5 ms·

Simulating the TS808 Tube Screamer in LTSpice

40 points by cushychicken 11y ago

j4mie 11y ago
Sorry if this is a stupid question (I don't know anything about LTSpice) but would it be possible to actually feed a guitar signal through this simulation and run it out into an amp, in real time?
- cdawzrd 11y ago
  No, LTSpice simulations are not real-time. However, there have been attempts to make that happen[1]! [1] http://www.livespice.org/ http://www.livespice.org/
  - earlz 11y ago
    Have you personally been able to get anything to actually work in livespice? I've observed that it's UI is so broken that it's practically unusable
    - dsharlet 11y ago
      Can you say what's broken about it? Is there something that prevents you from following the tutorial to build the low pass filter? A lot of people have problems with audio IO (the ecosystem of audio devices, drivers, OSes, etc. unfortunately is a minefield) but I haven't heard many complaints about the UI.
      - earlz 11y ago
        I'd have to open it up again and try to do something with it to remember.. I think it was like I'd drag something onto it, but then I couldn't delete it, and a few other unintuitive things.. and yea, I never could get the audio IO to actually work
        dsharlet 11y ago
        Delete should just be the delete key... I think there was also an item on the edit menu to delete the selection. I tried to make it behave as standard as possible (the usual windows shortcut keys for cut/copy/paste/delete/etc.). There's an annoying issue in the GUI framework I used where the right part of the UI has to be in focus for these things to work. Maybe that was the issue.
- jandrewrogers 11y ago
  No, these kinds of tools are not designed for real-time signal processing. It is possible to build real-time signal processing software that accurately models physical circuit behaviors, you just need to optimize for computational throughput and latency to make it practical. For audio processing in particular, accurately simulating physical circuit designs is resurrecting a lot of classic hardware in virtual form. CPUs have enough computational power now to implement very convincing circuit emulations in real-time.
  - BMarkmann 11y ago
    By "convincing", what sort of delay would you be looking at on reasonably powerful hardware? Enough to fool the ear into thinking it really was real-time? As a side question, are the sorts of modeling / math that would be done in a scenario like this amenable to offloading to a GPU (if the transformations would potentially be better handled by something like http://www.bealto.com/gpu-fft.html http://www.bealto.com/gpu-fft.html, for instance)? Is that a dumb question? :-)
    - klodolph 11y ago
      No, that's not a dumb question. I suspect that a successful port of a circuit simulator to the GPU would yield impressive performance gains. However, it would be far, far from easy. Disclaimer: I am a developer on a commercial circuit simulator. As for delay, 10ms is fairly reasonable.
    - dsharlet 11y ago
      Unfortunately, the math required for circuit simulation does not lend itself to parallel implementations, whether it be on a GPU or otherwise. I built LiveSPICE (linked elsewhere in the thread), so I fought a lot with this problem :) A small disclaimer on the following, I only attempted simulation of audio amp/effect pedal circuits, which are generally small. There might be ways to effectively parallelize (much) larger circuits. Another disclaimer, I worked on this 1+ years ago, so some of it is fuzzy. The basic issue is that circuit simulation is fundamentally a serial operation. The circuit state at time step n is a function of the circuit state at time step n-1. The relationship between timesteps is basically a non-linear system of equations, where the number of variables is the number of nodes in the circuit. For a typical guitar effect pedal, this might be ~50 variables. There's basically no opportunity for parallelism here. You can't parallelize across the non-linear systems, because you don't know what to solve at timestep n until you've solved for timestep n-1. Within each step, a 50 variable system of equations is probably too small to parallelize, even on the CPU. Note that the technique used here is generally Newton's method, which also is difficult to parallelize for the same reason. The bottom line is that as far as I know, circuit simulation (at least for audio) is entirely limited by single thread performance. As a side note, the fun part of LiveSPICE is that even a 50 variable non-linear system is far too big to solve at real time sample rates (48 kHz, plus oversampling to avoid aliasing artifacts). The trick is to observe that only a fraction of these variables are non-linear, most of them are related to the other variables linearly. LiveSPICE solves for these linear relationships once during initialization of the simulation, and eliminates them prior to solving the non-linear system. This turns a typical ~50 variable system into a ~5 variable non-linear system (plus a 45 linear relationships). This is a massive reduction in computation required (Newton's method involves repeatedly solving a linear system, which is O(n^3) for simple methods).
- cushychicken 11y ago
  Not a dumb question at all! I don't know about that, but I did find out by submitting this to reddit that you can use a WAV file as a time domain stimulus for LTSpice.
  - korethr 11y ago
    Neat! I had hoped such might be the case, but wasn't sure -- it's been a while since I played with LT spice. I'll have to take some recordings of my guitars to use in simulations the next time the pedal hacking bug bites.
- atomheart 11y ago
  Not sure how much your question is focused on LTSpice, but there are tons of applications and VST plugins that can simulate amplifiers and effect pedals in real time with "good enough" (some artists use them to record their albums) quality, e.g. Amplitube, Guitar Rig, Peavey Revalver, LePou, etc.
vibrolax 11y ago
For a circuit meant to be used with a guitar or similar instrument, it is practical for many purposes to use a much lower sampling rate, e.g. 22 KHz. The frequency content beyond 10 kHz is quite limited. I have used conventional LTspice to simulate several vacuum tube guitar/bass/microphone preamps I have built, for example [1]. I am tempted to try a real-time A-B comparison of the actual device with a livespice simulation. [1] http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreamplifier.html http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreampl...
- TheOtherHobbes 11y ago
  As soon as you create non-linearities in DSP you have to oversample to avoid aliasing. In fact it's usual in DSP distortion circuits to oversample by a factor of at least 4 or 8. (I"ve seen some devs use 256X oversampling...) I can't think of a reason why this wouldn't also be needed in an LTspice model. You can do distortion without it, but if you sample at 22k you're guaranteed to get that harsh halo around the midrange and high end that digital is famous for. Analog circuits don't have the same problem, because anything out-of-band simply gets rolled off. Unlike a digital system analog doesn't have a Nyquist limit beyond which frequency components reflect back, so a physical circuit should always sound smoother.
  - vibrolax 11y ago
    Good point, that nonlinear processes generate frequencies not present in the input. One of the biggest challenges in making analog circuits with good-sounding clipping distortion is designing proper lowpass and bandpass characteristics into the coupling between stages to roll off undesired artifacts. The advantage in being able to actually listen to a real-time simulation modeled at the level of resistors, capacitors, inductors, and tubes or transistors is that one would be able to immediately hear the effect of component value changes, rather than just see them on the transient or frequency response plots. Modeling the system at the level of DSP blocks would be more computationally efficient, but then one is left to derive the equivalent analog circuit. It's not rocket science, but it does add a level of indirection. In any case, being able to simulate 5 triode sections and the linear filters between them in real time at a 48 kHz sampling on a modern CPU very well might be good enough for rock and roll. My use case is a design tool, not to obtain a substitute for a physical device.

40 points by cushychicken 11y ago

j4mie 11y ago

Sorry if this is a stupid question (I don't know anything about LTSpice) but would it be possible to actually feed a guitar signal through this simulation and run it out into an amp, in real time?

cdawzrd 11y ago
No, LTSpice simulations are not real-time. However, there have been attempts to make that happen[1]! [1] http://www.livespice.org/ http://www.livespice.org/
- earlz 11y ago
  Have you personally been able to get anything to actually work in livespice? I've observed that it's UI is so broken that it's practically unusable
  - dsharlet 11y ago
    Can you say what's broken about it? Is there something that prevents you from following the tutorial to build the low pass filter? A lot of people have problems with audio IO (the ecosystem of audio devices, drivers, OSes, etc. unfortunately is a minefield) but I haven't heard many complaints about the UI.
    - earlz 11y ago
      I'd have to open it up again and try to do something with it to remember.. I think it was like I'd drag something onto it, but then I couldn't delete it, and a few other unintuitive things.. and yea, I never could get the audio IO to actually work
      - dsharlet 11y ago
        Delete should just be the delete key... I think there was also an item on the edit menu to delete the selection. I tried to make it behave as standard as possible (the usual windows shortcut keys for cut/copy/paste/delete/etc.). There's an annoying issue in the GUI framework I used where the right part of the UI has to be in focus for these things to work. Maybe that was the issue.
jandrewrogers 11y ago
No, these kinds of tools are not designed for real-time signal processing. It is possible to build real-time signal processing software that accurately models physical circuit behaviors, you just need to optimize for computational throughput and latency to make it practical. For audio processing in particular, accurately simulating physical circuit designs is resurrecting a lot of classic hardware in virtual form. CPUs have enough computational power now to implement very convincing circuit emulations in real-time.
- BMarkmann 11y ago
  By "convincing", what sort of delay would you be looking at on reasonably powerful hardware? Enough to fool the ear into thinking it really was real-time? As a side question, are the sorts of modeling / math that would be done in a scenario like this amenable to offloading to a GPU (if the transformations would potentially be better handled by something like http://www.bealto.com/gpu-fft.html http://www.bealto.com/gpu-fft.html, for instance)? Is that a dumb question? :-)
  - klodolph 11y ago
    No, that's not a dumb question. I suspect that a successful port of a circuit simulator to the GPU would yield impressive performance gains. However, it would be far, far from easy. Disclaimer: I am a developer on a commercial circuit simulator. As for delay, 10ms is fairly reasonable.
  - dsharlet 11y ago
    Unfortunately, the math required for circuit simulation does not lend itself to parallel implementations, whether it be on a GPU or otherwise. I built LiveSPICE (linked elsewhere in the thread), so I fought a lot with this problem :) A small disclaimer on the following, I only attempted simulation of audio amp/effect pedal circuits, which are generally small. There might be ways to effectively parallelize (much) larger circuits. Another disclaimer, I worked on this 1+ years ago, so some of it is fuzzy. The basic issue is that circuit simulation is fundamentally a serial operation. The circuit state at time step n is a function of the circuit state at time step n-1. The relationship between timesteps is basically a non-linear system of equations, where the number of variables is the number of nodes in the circuit. For a typical guitar effect pedal, this might be ~50 variables. There's basically no opportunity for parallelism here. You can't parallelize across the non-linear systems, because you don't know what to solve at timestep n until you've solved for timestep n-1. Within each step, a 50 variable system of equations is probably too small to parallelize, even on the CPU. Note that the technique used here is generally Newton's method, which also is difficult to parallelize for the same reason. The bottom line is that as far as I know, circuit simulation (at least for audio) is entirely limited by single thread performance. As a side note, the fun part of LiveSPICE is that even a 50 variable non-linear system is far too big to solve at real time sample rates (48 kHz, plus oversampling to avoid aliasing artifacts). The trick is to observe that only a fraction of these variables are non-linear, most of them are related to the other variables linearly. LiveSPICE solves for these linear relationships once during initialization of the simulation, and eliminates them prior to solving the non-linear system. This turns a typical ~50 variable system into a ~5 variable non-linear system (plus a 45 linear relationships). This is a massive reduction in computation required (Newton's method involves repeatedly solving a linear system, which is O(n^3) for simple methods).
cushychicken 11y ago
Not a dumb question at all! I don't know about that, but I did find out by submitting this to reddit that you can use a WAV file as a time domain stimulus for LTSpice.
- korethr 11y ago
  Neat! I had hoped such might be the case, but wasn't sure -- it's been a while since I played with LT spice. I'll have to take some recordings of my guitars to use in simulations the next time the pedal hacking bug bites.
atomheart 11y ago
Not sure how much your question is focused on LTSpice, but there are tons of applications and VST plugins that can simulate amplifiers and effect pedals in real time with "good enough" (some artists use them to record their albums) quality, e.g. Amplitube, Guitar Rig, Peavey Revalver, LePou, etc.

vibrolax 11y ago

For a circuit meant to be used with a guitar or similar instrument, it is practical for many purposes to use a much lower sampling rate, e.g. 22 KHz. The frequency content beyond 10 kHz is quite limited. I have used conventional LTspice to simulate several vacuum tube guitar/bass/microphone preamps I have built, for example [1]. I am tempted to try a real-time A-B comparison of the actual device with a livespice simulation. [1] http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreamplifier.html http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreampl...

TheOtherHobbes 11y ago
As soon as you create non-linearities in DSP you have to oversample to avoid aliasing. In fact it's usual in DSP distortion circuits to oversample by a factor of at least 4 or 8. (I"ve seen some devs use 256X oversampling...) I can't think of a reason why this wouldn't also be needed in an LTspice model. You can do distortion without it, but if you sample at 22k you're guaranteed to get that harsh halo around the midrange and high end that digital is famous for. Analog circuits don't have the same problem, because anything out-of-band simply gets rolled off. Unlike a digital system analog doesn't have a Nyquist limit beyond which frequency components reflect back, so a physical circuit should always sound smoother.
- vibrolax 11y ago
  Good point, that nonlinear processes generate frequencies not present in the input. One of the biggest challenges in making analog circuits with good-sounding clipping distortion is designing proper lowpass and bandpass characteristics into the coupling between stages to roll off undesired artifacts. The advantage in being able to actually listen to a real-time simulation modeled at the level of resistors, capacitors, inductors, and tubes or transistors is that one would be able to immediately hear the effect of component value changes, rather than just see them on the transient or frequency response plots. Modeling the system at the level of DSP blocks would be more computationally efficient, but then one is left to derive the equivalent analog circuit. It's not rocket science, but it does add a level of indirection. In any case, being able to simulate 5 triode sections and the linear filters between them in real time at a 48 kHz sampling on a modern CPU very well might be good enough for rock and roll. My use case is a design tool, not to obtain a substitute for a physical device.