General Definition of a Virtual Theatre Organ
It is assumed that readers of this page have some knowledge of Theatre Pipe Organs in general. This description is in no way intended to be the definitive article, but serves as an introduction to the uninitiated. Any bias is purely intentional!
Back in the days when electronics were more in their infancy, space travel was considered to be science fiction and computers came as a secondary heating plant, the VTPO as we know it was possible but totally impractical. Still, folks wanted to have an instrument that they could play almost any time they wanted, that sounded something like their favourite Theatre Pipe Organ, but didn't come with the burden of owning a theatre or 10-15 tons or more of pipework and the maintenance problems caused by that. So the whizz kids came up with the idea of the electronic organ - where the "works" were more or less all inside the console and it could be delivered by two people and didn't take several weeks to install. Waveforms were generated and shaped electronically and some manufacturers even got good at it but the sound was never quite authentic. Some might even say that it still isn't quite authentic, but that's another argument! In the author's opinion, many electronic organs sound "nice" and are desirable to own and play, but they never quite "made it" as authentic Theatre Organs.
Now we get to what a VTPO comprises. As with all other Theatre Organs a VTPO must have a console, even if it's just two pieces of wood or a desk, propping up a single keyboard. It must also have some means of controlling "voices" and preferably a means of adjusting expression - i.e. At least one volume control. This is where the similarity to a Theatre Pipe Organ can end except in what it finally sounds like.
A VTPO is generally accepted as an instrument that uses "sounds" that have been recorded from real pipes and have been stored electronically/digitally on one or more computers. The samples may have been processed before storage and some software will allow tuning of individual virtual pipes. It is often possible to apply tremulant to individual virtual ranks and some software even allows for introduction of wind/blower noise and other "ambience".
Some owners have built complete replicas of major manufacturers' consoles, whilst others are content to mount their keyboards on stands, with boxes of switches for voice selection. In fact it's not even necessary to use physical switches at all - some software allows the use of touch screens with excellent replicas of console layouts in graphic format. Full blown keyboards are also not absolutely necessary, as some software will allow playing from the host computer keyboard, but this is of course less desirable.
Control signals between keyboards/stop keys/touch screens/pedals/expression and the computer are generally sent by MIDI interfaces, although there are some other types of interface available and some are even "home brewed".
The final analogue output can be from the computer's built in sound card to a single loudspeaker or it can be via one or more specialist sound cards and to multiple banks of loudspeakers. Some constructors have even gone to the trouble of installing multiple speakers per rank. The possibilities are endless and of course headphones can be used for "silent" practice.
To record individual performances with more or less reliable playback, in the past, paper rolls or cards were used with special readers, which were a rareity. With a VTPO, software can often be used to capture every nuance of a player's performance, and play it back digitally via MIDI control signals, giving a perfect reproduction of the original. Performances can even be recorded onto small storage media (thumb drives, CDs etc.) and played back via a totally different VTPO.
In fact it is now possible to store a complete computer operating system along with VTPO software, sampled sounds and a player's performance on a single thumb drive, which is easily carried in the pocket and can be taken anywhere in the world and played back via any computer that has suitable sound capabilities.