Uncovering the waveforms of the SID

@DrWatson

Published in 

Commodore64

 · 23 Nov 2022

Document version 1.0, 19 Apr 1995.

After posting to comp.sys.cbm for info on an algorithm for reproducing the output of the noise-waveform, someone followed up with an article on how to do pseudo-random generators in hardware. The poster described a leftshifting register scheme with an exclusive-or gate as feed for the outshifted bit 0. This inspired me to examine the SID and especially the noise-waveform in more detail.

This document describes some of the things, I discovered in my search. I give algorithms for reproducing the pulse- and noise-waveforms. Introductory work on reproducing the waveforms corresponding to specific frequencies have been done, but rather than finish that job, I'd like to give you the info on the noise waveform first.

Sampling the waveform

The waveforms of the SID in the c64 and c128 can be examined, because the SID provides a 8-bit output register of the waveform of voice 3 in register $1b. The exact waveform can also be examined from the "start" of the waveform, because the test-bit (bit 3 in register $12 for voice 3) can be used to reset the random-waveform.

To examine the data, we want to be able to sample the output in a consistent way. First step is to establish the sampling-rate to be used. Of course the sampling-rate must depend on the frequency used in registers $0e and $0f, so our job is to determine the dependency between the frequency and the waveform "wavelength" in cycles, i.e. the number of cycles between the waveform-values change.

To this end, we want to read the waveform output of register $1b as fast as possible. With a REU it is possible to sample the value every cycle, and with the program "Cyclewise", which samples $10000 bytes of the waveform output into bank 0 of a REU, I have collected the data in table 1 which are valid for the noise waveform. The number in the first column is the frequency put into registers $0e and $0f. The second number is the number of cycles each value of the waveform at least lasted, while the third number is the number of cycles the first value ($fe) lasted when the sampling is done using the construct:

 	sta $d412	;Start waveform 
	stx $df01	;Start sampling

The STA and STX instructions take 4 cycles, but the data in the table suggests that the sampling is delayed only 3 cycles. Furthermore the first value count is probably 1.5 times longer than the waveform.

Table 1: Frequency, wavelength and initial delay

This leads to the conclusion that the frequencies can be generated with a loop like this for the noise waveform:

  void Frequency-generator(long freq) { 
    long delay=0x180000; 	/* C-notation for $180000 */ 
    long cycle=0; 
    for (;;;) { 		/* Repeat forever */ 
      delay= delay-freq; 
      if (delay<0) { 
        delay= delay+0x100000; 	/* C-notation for $100000 */ 
        waveform_output= calculate new value for waveform; 
      }; 
      waveform[cycle]= waveform_output; 
      cycle= cycle+1; 
    } 
  }

Furthermore, we notice that the highest frequency that is 100% cycle-aligned is $8000, which gives us a wavelength of 32 cycles. In the following discussion this frequency is assumed to be used.

Notice that the above program is not guaranteed to be 100% correct, since I haven't tested it. Specifically is the value of $180000 a quick hack and it might be $17ffff just as well.

The noise-waveform

The next step is to determine whether the noise waveform loops, since this knowledge is useful in respect to an algorithm based on manipulating an internal register. If the algorithm is based upon manipulation an internal register (by doing for instance shifting and exclusive-or), the values _have_ to restart sometime, because an internal register on n bits can only hold 2^n different values. In other words, we will try to establish whether the output stream from $d41b will repeat itself and if it doesn't in a set period of time, say n clockcycles, we will know that the internal register will be of at least log2(n/32) bits, since the value changes each 32 clockcycles (note: log2(x) is the same as log(x)/log(2)).

The program "Loopchecker" checks for loops with an algorithm like this:

  void Loopchecker() { 
    start_noise-waveform(); 

    /* We want to sample *inside* the potential loop */ 
    wait_a_while(); 

    /* Sample 256 values */ 
    for (i=0;i<256;i=i+1) 
      data[i]= peek($d41b); 

    /* Sample until those 256 values arrive again */ 
    do { 
      i=0; 
      while (peek($d41b) = data[i]) do 
        i=i+1; 
    while (i<256); 
  end;

This program will terminate if a sequence of 256 recorded bytes will appear again later. If the 256 bytes reappear, we can be quite certain that the waveform loops, since the chance of this happening with a totally random source is infinitely small (well below 1e-500).

Marko Makela and I hacked a loopchecker together over the IRC, and our results with a 16 cycle sampling-rate was that the computer terminated after approximately 2 minutes and 15 seconds. However, our results weren't consistent, partly because of the 16 cycle sampling frequency, which isn't completely cycle-aligned and partly because the reseting of a waveform with the TEST-bit does not reset the waveform immediately, but rather $2000-$8000 cycles later (this figure varies greatly, does anybody know of a way to reset the waveform fast?).

The "Loopchecker" program given below fixes these errors, and if you run this program, the computer will terminate after approximately 4 minutes and 32 seconds or 272 seconds. Firstly this means that the waveform repeats. Secondly, when the waveform changes every 32 cycles, this means that the length of the loop is approximately

  (272 sec * 980000 cyc/sec)/32 cyc/bytes ~= 8.0 Megabytes.

Thirdly, this implies an internal register length of log2(8.0 MB)= 23 bits. And finally, we must admit that it would be a difficult task to sample the entire 8 MB on a c64 with only 64Kb memory. It is possible to sample the lot in chunks and saving it on multiple disks, but this is not a trivial task. Notice that the data probably can't be packed much since they are random. It should be mentioned that by changing the delayloop after the noisewaveform is selected, it can be demonstrated that the waveform indeed loops after 8 megabytes of data nomatter where in the waveform cycle you are.

With the implication of an internal register of 23 bits, the next step is to find a pattern in the data which might hint the algorithm used to produce the data. Since we know a shifting and eor scheme can be used, it might be useful to take a look at the data in binary:

11111110 
11111100 
11111100 
11111100 
11111000 
11111000 
11111000 
11111000 
11110000 
11110000 
11100000 
11100000 
11100000 
11000000 
11000000 
11000000 
11000000 
10000001 New value for bit 0! 
10000001 
00000011 
00000011 
00000011 
00000110 
00000110 
00000100 
00000100 
00001100 
00001000 
00011000 
00011000 
00011000 
00110000 
00110000 
...

It should be quite clear that some kind of shifting scheme is used. Further examination of the data suggests that the internal register indeed is on 23 bits, where the mapping between the 8 bits in the output and the internal 23 bit register is like this:

Internal register bit number

	22 21 20 19 18 17 16 15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0

Output bit from register $1b

	 7     6           5        4     3           2        1     0

The first data from the output can be reproduced with this layout if the internal register is leftshifted, and the initial value of the internal register is:

	 1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  0  0  0

Now we need to explain the new bits appearing in bit 0 of the data. Even further examination of the data implies that an eor-gate is used to feed bit 0 of the internal register. I have found that the bits 22 and 17 of the internal register provides the feed for bit 0 through an eor-gate, which gives us the entire mechanism:

22<-21<-20<-19<-18<-17<-16<-15-14<-13<-12<-...<-8<-7<-6<-5<-4<-3<-2<-1<-0 
|                   |                                                   | 
+---->----->-----> eor ->----->----->----->----->----->----->----->---->+

This can also be expressed as a C-program like this:

/* Test a bit. Returns 1 if bit is set. */ 
long bit(long val, byte bitnr) { 
  return (val & (1<<bitnr))? 1:0; 
} 

 
/* Generate output from noise-waveform */ 
void Noisewaveform { 
  long bit22;	/* Temp. to keep bit 22 */ 
  long bit17;	/* Temp. to keep bit 17 */ 

  long reg= 0x7ffff8; /* Initial value of internal register*/ 

  /* Repeat forever */ 
  for (;;;) { 

    /* Pick out bits to make output value */ 
    output = (bit(reg,22) << 7) | 
	     (bit(reg,20) << 6) | 
	     (bit(reg,16) << 5) | 
	     (bit(reg,13) << 4) | 
	     (bit(reg,11) << 3) | 
	     (bit(reg, 7) << 2) | 
	     (bit(reg, 4) << 1) | 
	     (bit(reg, 2) << 0); 

    /* Save bits used to feed bit 0 */ 
    bit22= bit(reg,22); 
    bit17= bit(reg,17); 

    /* Shift 1 bit left */ 
    reg= reg << 1; 

    /(* Feed bit 0 */ 
    reg= reg | (bit22 ^ bit17); 
  }; 
};

Every loop in the above program provides a new value for the noise waveform in the variable "output". Notice that the value will repeat for several cycles, just like the real SID. All in all, I find that the above program can sufficiently reproduce the output from the noise-waveform in the SID.

When it comes to the quality of the pseudo-random generated numbers, one thing is clear: Since all bit-patterns of the 23 bits are generated, all values in the 8 bit output will appear equally many times over time. Furthermore, I believe this particular scheme is recognized as one of the better ones for doing pseudo-random numbers, and it is based on polynomiums, much like the CRCs used for checksumming.

But since the values are reproducible, you'll have to be carefull if you plan to use the noise waveform as a source for random numbers for games and such. Firstly, only reset the waveform when the games is loaded. And wait with the sampling until the user presses a key or fire, which will provide an offset into the stream, so the values will be different from each run. Furthermore, be sure to read the values at an appropriate sampling-rate, for instance every 32 cycle with a frequency of $8000. One prefered method is to do a table of say $2000 values before the game starts, so that voice 3 can be used in the game-music or for sound-effects in the game.

Pulse-waveform

At frequency $1000 this program generates the output:

for (i=0; i<pulsewidth; i++) 
  output= 0; 

for (;i<$1000;i++) 
  output= $ff;

The pulsewidth is the value presented in $d410+$d411 for voice 3. This program should be correct for all values of the pulsewidth, including 0 and $fff.

Final words

I have studied the other waveforms as well, but I want to get this out now, so the details will wait. I'd like somebody to verify the data for the noise-waveform, since I have only tried it on my c64 with a 6581 SID produced in week 22 of 1982. If someone would be a real nice dude, you could try to sample the output (or investigate it with an oscilloscope) from the loud-speaker and determine whether the true output is 8 bit.

If interest exists, I'll get myself together and examine the final details of the waveforms and the frequency mapping. Furthermore, investigation into the envelope could be interesting. As a final note, I have looked into the mixing of waveforms and some mysterious results have arisen - the results does not suggest a simple AND of the respective waveforms - rather it's strangely organized in bursts every 63 cycle, which suggests correlation with the VIC! I have seen similar correlation in the keyboard reading, which also is clearly associated with the VIC, so this is another topic open for examination. E-mail me if you want details.

--
Asger Alstrup, diku0748@diku.dk

[Cut here] 

Programs in assembler 
--------------------- 

;"Cyclewise" - records noise-waveform each cycle using REU. 
;---------------------------------------------------------- 

	* = $1000 

freq	= $8000		;Define frequency here 

cycle	jsr init	;Init screen 

	lda #$08	;Set testbit to reset the waveform. 
	sta $d412 

	jsr pause	;Give it time to settle - necessary on my machine. 

	lda #<freq 	;Set frequency 
	ldx #>freq 
	sta $d40e 
	stx $d40f 

	;Set up REU 

	ldx #<$d41b	;Define REU read address to be $d41b. 
	ldy #>$d41b 
	stx $df02 
	sty $df03 

	lda #$00 	;Record into $0000 
	sta $df04 
	sta $df05 
	sta $df06	;in bank 0. 

	sta $df07	;Transfer $10000 bytes. 
	sta $df08 

	lda #$00	;No interrupts from the REU. 
	sta $df09 

	lda #%10000000	;Fix c64 adress at $d41b. 
	sta $df0a 

	lda #$80 
	ldx #%10010000	;REU command: Do transfer from c64->REU. 

	;start waveform and do the sampling 

	sta $d412	;Enable noise-waveform 
	stx $df01	;and start recording. 
	jmp done	;After sampling, wrap it up 

 
;Loopchecker - checks for a loop in the noise-waveform. 
;------------------------------------------------------ 

	* = $1100 

block	= $2000 

loop	jsr init	;Init screen 

	lda #$08	;Set testbit to reset the waveform. 
	sta $d412 

	jsr pause	;Give it time to settle - necessary on my machine. 

	lda #$00 	;Set frequency to $8000 
	sta $d40e 
	lda #$80 
	sta $d40f 

	lda #$80 	;Start noise-waveform 
	sta $d412 

	jsr pause	;Wait a while, so we get well into the waveform 

	inc $d020	;Signal that the recording starts now 

	ldx #0		;Sample 256 bytes at 32 cycles 
l1	lda $d41b 
	sta block,x 
	bit $ffff 
	bit $ffff 
	bit $ffff 
	bit $ffff 
	nop 
	inx 
	bne l1 

	nop		;Wait exactly 1 value 
	bit $ea 
	bit $ffff 
	bit $ffff 

l2	ldy #$1d	;See if the 256 bytes repeat 
	ldx #$00 
l3	lda $d3ff,y	;(Trick to get 5 cycles execution time) 
	bit $ffff 
	bit $ffff 
	bit $ffff 
	bit $ffff 
	cmp block,x 
	bne l2 
	inx 
	bne l3 

	jmp done	;And exit if they do 

;Misc subroutines 
;---------------- 

	* = $1800 

	;Init screen 

init	sei		;Secure accurate timing by 
	lda #$00	;disabling sprites 
	sta $d015 
in1	lda $d011	;and screen. 
	bpl in1 
	and #$ef 
	sta $d011 
	rts 

	;Wait 50 frames so waveform can be reset 

pause	ldx #50 
p1	bit $d011 
	bpl p1 
p2	bit $d011 
	bmi p2 
	dex 
	bne p1 
	rts 

	;Wraps it up 

done	lda $d011	;Reenable screen 
	ora #$10 
	sta $d011 

	cli		;and exit. 
	rts 

[cut here]

Output from "cyclewise" with a frequency of $ffff (The data are correct: the wavelength is effectively 16 cycles for the first $10000 or so values):

Value (number of times)
$fe ($16), $fc ($30), $f8 ($40), $f0 ($20), $e0 ($30), $c0 ($40), $81 ($20), $03 ($30), $06 ($20), $04 ($20), $0c ($10), $08 ($10), $18 ($30), $30 ($20) ...