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A real-time netlist based audio circuit plugin
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README.md

RTspice

A netlist-based realtime audio circuit simulator

Requirements

Getting started

I recommend to obtain the dependencies through your distribution's package manager (Ubuntu has JACK on the default repo and the DBUS version on some PPAs, arch linux also has most of the dependencies on the default repo, and the rest can be installed from AUR). The JACK configuration can be tricky if your distribution depends on PulseAudio, however, the D-Bus version should make it easier to setup.

Once all dependencies are installed, git clone --recurse-submodules this repository into a directory of your choice, create a build directory mkdir build, configure the project with cmake .. -DCMAKE_BUILD_TYPE=Release and make it. Should compilation succeed, you can check that the simulation works with the compiled tests and run the program with ./rtspice and you'll be greeted with the blank entry screen:

home screen

You may then open a netlist file (.net extension), and if the netlist has a proper syntax, you should see the simulation screen:

sim screen

Basic information, such as the circuit's name, the number of nonzero elements in the modified admittance matrix and the number of modified nodes can be seen top left.

Information regarding JACK processing load, ports and connections lies top right. Once you've chosen the appropriate input and output connections, press 'Activate' to begin simulation.

For circuits containing parametrized components (i.e. variable resistors, capacitors or inductors), knobs will be visible on the bottom of the screen. The 'Log' checkbox switches the knob control pattern from linear to logarithmic.

How it works?

Traditional circuit simulation programs are based on modified nodal analysis, where the node voltages and some branch currents are the unknowns in a (usually sparse) linear system, and can be obtained through regular linear system solvers.

Circuits that contain dynamic components such as capacitors require modelling of the dynamic behavior based on some integration method, being reduced to sources and time-varying resistances. For now, we use trapezoidal integration for both capacitors and inductors.

Circuits that contain nonlinear components require solving a nonlinear system of equations, however using the Newton-Raphson method, this also reduces to solving iterations of linear systems.

Based on this, this program provides an interface for a computer's audio ports to act as signal sources or measurements for electronic circuits. A difficulty arises in the needed time to solve said linear systems rapidly enough to keep up with the sound samples: for instance, a 44.100 kHz sampling rate requires the solution to be calculated in less than 23 μs. Considering this, it can be expected this software to work better with modest-sized circuits, however, the actual size you'll be able to consistently simulate will depend on your hardware.

Netlist syntax

The circuits are described through a netlist, representing which components should be added and which nodes the components are connected to. Our netlist syntax is similar but not exactly the same as SPICE's:

  • The first line of the netlist is a special comment containing the circuit name:
    • My little circuit
  • Lines beginning with asterisks(*) are considered comments and are ignored:
    • *Some witty comment
  • The rest of the lines are considered statements and go towards the circuit definition:
    • RL 0 1 2.2k
  • Multiline statements can be made prepending the sequenced lines with +:
    • Vin NODE_A NODE_B
      +EXT IN

Components

The currently supported components are listed below:

Component Type Syntax Example Notes
DC Voltage V{ID} {NODE+} {NODE-} DC {VALUE} Vcc 1 0 DC 12 VALUE in Volts
Sinusoidal Voltage V{ID} {NODE+} {NODE-} SINE {AMPLITUDE} {FREQUENCY} {PHASE} Vac p 0 SINE 120 60 0 AMPLITUDE in Volts, FREQUENCY in Hertz and PHASE in degrees
External Voltage V{ID} {NODE+} {NODE-} EXT {INPUT} VIN 1 0 EXT INPUT INPUT defines the name of an input port to be connected
DC Current I{ID} {NODE+} {NODE-} DC {VALUE} Ibias b 0 DC 1u VALUE in Amperes, flows from NODE+ to NODE-
Sinusoidal Current I{ID} {NODE+} {NODE-} SINE {AMPLITUDE} {FREQUENCY} {PHASE} If p 0 SINE 1n 10k 90 AMPLITUDE in Amperes, FREQUENCY in Hertz and PHASE in degrees
External Current I{ID} {NODE+} {NODE-} EXT {INPUT} IIN 1 0 EXT I_INPUT INPUT defines the name of an input port to be connected
Linear Voltage Amplifier E{ID} {OUT+} {OUT-} {IN+} {IN-} {Av} E1 0 x 1 0 100
Linear Current Amplifier F{ID} {OUT+} {OUT-} {IN+} {IN-} {Ai} F1 2 9 h u -3
Linear Transconductance G{ID} {OUT+} {OUT-} {IN+} {IN-} {Gm} G1 6 0 1 0 10u Gm in Siemens
Linear Transresistance H{ID} {OUT+} {OUT-} {IN+} {IN-} {Rm} H5 8 0 t u -3 Rm in Ohms
Linear Resistor R{ID} {NODE_A} {NODE_B} {VALUE} Rload 0 X 22k VALUE in Ohms
Variable Resistor R{ID} {NODE_A} {NODE_B} EXT {MAX_VALUE} {PARAM} Rvol OUT 0 EXT 500k Volume MAX_VALUE in Ohms, PARAM defines the name of a knob
Linear Capacitor C{ID} {NODE_A} {NODE_B} {VALUE} Rbypass 23 A 10u VALUE in Farads
Linear Inductor L{ID} {NODE_A} {NODE_B} {VALUE} Lchoke vcc c 10m VALUE in Henrys
Ideal OPAMP U{ID} {OUT+} {OUT-} {IN+} {IN-} OPAMP U1 out 0 in out OPAMP Usually, OUT- should be grounded
Basic Diode D{ID} {ANODE} {CATHODE} IS={IS} N={N} D1 cut 0 IS=4.3n N=1.9 IS is saturation current in Amperes, N is the emission coefficient
Bipolar NPN Q{ID} {COLLECTOR} {BASE} {EMITTER} NPN IS={IS} BF={BF} BR={BR} Q1 c b e NPN IS=3.84e-14 BF=324.4 BR=8.29 BF is forward beta, BR is reverse beta
Bipolar PNP Q{ID} {COLLECTOR} {BASE} {EMITTER} PNP IS={IS} BF={BF} BR={BR} Q1 c b e PNP IS=3.84e-14 BF=324.4 BR=8.29
PROBE PROBE {NODE} PROBE OUT PROBE is how output variables are defined. For each PROBEd node, an output port becomes available to the user

Inputs, Outputs and Params

The netlist not only describes the circuit, but it also defines where the inputs are located. For each different {INPUT} for external voltages and currents, an input port will be available to connection, corresponding to that given source.

For potentiometers, each different {PARAM} corresponds to a knob that will be available on the control screen.

For the outputs, the special component PROBE marks a node voltage (or branch current) that will serve as an output port for the system.

TODO

  • Proper potentiometer

    • Currently one can emulate a potentiometer: Rpot X Y Z POT VAL PARAM as:
      RpotX X @XY VAL
      RpotY @XY Y EXT -VAL PARAM
      RpotZ Y Z EXT VAL PARAM
  • Subcircuit models (i.e. compensated OPAMP)

  • JFET and MOSFETs

  • Nonlinear dynamic components

  • A graphical circuit editor

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