Re: [Audacity-devel] IIR filters (aka 'Scientific Filter')

[Steve t. F. <ste...@gm...>]

On 5 January 2014 16:31, Bill Wharrie <bi...@go...> wrote:
>
> On 03/01/2014, at 2:01 PM, Gale (Audacity Team) <ga...@au...> wrote:
>
>>> Steve wrote:
>>>> On 3 January 2014 16:23, Federico Miyara wrote:
>>>>
>>>> Steve,
>>>>
>>>> Sorry, I messed it up a bit.
>>>>
>>>> What I was actually meaning is that starting with the formula for any
>>>> analog filter, particularly Butterworth, both Chebyshevs, Elliptic,
>>>> etc., the bilinear (not biquad) transform renders an IIR version whose
>>>> frequency response is similar (not identical but reaonably approximate)
>>>> to the original frequency response.
>>>>
>>>> Biquads are sort of bricks to build analog filters, they have at most
>>>> two poles and two zeros.
>>>>
>>>> Calling the effect Biquad would be misleading, since only 2nd order
>>>> Butterworth or Chebychev are actually Biquads. Higher order filters
>>>> (whatever name they have) are the combination of several biquads.
>>>
>>> Thanks for the clarification. That's clear.
>>>
>>> Re. the effect name, I see the benefit of it being next to the
>>> Equalization effect, so I agree that  "EQ IIR Filters" is a good
>>> choice.
>>
>> Thanks to Federico. I also agree with Federico that "EQ IIR Filters" may
>> be too general. It may be a marginal convenience to have Norm's effect
>> next to EQ, but might not users ask what the functional connection with
>> EQ is, and why these are simply not presets in EQ?
>>
>> My vote is to name as "IIR Filters" or "Classic IIR Filters", at least if
>> Nyquist
>> High Pass/Low Pass are retained (see below).
>
>
> I think the Nyquist HP/LP must be retained. They are much simpler and easier for the novice user to understand.
>
>>
>>
>>> Also, if the first order Butterworth Low Pass could be improved the
>>> perhaps it could replace the Low Pass / High Pass plug-ins.
>>
>> I like rationalising filters, but I agree we can't replace Low Pass and
>> High Pass unless the frequency response of Norm's Butterworth Low Pass
>> is more like Nyquist Low Pass (that is, unless Norm's effect has less high
>> frequency attenuation).
>>
>> But if we do replace Nyquist High Pass/Low Pass, wouldn't
>> "High Pass/Low Pass (IIR)" be an idea?  Easy for novices to find?
>>
>> Re Martyn's relabelling (
>> http://code.google.com/p/audacity/source/detail?r=13016 )
>> to "Maximum Passband Attenuation" and "Minimum Stopband Attenuation" -
>> although this seems more correct I have a slight qualm about removing the
>> "Ripple" word from the GUI. Isn't the "Ripple" word expected?  Would
>> "Ripple Level" or similar be better/shorter?
>
> FWIW the wikipedia article on Chebyshev filters uses "ripple" (or "equiripple") for both passband and stopband.
> https://en.wikipedia.org/wiki/Chebyshev_filter
> I'd like to keep "ripple" at least in reference to the passband. For the stopband I think we could use "ripple" as well, even if it is not strictly correct, as that is how it is generally described.


+1 to use the word "ripple" for type 1 passband. This appears to be
the usual terminology.

-1 to the word "ripple" for type 2 stopband. As someone previously
explained, the stopband ripple is infinite and is not the correct
term. I much prefer "Minimum Stopband attenuation" (is that the
correct capitalisation?)

Steve


>
> -- Bill
>
>>
>> One point: In Release build on Windows, if I open Scientific Filter in
>> Butterworth
>> type it initialises as it was before the commit (with the "Filter Type"
>> control
>> in the centre instead of moved leftwards). So when I switch to Chebyshev I,
>> "Maximum Passband Attenuation" covers the "Filter Order" control, and
>> in Chebyshev II, "Minimum Stopband Attenuation" covers "Cutoff".
>>
>> It displays correctly if I OK the effect on a Chebyshev type.
>>
>> PS Can we agree on "Cut-off" as per
>> http://bugzilla.audacityteam.org/show_bug.cgi?id=667#c0 ?
>>
>> I think the consensus on the -quality list is to hyphenate Cut-off (but not
>> "Rolloff").  If not, the Manual will have to adjust to say "cutoff" :=)
>>
>>
>>
>> Gale
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> --
>> View this message in context: http://audacity.238276.n2.nabble.com/IIR-filters-aka-Scientific-Filter-tp7561040p7561059.html
>> Sent from the audacity-devel mailing list archive at Nabble.com.
>>
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