[sdiy] Fast envelope follower circuit needed..

Dave Manley dlmanley at sonic.net
Fri Mar 29 01:43:19 CET 2013


You could also discard all negative samples (half wave).  The output goes thru LPF smoothing anyway,  why bother with full wave?

-------- Original Message --------
 From: rsdio at sounds.wa.com
 Sent: Thu, Mar 28, 2013 05:14 PM
 To: Synth-Diy List <Synth-diy at synth-diy.org>
 CC: 
 Subject: Re: [sdiy] Fast envelope follower circuit needed..

>
>On Mar 28, 2013, at 11:24, Tom Wiltshire wrote:
>> Hi Dan,
>>
>> On 28 Mar 2013, at 13:55, Dan Snazelle <subjectivity at hotmail.com>  
>> wrote:
>>> Though i work with c//arduino/AVRS not pics, this sounds very  
>>> appealing! Can anyone recommend a place to learn about writing C  
>>> code for an envelope follower? How do you do it? Shift signal to  
>>> 0-5, then......
>>
>> The first thing after that would be to do the full wave  
>> rectification. Either dump the sign bit, if you've got signed data,  
>> or invert the data if the highest bit is clear if it's not signed -  
>> same thing, different representation. Half-wave rectification just  
>> makes the signal more "lumpy" and isn't significantly easier  
>> digitally anyway.
>
>
>You can't dump the sign bit, at least not with twos-complement data,  
>which is what all digital signed data is. Maybe you're thinking of  
>conceptually dumping the negative symbol, but that's not the same as  
>dumping the sign bit. If you were to dump the sign bit, then the  
>negative part of the signal would not be inverted, as needed, but  
>would merely be shifted up into the positive range. That would make  
>an ugly, spiked signal that would introduce significant errors in  
>your envelope.
>
>The easiest thing to do with signed data is use abs() to get the  
>absolute value. Many 8-bit processors even have a native opcode to  
>compute abs(), but they're not merely masking out the sign bit as  
>would be done by and'ing with 0x7F.
>
>You are correct that inverting the data below the midpoint will work  
>with unsigned data.
>
>You can probably use almost exactly the same code for either signed  
>or unsigned.
>
>Brian Willoughby
>Sound Consulting
>
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