diff --git a/src/emu/machine/smpc.c b/src/emu/machine/smpc.c
index 3d5c673ac1d..d89331413d5 100644
--- a/src/emu/machine/smpc.c
+++ b/src/emu/machine/smpc.c
@@ -155,25 +155,6 @@ TODO:
 #define LOG_SMPC 0
 #define LOG_PAD_CMD 0
 
-#if 0
-/* TODO: move this into video functions */
-static int vblank_line(running_machine &machine)
-{
-	saturn_state *state = machine.driver_data<saturn_state>();
-	int max_y = machine.primary_screen->height();
-	int y_step,vblank_line;
-
-	y_step = 2;
-
-	if((max_y == 263 && state->m_vdp2.pal == 0) || (max_y == 313 && state->m_vdp2.pal == 1))
-		y_step = 1;
-
-	vblank_line = (state->m_vdp2.pal) ? 288 : 240;
-
-	return vblank_line*y_step;
-}
-#endif
-
 
 /********************************************
  *
@@ -273,9 +254,12 @@ static TIMER_CALLBACK( smpc_change_clock )
 	state->m_vdp2.dotsel = param ^ 1;
 	state->stv_vdp2_dynamic_res_change();
 
+	state->m_maincpu->set_input_line(INPUT_LINE_HALT, CLEAR_LINE);
 	if(!state->m_NMI_reset)
 		state->m_maincpu->set_input_line(INPUT_LINE_NMI, PULSE_LINE);
+	state->m_slave->set_input_line(INPUT_LINE_HALT, CLEAR_LINE);
 	state->m_slave->set_input_line(INPUT_LINE_RESET, ASSERT_LINE);
+	state->m_audiocpu->set_input_line(INPUT_LINE_HALT, CLEAR_LINE);
 
 	/* put issued command in OREG31 */
 	state->m_smpc.OREG[31] = 0x0e + param;
@@ -721,11 +705,16 @@ static void smpc_comreg_exec(address_space &space, UINT8 data, UINT8 is_stv)
 			if(LOG_SMPC) printf ("SMPC: Change Clock to %s (%d %d)\n",data & 1 ? "320" : "352",space.machine().primary_screen->hpos(),space.machine().primary_screen->vpos());
 
 			/* on ST-V timing of this is pretty fussy, you get 2 credits at start-up otherwise
-			   sokyugurentai threshold is 74 lines
-			   shanhigw threshold is 90 lines
-			   I assume that it needs ~100 lines, so 6666,(6) usecs. Obviously needs HW tests ... */
+			   My current theory is that SMPC first stops all CPUs until it executes the whole snippet for this,
+			   and restarts them when the screen is again ready for use. I really don't think that the system
+			   can do an usable mid-frame clock switching anyway.
+			   */
 
-			space.machine().scheduler().timer_set(attotime::from_usec(6666), FUNC(smpc_change_clock),data & 1);
+			state->m_maincpu->set_input_line(INPUT_LINE_HALT, ASSERT_LINE);
+			state->m_slave->set_input_line(INPUT_LINE_HALT, ASSERT_LINE);
+			state->m_audiocpu->set_input_line(INPUT_LINE_HALT, ASSERT_LINE);
+
+			space.machine().scheduler().timer_set(space.machine().primary_screen->time_until_pos(state->get_vblank_start_position()*state->get_ystep_count(), 0), FUNC(smpc_change_clock),data & 1);
 			break;
 		/*"Interrupt Back"*/
 		case 0x10:
diff --git a/src/emu/video/stvvdp2.c b/src/emu/video/stvvdp2.c
index 66a3a0b4d40..c51e66198e8 100644
--- a/src/emu/video/stvvdp2.c
+++ b/src/emu/video/stvvdp2.c
@@ -6252,6 +6252,28 @@ UINT8 saturn_state::get_odd_bit( void )
 	return machine().primary_screen->frame_number() & 1;
 }
 
+int saturn_state::get_vblank_start_position( void )
+{
+	int vblank_line;
+
+	vblank_line = (m_vdp2.pal) ? 288 : 240;
+
+	return vblank_line;
+}
+
+int saturn_state::get_ystep_count( void )
+{
+	int max_y = machine().primary_screen->height();
+	int y_step;
+
+	y_step = 2;
+
+	if((max_y == 263 && m_vdp2.pal == 0) || (max_y == 313 && m_vdp2.pal == 1))
+		y_step = 1;
+
+	return y_step;
+}
+
 /* TODO: these needs to be checked via HW tests! */
 int saturn_state::get_hcounter( void )
 {
diff --git a/src/mame/drivers/saturn.c b/src/mame/drivers/saturn.c
index 42b96d19286..b0fd8999c41 100644
--- a/src/mame/drivers/saturn.c
+++ b/src/mame/drivers/saturn.c
@@ -1520,15 +1520,10 @@ TODO:
 TIMER_DEVICE_CALLBACK_MEMBER(saturn_state::saturn_scanline)
 {
 	int scanline = param;
-	int max_y = machine().primary_screen->height();
 	int y_step,vblank_line;
 
-	y_step = 2;
-
-	if((max_y == 263 && m_vdp2.pal == 0) || (max_y == 313 && m_vdp2.pal == 1))
-		y_step = 1;
-
-	vblank_line = (m_vdp2.pal) ? 288 : 240;
+	vblank_line = get_vblank_start_position();
+	y_step = get_ystep_count();
 
 	//popmessage("%08x %d T0 %d T1 %d %08x",m_scu.ism ^ 0xffffffff,max_y,m_scu_regs[36],m_scu_regs[37],m_scu_regs[38]);
 
diff --git a/src/mame/drivers/stv.c b/src/mame/drivers/stv.c
index 23b814d7970..5f6bf13a7bc 100644
--- a/src/mame/drivers/stv.c
+++ b/src/mame/drivers/stv.c
@@ -658,20 +658,11 @@ bp 6001d22 (60ffef0)
 
 */
 
-READ32_MEMBER(saturn_state::astrass_hack_r)
-{
-	if(space.device().safe_pc() == 0x60011ba) return 0x00000000;
-
-	return m_workram_h[0x000770/4];
-}
-
 DRIVER_INIT_MEMBER(saturn_state,astrass)
 {
 	sh2drc_add_pcflush(machine().device("maincpu"), 0x60011ba);
 	sh2drc_add_pcflush(machine().device("maincpu"), 0x605b9da);
 
-	machine().device("maincpu")->memory().space(AS_PROGRAM).install_read_handler(0x06000770, 0x06000773, read32_delegate(FUNC(saturn_state::astrass_hack_r),this));
-
 	install_astrass_protection(machine());
 
 	DRIVER_INIT_CALL(stv);
diff --git a/src/mame/includes/stv.h b/src/mame/includes/stv.h
index 04ff831cd8c..071cb10ef70 100644
--- a/src/mame/includes/stv.h
+++ b/src/mame/includes/stv.h
@@ -151,7 +151,6 @@ public:
 	DECLARE_WRITE32_MEMBER(magzun_ioga_w32);
 	DECLARE_READ32_MEMBER(magzun_hef_hack_r);
 	DECLARE_READ32_MEMBER(magzun_rx_hack_r);
-	DECLARE_READ32_MEMBER(astrass_hack_r);
 	DECLARE_INPUT_CHANGED_MEMBER(key_stroke);
 	DECLARE_INPUT_CHANGED_MEMBER(nmi_reset);
 	DECLARE_INPUT_CHANGED_MEMBER(tray_open);
@@ -381,6 +380,8 @@ public:
 	int get_pixel_clock( void );
 	UINT8 get_odd_bit( void );
 	void stv_vdp2_dynamic_res_change( void );
+	int get_vblank_start_position( void );
+	int get_ystep_count( void );
 
 	void refresh_palette_data( void );
 	int stv_vdp2_window_process(int x,int y);