[sdiy] Digital 4 pole filter in an FPGA

[Scott Gravenhorst]

Jim Patchell <patchell at cox.net> wrote:
>I have been doing digital design since about 1975...using FPGAs is just 
>another phase of doing the same thing...instead of using 74xxx 
>logic...now I write in verilog.  There are a lot of books on Amazon.com 
>that cover this subject...although, I wouldn't really know which one 
>would be the best to begin with.
>
>And FPGA synths are a lot of fun...and there are more ways to skin this 
>cat than you can shake a leg at...

That's no exageration.  Ditto on the "fun" part.  If you want to get a taste of what
Verilog looks like, try this tutorial:

http://www.asic-world.com/verilog/veritut.html

It's last update was in December of 2008, so the maintainer is active.  There are LOADS
of simple examples.

After reading this tutorial, I was able to get some Hello World design projects
working.  I will admit that I am also a software developer from way back.  With some
study, practice and help from kind folks here and on a couple other forums, I've been
able to design MIDI synthesizers.  I also found very good supplemental information in
other tutorials and PDF papers I found on the web.  If you're into USENET, good sources
of information can be found on comp.arch.fpga and comp.lang.verilog.

Basic digital design knowledge is definately a good thing to have.  When I work in
Verilog, I find myself thinking in terms of registers (flipflops or RAM), OR gates, AND
gates, multiplexors, etc. just as if I were going to solder up some CD4xxx stuff. 
Verilog is used to describe how these little logic bits (I call it Logic Lego) are
connected together to make a working system.

>-Jim
>
>HL-SDK Synths wrote:
>> I'm interested as well, I have a question though, where did you learn 
>> all of this FPGA ...stuff? I've looked around on the web and I'm no 
>> computer science guy. Is it all hidden in the textbooks? I think FPGA 
>> synths are crazy cool!
>> 
>> On Tue, Dec 9, 2008 at 12:09 AM, Jim Patchell <patchell at cox.net 
>> <mailto:patchell at cox.net>> wrote:
>> 
>>     Hope y'all aren't getting tired of these progress reports.
>> 
>>     But it looks like I have got the basics of the four pole filter
>>     working (at least in the simulator).  It is going to take a bit more
>>     testing to verify that all of the data is going where and when it is
>>     supposed to...
>> 
>>     The filter has a four clock pipe delay through it...i.e. it takes
>>     four clock cycles to complete one sample...but since it is
>>     pipelined...it only takes 512 clocks to complete all 512 filters...
>> 
>>     One of the things I need to verify yet is that the resonance control
>>     works as it should.
>> 
>>     Now, also to give you an idea of exactly what I am doing, this is
>>     basically the same kind of filter you would make with an
>>     SSM2040...without, unfortunately, the non-linear distortion that is
>>     inherent in the SSM2040.  I am not sure if I will be able to add a
>>     non-linearity in the filter loop or not.  That may be some future
>>     experiment.
>> 
>>     Anyway...it is sure looking promising.
>> 
>>     -Jim
>>     ----------------------------------------------
>>     Visit http://www.noniandjim.com <http://www.noniandjim.com/>
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>> 
>
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-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
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