My intent with this article is not to denigrate wet sampling techniques. The point of this theoretical discussion is to ask, and one hopes answer, the question "Can comparable results be obtained with dry samples and convolution?"
First A Word About Signal Mixing
As Pykett has very convincingly argued*, virtual organs suffer from a phenomenon he describes as "signal mixing". This effect occurs when multiple signals are layered electronically and reproduced through a limited number of audio channels. In practice, a few ranks played together, especially if harmonically dissimilar, can sound reasonably acceptable when reproduced through headphones or a single pair of good quality speakers well positioned in a treated room. But, as more ranks are added, the sound gets harsher and ever more irritating. The solution is to segregate ranks (especially those with similar overtone structures) into separate audio channels. Subjectively, each doubling in the number of audio channels (all other things being equal) brings a comparable amount of sonic improvement. Obviously, the economic and space costs soon add up, so every user will need to decide where the point of diminishing returns sets in.
It has been posited that wet sampled organs do not suffer from signal mixing effects nor benefit from multiple audio channels. If correctly understood, I respectfully cannot accept this line of reasoning. True, reverberation can to an extent mask signal mixing and other defects. [The late Tom Hazelton quipped that reverberation "hides a multitude of sins".] Strip away the reverb tails from any wet sampled organ, and you still have individual ranks that when mixed electrically will progressively sound harsher, IMO.
How To Create A Convincing Virtual Room Using Convolution
It is true that wet samples can have convincing reverberation in part because each pipe is a unique generator occupying a unique spacial position. Can a dry sampled organ and convolver come close to this? I think they can. It does not seem necessary to have one IR instance per pipe to create a comparably complex sonic field. One key is not to mix the channels electrically into an Aux bus to drive the convolver. Instead, pick up the sound from a multi-channel audio system using a pair of good quality microphones. The multiple angles of incidence from the speakers, along with the comb filtering effects, will provide most of the randomization observed with wet sampling techniques. The more instances of convolved IRs, each with a unique microphone position when recorded, the more convincing the simulation. It matters that the reverberation signals emanate from as many unique channels as can be reasonably accommodated so that the sound field seems homogeneous and diffuse. It is very important that the listening room be treated to minimize its own signature, otherwise your brain will believe that something just isn't right (whether using wet samples or convolution).
An advantage to convolved reverberation compared with wet samples is that there is much more possibility to adjust the sound to suit taste and listening room acoustics.
Ralph Glasgal's findings are highly recommended reading:
http://www.ambiophonics.org/
Joe Hardy
* Pykett, "The End Of The Pipe Organ"; "Signal Mixing": http://www.pykett.org.uk/EndOfPipeOrgan.htm#Mixing
First A Word About Signal Mixing
As Pykett has very convincingly argued*, virtual organs suffer from a phenomenon he describes as "signal mixing". This effect occurs when multiple signals are layered electronically and reproduced through a limited number of audio channels. In practice, a few ranks played together, especially if harmonically dissimilar, can sound reasonably acceptable when reproduced through headphones or a single pair of good quality speakers well positioned in a treated room. But, as more ranks are added, the sound gets harsher and ever more irritating. The solution is to segregate ranks (especially those with similar overtone structures) into separate audio channels. Subjectively, each doubling in the number of audio channels (all other things being equal) brings a comparable amount of sonic improvement. Obviously, the economic and space costs soon add up, so every user will need to decide where the point of diminishing returns sets in.
It has been posited that wet sampled organs do not suffer from signal mixing effects nor benefit from multiple audio channels. If correctly understood, I respectfully cannot accept this line of reasoning. True, reverberation can to an extent mask signal mixing and other defects. [The late Tom Hazelton quipped that reverberation "hides a multitude of sins".] Strip away the reverb tails from any wet sampled organ, and you still have individual ranks that when mixed electrically will progressively sound harsher, IMO.
How To Create A Convincing Virtual Room Using Convolution
It is true that wet samples can have convincing reverberation in part because each pipe is a unique generator occupying a unique spacial position. Can a dry sampled organ and convolver come close to this? I think they can. It does not seem necessary to have one IR instance per pipe to create a comparably complex sonic field. One key is not to mix the channels electrically into an Aux bus to drive the convolver. Instead, pick up the sound from a multi-channel audio system using a pair of good quality microphones. The multiple angles of incidence from the speakers, along with the comb filtering effects, will provide most of the randomization observed with wet sampling techniques. The more instances of convolved IRs, each with a unique microphone position when recorded, the more convincing the simulation. It matters that the reverberation signals emanate from as many unique channels as can be reasonably accommodated so that the sound field seems homogeneous and diffuse. It is very important that the listening room be treated to minimize its own signature, otherwise your brain will believe that something just isn't right (whether using wet samples or convolution).
An advantage to convolved reverberation compared with wet samples is that there is much more possibility to adjust the sound to suit taste and listening room acoustics.
Ralph Glasgal's findings are highly recommended reading:
http://www.ambiophonics.org/
Joe Hardy
* Pykett, "The End Of The Pipe Organ"; "Signal Mixing": http://www.pykett.org.uk/EndOfPipeOrgan.htm#Mixing