Korg Poly800/EX800 Users

On Samstag 20 September 2008, korgpolyex800 wrote:
> Wow, now that I look at it in C, it definitely needs improvement.

If it's really implemented like you've commented, that would be so bad 
you should have heard as the longest cycle would be 9.  Maybe your 
assembler uses different mnemonics, but I think RLC should be "rotate 
circular", not "rotate through carry".  If so, then the C expression 
transforms to something more complicated:

m = ((m ? m : -m) << 1) + (m&0x80 ? 44 : 43);

Since you're not using a homogeneous recurrence (by virtue of adding 
43) zero is actually not a forbidden state and by excluding zero you 
indeed shorten the longest cycle from 128 to 121.  A better 
recurrence would therefore be:

m += (m&0x80 ? 44 : 43);

or equivalently:

; my pseudo random number generator - used for the LFO randomiser
; returns a random number in A

RANDOMIZE:      push    h ; save HL
                lxi     h, M_RANDOM ; point at the previous value
                mov     a, m ; put the seed value into A
RANDOMIZEA:     rlc ; rotate A circular
                adi     43 ; add 43 offset
                mov     m, a ; save in seed
                pop     h ; load HL
                ret ; return with pseudo random in A

The cycles associated with different seed values then are (seed before 
the colon, cycle length at the end in brackets):

0:	43, 129, 46, 135, 58, 159, 106, 255, 42, 127, 41, 125, 37, 117, 21, 
85, 213, 214, 216, 220, 228, 244, 20, 83, 209, 206, 200, 188, 164, 
116, 19, 81, 205, 198, 184, 156, 100, 243, 18, 79, 201, 190, 168, 
124, 35, 113, 13, 69, 181, 150, 88, 219, 226, 240, 12, 67, 177, 142, 
72, 187, 162, 112, 11, 65, 173, 134, 56, 155, 98, 239, 10, 63, 169, 
126, 39, 121, 29, 101, 245, 22, 87, 217, 222, 232, 252, 36, 115, 17, 
77, 197, 182, 152, 92, 227, 242, 16, 75, 193, 174, 136, 60, 163, 114, 
15, 73, 189, 166, 120, 27, 97, 237, 6, 55, 153, 94, 231, 250, 32, 
107, 1, 45, 133, 54, 151, 90, 223, 234, 0, 
[128]
2:	47, 137, 62, 167, 122, 31, 105, 253, 38, 119, 25, 93, 229, 246, 24, 
91, 225, 238, 8, 59, 161, 110, 7, 57, 157, 102, 247, 26, 95, 233, 
254, 40, 123, 33, 109, 5, 53, 149, 86, 215, 218, 224, 236, 4, 51, 
145, 78, 199, 186, 160, 108, 3, 49, 141, 70, 183, 154, 96, 235, 2, 
[60]
9:	61, 165, 118, 23, 89, 221, 230, 248, 28, 99, 241, 14, 71, 185, 158, 
104, 251, 34, 111, 9, 
[20]
30:	103, 249, 30, 
[3]
44:	131, 50, 143, 74, 191, 170, 128, 44, 
[8]
48:	139, 66, 175, 138, 64, 171, 130, 48, 
[8]
52:	147, 82, 207, 202, 192, 172, 132, 52, 
[8]
68:	179, 146, 80, 203, 194, 176, 140, 68, 
[8]
76:	195, 178, 144, 76, 
[4]
84:	211, 210, 208, 204, 196, 180, 148, 84, 
[8]
212:	212, 
[1]

There is a slight DC bias to the longest sequence and a strong one to 
all the others.  I think it would be easier to just put a random 
table into memory (if you can spare the 256 bytes).  That way you 
could even make this a general waveform by allowing to load new 
tables into memory and/or switch between different ones later on.


Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk]>+

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