Open Music Labs Forums

the pitch is set from the onboard oscillator, which also does the MIDI communication, so you can not change the oscillator without it messing up the MIDI. there are 2 other options: 1. send MIDI CC messages to do a frequency sweep, and then another one to turn frquency sweep off. this will pitch it down a bit, but only after note release, so its not a great option. 2. hack the code to add that MIDI CC control, or to read an analog pin and set the value based on that.

if you are interested in exploring option 2, i can give pointers. you will need an ISP programmer (i hear an arduino can be hacked to do this).

ok, i can give pointers to get things going. probably the first thing to do would be to get a toolchain setup, and make sure you can change the code. luckily the ADC pins are broken out to the midi channel tabs, so you can attach there, which will be a lot easier than trying to solder directly to the IC. that potentially gives you 4 controllers you can add.

do you have an AVR ISP programmer?

go here
http://wiki.openmusiclabs.com/wiki/MidiVamp1
and download the avr studio project. i used avrstudio 4.13, so hopefully whatever they have these days is backwards compatible. load up the project and see if it compiles.

So from the answers here,I would not be able to pitch up or down each of the 8 midi vampire 1 modules I want to buy so I can make a very thick fat analog synth with detuned osc(x 8)?Or is the limitation only for physical pitch control OUTSIDE midi control CCs?

pianokeyjoe wrote:So from the answers here,I would not be able to pitch up or down each of the 8 midi vampire 1 modules I want to buy so I can make a very thick fat analog synth with detuned osc(x 8)?Or is the limitation only for physical pitch control OUTSIDE midi control CCs?

it would have to be reprogrammed to accept external CC messages (or reprogrammed and rewired, as is being done here) to get the detune to work.

after code uploading is verified to work, we can start developing the pitch bending code. the main issue here is how to implement it. a good pitch bender would use a multiply, and adjust the pitch by a certain percentage. this would make the shift amount consistent regardless of the note played. unfortunately, a multiply takes up a lot of clock cycles, and uses different registers.

since there isnt much free processing time, i would suggest starting with a constant pitch offset. this means that lower frequencies will appear to pitch more than higher frequencies. it might sound interesting, and will take a lot less computation.

a quick way to prototype this, is to steal another CC, and put your pitch shifting code in. for example, stealing the pitch sweep CC. look for these lines in vamp1.asm:

mod13: ; frequency sweep

cpi r16,$0d
brne mod14
clr r16
subi r17,$40
sbc r16,r10
sts sweep_l,r17
sts sweep_h,r16
rjmp main ; go back to waiting

this code essentially does what we want already, it takes in the midi value, and converts it into a signed 16b value centered around 0. so values above 64 will pitch up, values below 64 will pitch down, and 64 will do nothing. we can just create a new variable, and reuse it:

mod13: ; frequency sweep

cpi r16,$0d
brne mod14
clr r16
subi r17,$40
sbc r16,r10
;sts sweep_l,r17 ;these variables commented out
;sts sweep_h,r16
sts pitch_l,r17 ; new variables added
sts pitch_h,r16
rjmp main ; go back to waiting

you must create and initialize these variables as well. all of the variables are kept in vamp1_defs.inc. this just tells the compiler where in memory to look for your variable. it can just get tacked on at the end of the list:

.equ pitch_l	= $0825
.equ pitch_h	= $0826

and initialization should go somewhere in the init: section at the top of vamp1.asm, probably the setup voices section

clr r16
sts pitch_l,r16 ; set to 0
sts pitch_h,r16

now that pitch just has to get added in whenever a note plays. there are 4 voices, and it has to be added to each one. there are also 2 conditions, wether the note was decaying or not, so there are actually 8 different note on sequences. they are voice1_on:, voice1_on0:, voice2_on:, etc. here is an example of the modification needed:

voice2_on: ; turn on voice2

lds r30,last_byte ; fetch note from lookup table
sts voice2,r30 ; store note number for turnoff sequence
lsl r30 ; adjust for 2byte fetch
ldi r31,$0c
lpm r16,z+ ; load it into the frequency register
lpm r17,z
lds r30,pitch_l ; fetch pitch bend value - new code
lds r31,pitch_h
add r16,r30 ; add in pitch value to current note - new code
adc r17,r31
cli
sts freq2_l,r16
sts freq2_h,r17
lds r16,pwm_start
sts pwm_depth2,r16 ; reset pwm depth
andi r23,$f3
ori r23,$04 ; set voice2 to attack
ori r24,$04 ; set voice2 trigger
sei
rjmp main ; go back to waiting

this currently only allows for the pitch to change when it is triggered, ie it will not act a 'bend' where you can modulate the note afterwards. that can be added later.