The standard behavior of Q on a shelving filter is to control the steepness of the shelf. There is a tradeoff between steepness and stability, so in certain designs too steep a shelf can make a filter that will "blow up" with some inputs. Also it can be useful aesthetically to have a gradual cutoff rather than a steep pass band on a filter.

richard duckworth wrote:
> peaking mode appears to work like a normal parametric eq, as you say, it boosts/cuts a range of frequencies around a centre frequency - the 'width' of the range is determined by the value of Q. 
> 
> 'shelving' mode appears to implement low & hi shelf filters. Your description of low shelving is correct, though a plot of the  frequency response of a euphonic shelf filter would appear to curve gently between the boosted ( or cut) portion and the area above Fc. 
> 
> I implemented a demo of pareq in QuteCsound with an fft display and widgets to vary the parameters: Fc seems to exhibit strange behaviour towards the extremities of the 20-20kHz frequency span. I'll try the others next. 
> All the best, Rich    
>  
> 
> 
> ________________________________
>  From: Dennis Raddle 
> To: csound@lists.bath.ac.uk 
> Sent: Monday, 26 December 2011, 22:55
> Subject: [Csnd] using parametric EQ
>  
> 
> Can someone explain the parametric EQ opcode to me (pareq). 
> 
> Does "peaking" mode mean that it boosts (or cuts) frequencies around a center frequency and leaves the rest of the spectrum intact?
> 
> What does "low shelving" and "high shelving" mean?
> 
> I'm guessing that low shelving means it boost or cuts everything below the corner frequency and leaves everything above it alone. And the shape of the boost or cut is flat. So the response would look like a stairstep. Is this right?
> 
> What does Q mean in the shelving mode?
> 
> Thanks,
> Dennis
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