Organ

The organ, sometimes called the queen of instruments, is in it classic form an instrument generating the tones by controlling airflow through several organ pipes. Those pipe organs today can usually be found in churchs. There also exist electronic versions or organs, which are more interesting for the audience here, as they have some similarities to a synthesizer.

An electronic organ usually consists of an enclosure containing all electronics to generate the tones and a keyboard. The size of those models differs, there exist large versions with a form comparable to an upright piano, and portable versions, which look a lot like the synthesizer keyboard we know.

Technically, the classic electronic organ works completely different than a synthesizer. An organ made in the 1960s is fully polyphonic, compared to the usually monophonic synthesizers of this time. This polyphony is achieved by using just square waves. One master clock is first divided into 12 semitones, which have a quite high pitch after those dividers. Those 12 semitones are then divided in a divider chain, each chain divides the frequency by 2. This can be done very easy with digital electronics, as just one flipflop is needed for one divider stage. So all basic tone generators always run in parallel. The keys now just switch on and off this signal to the rest of the audio chain.

Usually, an organ-sound has a simple envelope. If the key is pressed down, the sound starts. If the key is released, the sound stops. The first electronic organs worked exactly this way. By the time, organs got more different sound flavours, even piano sounds can be found on some analog products, so an envelope generator is needed. And to keep the polyphony, not just one envelope generator is needed, but one per key.

To create an own volume envelope per key, there is a very simple VCA connected to each key, just consisting of a capacitor, which is fully charged when idle and is connected to the amplitude of the VCA when the key is pressed down, which discharges the capacitor. If the key is released, the capacitor is recharged very quickly.

So for a 61-key organ, we now have 61 rough square wave oscillators and 61 VCAs including envelope generators. Compared to a synthesizer, this is a lot, but all those components are quite primitive, the oscillators don't know any more complex waveforms, the the envelope generators are of AR-type with fixed attack and release values. Between those 2 sets of units, a common unit is present, which forms the sound. This usually consists of filters, depending on the type, several different configurations for this filtering can be activated to create different sound flavours.

So the sound flexibility of an electronic organ is very small compared to a synthesizer, but this might be compensated by the very good polyphony.

Most electronic organs also contain a bass section, which is a real analog synthesizer, as the bass section is also monophonic and usually of a voltage-controlled type.

MIDI is nearly impossible for this classic electronic organ, as every key needs to switch the audio signal and the capacitor for the envelope, so a MIDI kit would need two switches for every key. If you see an organ using MIDI, it is some modern digital organ, which works more like a primitive digital synthesizer.