Using Devtronix generator-keyers on Schober organs
Introduction
Schober organs and many other electronic organs of that era use
"subtractive synthesis" to imitate the sounds of organ pipes.
Starting with a waveform such as a sawtooth wave which contains a rich
set of overtones (and sounds rather like an unmusical buzz), some
overtones are removed or reduced to produce a musical sound. This
is exactly analogous to such instruments as the violin, where the bow
and string produce a sawtooth wave and the resonances in the body of
the instrument reduce some overtones while preserving others.
In Schober organs, sawtooth waves, which contain both even and odd
numbered harmonics, are the basis for sounds such as diapasons, open
flutes, strings, and most reed stops. Square waves, which lack
even harmonics, are used as the basis for Bourdons, other stopped
flutes, and some reed stops such as the Clarinet. The stop
filters control the strength of the harmonics so that a Diapason sounds
different from a Trumpet, and Bourdon from a Clarinet, even though the
original sawtooth or square waves are the same in each case.
Schober tone generators either made sawtooth waveforms directly or by
adding up to seven square waves to make a very good
approximation. Square waves were produced by subtracting the even
harmonics from a sawtooth wave. To compensate for the generally
"low pass" nature of the stop filters, the resistors in the keyboards
were proportioned to pre-emphasize higher pitched notes. After
filtering, the loudness of the notes was approximately equal across the
whole keyboard.
In the original Schober circuit, the keyboard contacts directly
controlled the switching of sawtooth waves for each note. This
always introduces a small (but tolerable) click at the beginning
of a note, but the click became very objectionable and the action
erratic if the key contacts were oxidized or damaged.
Although the electronic keying in the Devtronix tone generators will
often solve keying problems on Schober organs, they also change
the sound of the organ. For the most part, I don't like the sound
with Devtronix generators as much as the original Schober sound.
Why the sound is different with Devtronix generators
There are four main differences: speed of attack, overtone
structure, scaling, and tuning. For flutes and diapasons, the
slower attack of the
Devtronix keying removes a certain crispness and even a suggestion of
Baroque "chiff" from the sound. The sound also doesn't contain as
many overtones as the original Schober generators (four octaves of even
harmonics at 8 foot pitch instead of as many as seven), which makes it
sound less natural, less full, and more "electronic", especially on
string stops such as the Gamba. Also, the scaling (loudness
at different pitches) is
wrong for flutes and diapasons, which are much too loud in the bass and
too soft in the treble.
In some respect, the Devtronix tone generators are more suitable for reeds.
Reed pipes have a relatively slow attack, similar to the Devtronix
keyers. The reduced overtones don't seem to have as much effect
on the reeds, perhaps because reed formant frequencies are typically
within four octaves of the fundamental. Since Devtronix scaling
does not pre-emphasize the treble, it actually works better with the "bandpass" filters for reeds
than with the "low pass" filters for flutes.
I haven't yet tried a Devtronix generator for the Pedal clavier.
I would expect fewer problems with scaling, since many Recital organs
already have Pedal
scaling similar to the Devtronix. Lack of harmonics may still be
a problem, though at 16 foot pitch the Devtronix generator has five
octaves
of even harmonics. It would be interesting to see how much the
difference is noticeable.
The other major annoyance, at least for me, is that the Devtronix
generator does not actually produce an equal tempered scale. This
can easily be demonstrated by first playing C and G together, then
playing D and A together. Beats can be clearly heard on the C to
G fifth, but there are none on the D to A fifth! Many pipe organs
are tuned in unequal but "well tempered" temperaments such as
Werkmeister, which favor commonly used keys over more remote
keys. Unfortunately, the top octave generator used in the
Devtronix boards mostly does the opposite; it's a sort of "reverse well"
temperament. Its best major keys are E, B flat, and B; the worst
are C, F, F# and G. I may try running the Devtronix keyers from
the top octave of the Schober generators or building a better digital top octave generator, and
there may even be some way to change the scaling, but those are future
projects.
Advantages of the Devtronix generators
To be fair, the Devtronix generators do have several advantages.
There is virtually no leakage through the keyers, so there is very
little background noise. The keying is much less sensitive to
contact resistance, and if desired the attack can be speeded up by
substituting smaller capacitors in the keying circuit. Tuning is also easily
accomplished since there is only one adjustment (but they will always be somewhat out of tune due to the faulty temperament).
Perhaps the best thing about having more than one set of generators is
chorus effect due to slight differences in tuning. This sounds
more like a pipe organ, and probably works best if each division plays
through a seperate speaker.
Tweaks
To get the best sound from a Devtronix generator, the output level
controls need to be set properly. If you are using only Devtronix
generators, you can probably just set all the output controls to
maximum. However, if that produces distortion or if you are still
using the Schober generators for anything besides 2 2/3', you will need
to adjust the Devtronix generator or generators for the best level and
the best square waves. I will describe the process for the case
where one keyboard has a Devtronix generator and the other does not.
Start by turning on the Swell to Great and Great to Swell
couplers. Turn on only the Contra Violone 16' (or whatever 16'
stop you have on the Great). Play middle C on each keyboard and
adjust the 16' output control on the Devtronix generator until both
keyboards play at the same loudness. Now turn off the Contra
Violone and turn on the Swell 16' Bourdon, which uses square wave
("WL") tone. Play middle C and adjust the 8' output control up
and down to find the place where there is an abrupt change in tone
quality. Compare the tone of middle C played from each keyboard,
and adjust the 8' control carefully for the best match in tone quality
between the two keyboards. This should produce a good
hollow-sounding "stopped pipe" tone, and the loudness should be the
same on both keyboards. Now turn off the Bourdon and select a
stop such as the Gamba or Dulciana which uses regular 8' tone, and make
sure the loudness of middle C is the same when played from both
keyboards. If not, adjust the 8' control make the loudness the
same, then go back and re-check the tone on the Bourdon. There
should be a setting which is nearly optimal for both.
Now turn on only an 8' stopped flute, and adjust the 4' output control
while playing middle C until you detect the sudden change in tone
quality; then adjust until the tone quality matches on both keyboards.
Turn off the 8' stop and check the loudness of a Swell 4' flute
by playing middle C from each keyboard. If necessary, adjust the
4' output control for the best tradeoff between the tone quality of the
8' stopped flute and the loudness of the Swell 4' flute.
Now turn on only a stopped flute on the Great (for example, the 4'
Chimney Flute) and adjust the 2' level control as above for best tone,
always using middle C. Then check the loudness of any 2' stop and
re-adjust if necessary.
Note that because of the difference in scaling between the Devtronix
and Schober generators, this procedure will give a different result
depending on which octave you use to compare the generators. I've
specified middle C because I think that gives the best tradeoff between
the two, but you may decide otherwise after trying it.
Conclusions
On the balance, I think the best thing to do is to clean the keyboard contacts
and use the original Schober generators. For keyboards that have
already been modified for Devtronix generators, this will require
replacing any keyboard resistors that were removed during the
modification.
At the moment however I'm still using a Devtronix generator on the
Swell. But I'd rather keep the Recital
sound as close to 100% Schober as I can, so I may change it back later.
Last updated May 3, 2007