[Rpcemu] Code not working correctly on RPCEmu

Timmermans, Andre andre.timmermans at atos.net
Thu Feb 23 05:18:37 PST 2012


I have just added support for Impulse Tracker's resonant filters
in my TimPlayer module and tought I'd try it on RPCEmu too.
On most of the files it sound just te same as on a real RPC
but in a few cases note on which the filter is applied becomes
garbage. As the equalizer uses not too dissimilar in form
(y0 = a0 * x0 + a1 * x1 + a2 * x2 + b1 * y1 + b2 * y2)
I tried using it on a normal song and bingo, the sound becomes
completely distorted.

My best guess is that there is some problem with SMULL or SMLAL,
probably in the upper 32-bit part, but just in cas
I have listed below a complete listing of the IT Filter.


; Voice_ITFilter
; Filter in the form y0 = a0 * x0 + b1 * y1 + b2 * y2
; where xi is in(-i*T), yi is out(-i*T), ai and bj are in 1.24 fixed point
; In  - R0  stream ptr
;       R11 buffer size (in nr of values), must be multiple of 2
;       R12 seq data ptr
; Out -

Voice_ITFilter FNAME
 DEFPROC "R0-R8,R11-R12"

 ; Do nothing if no SMLA support
 LDR     R1,[R12,#data_Status]
 TST     R1,#seq_status_SMLASupport
 TST     R1,#seq_status_disable_ITFilters
 ; Nothing to do if no filter
 LDRB    R1,[R0,#stream_flags]
 TST     R1,#stream_flag_filter
 ; R12 = ptr start, R11 = ptr end
 LDR     R12,[R0,#stream_pbuffer]
 ADD     R11,R12,R11,LSL #1
 ; R0 = old values and filter parameters
 ADD     R0,R0,#stream_filter_y1
 ; read saved values
 ; read filter parameters
 LDMIA   R0,{R3-R4,R5-R7}
 MOV     R8,#&8000

 ; read two samples
 LDR     R1,[R12]
 MOV     R2,R1,ASR #16
 MOV     R1,R1,LSL #16
 MOV     R1,R1,ASR #16
 ; process first sample
 SMULL   R4,R14,R7,R4
 SMLAL   R4,R14,R6,R3
 SMLAL   R4,R14,R5,R1
 MOV     R4,R4,LSR #24
 ORRS    R4,R4,R14,ASL #8
 ADDLT   R4,R4,#1 ; cf rounding error of shift for negative
 ; process second sample
 SMULL   R3,R14,R7,R3
 SMLAL   R3,R14,R6,R4
 SMLAL   R3,R14,R5,R2
 MOV     R3,R3,LSR #24
 ORRS    R3,R3,R14,ASL #8
 ADDLT   R3,R3,#1 ; cf rounding error of shift for negative
 ; Even if sum of coefficients is one, due to one negative coefficient
 ; samples may locally be amplified by up to 2
 ; so divide output by 2 to avoid saturation.
 ; We will mix these filtered note at twice the volume to compensate.
 MOV     R1,R3,ASR #1
 MOV     R2,R4,ASR #1
 ; clip values
 CMP     R1,R8
 SUBGE   R1,R8,#1
 CMN     R1,R8
 MOVLT   R1,R8
 CMP     R2,R8
 SUBGE   R2,R8,#1
 CMN     R2,R8
 MOVLT   R2,R8
 ; store output values
 MOV     R1,R1,LSL #16
 MOV     R1,R1,LSR #16
 ORR     R1,R1,R2,LSL #16
 STR     R1,[R12],#4
 CMP     R12,R11
 BLO     Voice_ITFilter_Loop
 ; save old values

 STMIA   R0,{R3-R4}


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