[Openal] Filter
Andy Patrick
Andy.Patrick at rebellion-derby.com
Fri Oct 12 01:46:25 PDT 2007
Many thanks Dan, that is extremely helpful information.
My next question then: I seem to recall that there is an (optional?)
automatic distance filter built into the OpenAL calculations (hence why
you can change the density of the air and the units-to-metres scale) but
can't seem to find a reference to it now, in the docs. What does that
do, by comparison to trying to use the manual filter to achieve a
distance filter effect?
Thanks again
Andy P
> -----Original Message-----
> From: openal-bounces at opensource.creative.com
> [mailto:openal-bounces at opensource.creative.com] On Behalf Of
> Daniel PEACOCK
> Sent: 11 October 2007 20:02
> To: openal at opensource.creative.com
> Subject: RE: [Openal] Filter
>
>
>
>
>
> Hi Andy,
>
> The 'Gain' parameter controls the volume at all frequencies
> (so setting this parameter to 0 will silence the sound), the
> Gain HF parameter controls the volume at high-frequencies
> (the reference high frequency is 5KHz).
> Setting the Gain HF to 0 should result in very muffled audio
> where all the high-frequencies have been filtered out.
>
> The EFX10Show application in the OpenAL SDK allows you to
> play with these
> parameters to see how they work. The parameters are in the
> box labelled
> 'Direct' on the right-hand-side of the Sound Properties dialogue box.
> NOTE : It is actually a little tricky to play with these
> parameter in the application due to the fact that they are in
> a linear 0 to 1 scale. (The parameters are in linear scale
> to match the rest of OpenAL).
>
> These parameters are actually being mapped to EAX Source parameters
> 'Direct' and 'DirectHF'. If you have the EAX 4.0 SDK
> installed you can
> use the ALEAX40Show test application to control the 'Direct'
> and 'Direct HF' parameters in a decibel scale (-10000mb to
> 0mb) which makes it easier to hear their effect.
>
> If you are using a linear interpolation to control the Gain
> HF parameter based on source to listener distance I would
> imagine that this would not sound too good and would result
> in very little audible change until your
> were practically at your defined maximum distance. A logarithmic
> interpolation would have the opposite affect - but may be too
> drastic for your needs - could be worth a try though.
>
> Dan
> Creative Labs, Inc.
>
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>
> "Andy Patrick"
>
> <Andy.Patrick at reb
>
> ellion-derby.com>
> To
> Sent by: "Luis"
> <h.k.1981 at gmail.com>,
> openal-bounces at op
> <openal at opensource.creative.com>
> ensource.creative
> cc
> .com
>
>
> Subject
> RE: [Openal] Filter
>
> 10/10/2007 01:49
>
> AM
>
>
>
>
>
>
>
>
>
>
>
>
>
> Hi Luis, thanks, yes I am reassigning the filter to the
> source after modifying it, however it still doesn't seem to
> change the sound very much.
>
> And I'm still wondering why a filter (for which I would
> expect to change cutoff and slope) instead has fixed cutoff
> and slope, and only allows you to change "gain" and "gainHF"
> and it's not clear what that is expected to do.
>
> Thanks
>
> Andy
>
>
> From: Luis [mailto:h.k.1981 at gmail.com]
> Sent: 10 October 2007 06:12
> To: Andy Patrick; openal at opensource.creative.com
> Subject: Re: [Openal] Filter
>
> Hi Andy.
> Just checking, are you reassigning the filter to the source
> after modifying its parameters? You must do this or the new
> parameters won't affect the source at all.
>
> Hope that helps
>
> On 10/4/07, Andy Patrick <Andy.Patrick at rebellion-derby.com> wrote:
> Hello all,
>
> We're looking at playing around with the per-voice filter
> in the Effects
> Extension, in preparation for occlusion etc. We'd also like
> to change the
> filter over distance using a method that offers more
> control than the
> in-built distance filter in OpenAL, which I seem to remember reading
> about but can't now find a reference to!
>
> I understand that the low pass filter cut-off (-3dB point)
> is fixed at
> 4KHz, and the slope is fixed at -6dB/octave. What I don't
> understand is
> how the available parameters - AL_LOWPASS_GAIN and
> AL_LOWPASS_GAINHF -
> relate to that. I assumed that GAIN would modify the gain
> to the left of
> the cut-off point, and GAINHF would modify the gain to the
> right of the
> cut-off point. However, my attempts at varying GAINHF from 0 to 1 by
> distance doesn't seem to change the sound very much, and
> based on this
> assumption setting both GAIN and GAINHF to 0 would silence
> the sound,
> which it doesn't.
>
> Could someone please explain what the intent of these
> settings is, and
> briefly suggest how I might be able to implement the
> effects I'm after?
> I'm using OpenAL 1.1 on the Windows platform, primarily.
>
> Many thanks
>
> Andy P
>
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> --
> Luis
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