[gst-devel] Help needed on changing frequencies of equalizer

[Stefan Kost]

Am 10.01.2010 00:37, schrieb Maarten Bosmans:
> 2010/1/8 Yogesh Marwaha <yogeshm.007 at gmail.com>:
>> It is centerfrequency which is troubling me. Take, for example,
>> upper frequency bands of winamp's equalizer (70, 180, 320,
>> 600, 1k, 3k, 6k, 12k, 14k, 16k).
> 
> In most applications it makes the most sense to view frequencies on a
> logarithmic scale.
> Something like 35, 70, 140, 280, 560, 1k1, 2k2, 4k4, 9k, 18k, for
> example. Note how the frequency is doubled every time.
> 
>> For the top-most band (16k),
>> I would use a bandwidth of 2000, which means 15000 - 17000,
>> then, what about those above 17000? To cover up all frequencies
>> I will have to use a bandwidth of 8000 with centerfrequency of
>> 16000 (12000 - 2000).
> 
> Couple of options:
>  - You could  use a shelf filter instead of a bandpass for the outer
> frequencies
>    (not sure how to do that in GStreamer though)


The eq already uses shelf filters for first and last band in last
gst-plugins-good for the 3/10 band versions. In the full parametric version it
is configurable.

Stefan

>  - Make sure that the bandwidth of the outer frequencies cover the
> whole audible spectrum (~ 20-20k)
>  - Accept the fact that you can't equalize frequencies above 17k,
> which for some applications may be entirebly reasonably and for other
> it isn't.
> 
>> I could not find any documentation about Q-factor of band. What
>> is it? Any details about that?
> 
> http://en.wikipedia.org/wiki/Q_factor
> 
> If you have bands with constant-Q width, the crossover frequency is
> the geometric mean of the centerfrequencies, not the arithmetic mean.
> So in my one-octave bands example where the bands are doubled, the
> bandwidth extends between 0.71 and 1.41 times the center frequency.
> 
> By reading the source (gstiirequalizer.c) though, it seems that the
> algorithm used in GStreamer uses bandwidths defined in Hz from center.
> So you'd have to convert from Q to Hz and probably shift the center
> frequency also.
> 
> Maarten
> 
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