According to Dr. Chowning, the Father of FM, one of the major innovations that Yamaha engineers brought to the table was the Feedback loop. Basically, routing the output back to the input to generate more complex tones. While this is similar to having additional modulators, it is not exactly the same thing – but will get you similar sonic results. In the reface DX each of the four Operators can be fedback on itself; this means a single operator can be a complex wave source. In the original DX7, for example, you had some ability to use Feedback. When all Operators are simply sine wave generators, if you wished to modulate a Carrier with a Sawtooth or Square Wave you would need to create a two Operator Modulator “stack” to generate that modulation wave to feed into the Carrier. Now any single Operator can generate the Sawtooth Wave or a Square Wave.

What this will translate into is you can build more complex waveshapes with fewer Operators; you can use your Operators to add fine details to your sound because it take less of them to create complex tones. You can simply build really phat sounds, quickly and easily. It’s something new to explore. Let’s begin with an Initialized Voice.

Press [FUNCTION]
Navigate to screen #4
Select “VOICE INIT”
Select “YES”
Press [ALGO]

This time lets select Algorithm #12… where each Operator is a Carrier (which means it is capable of audio output). Again, only Operator 1 has its Output Level set to 100, in an initialized Voice.

Playing the keyboard, you will hear the pure (and lonely) sine wave – no harmonics. Let’s apply some Feedback: Much like a microphone in front of a speaker column or an electric guitarist standing up close to the amplifier, Feedback is the process of signal traveling in a loop, output back to input, over and over again. What the engineers designed this time is a way to channel that energy specifically toward creating Sawtooth type Waves (those that contain every whole integer multiple of the Fundamental – ie, all harmonics) or toward creating Pulse type Waves (those missing certain odd numbered harmonics according to the width of the pulse). A Pulse Wave that is 1:2 (square) will be missing every other harmonic; it contains only the odd numbered harmonics. A pulse that is 1:3 will be missing every third harmonic, a pulse that is 1:4 will be missing every fourth harmonic and so on.

Press [FB] Feedback
You arrive at a screen showing the four Operators set to 0 (0 here means sine wave). As you swipe up in touch column 1 you will be sending the output of Operator 1 back into itself and thereby generating a Sawtooth Wave output. The higher the value toward 127, the brighter the result. The wave will contain all harmonics.

Notice that there is a “law of diminishing returns” in effect here: as you increase the value, the sound gets richer and brighter, however, once you exceed a certain value the sound starts to over modulate and thin out. Hint: when setting Output Level – use your ears, they will serve you better than the mathematics!

Try the opposite direction, again, as you increase the value by swiping down from the 0 centerpoint, the sound gets richer and rounder, as it tends toward the Square/Pulse Wave output. And again as you near the extreme the sound begins to thin out again. 

Extra Credit: We mentioned that only Carrier Operators can be heard – Modulators are only heard by how they impact the timbre of the Carrier. You will discover when working with Operators and creating basic wave shapes, it is useful to use the ability to switch Algorithms. When you want to hear your Operator you can switch to an Algorithm, like #12, where all Operators are Carriers and will output audio. The integrity of your programming within the Operator is maintained even though you switch it from Modulator to Carrier, for example. In general, the more complex the Modulator’s waveform the more radically it will influence the Carrier you route it through. If your Modulator is very complex, it may quickly result in generating so many sidebands (all frequencies) it will begin to approach non-musical sound (Noise). Noise contains all frequencies simultaneously. Sometimes the best sound is obtained through subtle use of this modulating “influence”… like applying vibrato to a violin or to a synth lead, sometimes a little is all you need. Applying too much can lead to a bit of chaos. The “Modulation Index” is a fancy way of saying the “amount” of modulation you are applying to the Carrier (Modulation Index = the Output LEVEL of the Modular). As we go forward, we’ll show you how to isolate the contribution of each of your Operators. Next we’ll look at the Amplitude Envelope Generator – how loudness or output is controlled over time. The old “ADSR” (Attack-Decay-Sustain-Release).

If you have any questions or comments about this article, please let us know in this forum thread.

With the MIDI CONTROL set to ON, the reface DX will generate/receive Control Change messages from many of its key parameters. This makes sense from both a performing and programming standpoint. If you are using your reface DX as a sound source, recording to your favorite DAW, you can use the ability to draw in controllers to make sonic changes. The real-time controllable parameters include:

Algorithm – there are 12 of these configurations of Modulators and Carriers. An algorithm is like a basic framework (recipe) in which the Operators relate to each other. The fact that each Operator is programmed independently and each Operator has its own EG, means by simply changing the algorithm you can change the role an Operator plays in the resulting sound. An Operator that was a Modulator (influencer) in one Algorithm, may become an audible Carrier in another and/or vice versa.  
cc80

Operator Output Level – the Output Level will change the amount  of influence one Operator has on another or itself, or will change the resulting volume output of the final sound. In a situation where all Operators are Carriers (audible) the Output Level will be understood as acting like the “drawbars” on a tone wheel organ. In a situation where an Operator is a Modulator, changing its output level will change the resulting timbre of the Operator it is influening.
Op1 = cc85
Op2 = cc102
Op3 = cc108
Op4 = cc114

Operator Feedback Level – the Output Level of an Operator can be fed back to its own input creating a Feedback Loop. This can be used to create more complex waveshapes without the need for a stack of Operators acting as Modulators. So a movement in Feedback Level will sound like timbre change. The roll of the Filter in an analog (subtractive) synthesis system would be timbre change, while this is roughly what it going on, the result will be quite a bit different. While a Low Pass (analog) Filter is removing high harmonics to affect timbre change, changing the Feedback Level will be more about changing the nature and relationships of the audible harmonics.
Op1 = cc86
Op2 = cc103
Op3 = cc109
Op4 = cc115

Operator Feedback Type – the Feedback Type can be toward the Sawtooth wave family (where all harmonics are represented) or toward the Square wave (pulse) family (where certain harmonics are missing according to the current pulse width ratio). Harmonics are whole integer multiples of the Fundamental pitch. If the original pitch (or Fundamental) is A110, then the whole integer multiples would be A220, E330, A440, C#550, E660, etc, etc. creating a sawtooth type waveform. If the original pitch is again A110 a square wave would produce just odd numbered harmonics – every other harmonic. As the pulse width narrows (the tone gets more pinched/nasal) the missing harmonics change accordingly… when the ON:OFF of the Pulse width extends to 1:3, 1:4 and so on every third and then every fourth harmonic would be missing. So real time control can be used to select the type of harmonic content you are controlling with the Feedback Level parameter.
Op1 = cc87
Op2 = cc104
Op3 = cc110
Op4 = cc116

Operator Frequency Mode – the Operator’s Frequency can be expressed in terms of a Ratio or it can be Fixed to a specific frequency. To understand “Ratio” is to understand musical harmonics. One Operator generating a Sine Wave: At Ratio = 1.00 the key you strike will play a sine wave at the Fundamental pitch. If you Coarse tune the Frequency Ratio to 2.00 the key you strike will play a sine wave at the second harmonic (one octave above the fundamental). If you Coarse tune the Frequency Ratio to 3.00, the key you strike will play a sine wave at the third harmonic (one octave and musical fifth above the fundamental), and so on. To understand “Fixed” is fairly easy – the Operator will create a specific pitch between 1.000Hz and 9,772Hz – and all keys reproduce that frequency. A Fixed Frequency of 1.000Hz is not heard as a musical tone, and 9772Hz is more than an octave above the highest note on the piano (4186.01Hz)
Op1 = cc88
Op2 = cc105
Op3 = cc111
Op4 = cc117

Operator Frequency Ratio (Coarse tune) –  The reface DX can reproduce the first 31 harmonics (plus they throw in the SubHarmonic at 0.500
Op1 = cc89
Op2 = cc106
Op3 = cc112
Op4 = cc118

Operator Frequency Ratio (Fine tune) – all the values between x.00 and y.00 fall inbetween the whole integer multiples – and therefore will have an entirely different relationship to our music. Some will just sound like horribly wrong, some will remind you of clangorous bell tones, some will have a pleasing effect others, not so much. Fine tuning will increase the possibility of frequency “beating” as it plays along side other Operators. Even in the role of a Modulator detuning the Frequency will have a profound affect on the resulting timbre. As a Carrier detuning can play tricks on our ears and give us some wonderful ‘aural illusions’ (you know about ‘optical illusions’, where your eyes get fooled – aural illusions are those that happen for your ears!)
Op1 = cc90
Op2 = cc107
Op3 = cc113
Op4 = cc119

POSSIBLITITIES
Many DAW programs, like Cubase, allow you to create automation by generating control data. In many cases you can use a controller or simply “draw” the controllers into a track. Each of the above parameters is available (MIDI CONTROL set to ON). Each of the parameters above has an assigned CC (Control Change) number allowing indivdual Operator control, except the Algorithm parameter which, when changed, affects all Operators together.

You can use an iOS App like Yamaha’s “Faders & Pad” (shown at left) to assign Control Change numbers to sliders and use the zero-gravity “gravity ball” to alter the sound in real time. Assigning Control Sliders is easy – you simply press the “LEARN” button, and move your finger on the reface DX’s Data Entry touch pad, and you’ve assigned that parameter to the Fader. You can set a MAX and MIN by simply having the App “learn” how far you want it to control a parameter (you don’t want to send a modulated 9772Hz Waveform flying around the room unnecessarily – your dog will move out!). 

The beautiful thing here is, you don’t have to know what exactly is going to happen, in fact, exploring what does happen is half the fun. Get an idea, try it! Or use it for practical purposes. Assign the Faders and use them directly, with out the gravity ball. Make one of them your Moduation Wheel, and setup others to control Output Levels and control timbre change in specific ways!

Got a cool idea about using control change with your reface DX? Got questions about this resource? Let us know here in the forum!