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Zee

Fri Jan 30 2009, 10:29PM

Registered Member #877 Joined: Tue Jul 03 2007, 09:15PM
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Posts: 21

Is anyone familiar to how things built into silicon (old computer chips to be precise) works?

The image above is a small (REALLY small) portion of the beloved soundchip of the Commodore 64, the 6581 SID. A so-called Voltage Controlled Resistor in the final mixer/filterstage of the chip.

There are people who are trying to understand, for the sake of emulation, the exact details how the audiosignal, generated inside the chips main waveform/envelope generators, is treated here before it leaves the chip and goes to your tv/amplifier.

What is also missing from the puzzle, are the values of the resistors (R), or the knowledge how to approximate what their values on the silicon could be.

Miscellaneous information:
-------------------------------
- 6-7um NMOS manufacturing process
- Mixed digital/analog in the same silicon
- Audiosignals around the silicon float at ~6 volts.
- Cutoff-signal is believed to be between 5-12 volts depending how the cutoff-registers of the chip are programmed during use

Wolfram

Fri Jan 30 2009, 11:26PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Unfortunately, I can't contribute with any useful information on the subject, but I was wondering if you have any links to where this is being discussed. I'm a big fan of the SID chip, and I actually have one on my desk now playing old SID tunes

, the SID itself is on the sub-board that is slightly visible in the photo.

Anders M.

Steve Conner

Sat Jan 31 2009, 12:53PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

From

wrote ...
The Filter was a classic multi-mode (state variable) VCF design. There was no way to create a variable transconductance amplifier in our NMOS process, so I simply used FETs as voltage-controlled resistors to control the cutoff frequency. An 11-bit D/A converter generates the control voltage for the FETs (it's actually a 12-bit D/A, but the LSB had no audible affect so I disconnected it!)

Sounds like plenty enough information to clone or emulate it. The exact values of the components are probably irrelevant, because they'll depend on the characteristics of the FETs on the chip. If I was doing it, I'd use discrete JFETs as a first attempt, and find the resistor values by trial and error to match the cutoff and resonance sweep ranges of a real SID.

If I was emulating it in software, I'd just make a state-variable filter and bung some square-law nonlinearities into it. Although having said that, the other two "R"s in the top right-hand corner might be for linearization.

If you really wanted to know the value of "R", you'd need to find what the resistivity (in "ohms per square") of the yellow stuff was. Again, this is a parameter of the particular IC manufacturing process the SID was made on.

Zee

Sun Feb 01 2009, 03:23AM

Registered Member #877 Joined: Tue Jul 03 2007, 09:15PM
Location:
Posts: 21

http://oms.wmhost.com/misc/ contains all the info I have gathered/produced so far (some are outdated/incorrect). One htm-link there leads to other areas, including emulation development, which aims to be as precise as it could be done, which is why the knowledge of the exact behaviour of the VCR is preferred. The current level of emulation makes some tunes play nearly identical compared to the real chip, while others sound more or less incorrect. While changes to the emulation code improves other tunes, it also breaks some. Ofcourse average listener not familiar to the SID propably never notices a difference. :)

Pretty much every bit of info that is online regarding the SID has been read over and over, but there are still errors, even on that interview of Mr.Yannes. The cutoff D/A has the MSB disconnected, not the LSB as he recalls, and the MSB state is also hardwired as high.

It also seems that the variations between different batches/revisions of the 6581 come from the manufacturing, since the logic itself is the same in each revision, minute differences only being in the buffering/protection of the pin connections to the outside world.

Sulaiman

Sun Feb 01 2009, 07:16PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Have a look at fig 6, a) here

for a typical fet vcr

Zee

Fri Feb 06 2009, 04:57PM

Registered Member #877 Joined: Tue Jul 03 2007, 09:15PM
Location:
Posts: 21

Would be interresting to try and build the filter/mixer to fiddle with, but I really have no idea what kind of FETs I should be using for something like that.

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