Pipes 2 is another one of those patches that has 2 stacks of three operators. The trouble with this patch is in the first stack. Op A has a ratio of 19, B is fixed frequency of 1Hz, and C is 2. I started out by trying to modulate the LFO rate with an oscillator and have the LFO modulate the pitch of another oscillator. I experimented with this in several different ways, and the results were too uncontrollable. The thing is you can get away with using up two oscillators for this kind of thing because you can always use a filter as an oscillator in the second stack. But there was no appeasing the gods on this one.
It took some reverse engineering to pull this one off. I started out by just listening to the first stack. I noticed two things: First, there was some modulating of the brightness caused by a high modulation ratio. That translates here into amplitude or FM Amount. Second, there was a very slight variation in pitch. This means that the second operator is itself acting as an LFO (which was obvious from the start, but how?) and is affecting both amplitude and carrier pitch.
I found this discovery to be exciting. All I have to do is use LFO1 to mod Osc1 FM amt and Osc1 Pitch in very subtle ways. The other stack will need two oscillators regardless, but by eliminating an operator between A and C, there's no need for the "4th Oscillator" effect.
With that in mind, it's almost a very simple patch. First set up the oscillators: Osc1 is 19:2, FM amt. 2. Osc2 8:1, FM amt. 0. Osc3 1:2, FM amt. 0. Next set the LFO rate to 2.02 Hz. This isn't a mistake: Originally the OP B fixed frequency is 1 Hz, but what we hear seems to be positive phase only, or a sine wave with only absolute values. Hence the LFO has to be twice the original setting to replicate the effect (like I always say, this isn't an exact science). We also need a MG to scale each effect according to the percussive chiff of real organ pipes (again, not an exact science!). Set MG A 25.1 ms, D 77.2, R 45.2. Another detail is the timing on all DX7 envelopes are slightly different--modeled after the air moving through pipes, I imagine. Let' set one more envelope, the filter envelope, to A 29.8, D 29.7, S -27 dB, R 23.5. The S level accounts for some amount of FM in the original patch, therefore it should be present here. Amp EG is a standard sustain envelope but accounting for pipe acoustics in the attack transient. Something else I find useful is altering the release transient so that it includes the sound of the pipe cutting off and leaving just a little sine wave action after the initial release.
Now we have everything set up. Let's go to the router. LFO1 to Osc1FMAmt, amount 75, scale 99 Mod Env. LFO1 to Osc1Pitch, amount 22, scale 99 MG. FilterEnv to Osc2 FMAmt, amount 59 (no scale). Osc2 to Osc3Pitch, amount 34, scale 82 MG.
For little details, I added a LPF to filter 1 because I thought some of the noisy components of the sound were a bit bright and harsh. Since you're using the filter EG elsewhere, I recommend turning the env amount all the way down. Chorus could also be used tastefully here, but it is optional. I set this the way I like it, disabled it, and then saved the patch so it will be there if I ever need it.
In conclusion, we're essentially "faking" the sound of a fixed-frequency FM op by setting the LFO to modulate the sonic components that we hear change. This is not a true substitute for the real deal, of course. But at the very least it help you miss it a little bit less!
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