Added stereo support

Added support for stereo sound in a way that allows the program to fall back on mono when necessary. This is not a breaking change.

98video.txt

@@ -19,14 +19,18 @@ Header:
 	Screen resolution spec (bit 4)
 	0x00	640x400 (pixel aspect 1:1)
 	0x10	640x200	(pixel aspect 1:2)
-	Unused (bit 5-7)
+	Audio channel spec (bit 5)
+	0x00	Mono
+	0x20	Stereo (if ADPCM, else also mono)
+	Unused (bit 6-7)
 	high byte uint8 Unused
 0x0A	uint4[16][3] colour table (rg br gb ...) for all 16 initial colours
 
 Frame Data:
 Audio Data:
 0x00	uint16 length of audio data (16-bit words, n)
-0x02	uint16[n] Audio data
+0x02	uint16[n] Audio data (mean level)
+var	uint16[n] Audio data (stereo difference, only if stereo specified, otherwise it is left out)
 ADPCM:
 The audio data is in 8-bit signed ADPCM format which expands to signed 16-bit PCM
 Buzzer audio:
@@ -38,6 +42,12 @@ ADPCM codec:
 	abs(data) <= 0x18: decrement bitshift by 1 after this sample.
 	abs(data) >= 0x68: increment bitshift by 1 after this sample.
 	(My decoder actually uses an acceleration table it creates at startup just to speed things up by using an acceptable amount of memory)
+Stereo encoding:
+	The mean level signal (m) is always included as this is the mono signal. Stereo is encoded through a difference signal (d), which uses the same ADPCM encoding as the mean level signal.
+	To get the right (r) and left (l) signals, we apply the following transformations:
+	l = m + d
+	r = m - d
+	This ordering is arbitrary.
 Video Frame Header:
 0x00	low byte  UUUU IRGB  planes to update (automatically determines how many plane sections and which planes they correspond to, U is reserved for 256 colour board compatibility)
 	high byte 0000 000P  update palette?

Makefile

@@ -29,4 +29,4 @@ $(PROCESS)/98videop.o	: $(BUILD)/$(OFILES)
 	ld -T doscom.lds -m i386pe -o $(PROCESS)/98videop.o $^
 
 $(BUILD)/$(OFILES)	: $(SOURCES)/$(ASMFILES)
-	as $< -o $@ -march=i386 -mtune=i386 --32
+	as $< -o $@ -march=i386 -mtune=i286 --32

doscom.lds

@@ -1,9 +1,15 @@
 /* Simple GNU ld linker script to build a DOS .COM executable */
 
+MEMORY
+{
+	segment (wxa) : ORIGIN = 0, LENGTH = 64K
+}
+
 SECTIONS
 {
     . = 0x100;
-	.text : { *(.text) }
-    .data : { *(.data) }
-    .bss : { *(.bss) }
+	.text ALIGN(0x10) : { *(.text) } >segment
+	.data ALIGN(0x10) : { *(.data) *(.rdata) } >segment
+	.bss (NOLOAD) :  ALIGN(0x10) { *(.bss) } >segment
+	/DISCARD/ : { *(.rdata$zzz) *(.reloc) } >segment
 }

src/98videoplayer.asm

@@ -23,7 +23,7 @@
 ;#FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
 ;#OTHER DEALINGS IN THE SOFTWARE.
 
-.arch i486
+.arch i286
 .intel_syntax noprefix
 .code16 ;#This is intended to run in real mode
 
@@ -221,6 +221,9 @@ video_read_end_magicnum: ;#Set various options
 	int 0x18		;#PC98 CRT BIOS: Set display options (640x200, display page 0, colour mode)
 	pop ax
 video_read_fullres:
+	and bx, 0x0020
+	shr bx, 5
+	mov cs:is_stereo, bl
 	and ax, 0x0007
 	mov cs:audiospec, ax
 	xor ax, ax
@@ -581,19 +584,25 @@ PROCEDURE_frameloop: ;#void frameloop(void)
 	mov cx, ax
 	push cx
 	mov cs:filebuffercurpos, si
+	mov bl, cs:is_stereo
+	test bl, bl
+	jz notstereo
+	shl ax, 1
+notstereo:
 	shl ax, 1
 	add ax, 4
 	call PROCEDURE_tryreadsection
 	
 	mov al, cs:using_86
 	mov si, cs:filebuffercurpos
-	cmp al, 0x01
-	jne frameloop_buzaudio
+	test al, al
+	jz frameloop_buzaudio
 	jmp frameloop_86audio
 	
 	;#Decode ADPCM for buzzer PCM
 frameloop_buzaudio:
 	pop bp
+	push bp
 	mov di, cs:samplebufferptr
 	push cs
 	pop es
@@ -643,14 +652,23 @@ frameloop_buzaudio_pushloop:
 	mov cx, 0x0010
 	rep stosb ;#Add padding to reduce clipping(?)
 	pop cx
+	pop ax
+	mov bl, cs:is_stereo
+	test bl, bl
+	jz no_skip_stereodata
+	shl ax, 1
+	add si, ax ;#Needed to skip over unused stereo difference data (buzzer audio is forced mono)
+no_skip_stereodata:
 	jmp frameloop_videodata_process
 	
 frameloop_86audio:
 	;#Decode ADPCM for 86 PCM
+	test bl, bl
+	jnz frameloop_86audio_stereo
 	pop di
 	mov dx, 0xA46C
 	mov cx, cs:adpcmshiftval
-frameloop_86audio_pushloop:
+frameloop_86audio_mono_pushloop:
 	lodsw
 	push ax
 	xor ah, ah
@@ -690,7 +708,104 @@ frameloop_86audio_pushloop:
 	out dx, al
 	mov cl, cs:[bx+accelerationtable_adpcm]
 	dec di
-	jnz frameloop_86audio_pushloop
+	jnz frameloop_86audio_mono_pushloop
+	jmp frameloop_videodata_process
+	
+frameloop_86audio_stereo:
+	pop bp
+	mov dx, 0xA46C
+	mov cl, cs:adpcmshiftval
+	mov ch, cs:adpcmshiftval_diff
+	mov bx, bp
+	shl bx, 1
+frameloop_86audio_stereo_pushloop:
+	lodsw
+	push ax
+	xor ah, ah
+	mov di, cx
+	and di, 0x00FF
+	shl di, 8
+	add di, ax
+	cbw
+	shl ax, cl
+	add ax, cs:lastsample
+	mov cs:lastsample, ax
+	mov cl, cs:[di+accelerationtable_adpcm]
+	mov al, [bx+si-2]
+	xor ah, ah
+	xchg cl, ch
+	mov di, cx
+	and di, 0x00FF
+	shl di, 8
+	add di, ax
+	cbw
+	shl ax, cl
+	add ax, cs:lastsample_diff
+	mov cs:lastsample_diff, ax
+	mov cl, cs:[di+accelerationtable_adpcm]
+	mov di, ax
+	mov ax, cs:lastsample
+	push bx
+	mov bx, ax
+	add ax, di
+	xchg al, ah ;#output left sample
+	out dx, al
+	xchg al, ah
+	out dx, al
+	sub bx, di
+	mov ax, bx
+	xchg al, ah ;#output right sample
+	out dx, al
+	xchg al, ah
+	out dx, al
+	pop bx
+	xchg cl, ch
+	pop ax
+	xchg al, ah
+	xor ah, ah
+	mov di, cx
+	and di, 0x00FF
+	shl di, 8
+	add di, ax
+	cbw
+	shl ax, cl
+	add ax, cs:lastsample
+	mov cs:lastsample, ax
+	mov cl, cs:[di+accelerationtable_adpcm]
+	mov al, [bx+si-1]
+	xor ah, ah
+	xchg cl, ch
+	mov di, cx
+	and di, 0x00FF
+	shl di, 8
+	add di, ax
+	cbw
+	shl ax, cl
+	add ax, cs:lastsample_diff
+	mov cs:lastsample_diff, ax
+	mov cl, cs:[di+accelerationtable_adpcm]
+	mov di, ax
+	mov ax, cs:lastsample
+	push bx
+	mov bx, ax
+	add ax, di
+	xchg al, ah ;#output left sample
+	out dx, al
+	xchg al, ah
+	out dx, al
+	sub bx, di
+	mov ax, bx
+	xchg al, ah ;#output right sample
+	out dx, al
+	xchg al, ah
+	out dx, al
+	pop bx
+	xchg cl, ch
+	dec bp
+	jnz frameloop_86audio_stereo_pushloop
+	mov cs:adpcmshiftval_diff, ch
+	xor ch, ch
+	add si, bx
 	
 frameloop_videodata_process:
 	mov cs:adpcmshiftval, cx
@@ -944,6 +1059,7 @@ frameloop_stopplaneread:
 	old_timer_vector_offset:	.word	0x0000
 	old_timer_vector_segment:	.word	0x0000
 	using_86:					.byte	0x00
+	is_stereo:					.byte	0x00
 	current_buzzer_shiftdown:	.byte	0x00
 	old_interrupt_mask:			.byte	0x00
 	current_sample_midpoint:	.word	0x0000
@@ -959,7 +1075,9 @@ frameloop_stopplaneread:
 	framesleft:					.word	0x0000
 	audiospec:					.word	0x0000
 	lastsample:					.word	0x0000
+	lastsample_diff:			.word	0x0000
 	adpcmshiftval:				.word	0x0000
+	adpcmshiftval_diff:			.word	0x0000
 	numframes_lo:				.word	0x0000
 	numframes_hi:				.word	0x0000 ;#32-bit to support very long videos
 	filehandle:					.word	0x0000