[sdiy] Fast envelope generation

[Scott Gravenhorst]

Tom Wiltshire <tom at electricdruid.net> wrote:
>Scott, Neil,
>
>On 18 Dec 2012, at 13:44, Scott Gravenhorst wrote:
>
>> 
>> When I've needed an exponential response for an envelope, I've used a single pole
>> lowpass IIR filt.  So far, I've used this only for the release phase of an ADSR, but it
>> works very nicely (in an FPGA) and I've not had a problem with 
>it's computational expense. > > BUT - perhaps I don't understand 
>what you are doing? Or am I confusing logarithmic and > 
>exponential? 
>
>No, I don't think there's any confusion, but my bet is you're 
>both on platforms with nice wide number formats. Scott's on FPGA, 
>and can make his variables as wide as he wants - 32, 48-bits, 
>whatever - and I bet he does. 
>
>The trouble with doing IIR on a little 8-bit uP is maintaining 
>accuracy. If you use long multi-byte variables, you finish up 
>needing to do long multi-byte multiplications - slow, especially 
>on a PIC with no multiply instruction. If you *don't* use long 
>variables, the accuracy goes to pot. 
>
>At one point I looked into IIR for this job, and I needed better 
>than 32-bit to get the range of delay times I needed. Though the 
>calculation is "simple", it finished up needing more lines of 
>code for the multiply than the simple table-lookup-with-interp. 
>The advantage of that method is that you can easily change the 
>times and the curves shapes without any worries about the 
>accuracy. 

Indeed, I tried using 18 bit arithmetic in an FPGA and found it to be lacking.  This particular
test wasn't an envelope, rather it was generating a slowly changing random signal, but the
principle is the same.  I wound up using 36 bit arithmetic - which in an FPGA wasn't time expensive
the way I did it, but it used more real estate (4 hardware 18x18 multipliers).  I wasn't aware that
this was being done in a 8-bitter, so I now see what the problem is and I would also have gone the
lookup table route as well if such a device was a requirement.


-- ScottG
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-- Scott Gravenhorst
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