Are IRs just eq?

[lppier]

I was just wondering, with the slew of new IR based acoustic amplification products out there - are IRs really just a very complex multi frequency sophisticated eq?

But if that is so, how do IR reverbs work?

Just asking from a knowledge perspective.

[rockabilly69]

Originally Posted by lppier

I was just wondering, with the slew of new IR based acoustic amplification products out there - are IRs really just a very complex multi frequency sophisticated eq?

But if that is so, how do IR reverbs work?

Just asking from a knowledge perspective.

nope they are captured tones.


https://www.premierguitar.com/the-wo...ulse-responses

[Doug Young]

Not just EQ, at least not usually. It's a way to capture the sonic signature of something. If it's a room, you can capture the sound and decay (reverb) of the room. They can be used to capture something like the distortion of an overdriven amp (ala the Kemper). You could also create an EQ in a DAW and capture that (so in that case , you're capturing an EQ curve, and the IR becomes an EQ), or a reverb, or various other effects.

When applied to a pickup, like ToneDexter, it ends up being closer to an EQ, at least as a simple way to think about it. James May mentioned a while back that is was reasonable to think of ToneDexter as a 1000-band EQ being set by a computer, and I've tried to use this analogy quite a few times - especially when people seem to think that ToneDexter is somehow playing back a recording. James or Cuki or Jon Fields could give you a better technical definition of the IR process.

[tadol]

Originally Posted by lppier

. a very complex multi frequency sophisticated eq?

Yup - thats basically it. Depending on who’s offering them, its about the degree of complexity, the total number of bands you’re breaking the frequency range into, and the sophistication of the algorithms that analyze that data and determine the problem frequencies, and the types and degree of modification offered to that signal in the processing both in the initial analyzation, as well as the secondary signal modification process. There’s a lot going on - and a lot of ways to achieve the end result - but, essentially, its as you say - a very complex multi frequency sophisticated eq.

[CosmicOsmo]

The big difference between an IR and a simple EQ is that IR’s are integrated over the time domain...it captures not only the frequency response in a single snapshot, but how it changes over time.

[lppier]

Originally Posted by CosmicOsmo

The big difference between an IR and a simple EQ is that IR’s are integrated over the time domain...it captures not only the frequency response in a single snapshot, but how it changes over time.

Oh can I liken it to a 1000-band eq that changes over time? (at least for the length of the impulse response? )

[dnf777]

Two questions:
Do they make 1000-band EQs with LED sliders?
Will that fit a standard rack system?

[jonfields45]

Skip the part about generating an IR and here is a different article which attempts to answer your questions...and more. It was written assuming the reader was not a math or engineering major.

An Introduction to the Mathematics of Impulse Responses

http://acousticir.free.fr/spip.php?article18

My IR algorithm is using 64K coefficients or frequency bands. Quite beyond any physical EQ.

The IR for reverb is dramatically longer in time which is usually achieved by using fewer bits per sample (lower SNR), a lower sampling rate (reduced frequency response), and fewer coefficients (lower resolution in the frequency domain), to make its implementation cost effective.

[Cuki79]

Originally Posted by lppier

But if that is so, how do IR reverbs work?

IR means Impulse Response. In the case of an IR reverb, it is the sum of all the echoes due to reflexion on the wall, ceiling, furniture...



As shown by the figure above, the perceived sound is the sum of the "direct sound" and the "echoes" which are replicas of the direct sound, delayed and attenuated.

One way to capture the Impulse Response (IR) of a room, is to go into the room, set a microphone and record when you clap your hand. The impulse will be the "clap", the response will be everything that follows in the recorded file. It's also called a reverb tail. (see figure below)



Now how does the convolution reverb work?

When you use a digital pedal (ex: Line 6 Helix, Tonedexter, Baggs VPDI or Fishman Aura), the signal from your guitar is sampled to let's say 48kHz, it means that your signal is measured every 20.8 microsecond. In fact, your signal has become a chain of "digital impulses" that are each delayed one another by 20.8 microseconds. To produce the reverb effect, the pedal will add subsequently to each impulse a reverb tail: the Impulse response. Of course the tail will be added with respect of the each impulse magnitude.

At the output of the digital FX, you will thus hear the direct sound AND the reverb tail.

What is the link with EQ?

Well, maybe you already experienced it, but some room "rings". In some room, you will have offending frequencies that pop out all the time. It's due to the fact that sound bounces on the walls again and again... Acoustic standing waves would build up at certain frequencies. It's called resonances. A room resonating at a given frequency is very similar to a parametric EQ set at this frequency with the gain set at +10dB. The given frequency content will be magnified by the EQ as it would have been by the room resonance.

Some room are known to color the tone in nice way, in this case it's the sum of all the room resonances and anti-resonances (notch) that give this "color".

How does it convey to acoustic guitar?

Undersaddle pickups mostly capture string tone because of their location under the saddle. As a result they don't capture the guitar top response, the box, the air in the box, the room...

If you take the spectral point of view (EQ), undersaddle pickups don't capture the resonances from the guitar top, from the back, from the air inside the box.

If you take the temporal point of view (reverb), undersaddle pickups don't capture, the acoustic waves that propagates in the wood or in the air inside the box.

At the end of the day, whatever point of view you choose, recording an acoustic IR makes as much sens as capturing a Room reverberation... Only it is much more difficult to achieve from an algorithm point of view.

You can check the website (not all articles are written by Jon) for more information:
http://acousticir.free.fr/

[lppier]

Wow thanks jonfields45 and Cuki79 for the detailed explanation.

So in the case of something like the tonedexter, other than the eq, it is also handling the "reverb/echos" that the mic perceives of the guitar itself right?

I also created an IR using Aaron's tonematch method: https://www.youtube.com/watch?v=ud3Rhqa2FUo
In that case I guess only the eq dimension is being taken care of? As it was a path going like this:

impulse wav file > output of audio interface > input of audio interface > daw recording the response

[perttime]

Perhaps I didn't fully understand all the answers.... but I have a fundamental question:

When you are enhancing the sound of a pickup with IR devices, are you "flavoring" the original signal from the pickup - or is that signal digitized and then processed in a computer?

[lppier]

Originally Posted by perttime

Perhaps I didn't fully understand all the answers.... but I have a fundamental question:

When you are enhancing the sound of a pickup with IR devices, are you "flavoring" the original signal from the pickup - or is that signal digitized and then processed in a computer?

It's a convolution process, I guess one could say that the response is mixed into the original signal.

[Cuki79]

Originally Posted by lppier

So in the case of something like the tonedexter, other than the eq, it is also handling the "reverb/echos" that the mic perceives of the guitar itself right?

Yes but they are all within few tens of ms. Because the "reverb/echos" are very short delayed (compares to a room which is bigger).

Originally Posted by lppier

I also created an IR using Aaron's tonematch method:
In that case I guess only the eq dimension is being taken care of?

Yes and No.

EQ plugins use IIR filters. IR convolution are FIR filters.

EQ plugins are thought from a spectrum point of view (maybe what you called "EQ dimension"). They have response called "Shelf", "Peak" (parametric), "Highpass", "lowpass"... They have a given shape in the spectrum domain that applies gain or attenuation at certain frequencies. However they also affect the "phase". It means that some frequencies will come later than others. This is never shown by the pluggin because most people don't understand "phase".

When you are using anEQ plugin like Fab filter Q to "match" two signal, you only equalize the balance between frequencies. So you are adjusting lows, mids, trebles... so it macthes. However you don't match the phase. Sio the phase is altered by default with the IIR phase response.

With IR pedals like Tonedexter, Voiceprint DI, Aura or Jon and my algorithm you create a filter that adjusts both amplitude and phase. It will affect the FEEL because the "attack" of the string will feel more natural.

Humans don't hear phase "in absolute".

So if you listen to two recordings, one processed with an EQ plugin and the orther with Tonedexter (or VPDI... etc) , you might not hear the difference.

HOWEVER as soon as you use an actual guitar and pluck the strings for real, the mechanical feedback your fingers will sens will be "naturally" correlated with the sound you hear from Tonedexter (or VPDI..etc) where as it will feel "wrongly" correlated with the EQ plugin.

Humans can sens phase correlation between different senses (earing and touch).

I have done some experiment with Doug Young on this subject. I sent him pure phase filters, that made no spectrum correction (flat sounding) but pure phase correction and asked him if he could "feel" it.

My 2 cents,
Cuki

[KevWind]

Originally Posted by lppier

Wow thanks jonfields45 and Cuki79 for the detailed explanation.

So in the case of something like the tonedexter, other than the eq, it is also handling the "reverb/echos" that the mic perceives of the guitar itself right?

I also created an IR using Aaron's tonematch method: https://www.youtube.com/watch?v=ud3Rhqa2FUo
In that case I guess only the eq dimension is being taken care of? As it was a path going like this:

impulse wav file > output of audio interface > input of audio interface > daw recording the response

Whoa lets back up. So while acknowledging jonfieds45 and Cuki79 excellent
technical information, in simple layman's terms ,,, an "impulse response" is exactly what it says and NO it is not an EQ...

They are two different concepts in sound:

An EQ either simply boosts or attenuate's (cuts) a given frequency or range of frequencies, and it does this regardless of how the input signal is generated.

An IR is a specific digital representation of the sound of a specific signal input,,, ( it is based on creating a short signal or "Impulse" and then writing a digital algorithm to represent the sound ("Response" to that signal input) of a specific room (which will be like a reverb) or a specific guitar sound when mic'ed , or specific pickup sound , or specific amp and speaker cab etc. etc.

So While yes can can use either or both to effect your sound IMO in order to get a basic understanding it is best NOT to commingle the concepts to start with.

[Cuki79]

Originally Posted by perttime

When you are enhancing the sound of a pickup with IR devices, are you "flavoring" the original signal from the pickup - or is that signal digitized and then processed in a computer?

All IR devices are digital. All have your signal digitilized.

I am pretty sure EVEN the original signal is digitilized. I am pretty sure the original signal is never kept fully analog.

How do I know that?

Because I tried and post the experiment 4 years ago:

https://www.acousticguitarforum.com/...d.php?t=483802

It is not worth the hassle. From an engineering point of view the gain does not exceed the costs and no IR pedal ever maker advertised on that.