Lamer Exterminator, or how a 22 year old malware can still piss you off.

As usual I need to step back for my obligatory back story.

-VIC-20 from 1981 to 1985
-C64 from 1986 to 1994 (even logged onto my University account with it!)
-Various PCs and Mac's from then on.

Didn't see the word Amiga there? Normal, only very rich kids had them where I came from. Only relatively recently did I start to indulge in that world, and what a 'fun' ride this still is. Amiga literate people can stop reading now, or continue and have a laugh at my expense...

Broadening my quest on early digital sound chips, a look at Paula (MOS8364) was inevitable, although as an 8bit DMA sample playback device, it didn't really fit the original intent of chipsounds. (no oscillators, classic samples are copyrighted, etc).

Nevertheless there are a bunch of things I plan to do following my current OCS Amiga research but I'll keep this for a series of future articles...
... So I got a bunch of gear and disks from various places.

1)Top is my ebay.co.uk bought PAL A500, which I recently upgraded and tweaked (more in a future article).
2)Middle is an Original NTSC Amiga (later re dubbed A1000), a loaner from XC3N which wasn't his in the 80's.
3)Bottom one is my most important Amiga and the first one I found in a recycling center for 20$ a few years ago: an NTSC B2000 (rev6.2a) with 1MB of chip and 2MB of 'fast/slow' ram, and a 'A2320 Display Enhancer' VGA card . This was originally a Video Toaster machine but that particular addon was already removed when I acquired it.  The most important feature of that particular Amiga for me is its generous amount of ram, since this allows transferring PC<->AMIGA floppy images transfers much easily (maybe the subject of a THIRD Amiga article)
I of course also have a weird collection of disks, some that have been recently transferred from the net, but most others are from the 'era' itself, including.... various bootlegged copies of Workbench.

Working on a complex platform that you are not familiar with is daunting for many people, but it is much more so when you get this horrible feeling that everything is flaky and unreliable. Machines that reboots randomly, Disk errors which suddenly appear on what were perfectly good disks just the day before. Since I dealt with 80s technology I just figured this was due to its age and condition...

But NO, it was ME .. the LAMER...  being exterminated!

Only this virus was made 22 years ago and it made me lose PRECIOUS RESEARCH TIME THIS YEAR, since I only figured out what was happening the day I downloaded X-Copy because of its advanced disk checking features .. or so I've read... but as I found out it also scans the boot sectors of each disks for known viruses.

This virus is apparently quite famous for Amiga people, it even has its own wikipedia page. In any case, I was quite happy to wipe all infected disks clean and redoing much of my workbench, utilities, and test disks. Yes you can laugh, but I don't recall ever getting a C64 virus in my day, so I laugh back.. boot disks .... Pffft shush...

And ... That A1000 loaner I got a month ago also came with a bunch of surprises of itself:

Yes XC3N, I've quarantined those for you for when you want to get it back.  :) Don't worry, those last ones didn't make me lose any data, I had learned my lesson by then.

Conclusion: I hate script kiddies, present and past.

patience

Oh yes I'm working on new stuff...

No updates for a while, its because I can't disclose my new stuff yet.
Sad because theres LOTS of research in there.

I'm hoping to post a glimpse at NAMM 2011.

New research on the SID ADSR

My previous post on the SID ADSR tables on the SID DIE left a question unanswered about the exact method by which the chip manages to apply the level changes in the envelopes at certain points in its decay/release stages.

Lucky for us, Frank Wolf took upon the challenge, and I'm quite priviledge to be allowed to publish his research on this blog.

So here is his analysis of that problem....

(high res)

6581 Envelope Generator (26.10.2010)
----------------------------------

In the upper part you can see the R-2R ladder; on the left is the MSB (Bit 7) of the 8 Bit counter.

Logarithmic Table
-----------------

Now take a look a the Table part! Here's the table in ASCII Form and reduced to the values on 'B' lines (please note the every second bit of the 8Bit counter is negated, i.e. the signal line is mirrored A<->B!)

B7 |B6 |B5 |B4 |B3 |B2 |B1 |B0  |Fixpoint|Signal line*
0   0   0   0   0   0   0   0     = 0x00    5
0   0   0   0   0   1   1   0     = 0x06    4
0   0   0   0   1   1   1   0     = 0x0E    3
0   0   0   1   1   0   1   0     = 0x1A    2
0   0   1   1   0   1   1   0     = 0x36    1
0   1   0   1   1   1   0   1     = 0x5D    0
1   1   1   1   1   1   1   1     = 0xFF    6

*The numbers of the signal lines have been chosen to match the numbers of the selector bits!

Every possible "Fixpoint" is compared to the 8 bit counter value keeping only one(!) of the seven signal lines "high" in case of equality. To be more exact: The lines are pulled down in case of inequality.

The 5 selector bits for the logarithmic table are generated by feeding each of the the signal lines 0 to 4 into a simple flip-flop to "set" it. Output of the flip-flops is active "low"!) Additionally both adjacent signal lines are also fed into the flip-flip to "reset" it; resulting in a "high" level output.


Example:

Selector bit|
Signal line | "Reset" signal lines
0             1, 6
1             0, 2
2             1, 3
3             2, 4
4             3, 5

Sustain value
-------------

The 4 bits from the sustain value register are connected to:

Sustain bit 0 to Counter bit compare 0 + 4
Sustain bit 1 to Counter bit compare 1 + 5
Sustain bit 2 to Counter bit compare 2 + 6
Sustain bit 3 to Counter bit compare 3 + 7

So they are (as already known and verified through tests) compared like:

0x0 (S) == 0x00 (Cnt)
0x1 (S) == 0x11 (Cnt)
0x2 (S) == 0x22 (Cnt)
.
.
.
0xE (S) == 0xEE (Cnt)
0xF (S) == 0xFF (Cnt)

Plogue chipsounds running on VMachine Hardware

Composite vs RGB (SNES)

VIC-20 (CR)'s bad waveform clipping.

Poor MOS6560, it could so much better outside a VIC-20!

For chipsounds 1.0, I got the help of eslapion from the Denial forum to analyze the RC filter output of the VIC-20. Eslapion got me to learn how to understand the link between the resistors and capacitors and how to extract the exact low pass filter coefficients that was needed to implement in DSP. It simply turns out to be a 1.6Khz one pole low pass filter. (third party confirmation here).

Such a low cutoff makes for one REALLY DULL sounding chip

But that's not the only sound coloration present on the real VIC. There is also quite a bit of clipping going on when all voices play at once. This clipping does add quite a bit of character when there are a combination of more than one pulse voices playing, as well as the noise channel.

Lets look at the phenomenon on a scope:







Quoting Eslapion:

"The specs sheet of the 6560 specifies the audio pin has an output impedance of 1kOhms. This impedance interacts with C9 (0.01uF) to create a 1st order low pass filter of 15.9kHz.

This is followed by R8 (1kOhms) and C10 (0.1uF) which combine to create another 1st order filter of 1.6kHz.

In other words, these two filters combine to provide a 20dB/decade attenuation from 1.6kHz to 16Khz and then 40 dB/decade above that.

Q1 acts as a current multiplier to ensure enough driving power on the final output but, as we can see, R27 acts as a pull down only device and there is no biasing. The output of the 6560, multiplied by Q1 pulls up and R27 pulls down.

Add to that, the fact that the volume control induces a DC offset.

DC offset values for register 36878: (average) (values picked after R8)
0:4.06
1:4.17
2:4.28
3:4.41
4:4.53
5:4.64
6:4.76
7:4.86
8:4.95
9:5.06
10:5.17
11:5.27
12:5.37
13:5.47
14:5.56
15:5.64

This DC offset, on the final output, is cancelled out by C11 which combines with R27 to create a high pass filter of 339Hz with 20dB/decade attenuation. Therefore, when no sound is playing there is no way to tell on the output that a DC level forms on the base input of Q1 as the volume is increased.

If the 6560 is going to pull up in DC when the volume rises then the maximum excursion of a sound wave is limited by a combination of the pull down current (set by R27) as well as the voltage feed which, in this specific case is 0 to 5volts. When the volume goes above a certain level, clipping of the positive side of the wave occurs as the 6560 attempts to pull up above 5V.

This clipping effect is even more intense when playing multiple voices simultaneously as their combined excursion adds to each other.

Obviously, the smallest amount of clipping occurs when the DC offset provides a DC level of 2.5V on the base of Q1. This is achieved when setting the volume level to 8.

Now, looking at the rise and fall time of the signal taken directly at pin 19 of the 6560, we see that the rise time is much greater than the fall time. Considering the effect of Q1, as seen by the 6560, R27 should appear as about 100 times greater.

This asymetrical voltage change is caused by the diode in the base of Q1 which can draw current on voltage rise but not give back any on the falling edge. This causes C9 and C10 to charge slowly as Q1 "steals" current used to charge these capacitors but not restore it when the 6560 discharges them. This assymetrical time constant is less dramatic on the actual output as C10 and C11 are still discharging at the very instant C9 appears to be empty."


Heres an audio capture form the final analog output of the VIC-20 CR:

Clipping sound

So there you have it!

The VIC emulation im working on for 2.0 includes that clipping and filtering and is starting to sound very close to the real thing.... stay tuned