Re: Jamin EQ Preecho, Linear Phase EQ considered harmful.

Greg,

    I have absolutely no idea what you're talking about ;-)  That's OK though
since I just do the GUI.  You'll have to get Steve to take a look at what you're
suggesting.  I can provide just about any slider or GUI control you want (within
reason) as long as Steve does the DSP work.  Or, conversely, you could try your
hand at it if you know C.  BTW, I was unable to open the attached postscript
files on my sister-in-law's Windoze system.  There may not be a problem with them
since it is a Micro$oft system (God, I hate Micro$oft).

Jan


On Sat, 24 Dec 2005 22:59 , Gregory Maxwell <[email protected]> sent:

>While using jamin to perform some EQ of some audio containing some
>very percussive vocals, I noticed what sounded like preecho...  So I
>decided to perform some tests, and sure enough, the IR shows some
>preecho...  I grabbed the Jamin source and I see that the EQ is linear
>phase, so there is no shock that there is pre-echo.
>
>I see there is an option to use an IIR for the crossover for the
>multiband compressor.. but no such option for the EQ.
>
>What was the design motivation behind using a linear phase filter for
>EQ in a mastering app?
>
>For the multiband compressor crossover it makes perfect sense, the
>transition should have low Q, the results are later summed so
>preringing usually vanishes, and perfect reconstruction is highly
>desirable (which would be impossible with a minimum phase crossover
>due to phase shiftage).
>
>For EQ, the subtracted audio will never be added back, so there will
>always be pre-echo with linear phase EQs and there is no risk of
>problems created by phase mismatch. The ear is far more sensitive to
>pre-ringing than postringing so linear phase is not really a good
>compromise for an EQ which is expected to be placed last in the signal
>chain..
>
>Also, the 'HDEQ' interface implies the ability to create filters which
>are steeper than can be created with the current filter length. 
>Allowing a longer linear filter would increase the risk of pre-echo
>and increase the delay.
>
>Would it be possible to provide an toggle option to use minimum phase
>filtering for the EQ? The multiband compressor should stay linear, but
>EQ should default to mode which will not risk creating pre-echo.
>
>An alternative would be to perform time-domain massaging of the
>filter's IR to produce asymmetric mixed filters which avoid pre-echo.
>Such code is available under the GPL in the pre-echo truncation
>functions in Denis Sbragion's DRC software
>(http://drc-fir.sourceforge.net/\).  The pre-echo truncation function
>filters the IR with a time and frequency dependant function and then
>corrects the departure from the desired magnitude response with a
>minimum phase filter.   If such a feature were provided it would be
>useful to provide sliders which control filter length and the amount
>of preringing suppression (from linear phase at one extreme to minimum
>phase at the other)... and augment the HDEQ display to show the
>delivered frequency and phase response (and perhaps a time domain
>view).
>
>
>Here is an example test with alternating +6/-6 db bands in the 30band EQ screen:
>Jamin test step function result:
>http://www.mindspillage.net/braindump/jamin.test.step.ps
>Jamin test high resolution octave/cycle plot:
>http://www.mindspillage.net/braindump/jamin.test.ps
>(thanks to Denis Sbragion for his great octave scripts for IR analysis)
>
>
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