THE CREATION OF THE SID-CHIP
This time we'll discuss the SID - the remarkable soundchip that
gives the C64 that fantastic sound.
When "Project C64" was first discussed at Commodore in Jan 1981,
the intention was to create a game computer. Those plans were
changed during the project and instead Commodore decided to build
a true home computer.
A good game computer must be able to generate good sound. And the
soundchip in the C64 can handle 3 voices at the same time spanning
an unusually wide range of frequencies.
The work on the C64 was to be carried out by two groups. One to
develop the graphics chip and one for the SID. (SID is short for
Sound Interface Device).
The work on the SID was begun in spring 1981 by Robert Yannes, a
young engineer who had been responsible for the design behind
the VIC-20. His team consisted of himself, two technicians and a
CAD-operator.
- I had worked with synthesizers and I wanted a chip like
those
in a synthesizer, Robert Yannes stated.
According to Yannes noone in the project team knew how to carry
out the project.
- Apart from the basic research, that I'd been doing since
higschool,
the actual development of the SID took just 4-5 months.
Yannes high expectations on the SID was the reason why the chip
got unusually high quality compared to those found in videogames.
The SID has a more precise frequency control and the independent
envelope-generator creates a more intense timbre.
The envelope-function enables the SID to imitate an instrument
more correct than other chips. Envelope is a tool for controlling
the volume-level on a voice without using the very volume-control.
Yannes originally incorporated a table in the SID, enabling it to
interpret musical tones into frequency-datas. This feature was
later removed, since it was too expensive.
The real capacity of the SID-chip is not fully known even today -
not even by its constructors! The reason for this is that the
the technical specification written while the chip was developed
accidently got incorrect.
An example of this is that the documentation states that the chip
can perform an logical AND on different waveforms.
Another error is in the filter-function.
- The filter was the last feature we added in, and there
was no
time to make it perfect, Yannes tells us. Computer simulations
told us the filter wouldn't work out very well. And it didn't.
I had no chance to correct the errors.
These errors in the documentation has many programmers unable to
use all possibilities of the chip.
Christer Rindeblad
