DOS: add PCS pulse width modulation driver.

config.txt

@@ -339,14 +339,24 @@
 # audio_driver = lptdss
 # audio_config_lpt = 1
 
+# The PC speaker pulse width modulation driver can play PCM sounds over the
+# hardware PC speaker. Like LPT audio, it can be computationally expensive.
+# Lower sample rates correspond to better sample quality, but rates under about
+# 18kHz have audible high pitched carrier frequencies. Different combinations
+# of audio_sample_rate and audio_buffer_samples can help reduce noise somewhat.
+# Supported sample rates (approximate): 5000, 6000, 7000, 8000, 9321, 10000,
+# 11000, 15500, 17000, 18643, 22500, 26500, 27500, 32000, 37286.
+
+# audio_driver = pcs
+
 # This driver can be used without a sound card to enable hardware PC speaker
 # sound effects without playing PCM. It is automatically selected if the
 # other audio drivers fail to initialize.
 
 # audio_driver = nopcm
 
-# To enable hardware PC speaker sound effects for any DOS audio driver,
-# enable this option. This is disabled by default for most drivers, but
+# To enable hardware PC speaker sound effects for most DOS audio drivers,
+# enable this option. This is disabled by default except with nopcm, but
 # may improve performance on slower machines.
 
 # audio_pc_speaker_use_hardware = 1

src/audio/audio_drivers.c

@@ -41,6 +41,7 @@ static const struct audio_driver * const available_drivers[] =
   &audio_driver_lpt_stereo1,
   &audio_driver_lpt_stereo2,
   &audio_driver_lpt_dss,
+  &audio_driver_pcs_pwm,
   &audio_driver_no_pcm,
 #endif
 #ifdef CONFIG_NDS

src/audio/audio_drivers.h

@@ -34,6 +34,7 @@ extern const struct audio_driver audio_driver_lpt_mono;
 extern const struct audio_driver audio_driver_lpt_stereo1;
 extern const struct audio_driver audio_driver_lpt_stereo2;
 extern const struct audio_driver audio_driver_lpt_dss;
+extern const struct audio_driver audio_driver_pcs_pwm;
 extern const struct audio_driver audio_driver_no_pcm;
 
 extern const struct audio_driver audio_driver_nds;

src/audio/djgpp/audio_lpt.c

@@ -17,6 +17,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include <math.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <dpmi.h>
@@ -177,8 +178,18 @@ static boolean init_audio_driver_lpt(unsigned mix_rate, unsigned irq_rate,
     return false;
   }
 
-  info("LPT audio initialized: %xh/%xh %uFr %uCh %zuHz (%uHz)", lpt_left_port,
-   lpt_right_port, audio.buffer_frames, channels, audio.output_frequency, irq_rate);
+  if(irq_handler == &pcs_pwm_handler)
+  {
+    info("PCS audio initialized: %.2f bit %uFr %uCh %zuHz (%uHz)\n",
+     log(current_div) / M_LN2, audio.buffer_frames, channels,
+     audio.output_frequency, irq_rate);
+  }
+  else
+  {
+    info("LPT audio initialized: %xh/%xh %uFr %uCh %zuHz (%uHz)\n",
+     lpt_left_port, lpt_right_port, audio.buffer_frames, channels,
+     audio.output_frequency, irq_rate);
+  }
   return true;
 }
 
@@ -233,6 +244,59 @@ static void quit_audio_driver_lpt(void)
   lpt_free_buffer();
 }
 
+/* PC speaker rates are even more limited than LPT rates due to
+ * issues with carrier frequencies, harmonics, and bus noise. */
+static uint16_t pcs_pwm_get_rate(int requested_rate)
+{
+  static const uint16_t supported_rates[] =
+  {
+    LPT_5K_HZ,  LPT_6K_HZ,  LPT_7K_HZ,  LPT_8K_HZ, PCS_9K_HZ,
+    LPT_10K_HZ, LPT_11K_HZ, LPT_15K_HZ, LPT_17K_HZ, PCS_18K_HZ,
+    LPT_22K_HZ, LPT_26K_HZ, LPT_28K_HZ, LPT_32K_HZ, PCS_37K_HZ
+  };
+  size_t i;
+
+  for(i = 0; i < ARRAY_SIZE(supported_rates) - 1; i++)
+    if(requested_rate < ((int)supported_rates[i] + supported_rates[i + 1]) >> 1)
+      break;
+
+  return supported_rates[i];
+}
+
+static void pcs_pwm_off(void)
+{
+  /* Clear PC speaker gate */
+  outportb(0x61, inportb(0x61) & 0xfc);
+  /* Reset mode to 3 */
+  outportb(0x43, 0xb6);
+}
+
+/* This driver is so similar to the LPT drivers that it is included here
+ * and shares most of the same code. */
+static boolean init_audio_driver_pcs_pwm(struct config_info *conf)
+{
+  unsigned rate = pcs_pwm_get_rate(conf->audio_sample_rate);
+  unsigned frames = MIN(conf->audio_buffer_samples, 32768);
+
+  /* Channel 2 mode: low byte, mode 0 "interrupt on terminal count", binary */
+  outportb(0x43, 0x90);
+  /* Raise PC speaker gate */
+  outportb(0x61, inportb(0x61) | 0x03);
+
+  if(!init_audio_driver_lpt(rate, rate, frames, 1, &pcs_pwm_handler))
+  {
+    pcs_pwm_off();
+    return false;
+  }
+  return true;
+}
+
+static void quit_audio_driver_pcs_pwm(void)
+{
+  quit_audio_driver_lpt();
+  pcs_pwm_off();
+}
+
 const struct audio_driver audio_driver_lpt_mono =
 {
   "LPT DAC (Mono)",
@@ -288,3 +352,17 @@ const struct audio_driver audio_driver_lpt_dss =
   init_audio_driver_lpt_dss,
   quit_audio_driver_lpt,
 };
+
+const struct audio_driver audio_driver_pcs_pwm =
+{
+  "PC Speaker pulse width modulated PCM",
+  "pcs",
+  false, /* Must be explicitly requested */
+
+  NULL,
+  &audio_player_pcs,
+  NULL,
+
+  init_audio_driver_pcs_pwm,
+  quit_audio_driver_pcs_pwm,
+};

src/platform/djgpp/interrupt.S

@@ -127,7 +127,7 @@ _timer_handler_tick:
 	je	3f			/*   goto nosound; */
 	/* sound(freq) */
 	movb	$1, _sfx_playing	/* sfx_playing = true; */
-	movb	$0xb6, %al		/* Channel 2, lo + hi, mode 2, binary */
+	movb	$0xb6, %al		/* Channel 2, lo + hi, mode 3, binary */
 	outb	%al, $0x43		/* PIT Mode/Command register */
 	movw	SFX_DIVIDEND_DX, %dx
 	movw	SFX_DIVIDEND_AX, %ax
@@ -230,6 +230,30 @@ _lpt_left_port:
 _lpt_right_port:
 	.word	0
 
+.balign 16
+.global _pcs_pwm_handler
+_pcs_pwm_handler:
+	pushl	%eax
+	pushl	%ds
+	pushl	%esi
+	movw	%cs:_int_ds, %ds
+	movl	_dos_audio_buffer_pos, %esi
+	movb	_current_div, %ah
+	movb	(%esi), %al		/* al = *pos++; */
+	incl	%esi
+	mulb	%ah			/* ax = al * ah; */
+	movb	%ah, %al		/* ax >>= 8; */
+	outb	%al, $0x42		/* write shifted sample to PCS reload */
+	cmpl	_dos_audio_buffer_end, %esi
+	jae	2f			/* if(pos >= end) goto 2; */
+1:
+	movl	%esi, _dos_audio_buffer_pos
+	popl	%esi
+	jmp	_timer_handler_var	/* goto timer_handler_var; */
+2:					/* (early predictors assume no jump) */
+	movl	_dos_audio_buffer, %esi	/* pos = dos_audio_buffer; */
+	jmp	1b
+
 .balign 16
 .global _lpt_mono_handler
 _lpt_mono_handler:

src/platform/djgpp/interrupt.h

@@ -61,6 +61,10 @@ MEGAZEUX_BEGIN_DECLS
 #define LPT_48K_HZ            47727 /* Divider 25 gets closest to 48000 */
 #define LPT_DSS_HZ            7000  /* For mixer; DSS outputs a fixed rate */
 
+#define PCS_9K_HZ             9321  /* Divider 128 -> sample >>= 1 */
+#define PCS_18K_HZ            18643 /* Divider 64 -> sample >>= 2 */
+#define PCS_37K_HZ            37286 /* Divider 32 -> sample >>= 3 */
+
 struct audio_sfx_data; /* audio_struct.h */
 
 extern const int int_lock_start;
@@ -81,6 +85,7 @@ extern uint16_t sfx_timer;
 extern uint8_t sfx_playing;
 
 /* Audio interrupt variables */
+extern const int pcs_pwm_handler;
 extern const int lpt_mono_handler;
 extern const int lpt_stereo1_handler;
 extern const int lpt_stereo2_handler;