[Openal] EFX Clarifications

[Daniel PEACOCK]

Hi,

> 1.  But how does that all relate to the cut-off?
> 2.  Is the cut-off particular to an implementation?
> 3.  If it is fixed, how does the adjustment of GainHF guarantee that
> the reference frequency is attenuated as indicated (to GainHF)?
>
> > Not in EFX.
>
> Is it variable in I3DL2 then?  Or do you mean there's no property to
> adjust it directly?  My understanding was that the GainHF value was
> used to determine it.
>
> > See http://www.iasig.org/pubs/3dl2v1a.pdf for details about the
> transfer function for a one-pole filter and how it relates to the
> reference high frequency and sample rate.
>
> So rather than having GainHF alter the cut-off, does it instead
> alter the coefficient(s) of the filter directly?

Yes, GainHF is one of the parameters used to computer a filter coefficient.

for example from the I3DL2 spec, g would be ...

g = 10^(gainHFdB / 10)

where gainHFdb = 20 * log10(gainHF)

> I didn't think the distance model would effect the accuracy of this
> calculation.  What would it look like for the other two models?  I
> imagine (distance - reference_distance) would be changed somewhat.

Well the problem is that the direct gain can be completely different when
switching from one distance model to another.   In the simple case, of no
distance model (AL_NONE) you would expect no air absorption either.

> On the effected system (the only one with hardware 3D sound support)
> I've got an old SB Live Value, with the most recent drivers (which
> are very old).  I suppose that means all these issues may be driver
> and card related.
>
> I just want to make sure the Generic Hardware device isn't at fault,
> since I'm relying on it for any systems that have hardware support
> but no specific OpenAL driver.
>
> > Depends on the soundcard you have installed.  If the device
> supports EAX 5 (hardware based X-Fi cards) it should work.  It
> should also work on most EAX 4 (hardware based Audigy cards).
>
> Okay, so this probably is the fault of my card.

The SBLive! supports EAX 2.0 so does explain why certain features are
unavailable.

> I just create a stereo source, then an effect slot loaded with
> reverb, and finally a mono source.  Then I set the mono source to
> send to the reverb effect, and mute the dry path.  I get no output,
> but that may be a driver/card issue too, I guess.

Yes that could be a driver bug.

> If anyone has a chance to test the Generic Hardware on a more recent
> card, could you let me know if all these parameters are working
correctly?

It is working on Audigy and X-Fi cards as far as I'm aware.

Dan