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File List | 1994-09-02 | 16.6 KB | 406 lines

;*************************************************************** ; ; Stereo chorus for the DSP56001 signal processor. ; Developed by Quinn Jensen (jensenq@qcj.icon.com). ; ; This program fragment implements a stereo "chorus" effect ; on a DSP56001 processor. Chorus adds depth and warmth to ; sound by creating the illusion that more instruments ; are involved in the sound than really are. It does this by ; mixing together a delayed version of the input with the input ; itself. This program uses the following signal flow. ; ; ; Left in ------+------- "dry" gain -----------> sum -----> Left out ; | ^ ; v | ; sum --> delay ---> "wet" gain -----+ ; ^ | ; | v - ; Right in -----+------- "dry" gain -----------> sum -----> Right out ; ; ; Note that the delay line output is negated before summing with the ; right input signal. This throws in 180 degrees of phase shift ; making for interesting results even with mono inputs ; (i.e. Left in == Right in). ; ; Chorus uses a delay time of between about 10 and 50ms in some commercial ; units. This program can be configured for longer delays. ; In the chorus effect, the delay time is slowly varied, adding a very ; subtle pitch shift. The depth and speed of the delay-time modulation ; are adjustable to taste. The greater the depth or speed, the greater ; the coloration of the signal. ; ; Variations in this algorithm are possible and encouraged. I came ; up with this code after studying the impulse response and characteristics ; of a commercial stereo chorus pedal and reading various articles in magazines ; and on usenet over the years. My somewhat dry TX81 synthesizer sounds pretty ; good with this and other effects I run on the 56001. ; ; A recent article with a pretty good not-too-technical overview of chorus ; and other effects: Gary Hall, "From the Top: Effects, the Essential Musical ; Spice," _Electronic Musician_, August 1991, pp. 62-68. ; ; I would enjoy seeing any improvements to the code. ; include "tdsg.a56" ;hardware specific initialization code ;*************************************************************** ; ; Data and constants ; ;*************************************************************** P:0076 dot ;remember where we were in P-space X:0020 org x:$20 ;put runtime variables in on-chip X-space ; A spreadsheet was used to calculate the following numbers ; Sample rate 32.5500 kc ; ; Delay time (4-52) 28.0000 ms delay time knob ; Depth (1-10) 10.0000 depth knob (I like it deep) ; Speed (1-10) 0.0000 speed knob (I like it slow) ; ; max depth +/- 24.0000 ms ; min delay 4.0000 ms ; max delay 52.0000 ms ; 1/2 cycle period 5.0000 s ; samples per 1/2 cyc 162750.0000 ; time delta/samp 0.2949 us ; offset samp/samp 0.0096 ; FFFF7E doff_i equ -130 ;initial delay offset (tap) 0.0096 ddeltaf equ 0.0096 ;delta-delay, per sample 027BBE dspeed_i equ 162750 ;number of samples per ;half cycle of triangle wave ;delay-time modulator ; ; Delay time (ms) tap tap delay ; ; 1 32.5500 1 0.03 ; 2 65.1000 2 0.06 ; 4 130.2000 4 0.12 ; 5 162.7500 8 0.25 ; 8 260.4000 16 0.49 ; 10 325.5000 32 0.98 ; 20 651.0000 64 1.97 ; 25 813.7500 128 3.93 ; 40 1302.0000 256 7.86 ; 50 1627.5000 512 15.73 ; 60 1953.0000 1024 31.46 ; 70 2278.5000 2048 62.92 ; 80 2604.0000 4096 125.84 ; 90 2929.5000 8192 251.67 ; 100 3255.0000 16384 503.35 ; ; The delay line is in off-chip X memory ; 002000 delay equ $2000 001000 dmax equ 4096 ;125 ms (probably way too long) ; ; doff and ddelta are 48-bit quantities ; X:0020 FFFF7E doff dc doff_i ;current delay distance Y:0020 org y:doff Y:0020 000000 dc 0 X:0021 org x:doff+1 X:0021 000000 ddelta dc 0 ;delta delay per sample Y:0021 org y:ddelta Y:0021 013A93 dc ddeltaf X:0022 org x:ddelta+1 X:0022 027BBE dspeed dc dspeed_i ;samples per half cycle of triangle modulator X:0023 000000 dtoggle dc 0 ;current sample count X:0024 delayout X:0024 000000 dc 0 ;current delay-line output Y:0000 org y:$0 P:0076 org p:dot ;go back to P-space ;*************************************************************** ; ; Initialization code ; ;*************************************************************** P:0076 hf_init P:0076 61F400 move #delay,r1 ;delay line input P:0077 002000 P:0078 05F421 movec #dmax-1,m1 ; P:0079 000FFF P:007A 71F400 move #doff_i,n1 ;distance to output P:007B FFFF7E P:007C 00000C rts ; ;*************************************************************** ; ; Sample-rate computations. Call chorus_compute at ; interrupt time when both left and right inputs are ; ready. ; ; fs = 32.552083 kHz ; ; x:<in_l left-channel input ; x:<in_r right-channel input ; x:<out_l left-channel output ; x:<out_r right-channel output ; ;*************************************************************** P:007D hf_comp P:007D 0D0066 jsr <saveregs ; ; output and input mix ; P:007E 253813 clr a #.4375,x1 ;clr a, get input scale P:007F 27401B clr b #.5,y1 ;clr b, get output scaler P:0080 46A400 move x:<delayout,y0 ;get delay out P:0081 4480BB macr y0,y1,b x:<in_l,x0 ;b = .5 * delay, x0=in_l P:0082 21E6CB macr x0,y1,b b,y0 ;b += .5 * in_l, y0=b P:0083 5704A3 macr x0,x1,a b,x:<out_l ;a = x1 * in_l, L = b P:0084 20CF00 move y0,b ;b = -y0 P:0085 44813E neg b x:<in_r,x0 ;x0 = in_r P:0086 2000CB macr x0,y1,b ;b += .5 * in_r P:0087 5705A3 macr x0,x1,a b,x:<out_r ;R = b, a += x1 * in_r ; ; delay line in ; P:0088 565900 move a,x:(r1)+ ; ; delay line length modulation. A simple triangle-wave modulator ; is used. A sine-wave modulator would be much better, especially ; with deeper and/or faster settings. ; P:0089 48A000 move l:<doff,a ;a = current offset P:008A 42A100 move l:<ddelta,x ;x = current delta P:008B 200020 add x,a P:008C 482000 move a,l:<doff ;new offset = a + x P:008D 219900 move a1,n1 P:008E 20AF00 move x1,b ;save delta for later use ; ; smoothly transition between delay-line offsets by ; interpolating the current sample with the previous or next ; one depending on whether the delay is currently getting longer or ; shorter. Otherwise, an obnoxious click results when the offset snaps ; to the next integral value. This does have a low-pass effect on the ; delayed signal path but it is not objectionable. ; P:008F 5EA000 move y:<doff,a ;compute |frac(doff)| P:0090 44F423 lsr a #$800000,x0 P:0091 800000 P:0092 254042 or x0,a #.5,x1 P:0093 218400 move a1,x0 P:0094 2000A4 mpy -x0,x1,a P:0095 21C60B tst b a,y0 ;y0 = 0.5 * |frac(doff)| P:0096 0AF0A3 jpl shorter ;on positive delta, the P:0097 0000A1 ;delay is shortening P:0098 205100 move (r1)- ;get previous sample P:0099 44E900 move x:(r1+n1),x0 P:009A 2F40D0 mpy y0,x0,a #.5,b ;scale P:009B 20595C sub y0,b (r1)+ ;compute scale for cur sample P:009C 21E600 move b,y0 P:009D 44E900 move x:(r1+n1),x0 ;get cur sample P:009E 2000D2 mac x0,y0,a ;scale and sum P:009F 0AF080 jmp endpan P:00A0 0000A7 P:00A1 shorter P:00A1 44E900 move x:(r1+n1),x0 ;get cur sample P:00A2 2F40D0 mpy y0,x0,a #.5,b ;scale P:00A3 20595C sub y0,b (r1)+ ;compute scale for next sample P:00A4 21E600 move b,y0 P:00A5 44E900 move x:(r1+n1),x0 ;get next sample P:00A6 2051D2 mac x0,y0,a (r1)- ;scale and sum P:00A7 endpan P:00A7 562400 move a,x:<delayout ;store resulting output ; ; update the triangle wave modulation (sinewave would be better) ; P:00A8 56A300 move x:<dtoggle,a ;decrement toggle count P:00A9 44F400 move #>1,x0 P:00AA 000001 P:00AB 200044 sub x0,a P:00AC 562300 move a,x:<dtoggle P:00AD 0AF0A7 jgt notogg ;time to toggle? P:00AE 0000B3 P:00AF 44A200 move x:<dspeed,x0 ;yes, negate delta and reset P:00B0 48A100 move l:<ddelta,a P:00B1 442336 neg a x0,x:<dtoggle P:00B2 482100 move a,l:<ddelta P:00B3 notogg P:00B3 bypass P:00B3 0D006F jsr <restregs P:00B4 00000C rts end Summary of psect usage section seg base last top used avail total ------------------------------------------------------------------------- Symbol Table ------------------------------------- ddelta 000021 ddeltaf 0.0096000000 ssi_int 000061 ssix 000010 notogg 0000B3 delayout 000024 delay 002000 init_stereo 000059 start 000040 in_rs 000003 in_ls 000002 in_r 000001 in_l 000000 dmax 001000 endpan 0000A7 doff 000020 doff_i FFFF7E dot 000076 mainloop 000060 hf_comp 00007D hf_init 000076 dtoggle 000023 saveregs 000066 savey 00000B savex 00000A saveb2 000009 saveb10 000008 savea2 000007 savea10 000006 dspeed 000022 dspeed_i 027BBE restregs 00006F out_r 000005 out_l 000004 bypass 0000B3 shorter 0000A1 m_scl1 00000F m_scl0 00000E m_scl 00C000 m_ssl1 00000D m_ssl0 00000C m_ssl 003000 m_hpl1 00000B m_hpl0 00000A m_hpl 000C00 m_ibl2 000005 m_ibl1 000004 m_ibl0 000003 m_ibl 000038 m_ial2 000002 m_ial1 000001 m_ial0 000000 m_ial 000007 m_ipr 00FFFF m_rdf 000007 m_tde 000006 m_roe 000005 m_tue 000004 m_rfs 000003 m_tfs 000002 m_if1 000001 m_if0 000000 m_if 000002 m_srie 00000F m_stie 00000E m_sre 00000D m_ste 00000C m_mod 00000B m_gck 00000A m_syn 000009 m_fsl 000008 m_sckd 000005 m_scd2 000004 m_scd1 000003 m_scd0 000002 m_scd 00001C m_of1 000001 m_of0 000000 m_of 000003 m_psr 00000F m_wl1 00000E m_wl0 00000D m_wl 006000 m_dc 001F00 m_pm 0000FF m_tsr 00FFEE m_sr 00FFEE m_crb 00FFED m_cra 00FFEC m_tx 00FFEF m_rx 00FFEF m_tcm 00000F m_rcm 00000E m_scp 00000D m_cod 00000C m_cd 000FFF m_r8 000007 m_fe 000006 m_pe 000005 m_or 000004 m_idle 000003 m_rdrf 000002 m_tdre 000001 m_trne 000000 m_tmie 00000D m_tie 00000C m_rie 00000B m_ilie 00000A m_te 000009 m_re 000008 m_woms 000007 m_rwi 000006 m_wake 000005 m_sbk 000004 m_wds2 000002 m_wds1 000001 m_wds0 000000 m_wds 000003 m_sccr 00FFF2 m_ssr 00FFF1 m_scr 00FFF0 m_stxa 00FFF3 m_stxh 00FFF6 m_stxm 00FFF5 m_stxl 00FFF4 m_srxh 00FFF6 m_srxm 00FFF5 m_srxl 00FFF4 m_dma 000007 m_hf1 000004 m_hf0 000003 m_hf 000018 m_hcp 000002 m_htde 000001 m_hrdf 000000 m_hf3 000004 m_hf2 000003 m_hcie 000002 m_htie 000001 m_hrie 000000 m_htx 00FFEB m_hrx 00FFEB m_hsr 00FFE9 m_hcr 00FFE8 m_pcd 00FFE5 m_pcddr 00FFE3 m_pcc 00FFE1 m_pbd 00FFE4 m_pbddr 00FFE2 m_pbc 00FFE0 m_bcr 00FFFE errors=0