Negative and positive values on a bipolar knob (sysex)

Hello,

i really try to get my head around sysex and LSB, MSB and what not ... it just makes my brain melt.

i need to have a knob sending sysex values which range from -99 to 99.

now, the sysex string i´m sending does fine with values from 0 to 99 but no negative values, no matter what i try.

the synth requests me to send two times (the same, i assume) parameternumber and a two values.

this is one string from the synth, catched with midi monitor.

00 F0 42 34 41 41 01 54 00 63 00 F7

and here´s what i´m sending:

midiout(0,{0xF0,0x42,0x34,0x41,0x41,0x01,0x54,00,x*99,00,0xF7})

i know it´s wrong, so how can i add the requested second value?

i allowed myself to attach a picture of the manual, where the part is marked yellow.

any help apprechiated.

cheers,
Martin.

hirnschmelze.jpg

Is there a corresponding Parameter Read command that you can send? You can examine the MachineDrum response to determine how it encodes negative parameter values.

I suspect that negative values are passed using two's compliment representation. Here is the Wikipedia article explaining how negative numbers are encoded:
http://en.wikipedia.org/wiki/Two%27s_complement

This might work:

decl lsb, msb;
if(x < 0){
  x = 0x4000 + x; // create two's compliment
  lsb = x & 0x7F;  // isolate lsb
  msb = (x >> 7) & 0x7F;  // isolate msb
} else {
  lsb = x & 0x7F; // positive x values
  msb = 0;
}
midiout(0,{0xF0,0x42,0x34,0x41,0x41,0x01,0x54,00,lsb,msb,0xF7});

Thanks,

this time it´s acutally a korg prophecy

i tried your code but had no luck with, strange thing though because the sysex sent bei the lemur looks exactly like the sysex sent from Prophecy.

so this should work, right?

hm ... even more confused now

cheers

one more thing.

This code reflects the positive value:

00  F0 42 34 41 41 01 1E 00  57 00 F7 

this one the negative

00  F0 42 34 41 41 01 1E 00  1D 01 F7  

both received from the prophecy,so they change the last byte to reflect either negative or positive values?

thanks for the link, will see how far i get, english is not my native language.

cheers

Cool. Making progress at least. The process of elimination is a good technique to use for problems like these.

Based on the info in your last post, it looks like it is not using two's compliment. So we have eliminated this possibility.

The next step is to collect some data so we can determine how the negative numbers are encoded. Try different parameter values, and write down the msb/lsb values in the corresponding sysex message.

hey there,

i think it´s like i said. last byte i either 00 (pos values) or 01 (negative values)

00 F0 42 34 41 41 01 54 00 52 01 F7

52 reflects a value -46

00 F0 42 34 41 41 01 54 00 1A 00 F7

1A reflects a value +26 on the korg

strange, looks like pos values are hex but negatives are not?

cheers

It looks like it is two's compliment, but the data field size is 8 bits rather than 14 bits. The same code that I posted above should work with small changes:

decl lsb, msb;
if(x < 0){
  x = 0x100 + x;   // create two's compliment, 8-bit
  lsb = x & 0x7F;  // isolate lsb 7 bits
  msb = 1;         // msb should always be 1 for neg. numbers
} else {
  lsb = x & 0x7F;  // positive x values
  msb = 0;
}
midiout(0,{0xF0,0x42,0x34,0x41,0x41,0x01,0x54,00,lsb,msb,0xF7});

i gave it a try, unfortunately it does not work, the prophecy does not respond in any way.

probably this is a problem of the bipolar knob?

i made a monitor with knob.x and the value never drops below 0, so how can your script start to work?

sorry if that´s totally stupid thinking, i try to understand this.

thanks for your effort.

cheers.

this code gives me a -1 when x of the knob is on the left and a +1 on the right.

decl lsb, msb;
    
if(x>0)
{
lsb =x;
msb = 0 & 0x7F;
    
} else 
{
lsb = x & 0x7F;       
msb = 1 & 0xF7;
}
 midiout(0,{0xF0,0x42,0x34,0x41,0x41,0x01,0x1E,00,lsb,msb,0xF7});

i tried to extend it with range and x*99 but had no luck.

cheers

I keep forgetting that x, in this context, refers to the Knob.x value. It has a range of 0 to 1.

Add a monitor object to observe the Knob.x value.

As you will see, Knob.x = 0.5 when the knob in the middle of its rotation range. To convert this to a bipolar value, subtract 0.5 from the x value, then scale as needed. I'd also recommend you convert the result to an integer using the floor() function.

 
//execution: on expression: floor((x-0.5)*198
decl y, lsb, msb;
y = floor((x-0.5)*198); // convert 0.0 to 1.0 range to -99 to +99 integer range
if(y<0){
  lsb = (0x100 + y) & 0x7F; // 8-bit two's compliment
  msb = 1;
}else{
  lsb = y;       
  msb = 0;
}
midiout(0,{0xF0,0x42,0x34,0x41,0x41,0x01,0x1E,00,lsb,msb,0xF7});

Man, excellent! works perfect, actually better then the knobs on the prophecy itself.

thanks a lot, you helped me a big deal.

cheers,
martin