VIC

MOS Technology VIC (6560/6561)

Overview

The VIC (Video Interface Chip), known formally as the MOS Technology 6560 in its NTSC version and the 6561 in its PAL version, is the combined video and sound chip at the heart of the Commodore VIC-20, released in 1981. As the machine's very name suggests, VIC stood for "Video Interface Computer," with the chip serving as the defining piece of hardware behind Commodore's first computer to sell more than a million units.

The chip was originally designed by Al Charpentier in 1977 for entirely different applications, including low-cost CRT terminals, biomedical monitors, and arcade-style video game hardware, but Commodore initially struggled to find a market for it. Before the VIC finally found its home in the VIC-20, engineer Robert Yannes, who would later go on to design the SID chip for the Commodore 64, fed improvements back into the design, most notably a stronger sound generator, refining it into the chip that would ultimately ship in Commodore's breakthrough machine.

Like several other integrated video/audio chips of its era, the VIC combines graphics generation and sound output on a single device. Its audio section provides three square-wave tone voices plus one white noise channel, arranged so that each of the three tone voices covers a different pitch range, informally referred to as bass, alto and soprano, alongside the separate noise generator used mainly for percussive effects.

Each voice's pitch is set by writing a value into its corresponding register, with the resulting frequency calculated by the system clock divided by 127 minus the register value. This particular divider arrangement produces a musical scale that is subtly, and audibly, out of tune compared to an ideal equal-tempered scale, a quirk widely credited with giving the VIC-20 its distinctive, slightly rough "dusty old piano" character that fans of the machine still recognise today.

Technical characteristics

• Manufacturer: MOS Technology
• Model: 6560 (NTSC) / 6561 (PAL)
• Channels: 3 square-wave tone voices (bass, alto, soprano) + 1 white noise channel
• Synthesis: Simple square-wave tone generation via frequency dividers, plus noise
• Envelope: None; each voice is simply on or off, with a single global volume control
• Volume: One shared master volume register, adjustable in 16 steps (0–15)
• Filter: None
• Tonal range: Roughly 38 usable notes per voice, with a mildly out-of-tune scale due to the chip's divider design
• Additional function: Also generates the VIC-20's video output (text and graphics modes)
• Computer: Commodore VIC-20

Composing with the VIC

Because the VIC offers no dedicated envelope hardware and only a single shared volume register affecting all voices at once, composers had comparatively little independent control over dynamics compared to later chips like the SID. Each of the three tone voices could simply be switched on or off at a given pitch, using the top bit of its register as an on/off flag, leaving the remaining seven bits to determine the note.

This shared, all-or-nothing approach to volume meant that fading a single voice in or out while others continued playing was not directly possible in hardware; any illusion of independent dynamics had to be created through rapid note or channel switching handled entirely in software. Combined with the chip's characteristically imprecise tuning, this gave VIC-20 music a raw, unmistakably lo-fi quality distinct from the cleaner, more controllable sound of its C64 successor.

Despite these constraints, the VIC-20's sound system was put to memorable use throughout its short but influential commercial life, and it remains a subject of interest within retrocomputing and chiptune circles today, with musicians occasionally revisiting the chip specifically to capture its uniquely imperfect, "dusty" tuning characteristics that later, more precise chips deliberately eliminated.

Historical importance

The VIC holds a notable place in Commodore's history as the direct predecessor to the far more famous VIC-II graphics chip used in the Commodore 64, and its sound architecture represents an important intermediate step between the earliest, most primitive computer sound systems and the far more sophisticated SID chip that would follow it just a year later. Studying the VIC's design also offers useful context for understanding the design lineage that led to the SID, since several members of the same MOS Technology engineering team, including Robert Yannes, worked on both chips.

Further reading

• MOS Technology VIC overview (Wikipedia)
• VIC-20 sound technical reference (Battle of the Bits Lyceum)
• Commodore VIC-20 technical overview (Commodore Info Page)