Re: [fluid-dev] [PATCH] Effect level clip

[Josh Green]

Hello David,

On Mon, 2007-09-17 at 04:48 -0500, David Hilvert wrote:
> On Mon, 17 Sep 2007 00:43:30 -0700
> Josh Green <address@hidden> wrote:
> 
> > I think I need to get a better idea of how your patch is helping to keep
> > Reverb and Chorus at a more constant output level, regardless of the
> > synth.gain setting.
> 
> If I recall correctly, effective gain was split in 1.0.7, for each effect
> module, into two general controls combining roughly multiplicatively -- one
> controlling data sent to the effect module (*_SEND), and one controlling the
> module's behavior (gain).  The *_SEND controls, in turn, are an additive
> combination of (at least) two inputs -- one specified within the soundfont, 
> and
> one specified by MIDI controllers (such as those found on vkeybd).  While the
> *_SEND logic produces a clipped value, ensuring that effect output does not
> considerably exceed dry output, the module gain control did not.
> 
> One way to resolve this would be to place a hard limit on the 1.0.7 effect
> gain; while doing this would serve to control output levels, it would also
> limit the usefulness of the control.  Instead combining all controls 
> additively
> into the *_SEND factor allows adjustment across the entire range of effect for
> any given (single) instrument, while preventing effect levels from
> significantly exceeding dry output.  This latter approach requires that the
> user-configurable gain levels be somehow combined into the *_SEND factors,
> which task has no immediately obvious implementation both trivial and clean.
> 
> My first patch was the most trivial I could manage; the second was less 
> trivial, 
> and perhaps only marginally cleaner from the perspective of computational 
> expense.
> 

I think I understood most of what you said there, but I think there is
still something fundamental I am missing.  As I mentioned previously,
I'm discovering how this thing works as I go along ;)

I realize my previous comment about the global reverb level not being
realtime, was wrong.  It should be a realtime effect (as it seems the
other reverb parameters are currently).

Prior to your patch, it looks like the fluid_revmodel_setlevel()
function was doing nothing (code was commented out).  I'm at a loss as
to how the reverb effect level was even working before (was it?).  From
the code which was commented out, it looks like it was at some point
modifying the 'wet' field of the revmodel structure.

The modulation of the Reverb send parameter is handled by the
fluid_voice_update_param() for voices for which this parameter changes,
which seems fine (how most other generators are processed).

The global reverb level on the other hand I would think would modify
parameters which affect the mix of the entire reverb buffer as handled
in fluid_revmodel_processreplace() and fluid_revmodel_processmix(), and
would not need to modify the reverb send on a per voice basis (or is
that where I'm missing something?).  If there was a need to do per voice
reverb changes based on the global reverb level, I would think it should
be calculated in fluid_voice_write().

Looking at the previously commented out code in
fluid_revmodel_setlevel() I see:

/*   fluid_clip(value, 0.0f, 1.0f); */
/*   rev->wet = value * scalewet; */
/*   fluid_revmodel_update(rev); */


My question is, what was wrong with this code?  It seems like it is
updating the wet mix parameter and then updating the rest of the reverb
parameters accordingly (when a change occurs), which affects the
fluid_revmodel_processreplace() and fluid_revmodel_processmix()
functions.

I guess I'm still not clear on the basic questions:
- What was wrong with the previous Reverb level code?
- What should be done to fix it?


Thanks for taking your time to help explain things, I just want to try
and get this stuff right, and not incur unnecessary processing overhead.

Best regards,
        Josh