Re: [Flightgear-devel] stall horn sound

[Maik J. <mj...@gm...>]

Hi Curt,
Curtis Olson schrieb am 07.11.2008 23:12:
> On Fri, Nov 7, 2008 at 2:55 PM, Curtis Olson wrote:
>
>     Question: has anyone noticed anything odd with the stall horn
>     sound in the default c172p in the current cvs (although it's
>     likely been an issue since we migrated to openal?)
>
>     When the stall horn goes off, it used to be a clean/constant tone,
>     however in openal it's warbly disjointed crackly sort of sound. 
>     It's enough to be very aurally annoying.
>
>     I almost wonder if this issue affects all our sounds, but because
>     the stall horn audio is such a clean sine wave with no extra noise
>     or "character", any little imperfection in the audio system really
>     jumps out when the sound is played.  This really seems like it
>     could be related to inconsistent positioning of the listener
>     versus the sound source?
>
>     The audio imperfections seem to be correlated with frame rate changes.
>
>     Anyone have any ideas on this?
>
>
> Replying to my own message here ...
>
> In src/Main/main.cxx the sound positions and velocities are updated.  
> I observe that the velocity is computed as (current position - last 
> position) / dt.  However, this approach leads to inconsistent 
> velocities because of some very subtle issues related to the flight 
> dynamics update loop.
>
> If I force the velocities to be zero, the sound quality *greatly* 
> improves, and not just with the stall horn.  The overall sound is much 
> more "rich" and vibrant (at least to my ears.)
Unfortunately I do not have a running flightgear now. I assume, that you 
are in the cockpit. Can you check what happens, if you change line 642 from
globals->get_soundmgr()->set_source_vel_all( model_vel );
to
globals->get_soundmgr()->set_source_vel_all( listener_vel );
This forces the velocities of all sounds to be the same as the listener 
velocity. In theory these two values should be exactly the same when you 
are in cockpit view. If the sound is "ok" after this, there is an issue 
with the order of view position calculation, model position calculation 
and sound calculation.
If the sound is not ok, it could be either a bug in the Doppler 
calculation (which kind you are using: USE_OPEN_AL_DOPPLER, 
USE_OPEN_AL_DOPPLER_WITH_FIXED_LISTENER or USE_SOFTWARE_DOPPLER?) or 
(again) a bug in the view position calculation or in the model position 
calculation. If you are not using USE_OPEN_AL_DOPPLER I am rather sure, 
that the viewer and the model position are not aligned, probably due to 
the order of updating these values.
>
> I'm not sure the best solution here since I'm sure we need correct 
> velocities to get correct doppler effects.
>
> I am trying to understand the code a bit more and I see that both the 
> listener and the sound velocities are rotated into a new coordinate 
> space.  Is this necessary?  It seems like the important thing is the 
> relative velocity of the sounds, not what coordinate system they are 
> transformed into?
No, not only the relative velocity is from interest. If you are in front 
of a mach 2 flying jet you will hear nothing from the jet, even with a 
relative speed of zero. But you can use any coordinate system as long as 
it is not moving compared to the air. If I remember correctly, we are 
using a coordinate system with its orient at the listener position in 
the same orientation as the aircraft.. Therefore we need only an offset 
to transform the sound source coordinates to this coordinate system and 
don't need to transform the sound source orientation.
>
> Is it worth giving up on doppler effect to get reasonable cockpit 
> sounds, or is doppler a big enough win to put up with really crappy 
> cockpit sound with lots of artifacts?
I think its worth to dig down this bug.
>
> It wouldn't be hard to add an --enable/disable-doppler configuration 
> option?
sure, but better fix the problem instead of adding a switch to choose 
between two problems. From my point of view the Doppler effect adds much 
realism to flightgear. As a workaround we could switch off the Doppler 
effect in all internal views. But there are other bug reports about the 
Doppler sound, maybe your observations lead us to the right track to fix 
some of them.
>
> Thought/comments?
>
> Regards,
>
> Curt.
> -- 
> Curtis Olson: http://baron.flightgear.org/~curt/ 
> <http://baron.flightgear.org/%7Ecurt/>
>   
Regards,
Maik