;***************************************************************
;
; Stereo chorus for the DSP56001 signal processor.
; Developed by Quinn Jensen
;
; 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