Ah, a nice question!:-)
The Commodore C64, as with the SX64 (Commodore 64 – Wikipedia) SID Chip (SID stands for Sound Interface Device) is a so-called .Custom Chip with the designation MOS6581, which was primarily responsible for sound output within the C64 computer, but also performed some I/O (single-output) tasks (seebelow).
By running it as its own custom chip, it can run the sound output largely independently of the main processor (MOS Technology 6510 – Wikipedia) and thus relieves it greatly from the hard real-time requirement of a sound output.
Small anecdote on the subject: The Sinclair ZX Spectrum (see also Sinclair ZX Spectrum 鈥?Wikipedia) of the first generation did not yet have a “Custom Chip” for its sound output, but its “Beeper“.
This was only connected to an output port of the internal gate array (ULA). but which itself did not produce waveforms.
Thus, the main processor (Zilog Z80 鈥?Wikipedia) of the Spectrum was responsible for the sound output, which wasted enormous computing time during a sound output.Therefore, spectrum games had only very rudimentary sound.
Only the Spectrum+ then had installed an AY-3-8910 鈥?Wikipedia soundchip from General Instruments / Yamaha, which could produce sound autonomously.Wherethis is the chip. SID was very limited (only 3 rectangular generators + 3 noise generators) + 1 ADSR Envelope Generator. So NO ring modulator as with the SID
But the SID could not only output sound, but also took care of some one-output tasks of the C64.
Concretely, the tasks of the SID (source SID – C64-Wiki):
- 3 sound generators (voices), frequency range from 0-4 kHz (16-bit resolution)
- 4 waveforms (saw tooth, triangle, rectangle with adjustable pulse width, “white” noise)
- 3 amplitude modulators, range up to 48 dB
- 3 envelope generators
- Synchronization of oscillators
- Ring modulation
- Programmable filters (low, belt, high pass, notch filter)
- Total volume setting in 16 levels
- 2 A/D converters (8 bits, low frequency)
- Random generator
- Audio input (filterable)
Here as an overview a block diagram of the SID (Source: File:SID Block Diagram.png)
Through the ring modulation – synthesizer Wiki he got his very characteristic sound.
But listen for yourself 😉
See also SID – C64-Wiki
It is also interesting that there is a small electronics (SIDFX) for the C64, on which you can plug 2 SID chips and thus extend the C64 from 3 to 6 hardware audio channels:
By the way, there is a SID as FPGA (Field Programmable Gate Array) replica!