Let’s open the hood and look at the main components of the FM engine – the dexed Operators. Operators are also called “oscillators” in other synthesizers. These are the primary tone generation sources within dexed. There are six of them in dexed, making it a “6-op (operator)” FM synth. Below, we’ll look at each element of an operator in dexed, as we lead up in this series towards designing our own sounds.
The first thing to notice in the operator section in dexed is the switch directly to the left of the operator number – in our case, operator 1. This unlabled switch is an on/off switch for the operator, a sine wave, in this section. When set to “off”, or the “up” position, completely stops any interaction of the oscillator with other operators, and stops audio output of the operator. It definitively removes the operator from the playing field. The default switch position of “down” enables the operator.
To the left of the Operator on / off switch is the Ratio / Fixed switch. This switch assigned the operator to either play at a fixed frequency or to play at a ratio, or mathematical relationship, between it and the other operators. When switched to “Fixed” and the light under “Fixed” is lit, then the Coarse, Fine, and Tune knobs to the left of the switch allow you to program an exact frequency to be played by the operator. As an example, if you wanted an operator to act as a constant LFO at 20Hz in a sound you were programming, then you could use this method to create that oscillation.
When this switch is set to Ratio, the operator will track along the MIDI keyboard as you play notes, playing different frequencies for different notes. If you wanted to create a sound with operators playing octaves apart, you could program an operator to play at a ratio of 1.0 and a second operator to play at a ratio of 2.0. The rest of the controls within an operator section define the shape of the Output Level of the operator, which is critically important in defining the interactions between operators in FM – where the magic happens.
Right below the Ratio / Fixed switch are the A Mod Sens, Key Vel, and Level parameters. Starting with the most basic parameter, Level sets the output level of the operator, programmable from 0 to 99. Key Vel is the velocity sensitivity for the operator. This parameter takes the velocity of the inbound MIDI note, and scales the output Level according to that velocity. To be clear, The MIDI velocity level is NOT added to the programmed level. A Mod Sens, the final knob in this group, determines the sensitivity the operator output has to Amplitude Modulation (A Mod). It can be programmed to a sensitivity of 0-3. Amplitude Modulation sources can be assigned and enabled at a Global parameter level in the PARM section of dexed – more on this in another installment.
The EG Levels and EG Rates are a programmable Envelope Generator (EG) for the amplitude of the operator. Dexed kindly supplies a graphic representation of the shape of the output that you program. There’s a bit of trickiness here that deserves mention – the EG in dexed, as well as in the DX7 series, programs the EG in RATE, rather than TIME. This means you need to “invert” your thinking a bit when programming envelopes. What is meant here is that there is an inverse relationship mathematically between rate and time. In a more traditional analog synth the Time knobs, as you increase their values, create longer periods of transition between the two levels that you are working with. So, turning the time knob to the right slows the transition between the two EG levels. In FM, it’s the opposite. The RATE knob, as you turn it to the right, speeds the transition between the two levels. So if you want to slow things down, you turn the knob to the left. It’s advised that you keep an eye on that envelope graphic – to confirm the changes to the EG are headed in the right direction for the sound you are designing.
The EG Levels can be scaled by a keyboard level “curve” of sorts that you have control over. If you imagine a line that represents the amount of modulation that the keyboard position, or MIDI note, contributes to the sound, the Breakpoint parameter determines which MIDI note is the fulcrum of the modulation about – and the L depth and R depth determine the amount of modulation that is added below and above that fulcrum, respectively. So, while many MIDI synths center the fulcrum at Middle C, or MIDI note 64, DX7 / dexed-style FM synthesis adds a little more flexibility.
Not to be left out, the EG rates can also be scaled, with the Rate Scaling parameter determining the amount, and the L curve and R curves determining the shape of the modulation effect, allowing four shapes: -LN, _EXP, + EXP, + LN. LN indicating a Linear shape and EXP an exponential, or curved shape. This modulation of EG rates can come in handy when creating a longer envelope for lower notes and a shorter envelope for higher notes – as in brass or percussive keyboard sounds.
The Operators within dexed are the main course. There are six main courses, and like a tasting menu at a restaurant, they will all interact to create the final experience. How the 6 operators interact is mapped on the algorithm and affected by the amount of feedback programmed. Later in this series, we’ll walk through examples of the algorithms, modify operator levels and feedback, and start to hear how this mob works together to create dynamic sounds that FM is famous for. For now, try to create a simple sound using Op1 only – it will be a boring sine wave – but by starting here with all other operators turned off, you can get some ideas about how the EG levels, rates, and Breakpoints work together to control the output level of the operator. Next installment, we’ll examine how operators interact via the algorithm, and take a look at the global modulation source section. Until next time – thank you for reading!
Resources
- The dexed plug-in:
- The sounds included with dexed:
- SoundEngine’s Rebuild: dexed (Sounds, documentation, and tutorials):
- Our custom programmed banks – Flywheel and Crankshaft: dexed:
- Our dexed series so far:
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