Flanger

MICrODEC: Stock Functions

This function implements a flanger. It is mono, taking in data from the left channel, and presenting the mono output on both the left and right channels.

Flanging is based on analog tape and reel recording techniques, where slight pressure is applied to the flange of the reel, slowing down the playback. This creates a time-varying delayed signal, which, when mixed with the original signal, has a swept filter effect. Any time-varying signal can be used, but in this case, we are using a sinusoidal signal, as it introduces the least amount of added frequency content (ramp waves are also common). This sinusoid is created by taking values from a look-up table stored in program memory. The frequency of the sinusoid is set by the pot (MOD1), and is variable from .084Hz to 10.84Hz.

Both the depth of the low frequency oscillator (LFO), and the offset delay time can be modified to change the quality of the sound. Since there is only one other built-in knob on the MICrODEC we crammed both functions onto MOD2. By pressing the pushbutton on MOD2, the functionality of the rotary encoder on MOD2 changes. In one state, it modifies the LFO depth, with rotations to the right increasing its amplitude (from 0ms to 6ms). In the other state, it modifies the offset delay, with rotations to the right increasing the delay time (from 0ms to 24ms). In both cases, the rotary encoder changes an 8b value in increments of 1, so it takes quite a few turns to go from one end to the other, although this also gives finer resolution.

Try the flanger with large LFO depth and some feedback, and you'll get what we've started calling "drunken sailor mode". It will make your guitar strings sound like rubber bands. And apologies for the awful user interface, this one is a good candidate for an extra mod knob, or MIDI control.

flanger_sine.asm

; program: flanger-16b-sine.asm
; UID = 000045 - unique id to eliminate conflicts between variables
; 16b address space
; mono data in on left channel, mono data out on both channels
; pot (MOD1) controls lfo frequency
; rotary encoder (MOD2) controls lfo depth and offset delay
; pushbutton (on MOD2) controls wether depth or offset is being modded
; offset delay variable from 90us to 23ms

; program overview
;
; data is read in from the codec and stored to sram.  data is read out of
; sram at a variable delay set by an lfo and an offset.  the lfo is
; generated from a 16b/512s half sinewave lookup table.  this is incremented
; with a 24b number to get very low frequencies.  the lfo rate is set via
; the adc, which is oversampled 256 times and deadbanded to get rid of
; glitches.  the lfo depth is created by multiplying the lfo signal with
; an 8b depth value, which is set via the rotary encoder.  the offset delay
; is an 8b value, which is also set with the rotary encoder, with the
; pushbutton selecting which function is currently active.

; register usage - may be redefined in other sections
;
; r0  multiply result lsb
; r1  multiply result msb
; r2  accumulation lsb
; r3  accumulation mlb
; r4  right/left lsb out/accumulation mhb
; r5  right/left msb out/accumulation msb
; r6  pushbutton state register
; r7  
; r8  adc accumulator fractional byte
; r9  adc accumulator lsb
; r10 adc accumulator msb
; r11 rotary encoder counter
; r12 lfo rate lsb
; r13 lfo rate msb
; r14 null register
; r15 switch sample counter
; r16 temporary swap register
; r17 temporary swap register
; r18 sine wave buffer/multiply msb
; r19 sine wave buffer/multiply msb
; r20 multiply swap register
; r21 multiply swap register
; r22 sinetable lookup address lsb
; r23 sinetable lookup address mlb
; r24 write address lsb
; r25 write address msb
; r26 sinetable lookup address mhb
; r27 sinetable lookup address msb
; r28 delay offset
; r29 lfo depth
; r30 jump location for interrupt lsb
; r31 jump location for interrupt msb
; t   rotary encoder edge detect indicator

; program starts here first time
; intialize registers
ldi r30,$05 ; increment z pointer to new jump location
clr r14 ; clear null register
ldi r28,$09 ; initialize delay offset
ldi r29,$06 ; intiialize lfo depth
reti ; finish with initialization and wait for next interrupt

; program starts here every time but first
; initiate data transfer to codec
sbi portb,portb0 ; toggle slave select pin
out spdr,r5 ; send out left channel msb
cbi portb,portb0

adiw r25:r24,$01 ; increment write address

wait1_000045: ; check if byte has been sent

in r17,spsr
sbrs r17,spif
rjmp wait1_000045
in r19,spdr ; recieve in left channel msb
out spdr,r4 ; send out left channel lsb

wait2_000045: ; check if byte has been sent

in r17,spsr
sbrs r17,spif
rjmp wait2_000045
in r18,spdr ; recieve in left channel lsb
out spdr,r5 ; send out right channel msb

;write left channel data to sram
out portd,r24 ; set address
sts porth,r25
out portg,r14 ; pull ce low,we low,and set high bits of address
ldi r17,$ff
out ddra,r17 ; set porta as output for data write
out ddrc,r17 ; set portc as output for data write
out porta,r18 ; set data
out portc,r19
sbi portg,portg2 ; pull we high to write
out ddra,r14 ; set porta as input for data lines
out ddrc,r14 ; set portc as input for data lines

wait3_000045: ; check if byte has been sent

in r17,spsr
sbrs r17,spif
rjmp wait3_000045
in r17,spdr ; recieve in right channel msb
out spdr,r4 ; send out right channel lsb

wait4_000045: ; check if byte has been sent

in r17,spsr
sbrs r17,spif
rjmp wait4_000045
in r17,spdr ; recieve in right channel lsb

; vco generation
movw r17:r16,r31:r30 ; store z register
;get sample 1
add r22,r12 ; increment sinetable address
adc r23,r13
adc r26,r14 ; r14 is cleared above
adc r27,r14
movw r31:r30,r27:r26 ; move to z register for data fetch
lsl r30 ; adjust pointer for 16b fetch
rol r31
andi r31,$03 ; limit value to 10b (512 samples x 2 bytes)
ori r31,$48 ; set to memory address location where table is stored
lpm r18,z+ ; get sine value lsb, increment z register
lpm r19,z ; get sine value msb
sbrc r27,$01 ; flip sign for half of the values
rjmp interpolate_000045
neg r18
adc r19,r14 ; r14 is cleared above
neg r19

interpolate_000045: ; multiply sample 1 by distance

movw r21:r20,r23:r22 ; get distance from sample 1
com r20 ; invert distance
com r21
mulsu r19,r21 ; (signed)Ah * (unsigned)Bh - multiply high bytes
movw r5:r4,r1:r0 ; store high bytes result for later
mul     r18,r20 ; (unsigned)Al * (unsigned)Bl ; multiply low bytes
movw r3:r2,r1:r0 ; store low byets for later
mulsu r19,r20 ; (signed)Ah * (unsigned)Bl - multiply middle bytes
sbc     r5,r14 ; r14 is cleared above - subtract sign bit
add     r3,r0 ; accumulate result
adc     r4,r1
adc     r5,r14 ; r14 is cleared above
mul r21,r18 ; (unsigned)Bh * (unsigned)Al - multiply middle bytes
add     r3,r0 ; accumulate result
adc     r4,r1
adc     r5,r14 ; r14 is cleared above

;get sample 2
adiw r27:r26,$01 ; set to next sample
movw r31:r30,r27:r26 ; move to z register for data fetch
lsl r30 ; adjust pointer for 16b fetch
rol r31
andi r31,$03 ; limit value to 10b (512 samples x 2 bytes)
ori r31,$48 ; set to memory address location where table is stored
lpm r18,z+ ; get sine value lsb, increment z register
lpm r19,z ; get sine value msb
sbrc r27,$01 ; flip sign for half of the values
rjmp interpolate1_000045
neg r18
adc r19,r14 ; r14 is cleared above
neg r19

interpolate1_000045: ; multiply sample 2 by distance

sbiw r27:r26,$01 ; reset address
movw r31:r30,r17:r16 ; restore z register
mulsu r19,r23 ; (signed)Ah * (unsigned)Bh - multiply high bytes
add r4,r0 ; accumulate result
adc r5,r1
mul     r18,r22 ; (unsigned)Al * (unsigned)Bl ; multiply low bytes
add r2,r0 ; accumulate result
adc r3,r1
adc r4,r14 ; r14 is cleared above
adc r5,r14
mulsu r19,r22 ; (signed)Ah * (unsigned)Bl - multiply middle bytes
sbc     r5,r14 ; r14 is cleared above - subtract sign bit
add     r3,r0 ; accumulate result
adc     r4,r1
adc     r5,r14 ; r14 is cleared above
mul r23,r18 ; (unsigned)Bh * (unsigned)Al - multiply middle bytes
add     r3,r0 ; accumulate result
adc     r4,r1
adc     r5,r14 ; r14 is cleared above

;set lfo depth - 8b value
ldi r16,$80 ; convert lfo to unsigned number
add r5,r16
movw r19:r18,r5:r4 ; move lfo signal to multiply register
mov r21,r29 ; move lfo depth to multiply register
mul r19,r21 ; (unsigned)ah * (unsigned)b
movw r5:r4,r1:r0
mul r21,r18 ; (unsigned)b * (unsigned)al
add     r4,r1
adc     r5,r14 ; r14 is cleared above

;add lfo to delay
movw r17:r16,r25:r24 ; move current location to read address
mov r20,r28 ; move delay offset to temporary register
clr r21 ; clear temporary high byte
lsl r20 ; multiply delay time by 4
rol r21
lsl r20
rol r21
sub r16,r20 ; remove delay offset
sbc r17,r21
sec ; set the carry bit so all values are reduced by 1 lsb
sbc r16,r5 ; remove lfo time
sbc r17,r14 ; r14 is cleared above

;get left channel sample 1 from sram
out portd,r16 ; set address
sts porth,r17
nop ; wait setup period of two cycles
nop
in r18,pina ; get data
in r19,pinc ; get data

;multiply sample 1 by distance
mov r20,r4 ; get distance from sample 1
mulsu r19,r20 ; (signed)ah * b
movw r5:r4,r1:r0
mul r18,r20 ; al * b
add r4,r1
adc r5,r14 ; r14 is cleared above
mov r3,r0

;get left channel sample 2 from sram
subi r16,$ff ; set to next sample
sbci r17,$ff ; done this way because there is no addi or adci
out portd,r16 ; set address
sts porth,r17
nop ; wait setup period of two cycles
nop
in r18,pina ; get data
in r19,pinc ; get data

;multiply sample 2 by distance
com r20 ; get distance to sample 2
mulsu r19,r20 ; (signed)ah * b
add r4,r0 ; accumulate result
adc r5,r1
mul r18,r20 ; al * b
add r3,r0 ; accumulate result
add r4,r1
adc r5,r14 ; r14 is cleared above

;check rotary encoder and adjust lfo depth
; although rotary encoder is externally debounced, it is done here again.
; pin1 is sampled on a transition from high to low on pin0.  if pin1 is
; high, a left turn occured, if pin1 is low, a right turn occured.
dec r11 ; check if time to sample rotary encoder
brne adcsample_000045 ; continue if not
ldi r17,$40 ; adjust sample frequency to catch all rising edges (1.5ms)
mov r11,r17
lds r17,pinj ; get encoder/pushbutton data
mov r16,r17 ; store pushbutton data to temporary register
eor r16,r6 ; check if pushbutton changed value
and r16,r17 ; check if rising edge
sbrc r16,$02 ; continue if not
rjmp pushbutton_000045 ; go to pushbutton routine if pressed
ldi r20,$80 ; mask off the high bit of the pushbutton state register
and r6,r20 ; pinj7 should be 0, as there is no i/o attached to it
or r6,r17 ; store pushbutton data for next sample comparison
sbrs r17,$02 ; check if pushbutton pressed
rjmp adcsample_000045 ; do not allow any parameter change while button pressed
sbrs r17,$00 ; check if pin0 is low
rjmp edge_000045 ; check if pin0 was low on previous sample
clt ;  clear state register if back high
rjmp adcsample_000045 ; finish off

edge_000045: ; check for falling edge

brts adcsample_000045 ; do nothing if edge was already detected
set ; set t register to indicate edge detected
ldi r21,$01 ; prepare for addition or subtraction
sbrs r6,$07 ; check which function is being modded
rjmp lfo_000045 ; do lfo function
sbrs r17,$01 ; check if pin1 is high
rjmp increment_000045 ; increment desired delay if right rotation
dec r28 ; decrement delay register else
cp r28,r21 ; r21 set to $01 above
brsh adcsample_000045 ; continue if not
mov r28,r16 ; set delay to min
rjmp adcsample_000045 ; finish off

increment_000045: ; increment desired delay register

add r28,r21 ; increment delay register
brcc adcsample_000045 ; check if overflow occured
ser r28 ; set delay to max

adcsample_000045: ; sample adc for lfo rate

lds r17,adcsra ; get adc control register
sbrs r17,adif ; check if adc conversion is complete
rjmp done_000045 ; skip adc sampling
lds r16,adcl ; get low byte adc value
lds r17,adch ; get high byte adc value
add r8,r16 ; accumulate adc samples
adc r9,r17
adc r10,r14 ; r14 is cleared above
ldi r17,$f7
sts adcsra,r17 ; clear interrupt flag
dec r15 ; countdown adc sample clock
brne done_000045 ; get delay time if its been long enough

deadband_000045: ; set the low value of the delay

lsr r10 ; divide adc value by 16
ror r9
ror r8
lsr r10
ror r9
ror r8
lsr r9 ; r10 is now empty
ror r8
lsr r9
ror r8
movw r17:r16,r9:r8 ; move adc sample to temporary register
ldi r21,$80 ; add in offset of min lfo rate ($0080)
add r16,r21
adc r17,r14 ; r14 is cleared above
sub r16,r12 ; find difference between adc sample and current lfo rate
sbc r17,r13
brsh check_000045 ; check for deadband if positive
neg r16 ; invert if negative
adc r17,r14 ; r14 is cleared above
neg r17

check_000045: ; check if difference is greater than deadband

cpi r16,$10 ; check if difference is less than 1 adc lsb
cpc r17,r14 ; r14 cleared above
brlo empty_000045 ; do nothing if less than 1 adc lsb
movw r13:r12,r9:r8 ; move adc sample to lfo rate register
add r12,r21 ; add in offset of min lfo rate ($0080)
adc r13,r14 ; r14 is cleared above

empty_000045: ; empty accumulation registers and finish off

clr r8 ; empty accumulation registers
clr r9
clr r10

switchsample_000045: ; check rotary switch

lds r16,pinj ; get switch data
andi r16,$78 ; mask off rotary switch
lsr r16 ; adjust switch position to program memory location
lsr r16
ldi r17,$02
add r16,r17
cpse r16,r31 ; check if location has changed
clr r30 ; reset jump register to intial state
mov r31,r16

done_000045:

reti ; return to waiting

lfo_000045: ; modify lfo depth parameter

sbrs r17,$01 ; check if pin1 is high
rjmp increment1_000045 ; increment desired delay if right rotation
sub r29,r21 ; decrement lfo depth register else
brcc adcsample_000045 ; check if underflow
clr r29 ; set depth to min
rjmp adcsample_000045 ; finish off

increment1_000045: ; increment desired delay register

add r29,r21 ; increment lfo depth register
brcc adcsample_000045 ; check if overflow occured
ser r29 ; set depth to max
rjmp adcsample_000045 ; finish off

pushbutton_000045: ; edge detect

ldi r20,$80 ; toggle msb to inidicate function change
eor r6,r20
and r6,r20 ; mask off the high bit
or r6,r17 ; store pushbutton data for next sample comparison
rjmp adcsample_000045 ; finish off

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MICrODEC: Stock Functions

Flanger