Samstag, 3. Mai 2008|
SID MOS 6581
RegisterTable C64 MIDI-Interface
The Sound Interface Device
was and still is the famous Sound-Chip of the even more infamous
Commodore C64 Home-Computer dated back in 1982.
This little 28 Pin Chip is remarkable in a few aspects:
The Cut-Off-Frequency rather depends on the external applied Capacitors. Recommended was always 2.2nF, but 470pF were used with the 6581, i.e. the smaler the capacity the less the (mainly LP) filtering !
Please note, that changing the capcitors from 0,47nF to 2,2nF does not "improve" the SIDs sound quality in any way ! You rather apply more low-pass-filtering to any existing Tune that - however - was not composed/programmed for that adjustment.
Furthermore you have a Resonance parameter which actually is more sth like a
signal feedback, which
results more in a kind of distortion, than the usual self-oscillation.
Nevertheless care should be taken as some combinations won't result in anything audible. Especially combinations with NOISE will lock up the corresponding OSC by filling it with
zeros - ergo: no sound at all.
MOS SID 6582
In 1986 Commodore's own Semiconductor company (confusingly called) "MOS" introduced a HMOS version of the SID called 6582. In contrast to the 6581 it needs only 9V for the Audio-Out-Amp instead of 12V. Though I haven't fully compared the two datasheets, this seems to be the only difference. I guess as a different type of MOS-technology has been used the sound might be different in nuances. Presumably on the filters. Personally I've never seen, nor knowingly heard a 6582 (yet).
CSG SID 8580
I assume that the 6582 was renamed, or even further re-engineered into the 8580 which was used in the white C64II and in later versions of the C128, too. Another reason might be that around that time MOS was changed into CSG (Commodore Semiconductor Group - this company has some history of it's on now e.g. law suits because of groundwater-contamination before it went into liquidation and was kind of re-opened as GMT Electronics by the old CSG management. The decontamination was still going on in Sep 2000)
Gone is the "little-crack-while-changing-volume" flaw which was extensively used to "play" the (usual two, or four bit) PCM-Samples.
Fix: solder a Resistor of about 330KOhm between audio-in-pin (26) and ground (e.g. 14). This introduces DC in the SID output that result in little cracks when switched by the VOL-Register.
Obviously this technical adjustment generally reduced the DC and low-frequency leakage. According to a spectral-analyses I did, it looks like the SID8580 got an additional 12dB High-pass filter on the output-amp at a cut-off-frequency of about 100Hz.
(see: 6582 Datasheet (1.5MB pdf) )
The SID 6581 R3 vs. R4 myth
Some ppl on the cbm newsgroups seem to experience "remarkable" differences in the production versions of the MOS SID 6581, MOS 6581R3/R4, CSG SID.
Excluding the "R5" CSG SID 8580, I have made parallel recordings with serveral SIDs of different week/years productions from different factories and revisions. Personally I can not tell a R4 from a R3, or a "R-none". At times they have output differences of about 2dB. Or different saturation and distortion, if more than one oscillator is run through the NMOS/HMOS FET filters, other than that I did/do not perceive any differences.
SID Comparisons here you find audio files that have different SIDs on the right and left channel. If you hear a simple monosignal "in the middle of your head" both SIDs sound exactly the same - without any difference. Also the signals show as a straight line on a vectorscope.
Some of my pseudo-stereo SID recordings (sSID):
The idea for an own Synthesizer based on the C64 Soundchip must have happened to me around 1990. However at this time I lacked the technical skills. I recall that my first few lines of assembler code did nothing more but driving the 2x20 LCD - this was back in December 1996 *ähem*
Update Samstag, 5. Oktober 2002
The hardware in general has been finished quite a long time ago, the basic problem is the development of the SOS (SID Operating System). Written on my Atari ST tranfered into the Flash-PROMs via my DIY ROM-PORT-Prommer and then discovering general addressing errors.
However some work on the first menus have been done, photos added and at the moment I can control 6 oscillators Polyphony via MIDI - mp3's added.
Update Mittwoch, 26. Mai 2004
At the beginning of 2003 I experienced quite some wild troubles with my ROM-code until I figured, that there are contact problems with the PLCC Flash-Proms in their sockets...
Now I finished quite some Menu and basic editing code the last 3 days and put in the last two SID's i.e. the SID'esizer now operates 12 polyphonic voices. However the MIDI-Mode ist still Single. Next will be my fist attempts on a LFO, to modulate the SID's parameters. OGGs added.