Cabinet simulation using biquad filters

My last post introduced tintamp and described the target hardware for the project. I’ll now fill that out a little bit with a description of one the building blocks from which I plan to construct the initial signal chain. For now the idea is to fairly simplistic DSP techniques from which I can develop and end-to-end signal chain, including an amplifier, tonestack and cabinet simulation. Once there is a complete signal chain then the hardware components needed to connect the delicate guitar signal to the target hardware can be tested. Likewise when more sophisticated software is written we have a benchmark to compare it to. There’s no point in sophistication just for the sake of it.

Ultimately I think only two blocks are needed to build a complete chain, a biquad filter and a waveshaper. On its own the biquad is sufficient to construct simple tonestacks and cabinet simulations while the waveshaper allows us to trivially model non-linear relationships between input and output voltages. A waveshaper is not insufficient to model a real valve’s behaviour for AC signals although it can be combined with a biquad filter that feeds back from its input to its output to add at least some modelling of dynamic behaviour.

So on that basis I coded up these basic building blocks (including a test suite) and set to work. I should at this point express my gratitude to Robert Bristow-Johnson for his Cookbook formulae for audio EQ biquad filter coefficients, all those filters compressed into such an easy to read document saved me an awful lot of work.

So, lets get back on topic and introduce a basic cabinet filter using only biquad filters. My starting point was a trace of the frequency response of the Condor cabsim from runoffgroove. I had no real reason to pick this cabinet response over any other but I happened to stumble across their graph first.

Based on the above graph picked out five biquad filters:

  • A -16dB partial notch filter at 400Hz to get the deep notch
  • A 6dB high boosting shelf filter at 400Hz to get the roller coaster effect
  • A single high pass filter at 60Hz
  • Two low pass filters each at 4000Hz

As of now I haven’t yet graphed the response of this cabinet simulation (although I have listened to it) and currently all the Q values are untuned and set to 0.7 . Tuning these will have a big effect on the mid range since in particular they change the shape of the curve around the notch.

Having got this far I won’t be using the runoffgroove graph as a reference any more. All further tuning will be by ear and, eventually, I’ll create other speaker models using graphs from real speakers. That said it will be while before I start improving the cabsim. At the moment one working cabsim is sufficient to get the rest of the signal chain in place so the focus has to be somewhere else for a while.

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