YM2151+ Routing
NB The description below refers to the first version of the YM2151+ patch. The version 2 patch is slightly different, but the text below is still relevant.
Rather than try and create the routing diagram for this patch, I've taken screen shots with the various different coloured cables showing in different pictures. This should help illustrate the routing for the patch. It is actually quite straightforward, but there's a lot of cables when you look at the entire patch, so lets break it down:
Click on the above picture to enlarge it. I've labelled the operators to help describe what's going on here.
This picture shows us the FM connections between the four operators.
We can see that the connections are as follows:
Operator 1's out is connected to an FmA input on Operator 1 and 2 (so Op1 is capable of self modulation.)
Operator 2's output is connected to an FmA input on Operators 1, 3 & 4.
Operator 3's output is connected to an FmA input on Operators 2, 3 & 4.
(so Op3 is capable of self modulation.)
Operator 4's output is not connected to any other Operators.
This configuration of connections is better understood through the following picture, prepared by Arikama:
If we now look at the next diagram, which shows the Audio cables of the patch, you can see that each operator is also sending its output to the Mix 4 mixer, Op A to Mix1, and so on (also clear from the above diagram).
So, the business of programming patches in the YM2151+ is a combination of deciding which Operators are going to modify which other Operator through the FmA1 & FmA2 controls on each Operator, and then deciding how much of each Operator will actually be audible in the final mix. Arikama provided a picture with the patch which details seven of the possible configurations. These are based on the possible configurations of the original patch, but there are actually may more than can be set up with this patch - these are just some of the possibilities.
These individual configurations are shown below, and I've explained how to set the first couple up so you can understand what's going on.
Algorithm 0
It shows Operator 1 modifying itself and Op2, which modulates Op3, which modulates itself and Op4 whose output is sent to the mixer. In other words we hear the output of Op4 only, but Op4's frequency is modulated by a complex signal resulting from the preceding modulation chain.
To implement this algorithm, we would turn up FmA1 on Op1 (so that Op1 self modulates), turn up FmA2 on Op2, so that Op2 is modulated by the output of Op1, turn up FmA1 and FmA2 on Op3 so that Op3 is modulated by itself and Op2 and finally turn up FmA2 on Op4 so that it is modulated by Op3.
All other FmA controls should be turned down and all channels on the mixer should then be turned down except channel 4, so that all we hear is the output from Op4.
Algorithm 1
Turn up FmA1 on Op1 and FmA2 on Op3 so that these two self modulate.
Turn up FmA1 & FmA2 on Op2 so that it is modulated by the output from both Ops 1 & 3. Turn up FmA1 on Op4 so that Op4 is modulated by Op2. Turn down all other FmA controls, and again, make sure that only channel 4 is turned up in the mixer.
Algorithm 2/3
This should be identical to Op 2 above, other than that FmA1 on Op2 should be turned down (so that it Op2 is no longer modulated by Op3), and FMa2 should be turned up on Op4 so that Op4 is now modulated by both Op2 & Op3. If you compare the above two diagrams you can see that only one change is really made - Op3 has been switched from sending its signal from Op2 to Op4.
(The reason this shows both Algorithm 2 & 3 is that the original had only one operator capable of self-modulation. Hence Algorithm 2 should only feedback Op3 and Algorithm 3 should feedback Op1 to fully replicate the original configurations of the chip.)
Algorithm 4
Again, only slightly different from the above - this time the only change is that Op2 no longer modulates Op4, and Channel 2 on the mixer should be opened up so that we hear the output from both Ops 2 & 4.
Algorithms 5-7
I don't think I need to spell the rest of these out so clearly - if you understood the above then you get the idea how changes to the FmA controls and the Mixer settings can bring about some radical changes in the configuration of the Modulation path for this patch.
Algorithm 5
One operator (#2) can modulate any or all of the other operators. You can mix 3 modulated output, but source of modulation is only one Operator. It's a simple structure and therefore not capable of creating a very complicated sound.
Algorithms 6 & 7
Again, no complex chaining of Operators here, so the output is quite simple. Each operator can only modulate one other operator. The final output is a mix of the individual operators , suitable for the creation of simple sounds: organ, Woodwind instrument etc.
More on FM
As well as the routing, the modulation amount and the relative pitch of the different operators in and FM patch, another important way in which the sound of a patch can be altered is by setting one or more of the operators to use a fixed pitch across the range of the keyboard. This means that each operator which is modulated by this operator will be modulated in exactly the same way, regardless of note - this can have a noticeable effect on the way the patch sounds, emphasising particular harmonics.
Raising the modulation over a certain level in many patches will cause such excessive distortion of the original signal that the sound 'degenerates' into noise - FM is a very good source of noise generation. Try increasing the modulation level in many of the presets I've provided and you'll hear the noise creep in quite quickly.
Other sources of modulation.
Looking at the diagram below, we can see how the patch also uses 'conventional' modulation sources to further modify the signal. Note first of all how the Amp for each Operator (always placed directly underneath each Operator) is modulated (by variable user-defined amounts) by the Note and Velocity outputs from the MVC. In each case, the Tmod (envelope segment time) inputs are connected to the Note and Velocity outputs, and the Lmod (envelope segment level) is modulated by Velocity only.
Furthermore, the frequency from the MVC passes through a Pitch Mod B module, which has the MVC frequecy as one modulation source and a MW Triangle LFO as the other, so the overall frequency of the Operators can be modulated in varying amounts by the Mod Wheel and also by the incoming frequency of the note itself.
Finally, it should be mentioned that after the Mixer the signal goes through the Poly Out module and then through a Delay module.
DX synth algorithms
Finally, here are the algorithm diagrams from the front panels of two classic Yamaha FM synths.
Both the DX1 & the DX7 were 6 operator FM synths with 32 'hard coded' algorithms to choose from. 32 is only a small fraction of the total number of algorithms possible in a 6 operator synth, but its a god starting point, nonetheless!
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| DX1 Front Panel Algorithms | DX7 Front Panel Algorithms |
As you can see the algorithms for both synths were actually same. See the August & September 2001 issues of Sound On Sound for a fascinating history of the DX series synths.
Thanks to www.kratzer.at for the pictures.
If you have a clearer picture of the front panel legend, please let me know.