30-01-2023, 02:23 AM
(This post was last modified: 30-01-2023, 02:29 AM by Lucien Nunes.)
Tuning is simple on a divider organ like this. There are only 12 oscillators in the organ, one on each of those 12 PCBs, tuned with coils. Lay the scale on those and.... you're done. The divider stages, 7 on each PCB, each halve the frequency to generate the next octave down, thus the 'G' oscillator tunes all the G's, etc. Stability of tuning was the main advantage of a divider organ, compared to one that has a separate oscillator per frequency, but also its main disadvantage, because all frequencies divided from one oscillator are locked together in phase.
In a pipe organ, all the octaves of all the ranks of a given note are tuned true, but never perfectly. There is always a very, very slow beat between them, which gives complex texture to the sound as the phase gradually drifts. If you have say ten ranks of pipes, each with five octaves of G's, then you have 50 'G'pipes more or less in tune but all slowly beating with each other. On an individual-oscillator organ, you would need 50 'G' oscillators to simulate that. When you put on a Diapason at 8ft and Flutes at 8ft and 4ft, you get three oscillators just as you would get three pipes. There have been such beasts, but only a few. I have a 4-rank Miller organ with 6, 7, 8 and 9 octaves in its extended ranks, totalling 29 oscillators per letter name of note rather than 50. It actually offers eight tone colour families, but you are only permitted to select stops from four families at any one time, because there are only four ranks of oscillators, in order to stay faithful to the free-phase concept. Even so, its generator cabinet is the size of a large wardrobe and contains 193 valves.
The next best thing is one oscillator per frequency, typically 85 or so. Here, the individual notes can beat with each other but all the stops are locked together in phase because they are sourcing their necessary component pitches from the same rank of oscillators.
The least desirable - musically speaking - is the divider, where all octaves of all stops of a given note name are locked in phase. Every single G in the organ comes from the same reference, making the sound relatively dull and lifeless. Why, other than saving the cost of six dozen coils and the time to set them up, was it such a popular method? After all, each divider stage requires one valve so you might as well use it as an oscillator instead. The answer is mainly in the tuning stability. The 12 top-octave oscillators all lie within a frequency ratio of 1.9 : 1, so are made with similar components that track well as they drift. In contrast, the oscillators in a 9-octave rank span a ratio of 512 : 1, hence have very different component values that do not track nearly as well, and require more elaborate engineering to keep them acceptably in tune.
Ultimately, true free-phase organs with hundreds of oscillators remained expensive and luxurious, and were vastly outnumbered by divider organs in all eras - valve, transistor and IC. By the time we get to the 1970s, there will be only one oscillator running in the MHz, divided down by peculiar-looking ratios in a custom 'Top Octave Generator' IC to create the 12 semitones, each of which is then divided in the customary fashion as above. We'll see in later posts what was done to disguise the thin, dull sound of a locked-phase generator system.
What you are seeing in the above pics of the Burge, are lots of adjustments for amplitude rather than frequency. There are often two layers of adjustments, one to equalise the generator outputs across the compass, and another to select the fraction of each of those amplitudes to use for each stop. In some organs it's done by presets, in others by fixed resistors selected on test. Compton had a neat system of phosphor-bronze springs and busbars; you form a hook in each resistor lead and hook it between the spring and the busbar. The voicer has a box of pre-hooked resistors of 20 different values that produce 2dB increments from which to choose. It takes barely a second to change them, but once the organ is voiced, nosey twiddlers are discouraged from interfering because there's nowhere to poke their tweaker. Other adjustments made on each stop on a subtractive organ like this, are to the filter tuning, for which there are usually assorted variable L's and C's.
It's important to note a couple of fundamental differences between this Burge and the Comptons we were looking at earlier. Obviously, this uses electronic generators where the Compton uses electromechanical. But, also this has subtractive voicing, i.e. it forms different sounds by filtering out bits of a complex waveform, whereas the Compton is additive, combining different harmonics of the desired frequency to achieve the same result. Further, this has audio keying, where all the generators produce all frequencies constantly, which are routed to the filters via the key contacts. The Compton has generator keying, where the keys switch DC control voltages proportional to the amplitude of the relevant harmonic components required from the generators. This is a significant advantage of the Compton over virtually all competing systems, i.e. that it is a voltage controlled synthesiser, where envelope shaping (attack, decay, sustain, release) can be applied to the control signals. It is near impossible with something like a Hammond, which uses audio keying like the Burge, resulting in abrupt switching of the tones. Another Compton advantage is the excellent tremulant, a true FM trem that is not available on an individual oscillator or divider organ like this. But the Burge generators and voicing produce a better and more accurate harmonic spectrum. No one system has it all.
In a pipe organ, all the octaves of all the ranks of a given note are tuned true, but never perfectly. There is always a very, very slow beat between them, which gives complex texture to the sound as the phase gradually drifts. If you have say ten ranks of pipes, each with five octaves of G's, then you have 50 'G'pipes more or less in tune but all slowly beating with each other. On an individual-oscillator organ, you would need 50 'G' oscillators to simulate that. When you put on a Diapason at 8ft and Flutes at 8ft and 4ft, you get three oscillators just as you would get three pipes. There have been such beasts, but only a few. I have a 4-rank Miller organ with 6, 7, 8 and 9 octaves in its extended ranks, totalling 29 oscillators per letter name of note rather than 50. It actually offers eight tone colour families, but you are only permitted to select stops from four families at any one time, because there are only four ranks of oscillators, in order to stay faithful to the free-phase concept. Even so, its generator cabinet is the size of a large wardrobe and contains 193 valves.
The next best thing is one oscillator per frequency, typically 85 or so. Here, the individual notes can beat with each other but all the stops are locked together in phase because they are sourcing their necessary component pitches from the same rank of oscillators.
The least desirable - musically speaking - is the divider, where all octaves of all stops of a given note name are locked in phase. Every single G in the organ comes from the same reference, making the sound relatively dull and lifeless. Why, other than saving the cost of six dozen coils and the time to set them up, was it such a popular method? After all, each divider stage requires one valve so you might as well use it as an oscillator instead. The answer is mainly in the tuning stability. The 12 top-octave oscillators all lie within a frequency ratio of 1.9 : 1, so are made with similar components that track well as they drift. In contrast, the oscillators in a 9-octave rank span a ratio of 512 : 1, hence have very different component values that do not track nearly as well, and require more elaborate engineering to keep them acceptably in tune.
Ultimately, true free-phase organs with hundreds of oscillators remained expensive and luxurious, and were vastly outnumbered by divider organs in all eras - valve, transistor and IC. By the time we get to the 1970s, there will be only one oscillator running in the MHz, divided down by peculiar-looking ratios in a custom 'Top Octave Generator' IC to create the 12 semitones, each of which is then divided in the customary fashion as above. We'll see in later posts what was done to disguise the thin, dull sound of a locked-phase generator system.
What you are seeing in the above pics of the Burge, are lots of adjustments for amplitude rather than frequency. There are often two layers of adjustments, one to equalise the generator outputs across the compass, and another to select the fraction of each of those amplitudes to use for each stop. In some organs it's done by presets, in others by fixed resistors selected on test. Compton had a neat system of phosphor-bronze springs and busbars; you form a hook in each resistor lead and hook it between the spring and the busbar. The voicer has a box of pre-hooked resistors of 20 different values that produce 2dB increments from which to choose. It takes barely a second to change them, but once the organ is voiced, nosey twiddlers are discouraged from interfering because there's nowhere to poke their tweaker. Other adjustments made on each stop on a subtractive organ like this, are to the filter tuning, for which there are usually assorted variable L's and C's.
It's important to note a couple of fundamental differences between this Burge and the Comptons we were looking at earlier. Obviously, this uses electronic generators where the Compton uses electromechanical. But, also this has subtractive voicing, i.e. it forms different sounds by filtering out bits of a complex waveform, whereas the Compton is additive, combining different harmonics of the desired frequency to achieve the same result. Further, this has audio keying, where all the generators produce all frequencies constantly, which are routed to the filters via the key contacts. The Compton has generator keying, where the keys switch DC control voltages proportional to the amplitude of the relevant harmonic components required from the generators. This is a significant advantage of the Compton over virtually all competing systems, i.e. that it is a voltage controlled synthesiser, where envelope shaping (attack, decay, sustain, release) can be applied to the control signals. It is near impossible with something like a Hammond, which uses audio keying like the Burge, resulting in abrupt switching of the tones. Another Compton advantage is the excellent tremulant, a true FM trem that is not available on an individual oscillator or divider organ like this. But the Burge generators and voicing produce a better and more accurate harmonic spectrum. No one system has it all.
