[PD] Delay circuit feedback DSP error-- only when signal path leaves abstraction

Christof Ressi info at christofressi.com
Tue Feb 25 10:34:46 CET 2020

Hi William,

I second IOhannes and Alex that it's better to attach actual patches, 
but from looking at the picture it seems like you attempt to do your 
"external" feedback path via signal connections and this obviously 
doesn't work because it creates a DSP loop. The workaround is to use a 
pair of [s~]/[r~] for one side of the aux path, which "breaks" the DSP 
graph. Note that in your "inner" feedback path you're already doing 
this, that's why you don't get a DSP loop there.

Note that [s~]/[r~] is basically a delay line of a single block, unless 
you force the order with signal connections, see 


On 25.02.2020 09:50, William Huston wrote:
> Hi, I've built a classic stereo delay using [delwrite~] and [vd~], and 
> L/R crossed feedback. This works!
> Now, I want to insert an effect into the feedback path. (many 
> possibilities here: add some bitcrush, so each repeat gradually 
> becomes more distorted.... or a [hilbert~] type pitch shift, which 
> might create something like Shepard tones, etc).
> *This works when everything is within a single patch or abstraction. *
> However, when I try to allow the effect to be external to the 
> abstraction (like a SEND/RETURN insert on a mixer), when I try to 
> complete the circuit, I get "DSP loop detected".
> The two circuits are essentially IDENTICAL!
> This seems like a bug to me. Any ideas?
> I've attached pix of both the high-level schematic, along with a demo 
> patch. (I can upload the actual patch if that would be helpful).
> Thanks!
> BH
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