PC speaker/Timer: Mode 3 (square wave) and counter == 1 generates a l…

…ow frequency buzz. Fixes Paratrooper [#1492]
joncampbell123 · Apr 28, 2020 · 42f4d90 · 42f4d90
1 parent af6bb46
commit 42f4d90
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diff --git a/CHANGELOG b/CHANGELOG
@@ -1,4 +1,8 @@
 0.83.1
+  - PC speaker: Mode 3 (square wave) and a counter value
+    of 1 produces a low frequency square wave. Ulrasonic
+    frequencies do not begin until count == 2. This fixes
+    helicopter noises in Paratrooper.
   - dosbox-x.conf is now recognized as the default config
     file name in addition to dosbox.conf (Wengier)
   - Tandy DAC output fixed to slowly ramp last sample to

diff --git a/src/hardware/timer.cpp b/src/hardware/timer.cpp
@@ -454,6 +454,12 @@ static void write_latch(Bitu port,Bitu val,Bitu /*iolen*/) {
             else
                 p->set_next_counter(9999/*check this*/);
         }
+        else if (p->write_latch == 1 && p->mode == 3/*square wave, count by 2*/) { /* counter==1 and mode==3 makes a low frequency buzz (Paratrooper) */
+            if (p->bcd == false)
+                p->set_next_counter(0x10001);
+            else
+                p->set_next_counter(10000/*check this*/);
+        }
         else {
             p->set_next_counter(p->write_latch);
         }