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I am not an organist, but I have developed a growing interest in the organ as an instrument,
and how virtual pipe organs work to emulate them.

A little background...

The Pipe Organ

The pipe organ is a wind instrument, controlled by keyboards (manuals), pedals and stops. These controls direct air (the wind) through sets of pipes that are tuned to a specific sound.

The diagram on the illustrated the basic operation of a pipe organ.

  1. A blower fills a reservoir with pressurised air.
  2. This air flows into a 'wind trunk.'
  3. 'Stops' are used to select which pipe air is able to flow through, when a key or pedal is depressed.
  4. When a key or pedal is depressed, a 'pallet' (flap valve) opens, allowing air to flow from the wind trunk through the selected pipes.

Each organ pipe is only able to produce a single pitch, and the pipes are organised into sets called ranks, each of which has a common timbre and volume. Most organs have multiple ranks of pipes of differing timbre, pitch and loudness. A player is able to select and combine pipes (both singly or in combination) using of controls called stops, to create the desired 'sound'.

A pipe organ has one or more keyboards (called manuals) played by the hands, and a pedalboard played by the feet, each of which has its own set of stops. It is therefore possible to create different sounds for each of these using the sets of stops for the different manuals / pedalboard.

It is this combination of stops that allows an organ to play such a wide range of sounds, even a small organ with 'only' 30 stops can mathematically produce 1,073,741,823 possible combinations (source)!

In addition, the organ is unlike other keyboard instrument, the continuous supply of wind allows it to sustain notes for as long as the corresponding keys are depressed, unlike the piano and harpsichord whose sound begins to decay immediately after attack. 

 

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Electronic / digital organs

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Pipe organs are complex, expensive instruments, and an installation typically takes up a lot of space. This has lead to various attempts to utilise electronic and digital technology to replicate the sound of a pipe organ. 

The earliest Hammond organs where one of the first instruments to use electricity to both control and generate the sounds produced. The organs were closer in style to the US theatre organs rather than the more classical church organ though.

Various sampling and modelling techniques were developed to improve sound reproduction, using complex, custom onboard computers, but it was the advent of powerful, ubiquitous multimedia PCs (and Macs) that changed what was possible.

The Virtual Pipe Organ

At the heart of a Virtual Pipe Organ (VPO) is an computer program that takes full advantage of the processing power of today's computers, to provide very complex pipe organ modeling and per-pipe sound shaping, while managing the enormous polyphony necessary to model a pipe organ successfully.

The VPO software system integrates sound samples (recording of real organs), MIDI controllers and audio output, that can produce an extremely accurate representation of the sampled organ.

Readily available MIDI keyboard controllers can be used to build a manual stack, pedalboards can be 'midified', and touch screen monitors allow the user to control the virtual organ console (making a traditional console with its physical stop and couplers redundant). The de-coupling of the physical console layout from the software allows a user to 'change' the organ they want to play!

Audio interface and speaker options can scale from a small practice room, to a huge auditorium.

 

So why does a Virtual Pipe Organ sound so realistic?

The following explanation is taken from the Hauptwerk website (a leading supplier of VPO software)...

Hauptwerk's system is built around the philosophy of using at least one large sample per pipe, (typically 3-10 seconds of a recorded pipe), all including natural attack and release samples of each individual pipe, and recorded with a resolution of up to 24 bit 96kHz. Since all samples are recorded chromatically from a real pipe organ there is no pitch strecthing to "fill in" sections of the keyboard compass. Every note is an individual and unique pipe recording!

All samples are held in the computer memory (RAM) to achieve a much higher polyphony than is possible with disk-streaming, commonly used in most samplers. Unlike generic software or hardware samplers, Hauptwerk has complex physical and acoustic models specifically designed to reproduce the features and sound of a pipe organ, and is thus able to achieve much more realistic results. It is also designed for a much higher polyphonythan generic samplers and supports up to 32,768 simultaneous notes!

Many digital organs compromise the quality of the pipe samples to allow them to fitin a smaller amount of memory. This results in artificial (synthesized) attacks and releases and very short sustaining portions. These factors alone reduce the realism of the overall sound and randomness of a real pipe.

Hauptwerk combines long samples with complex organ modeling to achieve the most realistic sounding virtual organs in the world.

 

Does a Virtual Pipe Organ really work?

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In 2010 Salisbury Cathedral’s Father Willis organ was scheduled to be out of action for essential maintenance work.

Daniel Cook, the then cathedral’s organist, decided to temporarily install his own Hauptwerk VPO in the cathedral while this was being done.

The sample of cathedral's organ had just published, and so the VPO was able to play the cathedrals own organ, in it's original setting!

For more information, see:  www.salisburycathedral.org.uk