[sdiy] FIR or IIR Digital filters for the PIC

[Csaba Zvekan]

Highly interesting stuff, Scott.
I was looking at the dev. board (149$ not too bad)  http://www.xilinx.com/products/devkits/HW-SPAR3E-SK-US-G-image.htm
  . It's looking sweet . But how much is the Verilog compiler? I  
understand  it's kind of like C ?
But isn't FPGA a highly complicated matter? Or has it been tamed in  
the meantime?

Csaba


On Jun 5, 2008, at 5:15 PM, Scott Gravenhorst wrote:

> Csaba Zvekan <czvekan at gmail.com> wrote:
>> Hi group,
>>
>> Has anybody done some work on FIR and IIR digital filters with the  
>> PIC
>> or dsPIC family that can be used for SDIY ?
>> I would look at some codes and designs .
>
> Not for a PIC, but for DSP inside an FPGA.
>
> I tried to use a single stage IIR filter as the main filter in the  
> GateMan-I synth.
> While it works, it is sonically underwhelming.  It is, however, very  
> easy to tune.
> This filter does not have what we call a corner frequency, at least  
> not in the same
> way we describe analog filters like SVF or ladder.  It also doesn't  
> have resonance.
> The single stage IIR leaves the corner frequency at zero Hz and the  
> tuning parameter
> changes the rolloff slope.  Sonically, it's more like a simple tone  
> control than what
> we like to use as an audio filter in music.
>
> Two places I found this IIR filter very useful and appropriate are  
> the reflection
> filter in PolyDaWG/8, a Karplus-Strong tight string synth and as the  
> main processing
> element to implement portamento in all 3 of the GateMan synths.
>
> The reason I mention a single stage IIR is that it's simplicity will  
> be attractive to
> someone using a device like a PIC, but I warn that it is not a good  
> choice for a
> filter used as we often do in modular type synths.
>
> An FIR can be quite simple too, but is not so easily tuned, fixed  
> filters are fairly
> easy to implement.
>
> AVRsynth (google it) is probably a good place to look at digital  
> filter code for
> microprocessors, I don't know what type of filter is used in that  
> synth however.
>
> My favorite digital filter so far is the State Variable Filter.   
> While it sounds
> great (IMO), it does have some rather intense computational and  
> sample rate
> requirements.  I found it fairly simple to implement and it tunes  
> quite easily.
>
> I'd give references to my Verilog code, but it isn't that easy to  
> transmute into
> microprocessor type instructions.
>
> If you are interesed, here are links to the projects I've published:
>
> - http://www.fpga.synth.net/pmwiki/pmwiki.php?n=FPGASynth.GateManI
> - http://www.fpga.synth.net/pmwiki/pmwiki.php?n=FPGASynth.GateManII
> - http://www.fpga.synth.net/pmwiki/pmwiki.php?n=FPGASynth.DigitalWaveguide
> - http://www.fpga.synth.net/pmwiki/pmwiki.php?n=FPGASynth.GateManPoly
>
> These projects are all implemented on a Xilinx Spartan-3E Starter  
> Kit (a development
> board) with very minimal external hardware (the schematics for the  
> external hardware
> are posted with the projects and is the same hardware for all of  
> them).
>
> -- ScottG
>
> -----------------------------------------------------------------
>
> -- Scott Gravenhorst
> -- GateManPoly - FPGA Based Polyphonic MIDI LA/FM Synthesizer
> -- GateMan-III - FPGA Based Monophonic MIDI LA/FM Synthesizer
> -- PolyDaWG/8 - FPGA Based 8 Voice Polyphonic MIDI Synthesizer
> -- FatMan: home1.gte.net/res0658s/fatman/
> -- NonFatMan: home1.gte.net/res0658s/electronics/
> -- When the going gets tough, the tough use the command line.
>
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