Tip: save CPU by decreasing IR Length

[yek]

Some months Cliff talked about decreasing the length of IRs to avoid reflections. See quote at the bottom.

An nice side-effect of decreasing IR length is that CPU usage decreases.
In a preset with a two Hi-Res IRs, decreasing the length of both IRs from "MAX" to 512 samples, decreases CPU with 8%.
When using two UltraRes IRs, the savings are a little less, but still 5% or so.
And no loss of quality whatsoever, at least to my ears.

The "IR Length" parameter is currently available only on the Axe-Fx III.

"I've been experimenting with IR length lately and keep finding that I like a shorter length. So I gave some thought to it and I think the reason is that a shorter IR trims off the early reflections. A 1024 sample IR is over 20 ms. If there is a wall 5 ft. away that puts a reflection smack dab in the middle of the IR. The Redwirez IRs you can see (and hear) the ceiling reflection pretty clearly (ceiling was probably about 8 ft). Using a shorter IR removes that reflection. I've actually been turning down the IR length lately on my personal patches, typically 512 samples, as I find it makes the IR "clearer". While the push in the industry has been towards longer and longer IRs I'm not sure that's a good thing unless you are careful with your IR capture to ensure that you aren't capturing reflections. Some IRs, particularly ported bass speakers, may need the longer length to capture the low end with sufficient detail but the average guitar cab is probably fine at 512 or even less samples. Heck, prior to the original Axe-Fx some products were even using 128 samples"

 

[biskitboy]

Nice bit of info!

 

[warlockII]

I remember the original post, but forgot all about it. I'm going to have to give it a shot and see if I hear a difference. Thanks for the heads up!

 

[Rock Chalk]

Some months Cliff talked about decreasing the length of IRs to avoid reflections. See quote at the bottom.

An nice side-effect of decreasing IR length is that CPU usage decreases.
In a preset with a two Hi-Res IRs, decreasing the length of both IRs from "MAX" to 512 samples, decreases CPU with 8%.
When using two UltraRes IRs, the savings are a little less, but still 5% or so.
And no loss of quality whatsoever, at least to my ears.

The "IR Length" parameter is currently available only on the Axe-Fx III.

"I've been experimenting with IR length lately and keep finding that I like a shorter length. So I gave some thought to it and I think the reason is that a shorter IR trims off the early reflections. A 1024 sample IR is over 20 ms. If there is a wall 5 ft. away that puts a reflection smack dab in the middle of the IR. The Redwirez IRs you can see (and hear) the ceiling reflection pretty clearly (ceiling was probably about 8 ft). Using a shorter IR removes that reflection. I've actually been turning down the IR length lately on my personal patches, typically 512 samples, as I find it makes the IR "clearer". While the push in the industry has been towards longer and longer IRs I'm not sure that's a good thing unless you are careful with your IR capture to ensure that you aren't capturing reflections. Some IRs, particularly ported bass speakers, may need the longer length to capture the low end with sufficient detail but the average guitar cab is probably fine at 512 or even less samples. Heck, prior to the original Axe-Fx some products were even using 128 samples"

Oh man, I wanted this to work! As soon as I saw this post, I went directly to a preset that that uses two cabs that I've been wanting to mix two factory IRs in each one, but didn't have the CPU for. I turned all four IRs down to 512, and sure enough, it saved enough CPU to work. The thing is, when doing palm muted single notes on the E, A, and D strings, I immediately noticed the bouncy tube-like feel was gone! Could this just be power of suggestion? Possibly, will have to play with it some more.......don't know. Thanks for the suggestion though!

 

[Smittefar]

I think the sound suffers noticably, when I decrease IR length.

 

[reclavea]

I think the sound suffers noticably, when I decrease IR length.

Cliff is noting that he noticed that the sound improved when he shortened (truncated) the IR and was wondering why.

He thinks that the improvement to his ears must be attributed to a shorter IR trims out the early reflections.

He continues...

FractalAudio:
"I just noticed that I'm instinctively turning the IR length down lately and wanted to get to the bottom of why I thought it sounded better. I believe this is why."

He's says SOMETIMES shorter is better.

And regarding adjusting the IR by ear he says the average user may not be able to hear at that fine granularity.

But adds...

FractalAudio:
I’m going to work on an auto-correlation display over the coming days and see if that can help in identifying early reflections.

 

[yek]

I think the sound suffers noticably, when I decrease IR length.

Not questioning your experience, but I hear no difference at all. None. EQ output graphs are the same.
The basic sound doesn't change much (if any) with IR length.

 

[willowdale]

I never worry about CPU with my III

 

[Randalljax]

Not questioning your experience, but I hear no difference at all. None. EQ output graphs are the same.
The basic sound doesn't change much (if any) with IR length.

i feel i can tell a difference too
it may be the way the 3rd party irs are captured but when i move them
from Audition in Cab manage to save them in a cab slot there is an audible change
this is in the FM3 which is auto truncating to the 512 I am guessing

based on this thread when Ultrares comes to FM3 I would expect a 2 or 3% cpu bump but to me its worth it
to others maybe not

 

[Smittefar]

Not questioning your experience, but I hear no difference at all. None. EQ output graphs are the same.
The basic sound doesn't change much (if any) with IR length.

I hear a loss of dimensionality and depth. It may be well be loss of floor reflections and those things. If so, it turns out, I like that.

 

[Rock Chalk]

I hear a loss of dimensionality and depth. It may be well be loss of floor reflections and those things. If so, it turns out, I like that.

Me too......

 

[Randalljax]

I hear a loss of dimensionality and depth. It may be well be loss of floor reflections and those things. If so, it turns out, I like that.

if your capture is including the Room then yes you are losing a lot of those reflections
Its a much dryer rawer sound for lack of better word you can always add a reverb block with room or studio
but its not exactly the same

for me the multi mic captures are where i hear it most vs a 57 alone

 

[RB2021]

I go back and forth and it depends on the IR. I find you can probably tune for some of the difference. I clearly hear a difference with my headphones but it's about as dramatic as changing the depth knob a quarter turn on some amps. The result is different so as always use your ears. I have definitely experienced patches where I liked a shorter IR. I have also bought some third party IR mixes and I find that I tend to prefer them at Max length.