[sdiy] Fast envelope follower circuit needed..

Scott Gravenhorst music.maker at gte.net
Fri Mar 29 22:32:23 CET 2013

Tom Wiltshire <tom at electricdruid.net> wrote:
>Hi Damian,
>> I'm afraid I have to disagree with you on several points here.
>I thought this might run for a while yet ;)
>The key thing for me here is that flopping the bottom part of the 
>signal up to the top doesn't change any of the peak levels, 
>whatever happens to the frequency spectrum, and it's primarily 
>the peaks I'm interested in. However, it does obscure which peaks 
>are positive ones and which are negative, so maybe that's a 
>Perhaps I don't even need to bother rectifying the signal. Why 
>not just keep a check on the negative peaks like the positive 
>ones, and then use some average of the two as the final output. 
>JP said his signal might well be asymmetric (which is not at all 
>uncommon), so we can't assume the two envelopes will be the same. 
>It seems to me that most of your complaints about filtering 
>rectified signals apply pretty much equally to analog as digital. 
>Ok, so you haven't got aliasing to worry about, but you still 
>converted a lot of low frequency energy in the signal into high 
>frequency spikes which then get filtered away - after all, that's 
>the point. And there will be some residual ripple - that's a 
>fact, and that's something we see in the analog implementations. 
>Also it's worth pointing out that we don't have to follow the 
>rectifier (if we use one) with some kind of filter. 

POSSIBLE NAIVE POSTER WARNING...  And even if you did and used full wave rectification, the peak
detection part of the algorithm which would supply the filter with its input data removes the
spikes (and their associated transients) anyway - so aliasing due to transients becomes moot.  The
signal you filter is then a band limited signal with very small potential for alias artifacts due
to very low frequency harmonic components.

In my opinion, a digital approach to this sounds dandy.

-- ScottG
-- Scott Gravenhorst
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