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SoftDrive GV


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SoftDrive GV is a virtual-analog model of the Marshall "Guv'nor" and "Guv'nor GV-2 Plus" guitar distortion effects. The Guv'nor is a solid-state distortion pedal first issued in 1989 to recreate the sound of a cranked up Marshall stack in a small stomp-box format. It became rather popular in the 90's and is still used widely nowadays for its quality high-gain sound and a its tonal versatility. It was re-issued in the early 2000 in an updated housing and incorporating a new "Deep" control to mimic the effect of a JCM2000's "Deep" switch.

This VST model is a tribute to the original effect and models the circuits of both the "vanilla" Guv'nor and its modern GV-2 variant.

SoftDrive GV expands on the original design and brings in new possibilities unthinkable of outside of the digital realm. In the spirit of my prior creations, this plug-in also features a few things to visualize what's going on inside the box.

I hope that SoftDrive GV will find its uses in your digital guitar effect chain, just as the original Guv'nor did in my analog one.                       

                             
The Model
The overall effect structure is as follows:


    Input Filter
    OpAmp A (gain, filtering, clipping)
    OpAmp B (gain, filtering, clipping)
    ToneStack (LED clipping, filtering)
    Deep circuit (filtering)
    Output Filter (attenuation, filtering)


Each stage is modeled separately and using different techniques. A care is taken to make sure that no inter-stage dependency is lost during the decomposition.

The input filter is a simple high-pass filter digitized with Bilinear Transform.

Two OpAmp stages are treated as non-linear filters and digitized as non-linear state-space systems with trapezoidal integration rule. The OpAmp feedback is solved implicitly without adding any extra delay. The nonlinearity was carefully modeled after TL072 op-amps and is kept parametric to allow for online changes of power supply voltage and the open-loop gain. The open-loop gain affects the sharpness of the saturation region and the Q-factor of the feedback filter. The whole stage is re-modeled online upon the parameter change to allow for very fast computation. This optimization is very similar to the one employed in SoftAmp 3OD.

The OpAmp stages are followed by the LED (ligh-emitting diode) clipper and the tonestack. The LED clipper is inseparable from the tonestack and effectively forms another non-linear filter. It is modeled as a non-linear state space system. The LED clipper can be switched on and off in the VST plug-in. Note that GV-1 and GV-2 tonestacks are different.

The Deep circuit is a linear filter that has a narrow peak in the low frequencies as well as more subtle peak in high frequencies. It is modeled as a state-space system with trapezoidal integration rule.

The Output filter consists of the Level potentiometer shunted by capacitor. It's a low-pass filter with cut-off frequency dependent on the position of the Level pot. It's digitized with matched-z transform with an additional compensating single-zero filter to reduce the discretization error at high frequencies.

The topology of every stage is preserved during discretization to allow for smooth parameter changes without any artificial discontinuities in the output signal.

The whole model can be optionally oversampled 8x to minimize the aliasing.

There's an additional option to scale the time base of the discretized model. It results in linearly shifted frequency responses of all stages. For example it can be tweaked to obtain more pleasing sound with your particular instrument or to move the whole response down an octave to use on bass guitar or a synth.

All controls are carefully linearized to produce a linear change in audio (logarithmic) scale.

UI Controls
The effect's controls are broken down into 4 sections:

     Six control knobs of the original analog effect
     Four switches to toggle different stages
     Four sliders to tune the physical parameters
     Four buttons on top to control the VST plug-in



    1. Knobs

    Gain - input gain, also affects the pre-emphasis filters
    Level - output attenuation, frequency-dependent (see "Bright")
    Deep - low-end resonance, also gives a slight top-end boost (GV-2 only)
    Bass - low frequencies in the output signal
    Mid - middle frequencies in the output signal
    Treble - high frequencies in the output signal


2. Switches

    Mode - switch between GV-1 and GV-2 circuit models
    LEDs - toggle the LED clipping stage of the circuit
    Bright - toggle the treble roll-off of the "Level" control (The famous "C22 Mod")
    Power - toggle the whole effect on/off


3. Sliders

    Voltage - power supply voltage (the default is 9v)
    Fidelity - Op-Amp's open-loop gain non-linearly from 100 to 200000 (default is 200000)
    Scale - "time scale" of the model, effectively shifts the frequency response.
    Mix - dry/wet mix
                           

4. Buttons

    About
    Check for updates
    Toggle HQ mode (oversampling)
    Toggle Stereo mode



UI Visualization
The plug-in UI has 2 areas that visualize the model:

1. Overdrive Stages - features an icon for each stage where saturation occurs:

     OpAmp A
     OpAmp B
     LEDs                           


2. Filtering Stages - features two logarithmic plots that show the amplitude-frequency characteristics of the filtering stages:

    Pre  (Input filter + OpAmpA + OpAmpB)
    Post (Tonestack + Deep + Output filter)


                                                           
Installation:
Unzip the archive and put the "AXP SoftDrive GV.dll" or "AXP SoftDrive GV x64.dll" to
your VST folder. Make sure to select the one appropriate for your system (Win32 or x64).


Notes:

    Enabling the HQ (oversampling) mode introduces an 8 sample delay. It won't be compensated by the DAW. It will be fixed in future releases.



    This plug-in requires the Windows Imaging Component library. It's included in all Windows versions starting with Windows XP SP3. If you are using Windows XP SP2, you can get it as a stand-alone library here:
    http://www.microsoft.com/en-us/download/...aspx?id=32



    It is my hobby to develop VST audio effect models. I'm not getting paid for it and I use my own limited spare time to do this work. If you like my projects and would want to encourage the future work please consider doing something from the following list:

  - Tell your friends about my site and my plug-ins
  - Send me audio samples you've made with my plug-ins
  - Drop me a line or two with any comments and suggestions
  - Make a small donation through my web site


http://www.soft-amp.com/softdrive-gv

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