Chase H.Q.: Routine at F2B6

F2B6: Plays engine effect.

Used by the routine at play_turbo_sfx_128k.
engine_sfx_from_speed_128k	F2B6	LD HL,($A24A)	Load speed into HL
	F2B9	RR H	Bottom bit of H moves to carry -- is H now empty?
	F2BB	LD A,L	Speed low byte
	F2BC	RRA	Halve speed, shifting carry in as MSB
	F2BD	CPL	L = ~A -- why complement?
	F2BE	LD L,A	L = ~A -- why complement?
	F2BF	LD A,($A253)	Jump if in low gear
	F2C2	AND A
	F2C3	JR Z,esfs_1
	F2C5	SLA L
	F2C7	RL H
esfs_1	F2C9	SLA L
	F2CB	RL H
	F2CD	SLA L
	F2CF	RL H
	F2D1	LD A,($A23B)	Read tunnel_sfx
	F2D4	AND A	Set flags
	F2D5	LD DE,$0190	Non-tunnel tone value
	F2D8	LD A,$0F	Non-tunnel volume
	F2DA	JR Z,esfs_2	Jump if tunnel_sfx was zero (not in tunnel)
In tunnel.
	F2DC	LD DE,$0258	In-tunnel tone value
	F2DF	LD A,$0C	In-tunnel volume
esfs_2	F2E1	ADD HL,DE	-- speed value + tone?
	F2E2	LD ($A217),HL	Set channel C pitch (both fine and coarse)
	F2E5	LD ($A21D),A	Set channel C volume
	F2E8	LD A,($A21A)	Set mixer to enable tone C
	F2EB	AND $3B
	F2ED	LD ($A21A),A
	F2F0	RET	Return