The Quilcom QX7 is a free VST synthesizer instrument inspired by the amazingly successful Yamaha DX7, first made available in 1983.
The QX7 is not an emulation, nor do the presets copy the sound of the originals, but the architecture is very similar.
There are 6 identical “Operators”, OP1 to OP6. An operator is a simple tone generator with an envelope. Each operator has an FM input (it’s actually phase modulation, as in the original, but it’s called FM). The output from each Operator is routed either to the master output or to another Operator’s FM input. The actual routing patch is called the “Algorithm” allowing a wide variation in modulator/carrier configurations. Every algorithm has 1 feedback path wired in. The amount of feedback can greatly influence the sound since the amplitude of the feedback signal will affect the spectrum (timbre) of the sound dynamically. The QX7 also has a patch bay so you can experiment and create your own algorithms.
On the right-hand side you can choose Patch bay or Hardwired algorithms (the patch bay is then replaced by an algorithm selector). The 32 hard wired ones are identical to the original’s but in addition there is a User algorithm set to all 6 Ops stacked. If you press the View Algorithms button you’ll get to see a sheet showing the connections for each algorithm. This image is also in the download zip. This is very useful while programming and editing presets to keep track of which Operator does what.
- Each operator has the following features:
- Waveform select (the DX7 had just sine waves) with chromatic and fine tuning.
- Keyboard-based pitch or fixed frequency which can be set coarse and fine.
- Envelope ADSR with variable velocity response and operator output level.
Based on the “Centre key” that you can set, you can adjust the operator’s output to be higher or lower based on the keyboard number. In this way you can achieve a key scaling across your keyboard so that the higher and lower ends will sound different. This is useful for modelling real instruments for example. If the knob is centred there is no key scaling.
The key to rate knob provides a variable amount of shortening of the decay and release stages at higher key numbers. This allows for shorter notes as the pitch increases, like in a piano.
The LFO to amplitude knob sets the depth of AM from the single polyphonic LFO on the front panel. This modulation will affect the amplitude of any operator feeding the master output (a carrier) or the timbre of any operator connected as an FM source (a modulator).
The Pitch env LED/button will connect the operator to the single polyphonic pitch envelope’s output bus.
There is a single polyphonic LFO that provides the LFO bus for all the operators. This LFO has a knob to modulate the pitch of all the operators simultaneously and also a delay for the LFO start. There is a pitch envelope generator and each operator can be connected or not to the Pitch env bus. The Pitch env should be left off if not required to save CPU.
The Modwheel LFO and the Static FM LFO affect the whole instrument.
Unlike the DX7, the QX7 has inbuilt effects, chained from left to right, and they can be individually enabled. The QX7, like the DX7, produces a monaural (non-stereo) output internally from the operators but the effects chain runs in stereo and can give a huge boost to the sonic impression.
If you do get a stuck note there is a reset button.
Finally there is a level meter with a clip-hold LED.
The download zip includes the original user manual. Although the QX7 is very different in detail, there’s useful background material in the manual to expand further on the technique, should you not be familiar with FM synthesis principles.