PMD FM3Extend Tutorial

PMD FM3Extend Tutorial by pedipanol

One of the features of the OPN series is that the 3rd FM channel can be used to have independent outputs between oscillators. This is useful for 2 things: extending polyphony and create unique-sounding instruments. Not much games on the PC-98 did the latter, but it's a possibility. Since, I think most people want the extended polyphony, I'll cover this on the tutorial.

Many people when first hear about this feature think that they'll be able to have 3 more additional channels to the defaults of the chip, but that's not quite it. Yes, you can have additional notes playing at the same time, but it doesn't mean you'll be having 3 more FM channels or anything. In fact, if you use the full extension and set up accordingly, you'll have 4 sine waves, one of them can have the sound a bit different.

Defining what kind of extended sound you'll use

Algorithm will define the organization of the FM channels and the kind of modulation will be done. It's important to all the FM instruments of course, but it plays a significant role in FM3Extend. In PMD it's the first number in the instrument definition after the instrument's number, and can range from 0 to 7.
;   ALG FB
@ 0   0  7
In case you don't know what each number will do, this graph will explain:
https://dl.dropbox.com/s/qs98rhkgumgs6u9/OPN%20ALG.png (originally from MovieMovies1 NSFW explanation on FM Algorithms https://twitter.com/moviemovies_one/status/867199501769093120 )
The ones that will matter for extended polyphony are ALG 4, 6 and 7 (5 too, if you're brave enough). 0-3, as you can see, will make the operators modulate between themselves, having Operator 4 as a carrier, that will deliver the output sound. 4-7 will have more carriers, which means different notes can be played at the same time. Using these along the MUL definition of the instrument can end up with you creating an instrument that plays multiple notes, even though it's limited what you can do with those. Many composers did this. It's a similar concept for FM3Extend, but in this case you don't aren't limited to what the MUL can do.

Anyways, here's what each ALG will let you do:

ALG 4: 2 note polyphony, both "channels" doing 2-OP FM synthesis (the first one being able to use feedback);
ALG 6: 3 note polyphony, 1 channel doing 2-OP FM synthesis with feedback and 2 others playing only sine waves;
ALG 7: 4 note polyphony, 4 channels playing sine waves, one of them can deviate a bit using feedback.

Most composers would opt to use ALG 4, since it'll be like having an additional limited FM channel.
Once you've set up the instrument(s) that you will use for FM3Extend, we can start doing the stuff.

On the MML's header, add #FM3Extend X(Y[Z]) accordingly to how much polyphony you'll use. In the end, you'll be using the C X Y Z for the 3rd FM channel. BUT that doesn't mean each one will automatically use each operator (like Deflemask does), you gotta set some stuff up.

The s(x) command in PMD will define which operators the channel will be using. It can be used for all the FM channels, but it's most useful FM3Extend.
About the numbers that will replace (x):

1: Will set the channel to command OP1
2: Will set the channel to command OP2
4: Will set the channel to command OP3
8: Will set the channel to command OP4

To set the channel to command more than 1 operator, you gotta add the numbers up. For an example: if you want it to command OP 2 and 3 simultaneously, you use s6. You're free to change this at will during the song. You can have FM3 using the extended mode, then making it 1 channel again, and etc. Just be aware of what you're doing.

After that there's just 2 things you have to keep in mind while using FM3 extend:

- Using stereo commands will affect all the FM3Extend channels.
- Changing instruments will only affect the operators set up by the s(x) command, but the Feedback and ALG values will be overwritten.

Every other command will work as intended.

Hope this tutorial helps!
Feel free to message me if you have any questions or something wasn't clear.
You can find me on Twitter, Youtube and Soundcloud by the nick @pedipanol, but you can also mail me at pdpano@yahoo.com