Volume and bass knobs, for players without an amplifier

The cabinets ran the game at unity and shaped volume and tone outside it,
in an external amplifier and a 3-way crossover. That is why there is no
master volume anywhere in the original code and none in AUDIO.INI - an
operator turned a knob on an amp. A desktop player has no amp and no
crossover, and the recovered soundbanks are a good deal livelier than what
4.12 shipped with, so the game has to offer the two controls the pod got
from hardware.

RP412AUDIOVOLUME, 0.0 to 4.0, is the amplifier: a listener gain, which the
port had never set at all. RP412AUDIOBASS, 0.0 to 1.0, is the crossover's
low band. Both default to leaving the mix exactly as the pod played it, so
neither changes anything for anyone who does not go looking.

The bass trim is not a filter, and the reason is worth writing down: the
OpenAL we ship is Creative's, not OpenAL Soft, and it implements only
AL_FILTER_LOWPASS. It rejects highpass and bandpass outright. A bandpass
would have been the tidy answer, carrying the authored brightness model on
GAINHF and the trim on GAINLF across the single direct filter a source
gets. It is not on offer.

So the trim scales sample data as it loads, which suits how this low end is
actually built: the weight lives in discrete deep layer zones whose per-zone
tuning bakes out to a very low playback rate - thirteen zones below 8kHz,
three to five octaves under their recorded pitch, against four fifths of the
set at 22kHz and up. Baked rate is a dependable proxy for band, so pulling
down the low-rate zones is a real low-band trim and not a blunt cut. It eases
in below 22kHz and reaches full depth at 5.5kHz.

Caught while building this, and the reason for the probe: EFX_Initialize
checks alGetError after configuring the scratch filter, so asking for a
filter type the driver refuses leaves an error pending and takes the entire
bridge down - reverb included. The bandpass attempt did precisely that and
would have silently killed the reverb and brightness work. Initialize now
survives losing the filter and says so.

Builds clean, runs with both knobs set and with neither.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

MUNGA_L4/L4AUDIO.cpp

@@ -1112,6 +1112,8 @@ void
 	// Apply filter scale
 	//--------------------------------------------------------------------------
 	//
+	float direct_gainhf = 1.0f;
+
 	if (UseSourceBrightnessScale())
 	{
 		const MIDINRPNValue 	filter_resolution = 2;// HACK - should come from audio.ini
@@ -1137,19 +1139,25 @@ void
 		// initial-filter-cutoff (NRPN 21) and then only updated its own
 		// bookkeeping member -- the cutoff was never applied to anything, so
 		// authored brightness (ctl 5) was inert.  Route it through EFX instead.
-		// Direct sources take brightness alone, gated on use_brightness_scale;
-		// the distance rolloff belongs to the 3D paths.
+		// Direct sources take brightness alone; the distance rolloff belongs to
+		// the 3D paths.
 		//
-		if (EFX_Available())
-		{
-			const float cutoff_scale =
-				(float)midi_filter_cutoff / (float)MIDI_MAX_CONTROL_VALUE;
-			const float gainhf = EFX_CutoffScaleToGainHF(cutoff_scale);
+		direct_gainhf = EFX_CutoffScaleToGainHF(
+			(float)midi_filter_cutoff / (float)MIDI_MAX_CONTROL_VALUE
+		);
+	}
 
-			for (int i = 0; i < channelSet.count; i++)
-			{
-				EFX_SetSourceLowpassGainHF(channelSet.sources[i], gainhf);
-			}
+	//
+	// Applied OUTSIDE the brightness gate: a source that does not use brightness
+	// still has to be told, because the same call carries the player's bass trim.
+	// At unity on both axes it detaches the filter, so this costs nothing in the
+	// default configuration.
+	//
+	if (EFX_Available())
+	{
+		for (int i = 0; i < channelSet.count; i++)
+		{
+			EFX_SetSourceLowpassGainHF(channelSet.sources[i], direct_gainhf);
 		}
 	}