EARLWOOD WOBBLE

This was originally known as the “Earlwood Tremulant”. The term was coined by Paul Stratman, to refer to an effect I introduced in the Earlwood Organ No.1 to simulate random movement in the pipe tones and in the organ choruses and so avoid a sterile sound. The following description is based on an email I sent to him:

I have applied the variation as a low frequency VIBRATO, but only about 3 cents. This is not usually enough to cause a perception of pitch change, but I beat it against the same sample (without the vibrato applied) at a very different volume. So it is causing actual beats, and these are perceived as a variation in VOLUME. What is more, because it is a vibrato (and not a simple, constant pitch change), the beats are forever changing from a maximum rate to zero, and back to the maximum rate, and so on. So I have avoided any suggestion of a rhythmic, cyclical effect. It does sound random.

I initially applied this to the Great Principal 8 Preset on its own. One would not expect to perceive it with such a slight pitch variation, but when playing it with reverberation, it is in effect beating against "an earlier version of itself" depending on the behaviour of the reverberation parameters. (I don't really like what the Fluidsynth reverb does with it, so there is a second reason why I don't like the Fluidsynth reverb). So when playing just on the Great Principal 8 you can hear it, if playing with reverberation. You can also hear it when adding the Great Octave 4, with or without reverberation, because now there is a beating between the 2nd Harmonic of the Principal 8 (which is having that pitch variation) and the Fundamental of the Octave 4, which has no pitch variation. In the case of the Gt Stopped Diapason 8, I used two Instruments, both based on the same samples, but applying the pitch variation to the "secondary" Instrument, and setting its volume quite low. I did the same for the Sw Rohrflute 8.

So if you want to impart this effect to just the one stop, basically, you use the second method, involving two instruments. But if you want it to affect the chorus, you can either apply it to (the) one instrument, or deal with it at the Preset level. If you want it to affect both, then you apply it at the Instrument level, but you apply it to the "primary" instrument and not to the low-level one.

Notice further that there are three variables to play with: the amount of pitch change, the low frequency rate (of that pitch change), and the difference in volume between the tone that is being varied and the one that is not being varied. I haven't played around much with these possibilities, and was pretty satisfied with what I came up with almost first go, except that I was a bit surprised at how much lower one of the two tones needs to be in volume, below the other one. I really think that these kinds of effects should be very subtle, almost inaudible, and one can control that very well by doing it in the instrument stage, with full control over the difference in volume between the two tones.

Because this effect is applied initially as a PITCH modulation, the final effect will mean slower beats low in the compass and faster beats high in the compass. I think this adds to the randomness, and may correspond to what happens in practice, anyway. A further point that I could have made, is that so far (and I really do think that simplicity is a virtue), I have applied this effect only in a global fashion, but it is still possible to apply it separately in the various zones (splits) if one really wants to. I have not taken the time to explore the possibilities in a comprehensive fashion, so there is probably ample opportunity to “tweak” the effect, according to individual taste.

My closing comment is that I really would like to impart a FURTHER effect, although in view of what has been achieved with the above, I could probably live without it. And that further effect is the perceptible change in pitch which occurs in pipe organs due to movement of the bellows, which movement probably executes Simple Harmonic Motion (i.e. a sine wave). It is cyclic, and fairly subtle and is not a tremulant or vibrato, because it is more momentary than constant - it relates to the demands being made on the wind supply at any given moment. It has almost been eliminated by the use of double-rise bellows, which I believe were introduced in order to eliminate it! But I think I do miss it if it can't be perceived at all! (Don't we create rods for our own backs?)

(Added February 2010 with the following addition in July 2011):

Later consideration of this effect led me to refine it somewhat. It was pointed out to me by an experienced organist friend that on some stops the effect sounded a bit like an unsteady wind supply, and as such a bit disagreeable. It occurred to me that what was needed was a faster beating rate, and that in those cases where two tones are being beated against each other, this faster rate could be achieved by having the low-order harmonics removed from the low-level tone (Instrument) which is being beated against the main tone. (I also remove the high-order harmonics). The spreadsheet techniques I use when making synthesized looped samples make this removal of harmonics a very simple task. The downside is that a whole new set of samples needs to be made, corresponding to the main set for a given Instrument, but using only Harmonics 5 to 8 in most cases, and perhaps Harmonics 3 to 5 for very high notes. This does not double the total sample size for a given Preset, however, because it is possible to make these secondary samples much shorter than the primary ones, without any real loss.

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