|Sound Interface Device|
1.) Triangle Wave, Sawtooth Wave, Pulse Wave, Combined Wave or White Noise
The Sound Interface Device (SID) is an analog synthesizer chip designed by Robert "Bob" Yannes for the Commodore 64 computer. It came in two models, 6581 and 8580.
On each of the 3 voices, you can choose
- pitch up to 3848 Hz (A♯7),
- ADSR, and
- waveform: triangle, sawtooth, pulse, noise, or combinations of the first three (though with differences per model). The combined triangle and pulse wave resembles a guitar or slap bass. The duty cycle of each pulse-based wave can be set with a precision of 0.0244140625%, ring modulation added to triangle-based waves, and Hard Sync to any wave. Percussion was initially made using one plain noise, but from 1985 to 1987, Rob Hubbard and We M.U.S.I.C. pioneered rapidly switching waveform for more thumping percussion. However, it was somewhat lacking compared to the "deep" percussion of POKEY or AY chips, and some Commodore 64 composers like Adam Gilmore have expressed their concerns about it.
One single low-pass, band-pass, high-pass or band-reject filter can be applied to any combination of voices. A resonance from 0 to 15 and a cutoff frequency from 0 to 2047 can be chosen. Unfortunately, these values are not in any standard like Hz; worse even, they vary with every SID chip and even every computer it is socketed on. As a result, gamers often heard sections muffled or louder than intended. Most composers, including Rob Hubbard, Martin Galway, Chris Hülsbeck, Jeroen Tel and even Yannes expressed their concern in interviews, and some, like Ben Daglish, Fred Gray and Chris Grigg, used the filter very sparingly or purposely avoided it. A very few games allow the gamer to alter the games' filter settings.
Both SID models can play samples in different ways, unforgettably the speech in Impossible Mission (C64). However, samples are always very CPU-intensive, 6000 Hz on average and usually limited to title screens.
Besides sound, SID also provides the potentiometers of connected mice or paddles. The current output of voice 3 can be read back: Noise was often used as random number generator, and the ADSR output was sometimes written back to a modulatable register, usually the cutoff frequency.
The slap bass sounds thin, and the other combined waves very quiet.
Changing SID's master volume and toggling the filter produce a click; the greater the change, the louder. A very few programmers worked around it by changing the volume gradually rather than at once. On the upside, it allows to play 4-bit PCM samples on a 4th voice. In the late 1980s, it was very popular to play digi drums over 3 SID voices.
When arrangers wanted to turn the filter off without click, they actually left it on and instead set the cutoff frequency to a bound where it made the least difference.
In 1986, the only major revision of SID appeared. It started prevailing in the early 1990s.
The slap bass sounds fuller and the other combined waves much louder.
The filter has become consistent. However, the lowest possible cutoff frequency has become mute, hence some songs arranged for 6581 sound choppy on 8580.
Clicks have become quiet or even mute. This can be fixed by soldering a resistor, but by 1997, methods to reliably play samples replacing one of the 3 SID voices were being discovered.
8580 arrangers forgot about the clicks, hence some of their songs click annoyingly on 6581.
This section demonstrates the above differences using recordings from up to 4 real C64s (labelled R2, R3, R4 and R5) by Stone Oakvalley's Authentic SID Collection (SOASC=):
|Feature||Song||Game||6581 recording||8580 recording|
|Samples on 4th voice||Theme||Arkanoid (C64)||R2||R5|
|Slap bass||Underground||Labyrinth: The Computer Game (C64)||R2||R5|
|Other combined waves||Theme||Impossible Mission II (C64)||R2||R5|
|Filter minimum||Main Menu||The Great Giana Sisters (C64)||R4||R5|
Listen to the bass of High Score - Wizball (C64):
Do you hear the bass go deeper and deeper with every C64? Now this may be matter of taste and no big deal, but listen to Insects In Space by the same composer and skip to the melody at 1:06:
Do you hear how in the first recording, the melody has a wah-wah sound, and in the second, it almost hasn't? Martin Galway must have put some effort in the wah-wah and wanted it to be heard. It could also mean that Stone Oakvalley's R4 is closest to Galway's C128D.
Sometimes, the melody itself can vanish or stick out unbearably:
Though not as unbearable as one of the following at 0:20, 0:44 and 1:15:
Sometime in 1982 or 1983, David Thiel bought 6581s from Commodore and had two wired on an arcade sound board. While he was testing, Gottlieb's buyer told him that Commodore preferred all 6581s for themselves.
To this day, the analog parts of SID are not fully researched. For example, emulators give Title - Bad Cat unrealistic volume differences.
The most popular and accurate SID emulator is reSID (in C++ and under GPL) and used in most C64 emulators, SID players and cross-platform editors. Many of them allow to choose the clock frequency, model and filter (called bias, curve, strength or distortion). reSID is still being developed directly as part of the VICE emulator.