creation functions) and route the output of the track that is on, into the input of another track with my EQ plugin loaded, record the OUTPUT of the EQ plugin track, then import that audio file back into ReaVerb for convolution into an IR. It all happens in the DAW, no need of for external patching. 

 

EDIT: does IRU let you export a sweep file instead of doing it in real time?

[saemola]

I can export the sweep from IRU, process the file in Pro Tools, but then I need to deconvolve it.

I can't reimport it into IRU as it needs to be a .SDIR file, and for the life of me I can't find a way to convert .WAV to .SDIR (though I can find the other way around, which I'm currently doing with the XLD app).

 

I'm giving Reaper a shot right now!

[SaschaFranck]

If you stay within a DAW, there's no need to use any IR creation utilities. Send a dirac IR through whatever you want to be captured, bounce it and possibly trim it. Works very well.

Here's a bunch of dirac IRs in all common formats (mono, stereo, various samplerates, various bit depths):

http://www.saschafranck.de/tmp/diracs.zip

 

Fwiw, also works nicely in case you want to capture an algorithmic reverb in an IR (or mix them), just anything dynamic and modulating won't be captured.

 

[Ilya-V]

47 minutes ago, SaschaFranck said:

Send a dirac IR

 

 

This is brilliant!

Thank you!

[SaschaFranck]

Fwiw, just to add this: A "dirac" IR is kinda like a "neutral" IR as a spike (sometimes also used to capture things in the analog world, but for these cases, usually a sweep and a deconvolving software is the better choice). When you load such a dirac into any IR block/plugin, you should notice no effect. But once you send it through whatever it might be (in our case perhaps any kind of cab sims, an EQ and what not) and do a mixdown, the sonic properties of the things you've sent the dirac through will be applied to the resulting mixdown file, which in return can be used as an IR on its own.

 

Fwiw #2: You might get away fine with placing the dirac straight on a beat "one", bounce a very short passage from there and load the resulting file straight into, say, the Helix. But for whatever reasons, I noticed that sometimes this file wouldn't mix properly with other IRs (causing phase issues and such), so I'm now checking each and every bounced file, trim the beginning manually (just at the first sample into the first transient peak, stoie this from Ownhammer and RedWirez, they seem to be trimmed that way as well) and shorten it to 1024/2048 samples (usually just 1024), so I know it'll sound identical in the Helix, compared to an IR loading plugin in my DAW.

 

Fwiw #3: I'm using this very method to mix my own IRs since a long time already. I usually split an "un-cab-ed" recording (from the Helix or whatever I might be using) through multiple mixer channels of my DAW (in my case Logic) via busses, each loaded with an IR plugin, EQs, exciters and sample delays (can do some wicked things when mixing IRs). This allows me to quickly blend together some single cab/mic IRs every bit the same as you'd do when mixing a multi-mic'ed take, in case the IR maker isn't supplying IR mixes on their own.

 

[saemola]

Sascha, what a great idea!

I tested your Dirac and it's as transparent as can be: if place it on a parallel path, raise the level by 6.2dB and flip phase, I get virtually complete cancellation.

I'm gonna use this for all my EQ sampling needs. Thanks so much!

[SaschaFranck]

You're welcome. As said, this technique can be used for a plethora of things.

For instance, I also used it to create an IR out of a mix of an acoustic guitar IR and a match EQ (with a great sounding nylon string as the source) to enhance my cheesy nylon strings piezo.

Or, as a more simple thing, I lowcutted pretty much all my live IRs drastically, even if I didn't do much else to them.

I'm also experimenting a bit with pickup modifying IRs and such.

Plenty of options, plenty of fun.

[saemola]

Acoustic modelling would be my next step.

Pickups modifiers are exactly what I'm trying to achieve right now, but I sample FabFilter as it's simply a much better EQ than what's on the Helix.