[Openal] Effects/filter chain lacking

Thu May 1 15:44:30 PDT 2008

Hi Tim,

>> Yes, that’s much better.  I didn’t realize you could use a direct filter
>> and an auxiliary send at the same time.

Gotcha.   That is definitely do-able.

>> That puts me much closer to where I want to be.

Great.

>>The current system makes sense for reverb effects, however what if I
>>wanted to use the compressor effect?  Here I don’t want the original
>>sound to be heard at all, I only want the “compressed” version to go to
the output.

In this scenario, you would attach a (direct) Filter to the Source with
Gain and Gain HF set to 0 (to mute the dry path), and attach a (send)
Filter to the Source to feed the global effect loaded with the Compressor
effect.

NOTE : The effects are 'global' and all of them output stereo only, except
for the reverb which outputs multi-channel.   This means that they will not
preserve any 3D positional information.  e.g If you feed a 3D positioned
sound to the compressor effect and mute the direct path, all you will get
is a stereo output of the compressed sound.

We are working on an "EFX2.0" extension that adds the ability to do Insert
Effects which allow you to transform audio data on a Source *before* the 3D
Environmental Audio pipeline.   With EFX 2.0 you would be able to compress
a sample and position it in 3D, filter it, and send it to a few reverb
effects, if you wanted to.

Dan

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             "Tim Sorrells"                                                
             <tsorrells at breaka                                             
             wayltd.com>                                                To 
                                       "Daniel PEACOCK"                    
             05/01/2008 02:52          <dpeacock at creativelabs.com>         
             PM                                                         cc 
                                       <openal at opensource.creative.com>    
                                                                   Subject 
                                       RE: [Openal] Effects/filter chain   
                                       lacking                             

Yes, that’s much better.  I didn’t realize you could use a direct filter
and an auxiliary send at the same time.  That puts me much closer to where
I want to be.

I understand what’s going on a little better.  I was thinking that when an
auxiliary effect was applied to would replace the existing sound instead of
add to it, but that is what the diagram in the documentation shows.

The current system makes sense for reverb effects, however what if I wanted
to use the compressor effect?  Here I don’t want the original sound to be
heard at all, I only want the “compressed” version to go to the output.

From: Daniel PEACOCK [mailto:dpeacock at creativelabs.com]
Sent: Wednesday, April 30, 2008 6:06 PM
To: Tim Sorrells
Cc: openal at opensource.creative.com
Subject: RE: [Openal] Effects/filter chain lacking

Hi Tim,

Your X-Fi drivers are pretty old - there are newer ones available from
http://us.creative.com/support/downloads (released March 14th 2008).

It sounds like you want a filtered direct path *and* a filtered send to the
reverb effect (something which the EFXFilter example doesn't actually
demonstrate). To do this you need to add a filter to the direct path in
addition to the filter applied to the effect send used in step (4).

To do this in the EFXFilter example, add this line of code ...

alSourcei(uiSource, AL_DIRECT_FILTER, uiFilter);

immediately before the Play call in the PlayAuxiliaryFilter() function.

Dan
Creative Labs, Inc.

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                                                                   Subject 

                                                      RE: [Openal]         
                                                      Effects/filter chain 
                                                      lacking              

WinXP SP 2
Creative SB X-Fi [C8E0]
Driver: 5.12.6.1187 Date: 8/17/2006
OpenAL: 1.1 Summer 2007

My application is doing roughly what EFXFilter is doing, and I can
replicate the problem with EFXFilter, so just use that as an example of the
EFX calls I’m making. The only modifications I’ve made to that sample is to
change the sound played to one of mine, and setting the GAINHF to 0.1 to
more effectively filter out higher pitched sounds. When I run that sample
what I hear for 3 (reverb only) and 4 (filter and reverb) is similar. What
I expect 4 to sound like is closer to 2 (filter only). My desire is to
filter the sound first, then apply reverb to the result of the filter,
which means I shouldn’t hear any of the high pitched, tinkly sounds in my
sample, but I do.

From: Daniel PEACOCK [mailto:dpeacock at creativelabs.com]
Sent: Wednesday, April 30, 2008 4:51 PM
To: Tim Sorrells
Cc: openal at opensource.creative.com
Subject: Re: [Openal] Effects/filter chain lacking

Hi Tim,

> I’m looking through the Effects Extension documentation and there’s
> a lot of discussion which seems to ultimately boil down to one
> parameter which is AL_AUXILIARY_SEND_FILTER which allows you to
> specify the effect and filter applied to a source. Perhaps I’m
> missing something but this only allows the sound designer to specify
> at most one effect and one filter.

The AL_AUXILIARY_SEND_FILTER is used to set-up the sends from a Source to
one of the global effects. Depending upon your hardware you may be able to
configure a Source to send to multiple global effects. For example, an X-Fi
soundcard allows you to create up to 4 global effects, and configure a
Source to have a filtered effect send to each of the global effects.

The AL_DIRECT_FILTER is used to apply a filter to the direct (dry) path of
a Source.

> But what happens when you want
> to apply 2 effects and a filter to a source?

A Source can have a one filter on the dry path (that will have no impact on
the effect sends), and (on X-Fi) up to 4 filtered sends to 4 different
effect slots (none of which will affect the dry path).

> I really want to be able to construct a chain so that I can specify
> something like this: Source->Filter->Effect1->Effect2->Output.

The current specification does not allow chaining of the auxiliary (global)
effects. But you can do this ...

Source->Filter---------->Output (dry path)
|--->Filter->Effect1->Output (send path to Aux Effect 1)
|--->Filter->Effect2->Output (send path to Aux Effect 2)

> I’d
> also like to attach an effect to the master so that every source is
> affected by that without me manually telling it so.

There are no master effects I'm afraid - just global effects.

> Even using the AL_AUXILIARY_SEND_FILTER with a reverb and a low pass
> filter it’s obvious the reverb effect is bypassing the low-pass
> filter.

That doesn't sound right. What soundcard are you using? Do you have up to
date drivers and OpenAL components? Which OpenAL device are you using? What
EFX calls are you making exactly?

> This differs from the graph on page 16 in the
> Effects Extension documentation which puts the filter first, and
> then the effect. Assuming this problem isn’t specific to my
> platform, you can hear this yourself by using the EFXFilter sample.

The EFXFilter sample sounds as expected on my X-Fi card.

You should hear the following ...

1)Original sound (no filters and no sends)
2)low-pass filtered sound with no effects
3)original sound feeding a reverb effect
4)original sound with a filtered send to the reverb effect (just the reverb
should sound muffled)

Dan
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