[Openal] Filter

Thu Oct 11 12:01:49 PDT 2007

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>,        
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                                                                   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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